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The African humid period (AHP) (also known by other names) is a climate period in Africa during the late Pleistocene and Holocene geologic epochs, when northern Africa was wetter than today. The covering of much of the
Sahara , photo = Sahara real color.jpg , photo_caption = The Sahara taken by Apollo 17 astronauts, 1972 , map = , map_image = , location = , country = , country1 = , ...
desert by grasses, trees and lakes was caused by changes in Earth's orbit around the Sun; changes in vegetation and dust in the Sahara which strengthened the African monsoon; and increased
greenhouse gas A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...
es. During the preceding
Last Glacial Maximum The Last Glacial Maximum (LGM), also referred to as the Late Glacial Maximum, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent. Ice sheets covered much of Northern North America, Northern Eur ...
, the Sahara contained extensive
dune A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, f ...
fields and was mostly uninhabited. It was much larger than today, but its lakes and rivers such as
Lake Victoria Lake Victoria is one of the African Great Lakes. With a surface area of approximately , Lake Victoria is Africa's largest lake by area, the world's largest tropical lake, and the world's second-largest fresh water lake by surface area after ...
and the
White Nile The White Nile ( ar, النيل الأبيض ') is a river in Africa, one of the two main tributaries of the Nile, the other being the Blue Nile. The name comes from the clay sediment carried in the water that changes the water to a pale color. ...
were either dry or at low levels. The humid period began about 14,600–14,500 years ago at the end of
Heinrich event 1 Heinrich may refer to: People * Heinrich (given name), a given name (including a list of people with the name) * Heinrich (surname), a surname (including a list of people with the name) *Hetty (given name), a given name (including a list of p ...
, simultaneously to the Bølling–Allerød warming. Rivers and lakes such as
Lake Chad Lake Chad (french: Lac Tchad) is a historically large, shallow, endorheic lake in Central Africa, which has varied in size over the centuries. According to the ''Global Resource Information Database'' of the United Nations Environment Programme, ...
formed or expanded, glaciers grew on
Mount Kilimanjaro Mount Kilimanjaro () is a dormant volcano in Tanzania. It has three volcanic cones: Kibo, Mawenzi, and Shira. It is the highest mountain in Africa and the highest free-standing mountain above sea level in the world: above sea level and ab ...
and the Sahara retreated. Two major dry fluctuations occurred; during the Younger Dryas and the short
8.2 kiloyear event In climatology, the so-called "8.2-kiloyear event" was a sudden decrease in global temperatures that occurred approximately 8,200 years before the present ( BP), that is, 6,251 BC. It defines the start of the Northgrippian age in the Holocene ...
. The African humid period ended 6,000–5,000 years ago during the Piora Oscillation cold period. While some evidence points to an end 5,500 years ago, in the
Sahel The Sahel (; ar, ساحل ' , "coast, shore") is a region in North Africa. It is defined as the ecoclimatic and biogeographic realm of transition between the Sahara to the north and the Sudanian savanna to the south. Having a hot semi-arid c ...
, Arabia and East Africa, the period appears to have taken place in several steps such as the
4.2-kiloyear event The 4.2-kiloyear (thousand years) BP aridification event (long-term drought) was one of the most severe climatic events of the Holocene epoch. It defines the beginning of the current Meghalayan age in the Holocene epoch. Starting around 2200&n ...
. The AHP led to a widespread settlement of the Sahara and the Arabian Deserts, and had a profound effect on African cultures, such as the birth of the Ancient Egyptian civilization. People in the Sahara lived as
hunter-gatherer A traditional hunter-gatherer or forager is a human living an ancestrally derived lifestyle in which most or all food is obtained by foraging, that is, by gathering food from local sources, especially edible wild plants but also insects, fungi, ...
s and domesticated cattle, goats and sheep. They left archaeological sites and artifacts such as one of the oldest ships in the world, and rock paintings such as those in the Cave of Swimmers and in the Acacus Mountains. Earlier humid periods in Africa were postulated after the discovery of these rock paintings in now-inhospitable parts of the Sahara. When the period ended, humans gradually abandoned the desert in favour of regions with more secure water supplies, such as the
Nile Valley The Nile, , Bohairic , lg, Kiira , Nobiin: Áman Dawū is a major north-flowing river in northeastern Africa. It flows into the Mediterranean Sea. The Nile is the longest river in Africa and has historically been considered the longest rive ...
and Mesopotamia, where they gave rise to early complex societies.


Research history

Herodotus in 440 BC and
Strabo Strabo''Strabo'' (meaning "squinty", as in strabismus) was a term employed by the Romans for anyone whose eyes were distorted or deformed. The father of Pompey was called "Pompeius Strabo". A native of Sicily so clear-sighted that he could see ...
in 23 AD discussed the existence of a greener Sahara, although their reports were at first questioned owing to their anecdotal nature. In 1850 the researcher Heinrich Barth discussed the possibility of past climate change leading to increased wetness in the Sahara after discovering
petroglyph A petroglyph is an image created by removing part of a rock surface by incising, picking, carving, or abrading, as a form of rock art. Outside North America, scholars often use terms such as "carving", "engraving", or other descriptions ...
s in the Murzuq Desert, and further discoveries of petroglyphs led desert explorer László Almásy to coin the concept of a ''
Green Sahara The African humid period (AHP) (also known by other names) is a climate period in Africa during the late Pleistocene and Holocene geologic epochs, when northern Africa was wetter than today. The covering of much of the Sahara desert by grasses, ...
'' in the 1930s. Later in the 20th century, conclusive evidence of a past greener Sahara, the existence of lakes and higher Nile flow levels was increasingly reported and it was recognized that the Holocene featured a humid period in the Sahara. The idea that changes in Earth's orbit around the Sun influence the strength of the monsoons was already advanced in 1921, and while the original description was partly inaccurate, later widespread evidence for such orbital controls on climate was found. At first it was believed that humid periods in Africa correlate with glacial stages ("
pluvial hypothesis In geology and climatology, a pluvial is either a modern climate characterized by relatively high precipitation or an interval of time of variable length, decades to thousands of years, during which a climate is characterized by relatively high ...
") before
radiocarbon dating Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon. The method was dev ...
became widespread. The development and existence of the African humid period has been investigated with archaeology, climate modelling and
paleoproxies In the study of past climates ("paleoclimatology"), climate proxies are preserved physical characteristics of the past that stand in for direct meteorological measurements and enable scientists to reconstruct the climatic conditions over a longer ...
, with archaeological sites,
dune A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, f ...
s and deposits left by lakes,
aeolian Aeolian commonly refers to things related to either of two Greek mythological figures: * Aeolus (son of Hippotes), ruler of the winds * Aeolus (son of Hellen), son of Hellen and eponym of the Aeolians * Aeolians, an ancient Greek tribe thought to ...
deposits and
leaf wax Epicuticular wax is a coating of wax covering the outer surface of the plant cuticle in land plants. It may form a whitish film or bloom on leaves, fruits and other plant organs. Chemically, it consists of hydrophobic organic compounds, mainly str ...
in the sea and wetlands playing an important role.
Pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...
, lake deposits and former levels of lakes have been used to study the ecosystems of the African humid period, and
charcoal Charcoal is a lightweight black carbon residue produced by strongly heating wood (or other animal and plant materials) in minimal oxygen to remove all water and volatile constituents. In the traditional version of this pyrolysis process, cal ...
and leaf impressions have been used to identify vegetation changes. The time 6,000 years ago has received particular attention, especially since that period of the AHP has been used as an experiment in the Paleoclimate Modelling Intercomparison Project.


Research issues

While the precipitation changes since the last glacial cycle are well established, the magnitude and timing of the changes are unclear. Depending on how and where measurements and reconstructions are made, different beginning dates, ending dates, durations and precipitation levels have been determined for the African humid period. The amounts of precipitation reconstructed from
paleoclimate Paleoclimatology (British spelling, palaeoclimatology) is the study of climates for which direct measurements were not taken. As instrumental records only span a tiny part of Earth's history, the reconstruction of ancient climate is important to ...
records and simulated by climate modelling are often inconsistent with each other; in general, the simulation of the Green Sahara is considered a problem for earth system models. Erosion of lake sediments and carbon
reservoir effect Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon. The method was ...
s make it difficult to date when they dried up. Vegetation changes by themselves do not necessarily indicate precipitation changes, as changes in seasonality, plant species composition and changes in land use also play a role in vegetation changes.
Isotope ratio The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...
s such as the hydrogen/ deuterium ratio that have been used to reconstruct past precipitation values likewise are under the influence of various physical effects, which complicates their interpretation.


Terminology

Earlier humid periods are sometimes known as "African humid periods" and a number of dry/wet periods have been defined for the Central Africa region. In general, these types of climate fluctuations between wetter and drier periods are known as " pluvials" and " interpluvials", respectively. Because the AHP did not affect all of Africa, Williams ''et al.'' 2019 recommended that the term be dropped. Other terms that have been applied to the Holocene AHP or correlative climate phases are "Holocene humid period", which also covers an analogous episode in Arabia and Asia; "early to mid-Holocene humid episode"; "Holocene Pluvial"; "Holocene Wet Phase"; "" in Central Africa; "Makalian" for the Neolithic period of northern Sudan; "Nabtian Wet Phase" or "Nabtian period" for the 14,000–6,000 humid period over the Eastern Mediterranean and Levant; "
Neolithic pluvial The Neolithic period, or New Stone Age, is an Old World archaeological period and the final division of the Stone Age. It saw the Neolithic Revolution, a wide-ranging set of developments that appear to have arisen independently in several parts ...
"; "Neolithic Subpluvial"; "Neolithic wet phase"; "" of the Western Sahara 6,500 – 4,000 years before present; "Subpluvial II" and "" in the Central Sahara 14,000 – 7,500 years before present. The terms "" and have been applied to the dry period in the
last glacial maximum The Last Glacial Maximum (LGM), also referred to as the Late Glacial Maximum, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent. Ice sheets covered much of Northern North America, Northern Eur ...
, the latter is equivalent to the "Kanemian"; "Kanemian dry period" refers to a dry period between 20,000 and 13,000 years before present in the
Lake Chad Lake Chad (french: Lac Tchad) is a historically large, shallow, endorheic lake in Central Africa, which has varied in size over the centuries. According to the ''Global Resource Information Database'' of the United Nations Environment Programme, ...
area.


Background and beginning

The African humid period took place in the late Pleistocene and early-middle Holocene, and saw increased precipitation in Northern and Western Africa due to a northward migration of the tropical rainbelt. The AHP is the most profound climate change of the low latitudes during the last 100,000 years and stands out within the otherwise relatively climatically stable Holocene. It is part of the so-called
Holocene climatic optimum The Holocene Climate Optimum (HCO) was a warm period that occurred in the interval roughly 9,000 to 5,000 years ago BP, with a thermal maximum around 8000 years BP. It has also been known by many other names, such as Altithermal, Climatic Optimu ...
, during which summers in the Northern Hemisphere were warmer than today. Liu ''et al.'' 2017 subdivided the humid period into an "AHP I" which lasted until 8,000 years ago, and an "AHP II" from 8,000 years onward, with the former being wetter than the latter. The African humid period was not the first such phase; evidence for about 230 older such "green Sahara"/wet periods exist going back perhaps to the first appearance of the Sahara 7–8 million years ago, for example during Marine Isotope Stage 5 a and c. Earlier humid periods appear to have been more intense than the AHP of the Holocene, including the exceptionally intense
Eemian The Eemian (also called the last interglacial, Sangamonian, Sangamonian Stage, Ipswichian, Mikulin, Kaydaky, penultimate,NOAA - Penultimate Interglacial Period http://www.ncdc.noaa.gov/global-warming/penultimate-interglacial-period Valdivia or Ri ...
humid period which provided the pathways for early humans to cross Arabia and Northern Africa and which, together with later moist periods, has been linked to expansions of the Aterian populations. Such humid periods are usually associated with
interglacial An interglacial period (or alternatively interglacial, interglaciation) is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age. The current Holocene in ...
s, while glacial stages correlate to dry periods. The Bølling-Allerød warming appears to be synchronous with the onset of the African humid period as well as to increased humidity in Arabia. Later, in the Blytt–Sernander sequence the humid period coincides with the Atlantic period.


Conditions before the African humid period

During the
Last Glacial Maximum The Last Glacial Maximum (LGM), also referred to as the Late Glacial Maximum, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent. Ice sheets covered much of Northern North America, Northern Eur ...
, the Sahara and Sahel had been extremely dry with less precipitation than today as reflected by the extent of dune sheets and water levels in
closed lake An endorheic lake (also called a sink lake or terminal lake) is a collection of water within an endorheic basin, or sink, with no evident outlet. Endorheic lakes are generally saline as a result of being unable to get rid of solutes left in the l ...
s. The Sahara was much larger, extending farther south, a difference of 5° latitude. Dunes were active much closer to the equator, and rainforests had retreated in favour of
afromontane The Afromontane regions are subregions of the Afrotropical realm, one of the Earth's eight biogeographic realms, covering the plant and animal species found in the mountains of Africa and the southern Arabian Peninsula. The Afromontane regions ...
and
savannah A savanna or savannah is a mixed woodland-grassland (i.e. grassy woodland) ecosystem characterised by the trees being sufficiently widely spaced so that the Canopy (forest), canopy does not close. The open canopy allows sufficient light to rea ...
landscapes as temperatures, rainfall, and humidity decreased. There is little and often equivocal evidence of human activity in the Sahara or Arabia at that time, reflecting its drier nature. The aridity during the Last Glacial Maximum appears to have been the consequence of the colder climate and larger polar ice sheets, which squeezed the monsoon belt to the equator and weakened the West African Monsoon. The atmospheric water cycle and the Walker and Hadley circulations were weaker as well. Exceptional dry phases are linked to Heinrich events when there are a large number of
iceberg An iceberg is a piece of freshwater ice more than 15 m long that has broken off a glacier or an ice shelf and is floating freely in open (salt) water. Smaller chunks of floating glacially-derived ice are called "growlers" or "bergy bits". The ...
s in the North Atlantic; the discharge of large amounts of such icebergs between 11,500 and 21,000 years before present coincided with droughts in the subtropics. Before the onset of the AHP, it is thought that the
Lake Victoria Lake Victoria is one of the African Great Lakes. With a surface area of approximately , Lake Victoria is Africa's largest lake by area, the world's largest tropical lake, and the world's second-largest fresh water lake by surface area after ...
, Albert,
Edward Edward is an English given name. It is derived from the Anglo-Saxon name ''Ēadweard'', composed of the elements '' ēad'' "wealth, fortune; prosperous" and '' weard'' "guardian, protector”. History The name Edward was very popular in Anglo-Sa ...
, Turkana and the
Sudd The Sudd (' or ', Dinka language, Dinka: Toc) is a vast swamp in South Sudan, formed by the White Nile's ''Mountain Nile, Baḥr al-Jabal'' section. The Arabic language, Arabic word ' is derived from ' (), meaning "barrier" or "obstruction". The ...
swamps had dried out. The White Nile had become a seasonal river whose course along with that of the main Nile may have been dammed by dunes. The
Nile Delta The Nile Delta ( ar, دلتا النيل, or simply , is the delta formed in Lower Egypt where the Nile River spreads out and drains into the Mediterranean Sea. It is one of the world's largest river deltas—from Alexandria in the west to Po ...
was partially dry, with sandy plains extending between ephemeral channels and exposed seafloor, and it became a source of sand for ergs farther east. Other lakes across Africa, such as
Lake Chad Lake Chad (french: Lac Tchad) is a historically large, shallow, endorheic lake in Central Africa, which has varied in size over the centuries. According to the ''Global Resource Information Database'' of the United Nations Environment Programme, ...
and
Lake Tanganyika Lake Tanganyika () is an African Great Lake. It is the second-oldest freshwater lake in the world, the second-largest by volume, and the second-deepest, in all cases after Lake Baikal in Siberia. It is the world's longest freshwater lake. ...
, also had shrunk during this time, and both the Niger River and
Senegal River ,french: Fleuve Sénégal) , name_etymology = , image = Senegal River Saint Louis.jpg , image_size = , image_caption = Fishermen on the bank of the Senegal River estuary at the outskirts of Saint-Louis, Senegal ...
were stunted.


Early humidity increases

Whether some parts of the desert such as highlands like the Red Sea Hills were reached by the westerlies or weather systems associated with the
subtropical jet stream Jet streams are fast flowing, narrow, meandering air currents in the atmospheres of some planets, including Earth. On Earth, the main jet streams are located near the altitude of the tropopause and are westerly winds (flowing west to east). ...
—and thus received precipitation—is contentious. It is only clearly supported for the Maghreb in northwestern Africa, though river flow/ terrace formation and lake development in the Tibesti and Jebel Marra mountains and residual Nile flow may be explained in this way. The highlands of Africa appear to have been less affected by drought during the last glacial maximum. The end of the glacial drought occurred between 17,000 and 11,000 years ago, with an earlier beginning noted in the Saharan mountains (possibly) 18,500 years ago. In southern and central Africa earlier starts 17,000 and 17,500 years ago, respectively, may be linked to
Antarctic The Antarctic ( or , American English also or ; commonly ) is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica, the Kerguelen Plateau and other ...
warming, while Lake Malawi appears to have been low until about 10,000 years ago. High lake levels occurred in the Jebel Marra and Tibesti Mountains between 15,000 and 14,000 years ago and the youngest stage of glaciation in the High Atlas mountains took place at the same time as the early African humid period. Around 14,500 years ago, lakes started to appear in the arid areas.


Onset

The humid period began about 15,000-14,500 years ago. The onset of the humid period took place almost simultaneously over all of Northern and Tropical Africa, with impacts as far as Santo Antão on
Cape Verde , national_anthem = () , official_languages = Portuguese , national_languages = Cape Verdean Creole , capital = Praia , coordinates = , largest_city = capital , demonym ...
. In Arabia, wet conditions apparently took about two millennia to advance northward, a gradual advance is supported by tephrochronological data. Lake Victoria reappeared and overflowed; Lake Albert also overflowed into the
White Nile The White Nile ( ar, النيل الأبيض ') is a river in Africa, one of the two main tributaries of the Nile, the other being the Blue Nile. The name comes from the clay sediment carried in the water that changes the water to a pale color. ...
15,000–14,500 years ago and so did
Lake Tana Lake Tana ( am, ጣና ሐይቅ, T’ana ḥāyik’i; previously Tsana) is the largest lake in Ethiopia and the source of the Blue Nile. Located in Amhara Region in the north-western Ethiopian Highlands, the lake is approximately long and wid ...
, into the Blue Nile. The White Nile flooded part of its valley and reconnected to the main Nile. In Egypt widespread flooding by the "Wild Nile" took place; this "Wild Nile" period led to the largest recorded floods on this river, sedimentation in floodplains, and probably also impacted human populations along the river. Even earlier, 17,000–16,800 years ago,
meltwater Meltwater is water released by the melting of snow or ice, including glacial ice, tabular icebergs and ice shelves over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reducing. Meltwater can be ...
from glaciers in Ethiopia – which were retreating at that time – may have begun to increase the flow of water and sediment in the Nile. In the
East African Rift The East African Rift (EAR) or East African Rift System (EARS) is an active continental rift zone in East Africa. The EAR began developing around the onset of the Miocene, 22–25 million years ago. In the past it was considered to be part of a ...
water levels in lakes began to rise by about 15,500/15,000-12,000 years ago; Lake Kivu began overflowing into Lake Tanganyika by about 10,500 years ago. About the same time that the AHP started, the cold glacial climate in Europe associated with Heinrich event 1 ended with climate changing as far as Australasia. A warming and retreat of sea ice around Antarctica coincides with the start of the African humid period, although the Antarctic Cold Reversal also falls into this time and may relate to a drought interval recorded in the Gulf of Guinea.


Causes

The African humid period was caused by a stronger West African Monsoon directed by changes in
solar irradiance Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/ ...
and in albedo feedbacks. These led to increased moisture import from both the equatorial Atlantic into West Africa, as well as from the
North Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Africa, Europe and ...
and the Mediterranean Sea towards the Mediterranean coasts of Africa. There were complex interactions with the atmospheric circulation of the extratropics and between moisture coming from the Atlantic Ocean and the Indian Ocean, and an increased overlap between the areas wetted by the monsoon and those wetted by
extratropical cyclone Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of ...
s. Climate models indicate that changes from a dry to a green Sahara and back have threshold behaviour, with the change occurring once a certain level of insolation is exceeded; likewise, a gradual drop of insolation often leads to a sudden transition back to a dry Sahara. This is due to various feedback processes which are at work, and in climate models there is often more than one stable climate-vegetation state. Sea surface temperature and
greenhouse gas A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...
changes synchronized the beginning of the AHP across Africa.


Orbital changes

The African humid period has been explained by increased
insolation Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/m ...
during Northern Hemisphere summer. Due to precession, the season at which Earth passes closest to the Sun on its elliptical orbit – the perihelion – changes, with maximum summer insolation occurring when this happens during Northern Hemisphere summer. Between 11,000 and 10,000 years ago, Earth passed through the perihelion at the time of
summer solstice The summer solstice, also called the estival solstice or midsummer, occurs when one of Earth's poles has its maximum tilt toward the Sun. It happens twice yearly, once in each hemisphere ( Northern and Southern). For that hemisphere, the summer ...
, increasing the amount of solar radiation by about 8%, resulting in the African monsoon becoming both stronger and reaching farther north. Between 15,000 and 5,000 years ago, summer insolation was at least 4% higher than today. The obliquity also decreased during the Holocene but the effect of obliquity changes on the climate is focused on the high latitudes and its influence on the monsoon is unclear. During summer, solar heating is stronger over the North African land than over the ocean, forming a low pressure area that draws moist air and precipitation in from the Atlantic Ocean. This effect was strengthened by the increased summer insolation, leading to a stronger monsoon that also reached farther north. The effects of these circulation changes reached as far as the subtropics. Obliquity and precession are responsible for two of the foremost Milankovich cycles and are responsible not only for the onset and cessation of ice ages but also for monsoon strength variations. Southern Hemisphere monsoons are expected to have the opposite response of Northern Hemisphere monsoons to precession, as the insolation changes are reversed; this observation is borne out by data from South America. The precession change increased seasonality in the
Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...
while decreasing it in the Southern Hemisphere.


Albedo feedbacks

According to climate modelling, orbital changes by themselves cannot increase precipitation over Africa enough to explain the formation of the large desert lakes such as
Lake Megachad Lake Chad (french: Lac Tchad) is a historically large, shallow, endorheic lake in Central Africa, which has varied in size over the centuries. According to the ''Global Resource Information Database'' of the United Nations Environment Programme, ...
or the northward expansion of vegetation unless ocean and land surface changes are factored in. Decreasing albedo resulting from vegetation changes is an important factor in the precipitation increase. Specifically, increased precipitation increases the amount of vegetation; vegetation absorbs more sunlight and thus more energy is available for the monsoon. In addition, evapotranspiration from vegetation adds more moisture, although this effect is less pronounced than the albedo effect. Heat fluxes in the soil and evaporation are also altered by the vegetation. Reduced dust generation from a wetter Sahara influences the climate by reducing the amount of light absorbed by dust and also modifying cloud properties, making them less reflective and more efficient at inducing precipitation. In climate models, reduced amounts of dust in the troposphere together with vegetation changes can often but not always explain the northward expansion of the monsoon. There is not universal agreement on the effects of dust on precipitation in the Sahel, however, in part because the effects of dust on precipitation may be dependent on its size. In addition to raw precipitation changes, changes in precipitation seasonality such as the length of dry seasons need to be considered when assessing the effects of climate change on vegetation, as well as the fertilizing effects of increased carbon dioxide concentrations in the atmosphere. Other sources of albedo changes: * Changes in soil properties result in changes in the monsoon; replacing desert soils with
loam Loam (in geology and soil science) is soil composed mostly of sand (particle size > ), silt (particle size > ), and a smaller amount of clay (particle size < ). By weight, its mineral composition is about 40–40–20% concentration of sand–sil ...
y ones results in increased precipitation, and soils that are wet or contain
organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ Chemistry * Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...
matter reflect less sunlight and accelerate the moistening process. Desert sand changes also modify the albedo. * Albedo changes caused by lakes and wetlands can alter precipitation in climate models.


Intertropical Convergence Zone changes

Warmer extratropics during summer may have drawn the
Intertropical Convergence Zone The Intertropical Convergence Zone (ITCZ ), known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the thermal e ...
(ITCZ) northward, resulting in precipitation changes. Sea surface temperatures off North Africa warmed under orbital effects and through weaker trade winds, leading to a northward movement of the ITCZ and increasing moisture gradients between land and sea. Two temperature gradients, one between a cooler Atlantic during spring and an already warming African continent, the other between warmer temperatures north of 10° latitude and cooler south, may have assisted in this change. In Eastern Africa, ITCZ changes had relatively little effect on precipitation changes. The past position of the ITCZ in Arabia is also contentious.


Precipitation changes in East Africa

The African humid period that took place in
East Africa East Africa, Eastern Africa, or East of Africa, is the eastern subregion of the African continent. In the United Nations Statistics Division scheme of geographic regions, 10-11-(16*) territories make up Eastern Africa: Due to the historical ...
appears to have been caused by different mechanisms. Among the proposed mechanisms are decreased seasonality of precipitation due to increased dry season precipitation, shortening of the dry season, increased precipitation and increased inflow of moisture from the Atlantic and Indian Oceans. The Atlantic moisture inflow was in part triggered by a stronger West African and Indian monsoon, perhaps explaining why the effects of the AHP extended into the Southern Hemisphere. The behaviour of the easterly trade winds is unclear; increased moisture transport by easterly trade winds may have aided in the development of the AHP but alternatively a stronger Indian Monsoon that draws easterly winds away from East Africa may have occurred. Changes in the
Congo Air Boundary Congo or The Congo may refer to either of two countries that border the Congo River in central Africa: * Democratic Republic of the Congo, the larger country to the southeast, capital Kinshasa, formerly known as Zaire, sometimes referred to ...
or increased convergence along this boundary may have contributed; the Congo Air Boundary would have been shifted east by the stronger westerly winds directed by lower atmospheric pressure over Northern Africa, allowing additional moisture from the Atlantic to reach East Africa. The parts of East Africa that were isolated from Atlantic moisture did not become significantly wetter during the AHP although at one site in Somalia the seasonality of precipitation may or may not have decreased. Various contributing factors may have led to the increased humidity in East Africa, not all of which were necessarily operating simultaneously during the AHP. That the "African humid period" reached this part of Africa has been doubted. Finally, increased greenhouse gas concentrations may have been involved in directing the onset of the AHP in tropical southeastern Africa; there, orbital changes would be expected to lead to climate variations opposite to those in the Northern Hemisphere. The pattern of humidity changes in south-eastern Africa are complex.


Additional factors

* Climate change in the far northern latitudes may have contributed to the onset of the AHP. The shrinkage of the Scandinavian and the Laurentide Ice Sheets occurred at its beginning, and in climate models, a retreat of the ice sheets is often required to simulate the humid period. Their existence might also explain why the AHP did not start immediately with the early insolation peak, as still existing ice sheets would have cooled the climate. * Sea surface temperature changes in the Atlantic influence the African monsoon and may have influenced the onset of the AHP. Weaker trade winds and higher
insolation Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/m ...
would lead to warmer sea surface temperatures, increasing precipitation by increasing moisture gradients between land and sea. Changes in North Atlantic temperature gradients were also involved. * Warming of the Mediterranean Sea increases the amount of Sahel precipitation; this effect is responsible for the recent anthropogenic global warming mediated increase in Sahel precipitation. Warmer sea surface temperatures there might also explain the increased precipitation recorded in the Mediterranean and increased intensity of precipitation reconstructed from former rivers in the Sahara during the AHP. * Increased precipitation during winter is correlated with a larger spatial extent of Mediterranean precipitation and might have aided in the establishment of the AHP, especially in North Africa including Morocco and
Northern Egypt Lower Egypt ( ar, مصر السفلى '; ) is the northernmost region of Egypt, which consists of the fertile Nile Delta between Upper Egypt and the Mediterranean Sea, from El Aiyat, south of modern-day Cairo, and Dahshur. Historica ...
, around the northern Red Sea, in the Tibesti and in northern Arabia and generally at higher latitudes where the monsoon did not arrive. This precipitation may have extended to other parts of the Sahara; such would have led to the areas of summer and winter precipitation overlapping and the dry area between the monsoonal and westerlies-influenced climate zones becoming wetter or disappearing altogether. Such changes in Mediterranean-derived precipitation may correlate with changes in the
North Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Africa, Europe and ...
and Arctic Oscillations. *
Trough Trough may refer to: In science * Trough (geology), a long depression less steep than a trench * Trough (meteorology), an elongated region of low atmospheric pressure * Trough (physics), the lowest point on a wave * Trough level (medicine), the l ...
-mediated northward transport of moisture during autumn and spring has also been proposed to explain the increased precipitation and its underestimation by climate models. In one climate model, increased northward moisture transport by such troughs increases autumn rainfall in the Sahara, especially in the mid-Holocene and when the climate is already moister than usual there. * Weaker
subtropical anticyclone The horse latitudes are the latitudes about 30 degrees north and south of the Equator. They are characterized by sunny skies, calm winds, and very little precipitation. They are also known as subtropical ridges, or highs. It is a high-pressure ...
s were proposed as an explanation during the 1970s–1980s. * In montane regions such as the Meidob volcanic field cold temperatures after the
last glacial maximum The Last Glacial Maximum (LGM), also referred to as the Late Glacial Maximum, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent. Ice sheets covered much of Northern North America, Northern Eur ...
may have reduced
evaporation Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...
and thus allowed an early onset of humidity. * Changes in the Earth's
geomagnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic fi ...
may be linked to the humidity changes. * Increased moisture supply from larger lakes like
Lake Megachad Lake Chad (french: Lac Tchad) is a historically large, shallow, endorheic lake in Central Africa, which has varied in size over the centuries. According to the ''Global Resource Information Database'' of the United Nations Environment Programme, ...
may have increased the precipitation, although this effect is probably not adequate to explain the entire AHP. A similar role has been attributed to the extensive wetlands, drainages and lakes in the Eastern Sahara and to the ecosystem in general. * Two high elevation winds, the African Easterly Jet and the Tropical Easterly Jet modulate atmospheric air flows over Africa and thus also the amount of precipitation; the Tropical Easterly Jet comes from India and is powered by temperature gradients between the tropics and the subtropics while the African Easterly Jet is powered by temperature gradients in the
Sahel The Sahel (; ar, ساحل ' , "coast, shore") is a region in North Africa. It is defined as the ecoclimatic and biogeographic realm of transition between the Sahara to the north and the Sudanian savanna to the south. Having a hot semi-arid c ...
. A stronger West African Monsoon resulted in a weaker African Easterly Jet and thus decreased transport of moisture out of Africa. * Increased atmospheric carbon dioxide concentrations may have played a role in triggering the AHP, especially its extension across the equator, as well as its resumption after the Younger Dryas and Heinrich event 1 through increased sea surface temperatures. * In some parts of the Sahara increased water supply from montane regions may have assisted in the development of moist conditions. * Larger forests in Eurasia may have led to a northward shift of the ITCZ. * Other proposed mechanisms involve convection occurring above the atmospheric boundary layer, increased latent heat fluxes, low pressure in northwestern Africa drawing moisture into the Sahara, changes in the solar cycles and complex atmospheric flow phenomena.


Effects

The African humid period extended over the Sahara as well as eastern, southeastern and equatorial Africa. In general, forests and woodlands expanded through the continent. A similar wet episode took place in the tropical Americas, China, Asia, India, the
Makran Makran ( fa, مكران), mentioned in some sources as Mecran and Mokrān, is the coastal region of Baluchistan. It is a semi-desert coastal strip in Balochistan, in Pakistan and Iran, along the coast of the Gulf of Oman. It extends westwards, ...
region, the Middle East and the
Arabian Peninsula The Arabian Peninsula, (; ar, شِبْهُ الْجَزِيرَةِ الْعَرَبِيَّة, , "Arabian Peninsula" or , , "Island of the Arabs") or Arabia, is a peninsula of Western Asia, situated northeast of Africa on the Arabian Plate ...
and appears to relate to the same orbital forcing as the AHP. An early Holocene monsoonal episode extended as far as the Mojave Desert in North America. In contrast, a drier episode is recorded from much of South America where Lake Titicaca, Lake Junin, the discharge of the
Amazon River The Amazon River (, ; es, Río Amazonas, pt, Rio Amazonas) in South America is the largest river by discharge volume of water in the world, and the disputed longest river system in the world in comparison to the Nile. The headwaters of t ...
and water availability in the Atacama were lower. The discharge of the
Congo Congo or The Congo may refer to either of two countries that border the Congo River in central Africa: * Democratic Republic of the Congo, the larger country to the southeast, capital Kinshasa, formerly known as Zaire, sometimes referred to a ...
, Niger, Nile, Ntem,
Rufiji Rufiji may refer to: * Rufiji Delta, a region in Tanzania * Rufiji District, in the Pwani Region of Tanzania * Rufiji River The Rufiji River lies entirely within Tanzania. It is also the largest and longest river in the country. The river is fo ...
, and Sanaga rivers increased. Runoff from Algeria, equatorial Africa, northeastern Africa and the western Sahara was also larger. Changes in the morphology of the river systems and their
alluvial plain An alluvial plain is a largely flat landform created by the deposition of sediment over a long period of time by one or more rivers coming from highland regions, from which alluvial soil forms. A floodplain is part of the process, being the sma ...
s occurred in response to the increased discharge, and the Senegal River expanded its riverbed, breached dunes and re-entered the Atlantic Ocean.


Flora and fauna of the Sahara

During the African humid period, lakes, rivers, wetlands and vegetation including grass and trees covered the Sahara and
Sahel The Sahel (; ar, ساحل ' , "coast, shore") is a region in North Africa. It is defined as the ecoclimatic and biogeographic realm of transition between the Sahara to the north and the Sudanian savanna to the south. Having a hot semi-arid c ...
creating a "Green Sahara" with a land cover that has no modern analogues. Evidence includes pollen data, archaeological sites, evidence of faunal activity such as
diatom A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...
s,
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s, ostracods,
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s and snails, buried river valleys, organic-rich mats,
mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...
s, evaporites as well as travertines and tufas deposited in subaqueous environments. The vegetation cover then extended over almost all of the Sahara and consisted of an open grass savannah with
shrub A shrub (often also called a bush) is a small-to-medium-sized perennial woody plant. Unlike herbaceous plants, shrubs have persistent woody stems above the ground. Shrubs can be either deciduous or evergreen. They are distinguished from trees ...
s and trees. In general, the vegetation expanded northward to 2730° northern latitude in West Africa with a Sahel boundary at about 23° north, as the Sahara was populated by plants that today often occur about  (lower estimate). farther south. The northward movement of vegetation took some time and some plant species moved faster than others. Plants that perform C3 carbon fixation became more common and the fire regime of the vegetation changed. Forests and plants from the humid tropics were concentrated around lakes and rivers. The landscape during the AHP has been described as a mosaic between various vegetation types of semi-desert and humid origin rather than a simple northward displacement of plant species, and some brown or yellow vegetation communities persisted. There was no southward displacement of Mediterranean plants during the Holocene and on the Tibesti Mountains cold temperatures may have restricted the expansion of tropical plants. Pollen data often show a dominance of grasses over humid tropics trees. The tree ''
Lophira alata ''Lophira alata'', commonly known as azobé, ekki or the red ironwood tree, is a species of plant in the family Ochnaceae. It is found in Cameroon, the Republic of the Congo, the Democratic Republic of the Congo, Ivory Coast, Equatorial Guinea, G ...
'' and others may have spread out of the African forests during the AHP, and the '' Lactuca'' plants may have split into two species under the effects of the AHP and other climate changes in Africa during the Holocene. The Sahara climate did not become entirely homogeneous; its central-eastern parts were probably drier than the western and central sectors and the
Libyan sand sea The Great Sand Sea is an approximately erg (landform), sand desert (erg) in the Sahara between western Egypt and eastern Libya in North Africa. Some 74% of the area is covered by sand dunes. Geography The Great Sand Sea stretches about from no ...
was still a
desert A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About on ...
although pure desert areas retreated or became arid/ semiarid. An arid belt may have existed north of 22° latitude, or the vegetation and the African monsoon might have reached 28–31° northern latitude; in general conditions between 21° and 28° northern latitude are poorly known. Dry areas may have persisted in the
rain shadow A rain shadow is an area of significantly reduced rainfall behind a mountainous region, on the side facing away from prevailing winds, known as its leeward side. Evaporated moisture from water bodies (such as oceans and large lakes) is carrie ...
s of mountains and could have supported arid climate vegetation, explaining the presence of its pollen in sediment cores. In addition, north–south gradations in vegetation patterns have been reconstructed from charcoal and pollen data. Fossils record changes in the animal fauna of the Sahara. This fauna included
antelope The term antelope is used to refer to many species of even-toed ruminant that are indigenous to various regions in Africa and Eurasia. Antelope comprise a wastebasket taxon defined as any of numerous Old World grazing and browsing hoofed mammals ...
s, baboons, cane rats, catfish, clams,
cormorant Phalacrocoracidae is a family of approximately 40 species of aquatic birds commonly known as cormorants and shags. Several different classifications of the family have been proposed, but in 2021 the IOC adopted a consensus taxonomy of seven ge ...
s, crocodiles, elephants, frogs, gazelles,
giraffe The giraffe is a large African hoofed mammal belonging to the genus ''Giraffa''. It is the tallest living terrestrial animal and the largest ruminant on Earth. Traditionally, giraffes were thought to be one species, ''Giraffa camelopardalis ...
s, hartebeest,
hare Hares and jackrabbits are mammals belonging to the genus ''Lepus''. They are herbivores, and live solitarily or in pairs. They nest in slight depressions called forms, and their young are able to fend for themselves shortly after birth. The ge ...
s, hippos, molluscs, Nile perches,
pelican Pelicans (genus ''Pelecanus'') are a genus of large water birds that make up the family Pelecanidae. They are characterized by a long beak and a large throat pouch used for catching prey and draining water from the scooped-up contents before s ...
s, rhinoceroses, snake-eagles, snakes, tilapia, toads, turtles and many more animals, and in Egypt there were African buffaloes,
spotted hyena The spotted hyena (''Crocuta crocuta''), also known as the laughing hyena, is a hyena species, currently classed as the sole extant member of the genus ''Crocuta'', native to sub-Saharan Africa. It is listed as being of least concern by the IUC ...
s, warthogs, wildebeest and zebra. Additional birds include brown-necked raven, coot, common moorhen, crested grebe, glossy ibis, long-legged buzzard, rock dove, spur-winged goose and tufted duck. Large herds of animals lived in the Sahara. Some animals expanded over the whole desert, while others were limited to places with deep water. Earlier humid periods in the Sahara may have allowed species to cross the now-desert. A reduction in open grasslands at the beginning of the AHP may explain a population bottleneck in cheetahs at the start of the humid period, while the humid period led to the expansion of some animal populations such as
Hubert's multimammate mouse Hubert's multimammate mouse, or Hubert's mastomys (''Mastomys huberti'') is a species of rodent in the family Muridae found in Burkina Faso, Gambia, Mali, Mauritania, Nigeria, and Senegal, and possibly Guinea, Guinea-Bissau, and Niger. Its nat ...
.


Lakes and rivers of the Sahara

A number of lakes formed or expanded in the Sahara and the Hoggar and Tibesti Mountains. The largest of them was
Lake Chad Lake Chad (french: Lac Tchad) is a historically large, shallow, endorheic lake in Central Africa, which has varied in size over the centuries. According to the ''Global Resource Information Database'' of the United Nations Environment Programme, ...
which increased to at least ten times its present-day size to form Lake Megachad. This enlarged Lake Chad reached dimensions of in north–south and east–west direction respectively, covering the Bodélé Depression and perhaps as much as 8% of the present-day Sahara desert. It influenced the climate itself; for example rainfall would have been reduced at the centre of the lake and increased at its margins. Lake Chad was possibly fed from the north by rivers draining the Hoggar (Taffassasset drainage) and Tibesti Mountains, from the Ennedi Mountains in the east through the "Eastern palaeorivers" and from the south by the Chari- Logone and Komadugu Rivers. The Chari River was the main tributary while the rivers draining the Tibesti formed alluvial fans/the Angamma river delta at their entry into northern Lake Chad. Skeletons of elephants, hippos and hominins have been found in the Angamma delta, which is the dominant shoreline feature of northern Lake Chad. The lake overflowed into the Niger River during highstand through the Mayo Kebbi and the
Benue River The Benue River (french: la Bénoué), previously known as the Chadda River or Tchadda, is the major tributary of the Niger River. The river is approximately long and is almost entirely navigable during the summer months. The size of its bas ...
, eventually reaching the Gulf of Guinea. Older dune systems were submerged by Lake Chad. Among the large lakes which may have formed in the Sahara are
Lake Megafezzan A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...
in Libya and Lake Ptolemy in Sudan. Quade ''et al.'' 2018 raised some doubts about the size and existence of some of these lakes such as Lake Ptolemy, Lake Megafezzan,
Lake Ahnet-Mouydir A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...
, especially for Lake Megafezzan. Other lakes are known from Adrar Bous in Niger, Era Kohor and
Trou au Natron Trou au Natron (French: "hole of natron") or ( Teda: "big hole") is a volcanic caldera of the Tibesti Massif in the nation of Chad in Northern Africa. The volcano is extinct. It is unknown when it last erupted. Its volcano number is 0205–01. T ...
in the Tibesti Mountains, I-n-Atei in the Hoggar, at Ine Sakane and in Taoudenni in Mali, the Garat Ouda and Takarkori Lakes in the Acacus Mountains, Chemchane in
Mauretania Mauretania (; ) is the Latin name for a region in the ancient Maghreb. It stretched from central present-day Algeria westwards to the Atlantic, covering northern present-day Morocco, and southward to the Atlas Mountains. Its native inhabitants, ...
, at Sebkha Mellala close to Ouargla in Algeria, at Bilma, Dibella, Fachi and Gobero in the Ténéré, in Niger and at "Eight Ridges", El Atrun, Lake Gureinat, Merga, "Ridge", Sidigh, at Wadi Mansurab, Selima and
Oyo Oyo can refer to: Places Nigeria * Oyo Empire, a former Yoruba state that covered parts of Nigeria and Benin, or the capital city * Oyo State, a present-day state of Nigeria named after the Oyo Empire * Oyo, Oyo State, a city founded in the 18 ...
in Sudan. Lake Yoa of the Lakes of Ounianga overflowed, either above surface or underground. Mosaics of small lakes developed in some regions. Wetlands also expanded during the AHP, but both their expansion and subsequent retreat were slower than that of lakes. In some parts of the Sahara ephemeral lakes formed such as at
Abu Ballas Abu Ballas (the pottery hill) is an archaeological site in the Libyan Desert of Egypt. It lies about south-west of the Dakhla Oases and consists of two isolated sandstone cones in the otherwise flat desert. Both hills are covered with Egyptian p ...
,
Bir Kiseiba Bir Kiseiba is a Neolithic archaeological site in Egypt, dating from approximately 11,000-5,000 BP, that lies approximately 250 km west of the Nile in Lower Nubia. Excavated by Fred Wendorf, Romauld Schild, and Angela Close, Bir Kiseiba ...
, Bir Sahara, Bir Tarfawi and Nabta Playa in Egypt, which may relate to later Egyptian religions, or
swamp A swamp is a forested wetland.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p. Swamps are considered to be transition zones because both land and water play a role in ...
-lakes such as at Adrar Bous close to the
Air Mountains The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...
. Ephemeral lakes developed between dunes, and a "freshwater archipelago" appears to have existed in the Murzuq basin. All these lake systems left fossils such as fish, limnic sediments and fertile soils that were later used for agriculture (El Deir, Kharga Oasis). Finally, crater lakes formed in volcanic fields and sometimes survive to this day as smaller remnant lakes such as Malha crater in the Meidob volcanic field. Potentially, the increased availability of water during the AHP may have facilitated the onset of
phreatomagmatic Phreatomagmatic eruptions are volcanic eruptions resulting from interaction between magma and water. They differ from exclusively magmatic eruptions and phreatic eruptions. Unlike phreatic eruptions, the products of phreatomagmatic eruptions conta ...
eruptions such as
maar A maar is a broad, low-relief volcanic crater caused by a phreatomagmatic eruption (an explosion which occurs when groundwater comes into contact with hot lava or magma). A maar characteristically fills with water to form a relatively shallow ...
formation in the Bayuda volcanic field, although the chronology of volcanic eruptions there is not well known enough to substantiate a link to the AHP. The large
Tamanrasset River The Tamanrasset River is an enormous palaeoriver believed to have flowed through West Africa as recently as 5000 years ago during the African humid period. The Tamanrasset River basin is thought to have been comparable with the present-day Gang ...
flowed from the
Atlas Mountains The Atlas Mountains are a mountain range in the Maghreb in North Africa. It separates the Sahara Desert from the Mediterranean Sea and the Atlantic Ocean; the name "Atlantic" is derived from the mountain range. It stretches around through Moroc ...
and Hoggar westward towards the Atlantic and entered it in the Bay of Arguin in
Mauretania Mauretania (; ) is the Latin name for a region in the ancient Maghreb. It stretched from central present-day Algeria westwards to the Atlantic, covering northern present-day Morocco, and southward to the Atlas Mountains. Its native inhabitants, ...
. It once formed the 12th largest watershed in the world and left a submarine canyon and riverine sediments. Together with other rivers it formed estuaries and mangroves in the Bay of Arguin. Other rivers in the same area also formed submarine canyons, and sediment patterns in marine sediment cores and the occurrence of submarine landslides in the area have been related to the activity of these rivers. Rivers such as the Irharhar in Algeria, Libya and Tunisia and the Sahabi and Kufra rivers in Libya were active during this time although there is some doubt that they had perennial flow; they appear to have been more important in earlier humid periods. Small watersheds, wadis and rivers discharging into endorheic basins such as Wadi Tanezzuft also carried water during the AHP. In Egypt, some rivers active during the AHP are now
gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...
ridges. In the Air, Hoggar and Tibesti Mountains, the so-called "Middle Terrace" was emplaced at this time. The rivers of the Sahara, lakes and their watersheds may have acted as pathways for the spread of humans and animals; the rivers were often connected to each other by alluvial fans. Proposed examples of animals that spread through rivers are the Nile crocodile and the fish '' Clarias gariepinus'' and '' Tilapia zillii''. It is possible that the name Tassili n'Ajjer, which means "plateau of the rivers" in
Berber Berber or Berbers may refer to: Ethnic group * Berbers, an ethnic group native to Northern Africa * Berber languages, a family of Afro-Asiatic languages Places * Berber, Sudan, a town on the Nile People with the surname * Ady Berber (1913–196 ...
, is a reference to past river flows. On the other hand, intense flows of these rivers may have made their shores dangerous to humans and thus created additional impetus for human movement.


Humans of the Sahara

Conditions and resources were ripe for first
hunter-gatherer A traditional hunter-gatherer or forager is a human living an ancestrally derived lifestyle in which most or all food is obtained by foraging, that is, by gathering food from local sources, especially edible wild plants but also insects, fungi, ...
s, fishermen and, later, pastoralists, which arrived in the Sahara at the time where lakes developed. They may have come either from the north ( Maghreb or Cyrenaica) where the Capsian culture was located, the south (
Sub-Saharan Africa Sub-Saharan Africa is, geographically, the area and regions of the continent of Africa that lies south of the Sahara. These include West Africa, East Africa, Central Africa, and Southern Africa. Geopolitically, in addition to the List of sov ...
), or the east (
Nile Valley The Nile, , Bohairic , lg, Kiira , Nobiin: Áman Dawū is a major north-flowing river in northeastern Africa. It flows into the Mediterranean Sea. The Nile is the longest river in Africa and has historically been considered the longest rive ...
). The human population in the Sahara increased at the beginning of the AHP. Traces of human activity have been found in the Acacus Mountains where caves and
rock shelter A rock shelter (also rockhouse, crepuscular cave, bluff shelter, or abri) is a shallow cave-like opening at the base of a bluff or cliff. In contrast to solutional caves (karst), which are often many miles long, rock shelters are almost alway ...
s were used as basecamps for humans, such as the Uan Afuda cave and the Uan Tabu and Takarkori rock shelters. The first occupation in Takarkori took place between 10,000 and 9,000 years ago; about five millennia of human cultural evolution are recorded there. At
Gobero The Gobero archaeological site, dating to approximately 8000 BCE, is the oldest known graveyard in the Sahara Desert. The site contains important information for archaeologists on how early humans adapted to a constantly changing environment. Gobe ...
in the Ténéré desert a cemetery has been found, which has been used to reconstruct the lifestyle of these former inhabitants of the Sahara, and at Lake Ptolemy in Nubia humans settled close to the lake shore, using its resources and perhaps even engaging in leisure activities. At that time, many humans appear to have depended on water-bound resources, seeing as many of the tools left by the early humans are associated with
fishery Fishery can mean either the enterprise of raising or harvesting fish and other aquatic life; or more commonly, the site where such enterprise takes place ( a.k.a. fishing ground). Commercial fisheries include wild fisheries and fish farms, both ...
; hence this culture is also known as " aqualithic" although substantial differences between the cultures of various places have been found. The greening of the Sahara led to a
demographic Demography () is the statistical study of populations, especially human beings. Demographic analysis examines and measures the dimensions and dynamics of populations; it can cover whole societies or groups defined by criteria such as edu ...
expansion and especially in the Eastern Sahara human occupancy coincides with the AHP. Conversely occupation decreased along the Nile valley, perhaps due to the expansion of wetlands there and frequent large-scale flooding of the Nile delta. Humans were hunting large animals with weapons that have been found in archaeological sites and wild
cereal A cereal is any Poaceae, grass cultivated for the edible components of its grain (botanically, a type of fruit called a caryopsis), composed of the endosperm, Cereal germ, germ, and bran. Cereal Grain, grain crops are grown in greater quantit ...
s occurring in the Sahara during the AHP such as brachiaria,
sorghum ''Sorghum'' () is a genus of about 25 species of flowering plants in the grass family (Poaceae). Some of these species are grown as cereals for human consumption and some in pastures for animals. One species is grown for grain, while many othe ...
and urochloa were an additional source of food. Humans also domesticated cattle, goats and sheep; cattle domestication occurred especially in the more environmentally variable Eastern Sahara. Animal husbandry picked up in earnest around 7,000 years ago when domestic animals came to the Sahara, and a population boom may be linked to this change in cultural practice; cattle and goats spread southwestwards from northeasternmost Africa from 8,000 years before present. Dairying has been demonstrated in some locations and cattle-husbandry is supported by the frequent depiction of cattle in rock paintings. The
Dufuna canoe The Dufuna canoe is a dugout canoe discovered in 1987 by a Fulani cattle herdsman a few kilometers from the village of Dufuna in the Fune Local Government Area, not far from the Komadugu Gana River, in Yobe State, Nigeria. Radiocarbon dating of a s ...
, one of the oldest known ships in the world, appears to date to the Holocene humid period and implies that the waterbodies of that time were navigated by humans. The cultural units "Masara" and "Bashendi" existed in Dakhleh Oasis during the AHP. In the Acacus Mountains, several cultural horizons known as Early and Late Acacus and Early, Middle, Late and Final Pastoral have been identified while in Niger the Kiffian culture has been related to the beginning of the AHP. Ancient civilizations thrived, with farming and animal husbandry taking place in Neolithic settlements. Possibly, the domestication of plants in Africa was delayed by the increased food availability during the AHP, it only took place around 2,500 BC. Humans created
rock art In archaeology, rock art is human-made markings placed on natural surfaces, typically vertical stone surfaces. A high proportion of surviving historic and prehistoric rock art is found in caves or partly enclosed rock shelters; this type also ...
such as
petroglyph A petroglyph is an image created by removing part of a rock surface by incising, picking, carving, or abrading, as a form of rock art. Outside North America, scholars often use terms such as "carving", "engraving", or other descriptions ...
s and rock paintings in the Sahara, perhaps the largest density of such creations in the world. Scenes include animals and everyday life such as swimming which supports the presence of past wetter climates. One well-known such petroglyph location is the Cave of Swimmers in the Gilf Kebir mountains of Egypt; other well known sites are the
Gabal El Uweinat Mount Uwaynat or Gabal El Uweinat ( ar, جبل العوينات ', Arabic for 'Mountain of the springs') is a mountain range in the area of the Egyptian-Libyan- Sudanese tripoint. Because of thousands of prehistoric rock art sites, it is conside ...
mountains also of Egypt, Arabia and the Tassili n'Ajjer in Algeria where rock paintings from this time have been discovered. Humans also left
artifact Artifact, or artefact, may refer to: Science and technology * Artifact (error), misleading or confusing alteration in data or observation, commonly in experimental science, resulting from flaws in technique or equipment ** Compression artifact, a ...
s such as '' Fesselsteine'' and ceramics in what today are inhospitable deserts. North Africa together with East Asia is one of the first places where pottery was developed probably under the influence of increased availability of resources during the AHP. The humid period also favoured its development and spread in West Africa during the 10th millennium BC; the so-called "wavy line" or "dotted wavy-line" motif was widespread across Northern Africa and as far as
Lake Turkana Lake Turkana (), formerly known as Lake Rudolf, is a lake in the Kenyan Rift Valley, in northern Kenya, with its far northern end crossing into Ethiopia. It is the world's largest permanent desert lake and the world's largest alkaline lake. B ...
. These populations have been described as
Epipaleolithic In archaeology, the Epipalaeolithic or Epipaleolithic (sometimes Epi-paleolithic etc.) is a period occurring between the Upper Paleolithic and Neolithic during the Stone Age. Mesolithic also falls between these two periods, and the two are someti ...
,
Mesolithic The Mesolithic (Greek: μέσος, ''mesos'' 'middle' + λίθος, ''lithos'' 'stone') or Middle Stone Age is the Old World archaeological period between the Upper Paleolithic and the Neolithic. The term Epipaleolithic is often used synonymous ...
and Neolithic and produced a variety of
lithic Lithic may refer to: *Relating to stone tools **Lithic analysis, the analysis of stone tools and other chipped stone artifacts **Lithic core, the part of a stone which has had flakes removed from it **Lithic flake, the portion of a rock removed to ...
tools and other assemblages. In West Africa, the cultural change from the African
Middle Stone Age The Middle Stone Age (or MSA) was a period of African prehistory between the Early Stone Age and the Late Stone Age. It is generally considered to have begun around 280,000 years ago and ended around 50–25,000 years ago. The beginnings of pa ...
to the
Late Stone Age The Later Stone Age (LSA) is a period in African prehistory that follows the Middle Stone Age. The Later Stone Age is associated with the advent of modern human behavior in Africa, although definitions of this concept and means of studying it ar ...
accompanied the beginning of the AHP. Genetic and archaeological data indicate that these populations which exploited the resources of the AHP Sahara probably originated in
Sub-Saharan Africa Sub-Saharan Africa is, geographically, the area and regions of the continent of Africa that lies south of the Sahara. These include West Africa, East Africa, Central Africa, and Southern Africa. Geopolitically, in addition to the List of sov ...
and moved north after some time, after the desert got wetter; this may be reflected in the northward spread of Macrohaplogroup L and
Haplogroup U6 Haplogroup U is a human mitochondrial DNA haplogroup (mtDNA). The clade arose from haplogroup R, likely during the early Upper Paleolithic. Its various subclades (labelled U1–U9, diverging over the course of the Upper Paleolithic) are found ...
genomic lineages. In return, the AHP facilitated the movement of some Eurasian populations into Africa. These favourable conditions for human populations may be reflected in paradise myths such as the
Garden of Eden In Abrahamic religions, the Garden of Eden ( he, גַּן־עֵדֶן, ) or Garden of God (, and גַן־אֱלֹהִים ''gan-Elohim''), also called the Terrestrial Paradise, is the Bible, biblical paradise described in Book of Genesis, Genes ...
in The Bible and
Elysium Elysium (, ), otherwise known as the Elysian Fields ( grc, Ἠλύσιον πεδίον, ''Ēlýsion pedíon'') or Elysian Plains, is a conception of the afterlife that developed over time and was maintained by some Greek religious and philos ...
and the Golden Age in Classical Antiquity, and in the spread of the Nilo-Saharan languages.


Additional manifestations in the Sahara

The expanded vegetation and soil formation stabilized previously active
dune A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, f ...
s, eventually giving rise to the present-day draa dunes in the Great Sand Sea of Egypt for example, although there is uncertainty about whether this stabilization was widespread. Soil development and biological activity in soils are attested in the Acacus Mountains and the Mesak Settafet area of Libya, but evidence of soil formation/ pedogenesis such as bog iron are described from other parts of the Sahara as well. In the Selima Sand Sheet, the landscape underwent erosional truncation and
bioturbation Bioturbation is defined as the reworking of soils and sediments by animals or plants. It includes burrowing, ingestion, and defecation of sediment grains. Bioturbating activities have a profound effect on the environment and are thought to be a pr ...
. The Central and Southern Sahara saw the development of alluvial deposits while
sebkha A sabkha ( ar, سبخة) is a coastal, supratidal mudflat or sandflat in which evaporite-saline minerals accumulate as the result of semiarid to arid climate. Sabkhas are gradational between land and intertidal zone within restricted coastal ...
deposits are known from the Western Sahara. Lightning strikes into soil left lightning-altered rocks in parts of the Central Sahara. The increased precipitation also resulted in recharged aquifers such as the Nubian Sandstone Aquifer; presently, water from this aquifer maintains several lakes in the Sahara, such as the Lakes of Ounianga. Other groundwater systems were active at that time in the Acacus Mountains,
Air Mountains The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...
, in the Fezzan and elsewhere in Libya and the
Sahel The Sahel (; ar, ساحل ' , "coast, shore") is a region in North Africa. It is defined as the ecoclimatic and biogeographic realm of transition between the Sahara to the north and the Sudanian savanna to the south. Having a hot semi-arid c ...
. Raised groundwater tables provided water to plants and was discharged in depressions, lakes and valleys, forming widespread carbonate deposits and feeding lakes. The formation of lakes and vegetation reduced the export of dust from the Sahara. This has been recorded in
marine core A core sample is a cylindrical section of (usually) a naturally-occurring substance. Most core samples are obtained by drilling with special drills into the substance, such as sediment or rock, with a hollow steel tube, called a core drill. The h ...
s, including one core where dust export decreased by almost half. In coastal places, such as in Oman, sea level rise also reduced the production of dust. In the Mediterranean, a decreased dust supply was accompanied by increased sediment input from the Nile, leading to changes in marine sediment composition. Whether the strengthening of the monsoon enhanced or reduced upwelling off Northwestern Africa is debatable, with some research suggesting that the strengthening in upwelling decreased sea surface temperatures and increased the biological productivity of the sea, while other research suggests that the opposite occurred; less upwelling with more moisture. However, regardless of whether upwelling increased or decreased, it is possible that the strengthening of the monsoon boosted productivity off the coasts of Northern Africa because the increased river discharge delivered more nutrients to the sea.


Arabia

Precipitation in Dhofar and southwestern Arabia is brought by the African monsoon, and a change to a wetter climate resembling Africa has been noted in southern Arabia and Socotra from cave and river deposits. It possibly reached as far as Qatar. Holocene paleolakes are recorded at Tayma, Jubbah, in the Wahiba Sands of Oman and at
Mundafan Mundafan was a former lake in Saudi Arabia, within presently desert-like areas. It formed during the Pleistocene and Holocene, when orbitally mediated changes in climate increased monsoon precipitation in the peninsula, allowing runoff to form a l ...
. In the Rub al-Khali lakes formed between 9,000 and 7,000 years ago and dunes were stabilized by vegetation, although the formation of lakes there was less pronounced than in the Pleistocene. The Wadi ad-Dawasir river system in central Saudi Arabia became active again with increased river runoff into the Persian Gulf. Wadis in Oman eroded across LGM dunes and formed accumulation terraces. Episodes of increased river discharge occurred in Yemen and increased precipitation is recorded in the caves of Hoti, Qunf in Oman, Mukalla in Yemen and Hoq cave in Socotra. Increased precipitation resulted in increased groundwater flow, generating groundwater-fed lakes and carbonate deposits. Forests and wildfire activity expanded across parts of Arabia. Freshwater sources in Arabia during the AHP became focus points of human activity and herding activity between mountains and lowlands occurred. In addition,
karst Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant ro ...
ic activity took place on exposed coral reefs in the Red Sea and traces of it are still recognizable today. Increased precipitation has been also invoked to explain decreased salinities in the Red Sea. Archaeological sites such as
cairn A cairn is a man-made pile (or stack) of stones raised for a purpose, usually as a marker or as a burial mound. The word ''cairn'' comes from the gd, càrn (plural ). Cairns have been and are used for a broad variety of purposes. In prehis ...
s appeared with the beginning of the humid period. The humid period in Arabia did not last as long as in Africa, deserts did not retreat as much and precipitation may not have reached the central and northern part of the peninsula past Oman and the
Yemen Highlands Yemen (; ar, ٱلْيَمَن, al-Yaman), officially the Republic of Yemen,, ) is a country in Western Asia. It is situated on the southern end of the Arabian Peninsula, and borders Saudi Arabia to the north and Oman to the northeast and sha ...
; northern Arabia remained somewhat drier than southern Arabia, droughts were still common and the land and still produced dust. One study has estimated that the amount of rainfall in the Red Sea did increase to no more than . Whether some former lakes in Arabia were actually marshes is contentious.


East Africa

Nile discharge was higher than today and during the early African humid period, the Nile in Egypt flooded up to higher than it did recently before flood control. The increased flooding may have turned the Nile Valley marshy and inhospitable and could explain why many archaeological sites along the Nile were abandoned during the AHP, with violent conflicts reconstructed from the Jebel Sahaba archaeological site. Early after the Younger Dryas, the Blue Nile would have been the major source of waters for the Nile. Waters from the Nile filled depressions like the Fayum Depression to form a deep lake with anoxic bottom waters and reaching above sea level, probably once a geomorphic barrier was breached. Wetlands and anastomosing channels developed in the
Nile Delta The Nile Delta ( ar, دلتا النيل, or simply , is the delta formed in Lower Egypt where the Nile River spreads out and drains into the Mediterranean Sea. It is one of the world's largest river deltas—from Alexandria in the west to Po ...
as sediment supply increased. In addition, Nile tributaries in northwestern Sudan such as
Wadi Al-Malik Wadi Al-Malik is the bed of an extinct river in Sudan. Following the Central African Shear Zone, it stretches for 560 km from the lake of Umm Badr in North Kurdufan NE-trending to the great bend of the Nile near Al Dabbah Al Dabbah (also ...
, Wadi Howar and Valley of the Queens became active during the AHP and contributed sediments to the Nile. Wadi Howar was active until 4,500 years ago, and at the time often contained dune-dammed lakes,
swamp A swamp is a forested wetland.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p. Swamps are considered to be transition zones because both land and water play a role in ...
s and wetlands; it was the largest Saharan tributary of the Nile and constituted an important pathway into sub-Saharian Africa. Conversely it appears that
Lake Victoria Lake Victoria is one of the African Great Lakes. With a surface area of approximately , Lake Victoria is Africa's largest lake by area, the world's largest tropical lake, and the world's second-largest fresh water lake by surface area after ...
and Lake Albert were not overflowing into the White Nile for all of the AHP, and the White Nile would have been sustained by overflow from
Lake Turkana Lake Turkana (), formerly known as Lake Rudolf, is a lake in the Kenyan Rift Valley, in northern Kenya, with its far northern end crossing into Ethiopia. It is the world's largest permanent desert lake and the world's largest alkaline lake. B ...
. There appears to be a tendency over the course of the AHP for the discharge of the Blue Nile to decrease relative to that of the White Nile. The Blue Nile built an alluvial fan at its confluence with the White Nile, and
incision Incision may refer to: * Cutting, the separation of an object, into two or more portions, through the application of an acutely directed force * A type of open wound caused by a clean, sharp-edged object such as a knife, razor, or glass splinter ...
by the Nile reduced flooding risk in some areas which thus became available for human use.
Closed lake An endorheic lake (also called a sink lake or terminal lake) is a collection of water within an endorheic basin, or sink, with no evident outlet. Endorheic lakes are generally saline as a result of being unable to get rid of solutes left in the l ...
s in East Africa rose, sometimes by hundreds of metres. Lake Suguta developed in the
Suguta Valley The Suguta Valley, also known as the Suguta Mud Flats, is an arid part of the Great Rift Valley in Kenya (Africa), directly south of Lake Turkana. Location The Suguta valley today is one of the driest parts of Kenya, with annual rainfall below . ...
, accompanied by the formation of river deltas where rivers such as the
Baragoi River Baragoi is a market town in Kenya, lying north of Maralal and east of the Suguta Valley. It is located in Samburu County. The entire Baragoi division has a population of nearly 20,000 (1999 census) comprising mostly people from the Samburu and T ...
entered the lake. In turn, Lake Suguta overflowed into the
Kerio River The Kerio River is a river in Turkana County, Kenya. It flows northward into Lake Turkana. It is one of the longest rivers in Kenya, originating near the equator. Course The Kerio River rises on the north slopes of the Amasya Hills to the west ...
, this adding water to
Lake Turkana Lake Turkana (), formerly known as Lake Rudolf, is a lake in the Kenyan Rift Valley, in northern Kenya, with its far northern end crossing into Ethiopia. It is the world's largest permanent desert lake and the world's largest alkaline lake. B ...
where increased discharge by the Turkwel River led to the formation of a large river delta. Over half of the water in Lake Turkana came from the Omo River, a decline compared to present-day conditions. Lake Turkana itself overflowed on its northwestern side through the Lotikipi Swamp into the
White Nile The White Nile ( ar, النيل الأبيض ') is a river in Africa, one of the two main tributaries of the Nile, the other being the Blue Nile. The name comes from the clay sediment carried in the water that changes the water to a pale color. ...
. Deposits from this lake highstand form the Galana Boi
Formation Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondary ...
. This overflowing large lake was filled with
freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...
and was populated by humans; the societies there engaged in
fishery Fishery can mean either the enterprise of raising or harvesting fish and other aquatic life; or more commonly, the site where such enterprise takes place ( a.k.a. fishing ground). Commercial fisheries include wild fisheries and fish farms, both ...
but could probably also fall back on other resources in the region. The Ethiopian
Lake Abhe Lake Abbe, also known as Lake Abhe Bad, is a salt lake, lying on the Ethiopia- Djibouti border. It is one of a chain of six connected lakes, which also includes (from north to south) lakes Gargori, Laitali, Gummare, Bario and Afambo. The lak ...
expanded to cover an area of , much larger than the present-day lake, in the "Abhe IV"-"Abhe V" lake cycle. The enlarged lake covered a large area west of the present-day lake, present-day lakes Afambo, Gamari and Tendaho, reducing Borawli,
Dama Ale Dama or DAMA may refer to: Animals * Dama gazelle (''Nanger dama'') * ''Dama'' (genus) (fallow deer) ** Fallow deer (''Dama dama'') **Persian fallow deer (''Dama mesopotamica'') * Tammar wallaby or Dama wallaby (''Macropus eugenii'') Business, s ...
and Kurub to islands. The maximum water level was reached during the early Holocene as river discharge increased, but was later limited by partial overflow and did not rise above again. Deep thermal groundwater recharge occurred in the region. About 9,000 years of human occupation are documented at the lake. Archaeological sites indicate that people obtained resources from the lake and followed its rise and decline. The cultural traditions at Lake Abhe appear to be unusual by AHP/African standards. Lake Zway and Lake Shala in Ethiopia joined with
Lake Abiyata Lake Abijatta is an alkaline lake in Ethiopia. It lies in the Main Ethiopian Rift valley south of Addis Ababa, in the Abijatta-Shalla National Park. Overview According to the ''Statistical Abstract of Ethiopia for 1967/68'', the lake is 17 kilo ...
and Lake Langano to form a large waterbody which began overflowing into the Awash River. Other lakes that expanded include Lake Ashenge and
Lake Hayq Lake Hayq ( Amharic: ሐይቅ ሐይቅ, ) is a freshwater lake of Ethiopia. It is located north of Dessie, in the Debub Wollo Zone of the Amhara Region. The town of Hayq is to the west of the lake. Lake Hayq is 6.7 km long and 6 km w ...
also in Ethiopia, Lake Bogoria, Lake Naivasha and Lake Nakuru/
Lake Elmenteita Lake Elmenteita is a soda lake, in the Great Rift Valley, about 120 km northwest of Nairobi, Kenya. Geography Elmenteita is derived from the Maasai word , meaning "dust place", a reference to the dryness and dustiness of the area, espec ...
all in Kenya,
Lake Masoko A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...
in Tanzania, and a lake formed in the
caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...
of the Menengai volcano. Overflow of several of these lakes allowed animals including Nile crocodiles and fish to propagate to the individual lake basins. A large and deep Lake Magadi formed in the early Holocene, and in the Danakil Depression of Ethiopia freshwater conditions became established. Lakes formed in depressions on the mountains around Lake Kivu. Finally, δ18O variations in Lake Garba Guracha in the
Bale Mountains The Bale Mountains (also known as the Urgoma Mountains) are mountain ranges in the Oromia Region of southeast Ethiopia, south of the Awash River, part of the Ethiopian Highlands. They include Tullu Demtu, the second-highest mountain in Ethiopia ...
document the beginning and end of the AHP. Glaciers stopped retreating or briefly expanded in East Africa at the beginning of the AHP before continuing retreat. On
Mount Kilimanjaro Mount Kilimanjaro () is a dormant volcano in Tanzania. It has three volcanic cones: Kibo, Mawenzi, and Shira. It is the highest mountain in Africa and the highest free-standing mountain above sea level in the world: above sea level and ab ...
they may have expanded during the AHP after a phase during the Younger Dryas where the mountain was ice free, but the tree line also rose at that time, accompanied by soil formation. The wetter climate may have destabilized the neighbouring Mount Meru volcano, causing a giant landslide that removed its summit. Erosion in catchments of East Africa increased with the beginning of the humid period but then decreased even before its end, as the increased weathering led to the formation of soils, these in turn to the establishment of a vegetation cover that subsequently reduced additional erosion. Increased weathering resulted in the increased consumption of atmospheric during the AHP. Surprisingly, and contrary to the patterns expected from precessional changes, the
East African Rift The East African Rift (EAR) or East African Rift System (EARS) is an active continental rift zone in East Africa. The EAR began developing around the onset of the Miocene, 22–25 million years ago. In the past it was considered to be part of a ...
also experienced a wetter climates during the AHP, reaching as far south as Lake Rukwa and Lake Cheshi into the Southern Hemisphere. In the region of the African Great Lakes,
pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...
evidence points to the occurrence of forests including
rainforest Rainforests are characterized by a closed and continuous tree canopy, moisture-dependent vegetation, the presence of epiphytes and lianas and the absence of wildfire. Rainforest can be classified as tropical rainforest or temperate rainfores ...
vegetation due to the increased precipitation, while today they occur only in limited areas there. Denser vegetation also occurred at
Lake Turkana Lake Turkana (), formerly known as Lake Rudolf, is a lake in the Kenyan Rift Valley, in northern Kenya, with its far northern end crossing into Ethiopia. It is the world's largest permanent desert lake and the world's largest alkaline lake. B ...
, with wooden vegetation covering almost half of the dry land although grasslands remained dominant. Development of forest vegetation around the African Great Lakes created an interconnected environment where species spread, increasing biodiversity with effects on the future when the environment became fragmented. Vegetation cover also increased in the
Afar Afar may refer to: Peoples and languages *Afar language, an East Cushitic language *Afar people, an ethnic group of Djibouti, Eritrea, and Ethiopia Places Horn of Africa *Afar Desert or Danakil Desert, a desert in Ethiopia *Afar Region, a region ...
region and Ericaceae plants spread at high elevations. Forests and moisture-requiring vegetation expanded in the
Bale Mountains The Bale Mountains (also known as the Urgoma Mountains) are mountain ranges in the Oromia Region of southeast Ethiopia, south of the Awash River, part of the Ethiopian Highlands. They include Tullu Demtu, the second-highest mountain in Ethiopia ...
. Different types of vegetation, including dryland vegetation, existed at Lake Malawi and
Lake Tanganyika Lake Tanganyika () is an African Great Lake. It is the second-oldest freshwater lake in the world, the second-largest by volume, and the second-deepest, in all cases after Lake Baikal in Siberia. It is the world's longest freshwater lake. ...
however, and vegetation did not change much. A wetter climate is recorded from a soil in the Afar region. In East Africa, the AHP led to improved environmental conditions in terms of food and water supply from large lakes, allowing early human populations to survive and grow in size without requiring major changes in food gathering strategies. Pottery techniques such as the "dotted wavy line" and "Kanysore" are associated with fishing and foraging communities. Earlier wet and dry periods in East Africa may have influenced the
evolution of humans Human evolution is the evolutionary process within the history of primates that led to the emergence of '' Homo sapiens'' as a distinct species of the hominid family, which includes the great apes. This process involved the gradual development ...
and allowed their spread across the Sahara and into Europe.


Other parts of Africa and the rainforest realm

Lake Bosumtwi in Ghana rose during the AHP. Evidence there also suggests a decrease in wildfire activity took place. Tropical forests expanded in Cameroon Highlands and the Adamawa Plateau of Cameroon and moved upward at Lake Bambili also in Cameroon. The core of the
rainforest Rainforests are characterized by a closed and continuous tree canopy, moisture-dependent vegetation, the presence of epiphytes and lianas and the absence of wildfire. Rainforest can be classified as tropical rainforest or temperate rainfores ...
was probably unaltered by the African humid period, perhaps with some changes in species and an expansion of their area, although the peatlands of Central Congo started developing during the African humid period and peat continues to accumulate there to this day, albeit with a slowdown in the
Cuvette Centrale The Cuvette Centrale ( French: "Central Basin") is a region of forests and wetlands in the Democratic Republic of the Congo. Some definitions consider the region to extend into the Republic of the Congo as well. The Cuvette Centrale lies in the cent ...
after the end of the African humid period. On São Nicolau and Brava in the Cape Verde Islands, precipitation and erosion increased. In the
Canary Islands The Canary Islands (; es, Canarias, ), also known informally as the Canaries, are a Spanish autonomous community and archipelago in the Atlantic Ocean, in Macaronesia. At their closest point to the African mainland, they are west of Morocc ...
, there is evidence of a moister climate on
Fuerteventura Fuerteventura () is one of the Canary Islands, in the Atlantic Ocean, part of the North Africa region, and politically part of Spain. It is located away from the northwestern coast of Africa. The island was declared a biosphere reserve by UNES ...
, the laurel forests changed perhaps as a consequence of the AHP. Recharge of groundwater levels have been inferred from Gran Canaria also in the Canary Islands, followed by a decrease after the end of the AHP. Choughs may have reached the Canary Islands from North Africa when the latter was wetter.


Levant and Mediterranean

High latitude Africa has not undergone large scale changes in the past 11,700 years; the
Atlas Mountains The Atlas Mountains are a mountain range in the Maghreb in North Africa. It separates the Sahara Desert from the Mediterranean Sea and the Atlantic Ocean; the name "Atlantic" is derived from the mountain range. It stretches around through Moroc ...
may have blocked the monsoon from expanding further north. However, river valley and cave deposits showing a moister climate in southern Morocco, vegetation changes in the Middle Atlas, several floods in Tunisian rivers and ecosystem changes which impacted
steppe In physical geography, a steppe () is an ecoregion characterized by grassland plains without trees apart from those near rivers and lakes. Steppe biomes may include: * the montane grasslands and shrublands biome * the temperate grasslands, ...
-dependent rodents of Northern Africa have been linked to the AHP. In the Pleistocene and Holocene humidity in the Mediterranean is often correlated to humidity in the Sahara, and the early-mid Holocene climate of Iberia, Italy, Negev and Northern Africa was wetter than today; in Sicily wettening correlates with ITCZ changes in Northern Africa. Mediterranean precipitation is brought by Mediterranean
cyclone In meteorology, a cyclone () is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above (opposite to an anti ...
s and the westerlies; either increased precipitation from the westerlies, northward moisture transport from Africa or monsoonal precipitation extending into the Mediterranean may have rendered it wetter. The connection between the African Monsoon and Mediterranean precipitation is unclear and it was winter rainfall that increased predominantly, although separating monsoonal and non-monsoonal precipitation can be difficult. The Mediterranean Sea became less saline during the AHP, in part due to increased precipitation from the westerlies but also from increased river discharge in Africa, leading to the formation of sapropel layers when the increased runoff led to the Mediterranean becoming more stratified and eutrophied, with changes in the main water masses of the sea. The S1 sapropel layer is specifically associated with the AHP and with increased discharge of the Nile and other African rivers. These processes together with decreased dust transport by wind led to changes in the sediment patterns of the Mediterranean, and increased marine nutrient availability and food web productivity in the Mediterranean, which impacted the development of deep-sea corals. In the Levant, wetter conditions during the AHP are recorded from Jeita Cave in Lebanon and
Soreq Cave Avshalom Cave ( he, מערת אבשלום, Me'arat Avshalom), known in academic literature as Soreq Cave ( he, מערת שׂורק, Me'arat Soreq), ar, مغارة سوريك, Mghar Suriq) an popularly as Stalactites Cave ( he, מערת הנטי ...
in Israel while the
Dead Sea The Dead Sea ( he, יַם הַמֶּלַח, ''Yam hamMelaḥ''; ar, اَلْبَحْرُ الْمَيْتُ, ''Āl-Baḥrū l-Maytū''), also known by other names, is a salt lake bordered by Jordan to the east and Israel and the West Bank ...
and other southern European lakes were low during this period. This is unlike some earlier wet periods in the Sahara; possibly the stronger winter-summer insolation gradient in these earlier wet periods created a different moisture pattern than during the Holocene. The northern Mediterranean may have been drier, with more wildfire activity, during the AHP.


Southern Africa

The effects, if any, of the African humid period on Southern Africa have been unclear. Originally it was proposed that the orbitally driven changes would imply a dry period in Southern Africa which would have given way to moister conditions as the northern AHP ended, as the ITCZ should shift its average position between the two hemispheres. However, the lack of paleoclimatology data with sufficient time resolution from Southern Africa has made it difficult to assess the climate there during the AHP. More recently obtained paleoclimate data have suggested however that southern Africa was actually wetter during the AHP rather than drier, reaching as far as north-northwest Madagascar 23° south and as far as the catchment of the Orange River. The area between
Lake Tanganyika Lake Tanganyika () is an African Great Lake. It is the second-oldest freshwater lake in the world, the second-largest by volume, and the second-deepest, in all cases after Lake Baikal in Siberia. It is the world's longest freshwater lake. ...
and Lake Malawi has been interpreted as the limit of the AHP's influence. Conversely, and consistent with the opposite reaction pattern of the Southern Hemisphere, the Zambezi River reached its lowest discharge during the AHP, and the AHP did not reach southern or southeastern Africa. There may have been opposite changes in precipitation between southeast Africa and tropical East Africa, separated by a "hinge zone". Particular changes occurred in central southern Africa, where a dry period co-occurred with an expansion of Lake Makgadikgadi; presumably the lake during this dry interval was nourished by increased wetness over the Okavango River catchment in the Angolan Highlands due to the AHP; peatlands formed in Angola during the AHP. In general there is little consistency between Northern and Southern Africa in terms of hydrological changes during the Holocene, and nowhere are both the start and end of the AHP apparent. Orbitally-mediated changes in Northern Hemisphere climate affected the Southern Hemisphere through oceanic pathways involving sea surface temperatures. Additionally, wetter periods unrelated to the AHP may have occurred after deglaciation in Southern Africa.


Numerical estimates

Estimates of the exact amount of increased precipitation vary widely. During the African humid period, Saharan rainfall increased to , and values exceeding may have spread to 19–21° northern latitude. In the eastern Sahara, a gradient from increment in the north to in the south has been identified. An area with less than may have remained in the Eastern Sahara however, although its driest parts may have received 20-fold more precipitation than today. Precipitation in the Sahara probably reached no more than , with large uncertainty. Other reconstructed values of the precipitation increase indicate an annual increase of about in Africa, with strong regional variation. From lake levels precipitation increases of 20–33% or 50–100%/40-150% have been inferred for East Africa, with an increase of 40% reconstructed for Northern Africa. In the early Holocene, there appears to have been an eastward- and northward-decreasing trend of humidity. Additionally, at Tayma in Arabia a threefold increase appears to have occurred and precipitation in the Wahiba Sands of Oman may have reached .


Effect on other climate modes

The El Niño–Southern Oscillation is a major climate variability mode. Paleoclimatology records from Ecuador and the Pacific Ocean indicate that during the early and middle Holocene ENSO variability was suppressed by about 30–60%, which can be only partially explained through orbital forcing. The Green Sahara may have suppressed ENSO activity, forcing a La Niña–like climate state, in a climate model this is accompanied by decreased upwelling and deepening of the thermocline in the Eastern Pacific as the Walker circulation shifts westward. In addition, Atlantic Niño sea surface temperature patterns develop in the Atlantic Ocean. Remote effects of the AHP on the monsoons of the Northern Hemisphere have also been studied. In climate models, the strengthened and expanding monsoons of Africa and Asia alter the atmospheric circulation of the planet, inducing a wetter
East Asian Monsoon The East Asian Monsoon is a monsoonal flow that carries moist air from the Indian Ocean and Pacific Ocean to East Asia. It affects approximately one-third of the global population, influencing the climate of Japan, the Korean Peninsula, Taiwan, Ch ...
and drying across tropical South America and central-eastern North America. The reduced dust emission warms the North Atlantic and increases westerly flow into the North American Monsoon, strengthening it. The far-field precipitation changes reach as far as Europe and Australia. Discrepancies between modelled and reconstructed northward extension and precipitation in the Asian monsoon regions and the North American Monsoon area may be explained through these remote effects. Sun ''et al.'' 2020 proposed that the greening of the Sahara during the AHP can increase precipitation over the Middle East even if neither the African nor the Indian monsoons reach it. During spring, the increased vegetation forces anomalous atmospheric circulations that direct moisture transport from the Mediterranean, the Red Sea and eastern tropical Africa into the Middle East, increasing precipitation and agricultural productivity there. This could explain increased precipitation in the Middle East during the AHP: A wet climate occurred in the Middle East during the early Holocene, leading to the Ubaid period of settlement in Mesopotamia, followed by dry phases around 5,500 years ago and a concomitant reduction in simulated wheat yield. The Atlantic meridional overturning circulation (AMOC) transports heat from the Southern into the Northern Hemisphere and is implicated in starting the Holocene AHP an earlier AHPs after the end of an ice age. Various studies have been conducted to determine which effects reduced dust supply and the greening of the Sahara would have had on its intensity, with conflicting results on which effects would predominate. Increased heat transport either through the atmosphere or the ocean would result in warming in the Arctic.


Hurricanes and the AHP

One climate model has indicated that a greener Sahara and reduced dust output would have increased tropical cyclone activity, especially over the Atlantic but also in most other tropical cyclone basins. Changes in the intensity of the storms, decreases in wind shear, changes in atmospheric circulation and less dust in the atmosphere, which results in warmer oceans, are responsible for this phenomenon, despite an expected decrease of
tropical wave A tropical wave (also called easterly wave, tropical easterly wave, and African easterly wave), in and around the Atlantic Ocean, is a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which ...
activity over the Atlantic in climate models. The net effect could be a global increase in tropical cyclone activity, a westward shift within the ocean basins and in the Atlantic Ocean a shift towards later dates. While there are no good paleotempestology data for the time of the African humid period that could confirm or refute this theory and many of these records are specific for particular locations, hurricane activity including past strikes in Puerto Rico and in Vieques appear to correlate with the strength of the West African Monsoon and increased precipitation on the northern Yucatan Peninsula during the middle Holocene could be explained by increased hurricane activity during the AHP. On the other hand, at Grand Bahama Bank and the Dry Tortugas of
South Florida South Florida is the southernmost region of the U.S. state of Florida. It is one of Florida's three most commonly referred to directional regions; the other two are Central Florida and North Florida. South Florida is the southernmost part of th ...
a decrease of hurricane activity took place during the AHP and dust emission is not always anti-correlated to hurricane activity. Finally, the northward movement of the ITCZ during the AHP may have caused a corresponding northward movement of tropical cyclogenesis areas and storm tracks in the Atlantic Ocean, which could also explain decreased hurricane activity in the Bahamas and Dry Tortugas.


Fluctuations

Some gaps with less precipitation took place during the late glacial and the Holocene. During the Younger Dryas 12,500–11,500 years ago, the North Atlantic and Europe became much colder again and there was a phase of drought in the area of the African humid period, extending over both East Africa, where lake levels dropped in many places, southern Africa and West Africa. The dry interval extended to India and the Mediterranean where
dune A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, f ...
activity occurred in the Negev. At the end of the Younger Dryas, precipitation, lake levels and river runoff increased again, although south of the equator the return of humid conditions was slower than the relatively abrupt change to its north. Another dry phase took place about 8,200 years ago, spanning East Africa and Northern Africa as documented by various lines of evidence such as decreased water levels in lakes. It coincided with cooling in the Northern Atlantic, in surrounding landmasses such as Greenland and around the world; the drought may be related to the
8.2 kiloyear event In climatology, the so-called "8.2-kiloyear event" was a sudden decrease in global temperatures that occurred approximately 8,200 years before the present ( BP), that is, 6,251 BC. It defines the start of the Northgrippian age in the Holocene ...
which separates the Greenlandian and Northgrippian stages of the Holocene and lasted for about one millennium. The 8,200 year event has also been noted in the Maghreb, where it is associated with a transition of the Capsian culture as well as with cultural changes both in the Sahara and the Mediterranean; at the Gobero cemetery a population change occurred after this dry interruption but the occurrence of widespread cultural changes appears to be questionable. This episode appears to have been caused by the draining of ice-dammed lakes in North America although a low latitude origin has also been suggested. Cooling of the Northern Atlantic during Heinrich event 1 and the Younger Dryas associated with a weaker Atlantic meridional overturning circulation leads to atmospheric pressure anomalies that shift the Tropical Easterly Jet and precipitation belts south, making Northern Africa drier. Storm tracks shift north away from the Mediterranean. Earlier Heinrich events were also accompanied by drought in North Africa. Likewise, a weakening of moisture transport and a less eastward position of the Congo Air Boundary contributed to reducing precipitation in East Africa although some parts of southern Africa at Lake Malawi were wetter during the Younger Dryas. Many humidity fluctuations in the early Holocene appear to be caused by the discharge of
meltwater Meltwater is water released by the melting of snow or ice, including glacial ice, tabular icebergs and ice shelves over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reducing. Meltwater can be ...
from the Laurentide Ice Sheet into the Atlantic, which weakens the Atlantic meridional overturning circulation. Some dry periods in
marine core A core sample is a cylindrical section of (usually) a naturally-occurring substance. Most core samples are obtained by drilling with special drills into the substance, such as sediment or rock, with a hollow steel tube, called a core drill. The h ...
s in the Gulf of Guinea appear to coincide with events recorded in Greenland
ice core An ice core is a core sample that is typically removed from an ice sheet or a high mountain glacier. Since the ice forms from the incremental buildup of annual layers of snow, lower layers are older than upper ones, and an ice core contains ic ...
s. Other variations in precipitation observed in records have been attributed to solar activity changes, water levels of
Lake Turkana Lake Turkana (), formerly known as Lake Rudolf, is a lake in the Kenyan Rift Valley, in northern Kenya, with its far northern end crossing into Ethiopia. It is the world's largest permanent desert lake and the world's largest alkaline lake. B ...
for example appear to reflect the 11-year solar cycle. In
Lake Turkana Lake Turkana (), formerly known as Lake Rudolf, is a lake in the Kenyan Rift Valley, in northern Kenya, with its far northern end crossing into Ethiopia. It is the world's largest permanent desert lake and the world's largest alkaline lake. B ...
, water level fluctuations took place between 8,500 and 4,500 years before present, with highstands before 8,400, around 7,000 and between 5,500 and 5,000 and lowstands around 8,000, 10,000 and 12,000 years before present. In total, five separate highstands are recorded in
desert varnish Desert varnish or rock varnish is an orange-yellow to black coating found on exposed rock surfaces in arid environments. Desert varnish is approximately one micrometer thick and exhibits nanometer-scale layering. Rock rust and desert patina are ot ...
around the lake. The highstands appear to be controlled by sea surface temperature patterns in the Atlantic and Indian Oceans, but also by overflow of water from Lake Suguta and - intermittently - the
Chew Bahir Lake Chew Bahir (Amharic: ጨው ባሕር ''č̣ew bāhir'', "salty lake") or Lake Istifanos, also called Stefanie, Basso Naebor and Chuwaha, is a lake in southern Ethiopia, located on the southwestern end of the Southern Nations, Nationalities ...
basins into Lake Turkana, which themselves received water from additional lakes. Volcanic and tectonic phenomena occur at Lake Turkana, but do not have the magnitude required to explain large changes in lake level. Water level fluctuations have also been inferred for
Lake Chad Lake Chad (french: Lac Tchad) is a historically large, shallow, endorheic lake in Central Africa, which has varied in size over the centuries. According to the ''Global Resource Information Database'' of the United Nations Environment Programme, ...
on the basis of pollen data, especially towards the end of the AHP. In the Taoudenni lake fluctuations of about a quarter-millennium have been recorded and frequent droughts occurred in the Eastern Sahara. Other variations appear to have occurred 9,500–9,000 and 7,400–6,800 as well as 10,200, 8,200, 6,600 and 6,000 years before present; they were accompanied by decreased population density in parts of the Sahara, and other dry interludes in Egypt have been noted 9,400–9,300, 8,800–8,600, 7,100–6,900 and 6,100–5,900 years ago. The duration and severity of dry events is difficult to reconstruct and the impact of events like the Younger Dryas is heterogeneous even between neighbouring areas. During dry episodes, humans might have headed to waterbodies which still had resources, and cultural changes in the central Sahara have been linked to some dry episodes. Aside from fluctuations, a southward retreat of the humid period may have been underway after 8,000 years ago with a major drought around 7,800 years ago.


End

The African humid period ended about 6,000–5,000 years ago; an ending date of 5,500 years before present is often used. After vegetation declined, the Sahara became barren and was claimed by sand. Wind erosion increased in northern Africa, and dust export from the now-desert and from dried up lakes such as the Bodélé Basin grew; Bodélé today is the largest single source of dust on Earth. The lakes dried up,
mesic Mesic may refer to: * Mesic, North Carolina, a town in the United States * Mesic habitat, a type of habitat See also *Mesić (disambiguation) *Mešić Mešić is a Bosnian surname, a patronymic derived from the masculine given name '' Meša'', it ...
vegetation disappeared, and sedentary human populations were replaced by more mobile cultures. The transition from the "green Sahara" to the present-day dry Sahara is considered to be the greatest environmental transition of the Holocene in northern Africa; today almost no precipitation falls in the region. The end of the AHP but also its beginning could be considered a "climate crisis" given the strong and extended impact. Drying extended as far as the
Canary Islands The Canary Islands (; es, Canarias, ), also known informally as the Canaries, are a Spanish autonomous community and archipelago in the Atlantic Ocean, in Macaronesia. At their closest point to the African mainland, they are west of Morocc ...
and southeastern Iran, and there is evidence of climate change on São Nicolau,
Cape Verde , national_anthem = () , official_languages = Portuguese , national_languages = Cape Verdean Creole , capital = Praia , coordinates = , largest_city = capital , demonym ...
. The Piora Oscillation cold period in the Alps coincides with the end of the AHP; the period 5,600–5,000 years ago was characterized by widespread cooling and more variable precipitation changes around the world and was possibly forced by changes in solar activity and orbital parameters. Some changes in climate possibly extended into southeastern
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
, Central America and into South America. The
neoglacial The neoglaciation ("renewed glaciation") describes the documented cooling trend in the Earth's climate during the Holocene, following the retreat of the Wisconsin glaciation, the most recent glacial period. Neoglaciation has followed the hypsither ...
began. A major pan-tropical environmental change took place about 4,000 years ago. This change was accompanied by the collapse of ancient civilizations, severe drought in Africa, Asia and the Middle East and the retreat of
glaciers A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as ...
on
Mount Kilimanjaro Mount Kilimanjaro () is a dormant volcano in Tanzania. It has three volcanic cones: Kibo, Mawenzi, and Shira. It is the highest mountain in Africa and the highest free-standing mountain above sea level in the world: above sea level and ab ...
and
Mount Kenya Mount Kenya (Kikuyu: ''Kĩrĩnyaga'', Kamba, ''Ki Nyaa'') is the highest mountain in Kenya and the second-highest in Africa, after Kilimanjaro. The highest peaks of the mountain are Batian (), Nelion () and Point Lenana (). Mount Kenya is locat ...
.


Chronology

Whether the drying happened everywhere at the same time and whether it took place in centuries or millennia is unclear in part due to disagreeing records and has led to controversy, and such a disagreement on timing also exists with respect to the expected vegetation changes.
Marine core A core sample is a cylindrical section of (usually) a naturally-occurring substance. Most core samples are obtained by drilling with special drills into the substance, such as sediment or rock, with a hollow steel tube, called a core drill. The h ...
s usually indicate an abrupt change but not without exceptions while
pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...
data do not, perhaps due to regional and local differences in vegetation. Africa is a diverse landscape and groundwater and local vegetation can modify local conditions; groundwater-fed water bodies for example persisted longer than those nourished by rain. The debate on how quickly the Sahara formed goes back to 1849, when the Prussian naturalist Alexander von Humboldt suggested that only a quick drying could form the desert. Most recently, the idea has taken hold that the end of the African humid period occurred from north to south in a stepwise fashion. In the western Sahara and east Africa it ended within 500 years with a one-step drying 6,000 – 5,000 years ago north of the present-day monsoon belt. Farther south, precipitation decrease was more protracted and closer to the equator the AHP ended between 4,000 and 2,500 years ago. In East Africa, pronounced drying occurred between 4,500 and 3,500 years ago, centered on 4,000 years ago; Egypt during the Old Kingdom was still wetter than today. A later end in northeast Africa about 4,000 years ago may reflect the different configuration of landmasses and thus monsoon behaviour, while other research has found a westward propagating drying trend. Some evidence points to a two-phase change in climate with two distinct dry transitions caused by the existence of two different steps of insolation decrease at which climate changes. Distinct environmental changes may have occurred in Central Africa, Western Africa and East Africa. Finally, sometimes the
4.2 kiloyear event The 4.2-kiloyear (thousand years) BP aridification event (long-term drought) was one of the most severe climatic events of the Holocene epoch. It defines the beginning of the current Meghalayan age in the Holocene epoch. Starting around 220 ...
- the transition from the Northgrippian to the Meghalayan stage of the Holocene - is considered to be the true end of the AHP, especially in central Africa. Increased variability in precipitation may have preceded the end of the AHP; this is commonly observed before a sudden change in climate. In Gilf Kebir, between 6,300 and 5,200 years ago apparently a winter rainfall regime became established as the AHP ended. Later fluctuations in climate that produced brief humid spells also took place, such as a moister period between 500 BCE – 300 CE in Roman Northern Africa and along the
Dead Sea The Dead Sea ( he, יַם הַמֶּלַח, ''Yam hamMelaḥ''; ar, اَلْبَحْرُ الْمَيْتُ, ''Āl-Baḥrū l-Maytū''), also known by other names, is a salt lake bordered by Jordan to the east and Israel and the West Bank ...
and an earlier one 2,100 years before present in the western Sahel. By 2,700 years ago the central Sahara had become a desert and remained one until the present-day.


Sahara and Sahel

After a first brief lake level drop between 5,700 and 4,700 calibrated years ago that might reflect climate variability towards the end of the African humid period, water levels in
Lake Megachad Lake Chad (french: Lac Tchad) is a historically large, shallow, endorheic lake in Central Africa, which has varied in size over the centuries. According to the ''Global Resource Information Database'' of the United Nations Environment Programme, ...
decreased quickly after 5,200 years before present. It shrank to about 5% of its former size, with the deeper northern Bodele basin drying up entirely about 2,000-1,000 years ago as it was disconnected from the southern basin where its major tributary, the Chari River, enters Lake Chad. The dried out basin was now exposed to the
Harmattan The Harmattan is a season in West Africa that occurs between the end of November and the middle of March. It is characterized by the dry and dusty northeasterly trade wind, of the same name, which blows from the Sahara over West Africa into the ...
winds, which blow dust out of the dry lake bed, making it the single largest source of dust in the world. Dunes formed in the dried-up Sahara or began moving again after stabilizing during the AHP. The tropical vegetation was replaced by desert vegetation, in some places suddenly and in others more gradually. Along the Atlantic coast, the vegetation retreat was slowed by a stage of sea level rise that increased soil moisture levels, delaying the retreat by about two millennia. In Libya at Wadi Tanezzuft the end of the humid period was also delayed by leftover water in dune systems and in the Tassili mountains until 2,700 years ago, when river activity finally ceased. A brief moist pulse between 5,000 – 4,000 years ago in the Tibesti led to the development of the so-called "Lower Terrace". The Egyptian Sahara might still have been vegetated until 4,200 years ago, based on depictions of savanna environments in Fifth Dynasty tombs in Egypt. At Lake Yoa, which is groundwater-fed, vegetation decreased and became desert vegetation between 4,700–4,300 and 2,700 years ago, while the lake became hypersaline 4,000 years ago. However, the climate there may have been affected by the Tibesti Mountains and the end of the AHP thus delayed, and fossil groundwater left by the AHP nourishes the lake to this day. In the central Sahara, water resources in the mountains persisted longer.


East Africa and Arabia

In northern East Africa, water levels dropped rapidly about 5,500 years ago while in Hoti cave in Arabia a southward retreat of the Indian Monsoon took place about 5,900 years ago. Drying is also documented from Oman, and rivers and lakes of Arabia became intermittent or entirely dry. The Blue Nile basin became less moist with a noticeable decrease of Nile discharge about 4,000 years ago. Decreased discharge of the Nile led to the cessation of sapropel deposition and turbidite activity off its delta, the abandonment of river channels in its delta and upstream and increased seawater influence in the delta. Some data from Ethiopia and the
Horn of Africa The Horn of Africa (HoA), also known as the Somali Peninsula, is a large peninsula and geopolitical region in East Africa.Robert Stock, ''Africa South of the Sahara, Second Edition: A Geographical Interpretation'', (The Guilford Press; 2004), ...
indicate that drying there may have begun already 7,000–8,000 years ago or earlier. Reconstructions from
Lake Abiyata Lake Abijatta is an alkaline lake in Ethiopia. It lies in the Main Ethiopian Rift valley south of Addis Ababa, in the Abijatta-Shalla National Park. Overview According to the ''Statistical Abstract of Ethiopia for 1967/68'', the lake is 17 kilo ...
in Ethiopia suggest that the end of the African humid period took the form of severe droughts rather than a gradual decrease of precipitation. Drying in Arabia commenced about 7,000 calibrated years ago and there are large disparities in the timing between various parts of Arabia but a tendency towards an arid climate between 6,000 and 5,000 years ago has been observed which continued until 2,700 years ago. In the
Bale Mountains The Bale Mountains (also known as the Urgoma Mountains) are mountain ranges in the Oromia Region of southeast Ethiopia, south of the Awash River, part of the Ethiopian Highlands. They include Tullu Demtu, the second-highest mountain in Ethiopia ...
and the Sanetti Plateau of Ethiopia vegetation changes signalling a drier climate took place around 4,600 years ago. Forest cover in the area of the African Great Lakes decreased between 4,700 and 3,700 years ago, although drying at
Lake Victoria Lake Victoria is one of the African Great Lakes. With a surface area of approximately , Lake Victoria is Africa's largest lake by area, the world's largest tropical lake, and the world's second-largest fresh water lake by surface area after ...
had begun around 8,000 years ago, at Lake Rukwa 6,700 years ago, at Lake Tanganyika about 6,000 years ago and at Lake Edward major changes in lake chemistry consistent with drying are noted 5,200 years ago. There a minor recovery in vegetation took place between 2,500 and 2,000 years ago, followed by a much more rapid appearance of grasses accompanied also by substantial wildfire activity. This might have been the most severe drought of the Lake Edward region in the Holocene, with many lakes such as Lake George dropping significantly or drying up altogether. Other lakes such as Nakuru, Turkana, Lake Chew Bahir, Lake Abbe and Lake Zway also dropped between 5,400 and 4,200 years ago. Decreased vegetation cover in the catchment of the Blue Nile has been correlated with increased sediment transport in the river beginning 3,600 – 4,000 years ago. The end of the AHP at
Lake Turkana Lake Turkana (), formerly known as Lake Rudolf, is a lake in the Kenyan Rift Valley, in northern Kenya, with its far northern end crossing into Ethiopia. It is the world's largest permanent desert lake and the world's largest alkaline lake. B ...
occurred about 5,000-5,300 years before present, accompanied by a lake level decline and the cessation of overflow from other lakes in its area into Lake Turkana. Between 5,000 and 4,200,
Lake Turkana Lake Turkana (), formerly known as Lake Rudolf, is a lake in the Kenyan Rift Valley, in northern Kenya, with its far northern end crossing into Ethiopia. It is the world's largest permanent desert lake and the world's largest alkaline lake. B ...
became more saline and its water levels decreased below the level of outflow to the Nile. Towards the end of the AHP water temperatures in the lake and in other regional lakes appear to have increased, followed by a drop after its end possibly resulting from the
insolation Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/m ...
seasonality pattern that was in force at the time of the end of the AHP. The decrease of water levels in Lake Turkana also impacted the Nile and the Predynastic societies dependent on it.


Mediterranean

Libya and the Middle Atlas became gradually more dry, and drying in Morocco took place about 6,000 radiocarbon years ago, Drier conditions in Iberia accompanied the end of the African humid period between 6,000 and 4,000 years ago, perhaps as a consequence of increasingly frequent positive North Atlantic Oscillation episodes and the shift of the ITCZ. More complicated changes have been found for the northern margin of the Mediterranean. A
4.2 kiloyear event The 4.2-kiloyear (thousand years) BP aridification event (long-term drought) was one of the most severe climatic events of the Holocene epoch. It defines the beginning of the current Meghalayan age in the Holocene epoch. Starting around 220 ...
is recorded in dust records from the Mediterranean and might have been caused by changes in the circulation of the Atlantic Ocean.


Tropical West Africa

In Lake Bosumtwi the African humid period ended about 3,000 years ago after a brief moistening between 5,410 ± 80 years ago that ended 3,170 ± 70 years ago. This, earlier but similar changes off western Senegal and later but similar changes in the Congo Fan appear to reflect a southward shift of the precipitation zone over time. Some drying occurred simultaneously between the Sahel and the Gulf of Guinea. Some lakes in the Guineo-Congolian region dried out, while others were relatively unaffected. A general tendency towards a drier climate is observed in West Africa at the end of the AHP. There, dense vegetation became progressively thinner between 5,000 and 3,000 years ago, and major perturbations of the vegetation took place around 4,200 and 3,000–2,500 /2,400 calibrated years ago. A brief return of moister conditions took place 4,000 years ago while a substantial dry phase occurred between 3,500 and 1,700 years ago. Aridity became established between 5,200 and 3,600 years ago in the Sahara. In Senegal modern-type vegetation arose about 2,000 years ago.


Central Africa

Farther south at the
equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...
between 6,100 and 3,000 calibrated years before present
savannah A savanna or savannah is a mixed woodland-grassland (i.e. grassy woodland) ecosystem characterised by the trees being sufficiently widely spaced so that the Canopy (forest), canopy does not close. The open canopy allows sufficient light to rea ...
expanded at the expense of forests, with the transition possibly lasting until 2,500 calibrated years before present; a different time course estimate for the area between 4° southern and 7° northern latitude states that forest cover decreased between 4,500 and 1,300 years ago. In the Adamawa Plateau ( Cameroon), the Ubangui Plateau ( Central African Republic) and the Cameroon Volcanic Line montane forests disappeared at the end of the African humid period. In the Adamawa Plateau savanna has continuously expanded since 4,000 calibrated years ago. Such a change took also place in Benin and Nigeria between 4,500 and 3,400 calibrated years ago. In the
Congo Basin The Congo Basin (french: Bassin du Congo) is the sedimentary basin of the Congo River. The Congo Basin is located in Central Africa, in a region known as west equatorial Africa. The Congo Basin region is sometimes known simply as the Congo. It con ...
, there were changes in the composition and density of the forests rather than their extent. Many vegetation changes in the tropical regions were probably caused by a longer dry season and perhaps a smaller latitudinal range of the ITCZ.


Southern Hemisphere Africa

In the Southern Hemisphere at Lake Malawi drying began later – 1,000 years before present – as did the African humid period which there began only about 8,000 years ago. Contrarily, increased water levels in Etosha Pan ( Namibia) appear to relate to a southward movement of the ITCZ at the end of the AHP although
stalagmite A stalagmite (, ; from the Greek , from , "dropping, trickling") is a type of rock formation that rises from the floor of a cave due to the accumulation of material deposited on the floor from ceiling drippings. Stalagmites are typically ...
growth data in Dante Cave also in Namibia has been interpreted as indicating a wetter climate during the AHP.


Mechanisms

The end of the humid period appears to reflect the changes in
insolation Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/m ...
during the Holocene, as a progressive decrease of summer insolation caused the insolation gradients between Earth's hemispheres to decrease. However, the drying appears to have been much more abrupt than the insolation changes; it is not clear whether
non-linear In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many other ...
feedbacks led to abrupt changes in climate and it is also unclear whether the process, driven by orbital changes, was abrupt. Also, the Southern Hemisphere warmed and this resulted in a southward shift of the ITCZ; orbitally-driven insolation has increased over the Holocene in the Southern Hemisphere. As precipitation decreased, so did vegetation, in turn increasing the albedo and further decreasing precipitation. Furthermore, vegetation may have responded to increased variations in precipitation towards the end of the AHP although this view has been challenged. This could have directed sudden changes in precipitation, although this view has been cast in doubt by the observation that in many places the end of the African humid period was gradual rather than sudden. Plants at higher and lower latitudes might respond differently to climate change; for example more diverse plant communities might have slowed down the end of the AHP. Other proposed mechanisms: * Decreases in polar insolation through altered cosmic ray fluxes might promote the growth of
sea ice Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oce ...
and cooling at high latitudes, which in turn result in stronger equator-to-pole temperature gradients, stronger
subtropical anticyclone The horse latitudes are the latitudes about 30 degrees north and south of the Equator. They are characterized by sunny skies, calm winds, and very little precipitation. They are also known as subtropical ridges, or highs. It is a high-pressure ...
s and more intense upwelling in for example the Benguela current. * Changes in the circulation of high latitude oceans may have played a role, such as the potential occurrence of another
meltwater Meltwater is water released by the melting of snow or ice, including glacial ice, tabular icebergs and ice shelves over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reducing. Meltwater can be ...
/ ice rafting pulse around 5,700 years before present. The decreased insolation during the mid-Holocene may have made the climate system more sensitive to changes, explaining why earlier comparable pulses did not terminate the humid period for good. * There is evidence that glaciers in Tibet such as at Nanga Parbat expanded during the Holocene, especially towards the end of the AHP. In climate models, increased snow and ice on the Tibetan Plateau can lead to a weakening of the Indian and African monsoons, with the weakening of the former preceding that of the latter by 1,500–2,000 years. * Decreases in sea surface temperatures of the Indian Ocean may be involved in the drying of East Africa, but there is no agreement on the temperature records from that ocean. Moreover, there is no evidence of temperature changes in the Gulf of Guinea at the critical time that might explain the end of the AHP. * Additional feedback processes may have included the drying of soils and loss of vegetation after decreased rainfall, which would have led to wind-driven deflation of the soils. * An expansion of
sea ice Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oce ...
around Antarctica about 5,000 calibrated years ago may have provided another positive feedback. * The expanding dry belt of the Sahara pushed the regions of cyclogenesis in the Mediterranean northwest-northward, resulting in wind changes and precipitation regime changes in parts of Italy. * Climate change at high latitudes has been proposed as a cause for the end of the AHP. Specifically, about 6,000–5,000 years ago the Arctic became colder, with
sea ice Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oce ...
expanding, temperatures in Europe and off Northern Africa decreasing and the Atlantic meridional overturning circulation weakening. This cooling tendency may have weakened the Tropical Easterly Jet and thus reduced the amount of precipitation falling over Africa. The orbitally-induced changes of precipitation may have been modified by the solar cycle; specifically, solar activity maxima during the ending phase of the AHP may have offset the orbital effect and thus stabilized precipitation levels, while solar activity minima compounded the orbital effects and thus induced rapid decreases in water levels of
Lake Turkana Lake Turkana (), formerly known as Lake Rudolf, is a lake in the Kenyan Rift Valley, in northern Kenya, with its far northern end crossing into Ethiopia. It is the world's largest permanent desert lake and the world's largest alkaline lake. B ...
. At Lake Victoria on the other hand, solar variations appear to sometimes lead to drought and sometimes lead to wetness, probably due to changes in the ITCZ.


Potentially human-mediated changes

Major changes in vegetation in East Africa about 2,000 years ago may have been caused by human activity, including large-scale deforestation for iron production during the Iron Age. Similar changes have been observed on the Adamawa Plateau ( Cameroon) but later dating of archaeological sites has found no correlation between human expansion in Cameroon and environmental degradation. Similar rainforest degradation across Western African took place between 3,000 and 2,000 years ago and the degradation is also known as "third millennium rainforest crisis". Climate-mediated processes may have increased the impact of land use changes in East Africa. In the Sudanian and Sahelian savannah on the other hand human activity seems to have had little impact, and in Central Africa forest changes were clearly triggered by climate change with little or no evidence of anthropogenic changes. The question has led to intense debate among paleoecologists and archaeologists. While humans were active in Africa during the end of the African humid period, climate models analyzed by Claussen and colleagues 1999 indicate that its end does not need any human activity as an explanation although vegetation changes may have been induced by human activity. Later it was suggested that overgrazing may have triggered the end of the AHP around 5,500 years ago; human influence might explain why the Sahara became a desert without the accompanying onset of an ice age; usually the existence of a Sahara desert is associated with the expansion of high latitude glaciers. Later research has on the contrary suggested that human pastoralism may have actually delayed the end of the AHP by half a millennium as moving herds of animals driven by humans seeking good pasture conditions may lead to more balanced impacts of pastures on the vegetation and thus to greater vegetation quality. However, increased grazing has been invoked to explain the increase in dust emissions after the end of the AHP. The effects of grazing on vegetation cover are context-dependent and hard to generalize over wider regions.


Global

A general drying tendency is observed in the northern tropics and between 5,000 – 4,500 calibrated years ago the monsoons weakened. Perhaps as a consequence of the end of the AHP,
Asian monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscil ...
precipitation declined between 5,000 and 4,000 years ago. A drought 5,500 years ago is recorded in Mongolia and eastern America, where drought conditions around 5,500–5,000 years ago occurred in places like Florida, New Hampshire and Ontario. A drying tendency is also noted in the
Caribbean The Caribbean (, ) ( es, El Caribe; french: la Caraïbe; ht, Karayib; nl, De Caraïben) is a region of the Americas that consists of the Caribbean Sea, its islands (some surrounded by the Caribbean Sea and some bordering both the Caribbean Se ...
and the Central Atlantic. Conversely, in South America there is evidence that the monsoon behaves in an opposite fashion consistent with precessional forcing; water levels in Lake Titicaca were low during the middle Holocene and began to rise again after the end of the AHP. Likewise, a trend towards increased wetness took place in the Rocky Mountains at this time although it was accompanied by a drier phase around
Lake Tahoe Lake Tahoe (; was, Dáʔaw, meaning "the lake") is a Fresh water, freshwater lake in the Sierra Nevada (U.S.), Sierra Nevada of the United States. Lying at , it straddles the state line between California and Nevada, west of Carson City, Nevad ...
, California and in the Western United States.


Consequences


Humans

As observed in archaeological sites, settlement activity decreased in the Sahara after the AHP. Population in Northern Africa decreased between 6,300 and 5,200 years ago over less than a millennium, beginning from the north. In inner Arabia many settlements were abandoned about 5,300 years ago. Some Neolithic people in the desert persisted for longer thanks to the exploitation of groundwater. Different human populations responded to the drying in diverse manners, with responses in the Western Sahara being distinct from those in the Central Sahara. In the Central Sahara, pastoralism replaced hunter-gatherer activity and a more nomadic lifestyle replaced semi-sedentary lifestyles as observed in the Acacus Mountains of Libya. Nomadic lifestyles also developed in the Eastern Sahara/ Red Sea Hills in response to the end of the AHP. There was a shift in domestic animal use from cattle to sheep and goats as these are more suited in arid climates, a change reflected in
rock art In archaeology, rock art is human-made markings placed on natural surfaces, typically vertical stone surfaces. A high proportion of surviving historic and prehistoric rock art is found in caves or partly enclosed rock shelters; this type also ...
from which cattle disappeared at this time. The development of irrigation systems in Arabia may have been an adaptation to the drying tendency. The decreased availability of resources forced human populations to adapt, in general fishing and hunting declined in favour of farming and herding. However, the effects of the end of the AHP on human food production have been subject to controversy. The warm episode and coinciding drought may have triggered animal and human migration to less inhospitable areas and the appearance of pastoralists where previously
fishery Fishery can mean either the enterprise of raising or harvesting fish and other aquatic life; or more commonly, the site where such enterprise takes place ( a.k.a. fishing ground). Commercial fisheries include wild fisheries and fish farms, both ...
-dependent societies had existed, as happened at Lake Turkana. Humans moved to the Nile, where the society of Ancient Egypt with pharaohs and pyramids was eventually forged by these climate refugees perhaps reflecting renewed exuberance; thus the end of the AHP can be considered responsible for the birth of Ancient Egypt. Lower water levels in the Nile also aided the settlement of its valley as has been observed at Kerma. A similar process may have led to the development of the
Garamantian The Garamantes ( grc, Γαράμαντες, translit=Garámantes; la, Garamantes) were an ancient civilisation based primarily in present-day Libya. They most likely descended from Iron Age Berber tribes from the Sahara, although the earliest k ...
civilization. Such human migrations towards more hospitable conditions along rivers and the development of irrigation also took place along the
Euphrates The Euphrates () is the longest and one of the most historically important rivers of Western Asia. Tigris–Euphrates river system, Together with the Tigris, it is one of the two defining rivers of Mesopotamia ( ''the land between the rivers'') ...
, Tigris and
Indus The Indus ( ) is a transboundary river of Asia and a trans-Himalayan river of South and Central Asia. The river rises in mountain springs northeast of Mount Kailash in Western Tibet, flows northwest through the disputed region of Kashmir, ...
, leading to the development of the
Sumerian Sumerian or Sumerians may refer to: *Sumer, an ancient civilization **Sumerian language **Sumerian art **Sumerian architecture **Sumerian literature **Cuneiform script, used in Sumerian writing *Sumerian Records, an American record label based in ...
and Harappan civilizations. Population shifts into mountain areas have also been reported for the
Air Mountains The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...
, Hoggar and Tibesti. In other places, such as the Acacus Mountains populations conversely remained in
oases In ecology, an oasis (; ) is a fertile area of a desert or semi-desert environment'ksar''with its surrounding feeding source, the palm grove, within a relational and circulatory nomadic system.” The location of oases has been of critical imp ...
and hunter-gatherers also stayed in the Horn of Africa. The Nile itself was not totally unaffected however; the
4.2 kiloyear event The 4.2-kiloyear (thousand years) BP aridification event (long-term drought) was one of the most severe climatic events of the Holocene epoch. It defines the beginning of the current Meghalayan age in the Holocene epoch. Starting around 220 ...
and the end of the AHP may be linked to the collapse of the Old Kingdom in Egypt when the Nile floods failed for three decades around 4,160 years before present and the final drying occurred. The ongoing decrease of precipitation after the end of the AHP could be the cause of the end of the Akkadian Kingdom in Mesopotamia. The end of the
Garamantian The Garamantes ( grc, Γαράμαντες, translit=Garámantes; la, Garamantes) were an ancient civilisation based primarily in present-day Libya. They most likely descended from Iron Age Berber tribes from the Sahara, although the earliest k ...
civilization may also relate to climate change although other historical events were probably more important; at Tanezzuft oasis after 1,600 years ago it certainly relates to the drying trend. In Central Africa, forests became discontinuous and
savannah A savanna or savannah is a mixed woodland-grassland (i.e. grassy woodland) ecosystem characterised by the trees being sufficiently widely spaced so that the Canopy (forest), canopy does not close. The open canopy allows sufficient light to rea ...
s formed in some places, facilitating the movement and growth of
Bantu Bantu may refer to: *Bantu languages, constitute the largest sub-branch of the Niger–Congo languages *Bantu peoples, over 400 peoples of Africa speaking a Bantu language *Bantu knots, a type of African hairstyle *Black Association for Nationali ...
speaking populations; these in turn may have affected the ecosystem. The vegetation changes may have aided in the establishment of agriculture. The relatively slow decline of precipitation gave humans more time to adapt to the changing climate conditions. Cultural changes may also have occurred as a consequence of climate change, such as changes in gender roles, the development of
elite In political and sociological theory, the elite (french: élite, from la, eligere, to select or to sort out) are a small group of powerful people who hold a disproportionate amount of wealth, privilege, political power, or skill in a group. D ...
s, the increased presence of human burials where formerly cattle burials predominated, as well as an increase of monumental architecture in the Sahara may have also been a response to increasingly adverse climates. A spread in cattle domestication at the time of climate change and as herders escaped the drying Sahara southwards may also relate to these events, although the details of the exact process by which cattle domestication spread are still controversial. Finally, changes in agricultural practices at the end of the AHP may be associated with the propagation of malaria and one of its causative pathogens ''
Plasmodium falciparum ''Plasmodium falciparum'' is a Unicellular organism, unicellular protozoan parasite of humans, and the deadliest species of ''Plasmodium'' that causes malaria in humans. The parasite is transmitted through the bite of a female ''Anopheles'' mosqu ...
''; in turn these may correlate with the origin of human genome variants such as sickle cell disease that are linked to malaria resistance.


Non-human

In the Sahara, animal and plant populations were fragmented and restricted to certain favoured areas such as moist areas of mountain ranges; this happened for example to fish and crocodiles which only persist in isolated water bodies. Mediterranean plants such as
cypress Cypress is a common name for various coniferous trees or shrubs of northern temperate regions that belong to the family Cupressaceae. The word ''cypress'' is derived from Old French ''cipres'', which was imported from Latin ''cypressus'', the ...
es too persist only in mountains, along with some
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s that may have also been stranded in mountains by the drying. The whip spider '' Musicodamon atlanteus'' is probably also a relic of past wetter conditions. The buffalo species ''
Syncerus antiquus ''Syncerus antiquus'', is an extinct species of bovid from the Pleistocene of Africa. It was one of the largest species in its family, potentially weighing up to . Due to this fact, it is sometimes known as the African giant buffalo. ''Syncerus an ...
'' probably went extinct from the increased competition of pastoralists triggered by the climate drying. Goat populations in Ethiopia shrunk during the droughts that followed the end of the AHP and
lion The lion (''Panthera leo'') is a large Felidae, cat of the genus ''Panthera'' native to Africa and India. It has a muscular, broad-chested body; short, rounded head; round ears; and a hairy tuft at the end of its tail. It is sexually dimorphi ...
habitat declined across Africa. The drying of the African Great Lakes region split gorilla populations into western and eastern populations, and a similar population split between the insect species '' Chalinus albitibialis'' and '' Chalinus timnaensis'' in Northern Africa and the Middle East may have also been caused by the expansion of deserts there. Some aquatic species disappeared from the Sahara. Giraffes, widespread in the Sahara during the AHP, may have been forced to migrate into the Sahel; this together with the separating effect of Lake Megachad may have influenced the development of giraffe subspecies. Climate change together with human impacts may have led to the extinction of a number of large mammals in Egypt. In northern Madagascar, wildlife declined after the end of the AHP even before the arrival of humans. On the other hand, the decline of tree cover may have grown the niche available to domestic animals and some drought-tolerant plant species may have expanded their range. The Dahomey Gap formed 4,500–3,200 years before present, correlative to the end of the AHP. The harbour porpoise declined in the Mediterranean due to a switch to
oligotrophic An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments. Oligotrophs are characterized by slow growth, low rates of ...
conditions as discharge from African rivers decreased.
Desert varnish Desert varnish or rock varnish is an orange-yellow to black coating found on exposed rock surfaces in arid environments. Desert varnish is approximately one micrometer thick and exhibits nanometer-scale layering. Rock rust and desert patina are ot ...
formed on exposed rocks in the Sahara and at Lake Turkana in East Africa.


Global climate

The shrinkage of subtropical wetlands probably led to a drop in atmospheric methane concentrations between 5,500 and 5,000 years ago, before
boreal Boreal may refer to: Climatology and geography *Boreal (age), the first climatic phase of the Blytt-Sernander sequence of northern Europe, during the Holocene epoch *Boreal climate, a climate characterized by long winters and short, cool to mild ...
wetlands expanded and offset the loss of subtropical wetlands, leading to a return of higher atmospheric methane concentrations. Conversely, increases in atmospheric methane concentrations, detected in Greenland
ice core An ice core is a core sample that is typically removed from an ice sheet or a high mountain glacier. Since the ice forms from the incremental buildup of annual layers of snow, lower layers are older than upper ones, and an ice core contains ic ...
s about 14,700 years ago, and atmospheric carbon dioxide decreases in the early Holocene may relate to the vegetation expansion caused by the AHP. Carbon dioxide concentration then increased after about 7,000 years as the biosphere began releasing carbon in response to increasing aridity. A sudden increase in the amount of land-originating dust in an oceanic drill core off
Cape Blanc Cap Blanc or Cape Blanc, meaning "White Cape", may refer to: * Ras Nouadhibou or Cap Blanc, a headland in Mauritania and Western Sahara * Cap Blanc rock shelter, a prehistoric limestone shelter in France * Cap Blanc (Ibiza), a beach on the Spanis ...
,
Mauritania Mauritania (; ar, موريتانيا, ', french: Mauritanie; Berber: ''Agawej'' or ''Cengit''; Pulaar: ''Moritani''; Wolof: ''Gànnaar''; Soninke:), officially the Islamic Republic of Mauritania ( ar, الجمهورية الإسلامية ...
, has been interpreted as reflecting the end of the AHP 5,500 years ago occurring in only a few centuries. Potentially, dried up lake basins became an important source for dust and silt-sized particles. Today, the Sahara is the single largest source of dust in the world, with far ranging effects on climate and ecosystems, such as the growth of the
Amazon rainforest The Amazon rainforest, Amazon jungle or ; es, Selva amazónica, , or usually ; french: Forêt amazonienne; nl, Amazoneregenwoud. In English, the names are sometimes capitalized further, as Amazon Rainforest, Amazon Forest, or Amazon Jungle. ...
. In one climate model, the desertification of the Sahara at the end of the AHP reduces the amount of heat transported in the atmosphere and ocean towards the poles, inducing cooling of especially in winter in the Arctic and an expansion of
sea ice Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oce ...
. Reconstructed temperatures in the Arctic indeed show a cooling, although less pronounced than in the climate model. Further, this climate transition in the climate model is accompanied by increased negative Arctic Oscillation states, a weaker subpolar gyre and increased precipitation and
cold air outbreak A cold wave (known in some regions as a cold snap, cold spell or Arctic Snap) is a weather phenomenon that is distinguished by a cooling of the air. Specifically, as used by the U.S. National Weather Service, a cold wave is a rapid fall in tem ...
s in much of Europe; such changes have also been observed in paleoclimate data. These findings imply that the vegetation state of the Sahara influences the Northern Hemisphere climate. In turn, this high latitude cooling may have further reduced precipitation over Africa.


Present-day situation

Presently, the African Monsoon still influences the climate between 5° south and 25° north latitude; the latitudes around 10° north receive the bulk of their precipitation from the monsoon during summer, with smaller amounts of rainfall occurring farther north. Thus farther north
desert A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About on ...
s can be found while the moister areas are vegetated. In the Central Sahara, annual precipitation reaches no more than . Even farther north, the margin of the desert coincides with the area where the westerlies bring precipitation; they also influence southernmost Africa.
Subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope move ...
of air over parts of Northern Africa is responsible for the existence of deserts, which is further increased by the radiative cooling over the desert. Climate variability exists to this day, with the Sahel suffering from droughts in the 1970s and 1980s when precipitation decreased by 30% and the flow of the Niger River and
Senegal River ,french: Fleuve Sénégal) , name_etymology = , image = Senegal River Saint Louis.jpg , image_size = , image_caption = Fishermen on the bank of the Senegal River estuary at the outskirts of Saint-Louis, Senegal ...
even more, followed by an increase of precipitation. The droughts are one of the most significant climate anomalies of the 20th century. Sea surface temperatures and feedbacks from land surface conditions modulate the strength of the monsoon and the droughts may have been triggered by sea surface temperature changes forced by anthropogenic aerosols. A large increase in dust fluxes after 1800 AD has been explained with changed agricultural practices. In East Africa the monsoon leads to two rain seasons in the equatorial area, the so-called "long rains" in March–May and the "short rains" in October–November when the ITCZ moves northward and southward over the region, respectively; in addition to the Indian Ocean-sourced precipitation there is also Atlantic- and Congo-sourced precipitation west of the Congo Air Boundary. In Arabia, the monsoon does not penetrate far from the Arabian Sea and some areas are under the influence of winter precipitation brought by
cyclone In meteorology, a cyclone () is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above (opposite to an anti ...
s from the Mediterranean Sea. East Africa is also under the influence of monsoon circulations.


Implications for future global warming

Some simulations of global warming and increased carbon dioxide concentrations have shown a substantial increase in precipitation in the Sahel/Sahara. This and the increased plant growth directly induced by carbon dioxide could lead to an expansion of vegetation into present-day desert, although it would be less extensive than during the mid-Holocene and perhaps accompanied by a northward shift of the desert, i.e. a drying of northernmost Africa. Such a precipitation increase may also reduce the amount of dust originating in Northern Africa, with effects on hurricane activity in the Atlantic and increased threats of hurricane strikes in the
Caribbean The Caribbean (, ) ( es, El Caribe; french: la Caraïbe; ht, Karayib; nl, De Caraïben) is a region of the Americas that consists of the Caribbean Sea, its islands (some surrounded by the Caribbean Sea and some bordering both the Caribbean Se ...
, the Gulf of Mexico and the
East Coast East Coast may refer to: Entertainment * East Coast hip hop, a subgenre of hip hop * East Coast (ASAP Ferg song), "East Coast" (ASAP Ferg song), 2017 * East Coast (Saves the Day song), "East Coast" (Saves the Day song), 2004 * East Coast FM, a ra ...
of the United States of America. The Special Report on Global Warming of 1.5 °C and the
IPCC Fifth Assessment Report The Fifth Assessment Report (AR5) of the United Nations Intergovernmental Panel on Climate Change (IPCC) is the fifth in a series of such reports and was completed in 2014.IPCC (2014The IPCC’s Fifth Assessment Report (AR5) leaflet/ref> As h ...
indicate that global warming will likely result in increased precipitation across most of East Africa, parts of Central Africa and the principal wet season of West Africa, although there is significant uncertainty related to these projections especially for West Africa. In addition, the end of the 20th century drying trend may be due to global warming. On the other hand, West Africa and parts of East Africa may become drier during given seasons and months. Currently, the Sahel is becoming greener but precipitation has not fully recovered to levels reached in the mid-20th century. Climate models have yielded equivocal results about the effects of anthropogenic global warming on the Sahara/Sahel precipitation. Human-caused climate change occurs through different mechanisms than the natural climate change that led to the AHP, in particular through increased inter-hemispheric temperature gradients. The direct effect of heat on plants may be detrimental. Non-linear increases in vegetation cover are also possible. One study in 2003 showed that vegetation intrusions in the Sahara can occur within decades after strong rises in atmospheric carbon dioxide but would not cover more than about 45% of the Sahara. That climate study also indicated that vegetation expansion can only occur if grazing or other perturbations to vegetation growth do not hamper it. On the other hand, increased irrigation and other measures to increase vegetation growth such as the Great Green Wall could enhance it. Plans to geoengineer the Sahara to increase its vegetation cover and precipitation have been proposed since the 19th century. The mechanisms and consequences of the AHP are important context to evaluate such proposals and their ramifications; precipitation may increase but the consumption of carbon dioxide would be small and there could be detrimental impacts on climate and dust fluxes in the far-field. Building large
solar farm Solar may refer to: Astronomy * Of or relating to the Sun ** Solar telescope, a special purpose telescope used to observe the Sun ** A device that utilizes solar energy (e.g. "solar panels") ** Solar calendar, a calendar whose dates indicate t ...
s in the Sahara desert would also act to decrease its albedo and may trigger similar climate responses. A greening of the Sahara on the one hand may allow agriculture and pastoralism to expand into hitherto unsuitable areas, but increased precipitation can also lead to increased water borne diseases and flooding. Expanded human activity resulting from a wetter climate may be vulnerable to climate reversals as demonstrated by the droughts that followed the mid-20th century wet period.


See also

* Sahara pump theory


Notes


References


Sources

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * ar * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *


External links

* * * * {{DEFAULTSORT:African humid period History of climate variability and change Holocene 2000s neologisms Pleistocene events Natural history of Africa Natural history of Asia Historical eras