An ice age is a period of long-term reduction in the temperature of
Earth's surface and atmosphere, resulting in the presence or expansion
of continental and polar ice sheets and alpine glaciers. Within a
long-term ice age, individual pulses of cold climate are termed
"glacial periods" (or alternatively "glacials" or "glaciations" or
colloquially as "ice age"), and intermittent warm periods are called
"interglacials". In the terminology of glaciology, ice age implies the
presence of extensive ice sheets in both northern and southern
hemispheres. By this definition, we are in an interglacial
period—the Holocene—of the ice age. The ice age began 2.6 million
years ago at the start of the
Pleistocene epoch, because the
Greenland, Arctic, and Antarctic ice sheets still exist.
1 Origin of ice age theory
2 Evidence for ice ages
3 Major ice ages
4 Glacials and interglacials
5 Positive and negative feedback in glacial periods
Positive feedback processes
Negative feedback processes
6 Causes of ice ages
6.1 Changes in Earth's atmosphere
6.1.1 Human-induced changes
6.2 Position of the continents
6.3 Fluctuations in ocean currents
6.4 Uplift of the Tibetan plateau and surrounding mountain areas above
6.5 Variations in Earth's orbit (Milankovitch cycles)
6.6 Variations in the Sun's energy output
7 Recent glacial and interglacial phases
7.1 Glacial stages in North America
7.2 Last Glacial Period in the semiarid Andes around Aconcagua and
8 Effects of glaciation
9 See also
11 External links
Origin of ice age theory
view • discuss • edit
Earliest sexual reproduction
Axis scale: million years
Orange labels: ice ages.
Human timeline and Nature timeline
In 1742 Pierre Martel (1706–1767), an engineer and geographer living
in Geneva, visited the valley of
Chamonix in the
Alps of Savoy.
Two years later he published an account of his journey. He reported
that the inhabitants of that valley attributed the dispersal of
erratic boulders to the glaciers, saying that they had once extended
much farther. Later similar explanations were reported from
other regions of the Alps. In 1815 the carpenter and chamois hunter
Jean-Pierre Perraudin (1767–1858) explained erratic boulders in the
Val de Bagnes in the Swiss canton of Valais as being due to glaciers
previously extending further. An unknown woodcutter from Meiringen
in the Bernese Oberland advocated a similar idea in a discussion with
the Swiss-German geologist
Jean de Charpentier
Jean de Charpentier (1786–1855) in
1834. Comparable explanations are also known from the Val de Ferret
in the Valais and the Seeland in western Switzerland and in
Goethe's scientific work. Such explanations could also be found in
other parts of the world. When the Bavarian naturalist Ernst von Bibra
(1806–1878) visited the Chilean Andes in 1849–1850, the natives
attributed fossil moraines to the former action of glaciers.
Meanwhile, European scholars had begun to wonder what had caused the
dispersal of erratic material. From the middle of the 18th century,
some discussed ice as a means of transport. The Swedish mining expert
Daniel Tilas (1712–1772) was, in 1742, the first person to suggest
drifting sea ice in order to explain the presence of erratic boulders
in the Scandinavian and Baltic regions. In 1795, the Scottish
philosopher and gentleman naturalist,
James Hutton (1726–1797),
explained erratic boulders in the
Alps by the action of glaciers.
Two decades later, in 1818, the Swedish botanist Göran Wahlenberg
(1780–1851) published his theory of a glaciation of the Scandinavian
peninsula. He regarded glaciation as a regional phenomenon.
Only a few years later, the Danish-Norwegian geologist Jens Esmark
(1762–1839) argued a sequence of worldwide ice ages. In a paper
published in 1824, Esmark proposed changes in climate as the cause of
those glaciations. He attempted to show that they originated from
changes in Earth's orbit. During the following years, Esmark's
ideas were discussed and taken over in parts by Swedish, Scottish and
German scientists. At the University of Edinburgh Robert Jameson
(1774–1854) seemed to be relatively open to Esmark's ideas, as
reviewed by Norwegian professor of glaciology Bjørn G. Andersen
(1992). Jameson's remarks about ancient glaciers in Scotland were
most probably prompted by Esmark. In Germany, Albrecht Reinhard
Bernhardi (1797–1849), a geologist and professor of forestry at an
academy in Dreissigacker, since incorporated in the southern
Thuringian city of Meiningen, adopted Esmark's theory. In a paper
published in 1832, Bernhardi speculated about former polar ice caps
reaching as far as the temperate zones of the globe.
In 1829, independently of these debates, the Swiss civil engineer
Ignaz Venetz (1788–1859) explained the dispersal of erratic boulders
in the Alps, the nearby Jura Mountains, and the North German Plain as
being due to huge glaciers. When he read his paper before the
Schweizerische Naturforschende Gesellschaft, most scientists remained
sceptical. Finally, Venetz convinced his friend Jean de
Charpentier. De Charpentier transformed Venetz's idea into a theory
with a glaciation limited to the Alps. His thoughts resembled
Wahlenberg's theory. In fact, both men shared the same volcanistic, or
in de Charpentier's case rather plutonistic assumptions, about the
Earth's history. In 1834, de Charpentier presented his paper before
the Schweizerische Naturforschende Gesellschaft. In the meantime,
the German botanist
Karl Friedrich Schimper
Karl Friedrich Schimper (1803–1867) was studying
mosses which were growing on erratic boulders in the alpine upland of
Bavaria. He began to wonder where such masses of stone had come from.
During the summer of 1835 he made some excursions to the Bavarian
Alps. Schimper came to the conclusion that ice must have been the
means of transport for the boulders in the alpine upland. In the
winter of 1835 to 1836 he held some lectures in Munich. Schimper then
assumed that there must have been global times of obliteration
("Verödungszeiten") with a cold climate and frozen water.
Schimper spent the summer months of 1836 at Devens, near Bex, in the
Alps with his former university friend Louis Agassiz
(1801–1873) and Jean de Charpentier. Schimper, de Charpentier and
possibly Venetz convinced Agassiz that there had been a time of
glaciation. During the winter of 1836/37, Agassiz and Schimper
developed the theory of a sequence of glaciations. They mainly drew
upon the preceding works of Venetz, de Charpentier and on their own
fieldwork. Agassiz appears to have been already familiar with
Bernhardi's paper at that time. At the beginning of 1837, Schimper
coined the term "ice age" ("Eiszeit") for the period of the
glaciers. In July 1837 Agassiz presented their synthesis before
the annual meeting of the Schweizerische Naturforschende Gesellschaft
at Neuchâtel. The audience was very critical and some opposed to the
new theory because it contradicted the established opinions on
climatic history. Most contemporary scientists thought that the Earth
had been gradually cooling down since its birth as a molten globe.
In order to overcome this rejection, Agassiz embarked on geological
fieldwork. He published his book Study on Glaciers ("Études sur les
glaciers") in 1840. De Charpentier was put out by this, as he had
also been preparing a book about the glaciation of the Alps. De
Charpentier felt that Agassiz should have given him precedence as it
was he who had introduced Agassiz to in-depth glacial research.
Besides that, Agassiz had, as a result of personal quarrels, omitted
any mention of Schimper in his book.
All together, it took several decades until the ice age theory was
fully accepted by scientists. This happened on an international scale
in the second half of the 1870s following the work of James Croll,
including the publication of Climate and Time, in Their Geological
Relations in 1875, which provided a credible explanation for the
causes of ice ages.
Evidence for ice ages
There are three main types of evidence for ice ages: geological,
chemical, and paleontological.
Geological evidence for ice ages comes in various forms, including
rock scouring and scratching, glacial moraines, drumlins, valley
cutting, and the deposition of till or tillites and glacial erratics.
Successive glaciations tend to distort and erase the geological
evidence, making it difficult to interpret. Furthermore, this evidence
was difficult to date exactly; early theories assumed that the
glacials were short compared to the long interglacials. The advent of
sediment and ice cores revealed the true situation: glacials are long,
interglacials short. It took some time for the current theory to be
The chemical evidence mainly consists of variations in the ratios of
isotopes in fossils present in sediments and sedimentary rocks and
ocean sediment cores. For the most recent glacial periods ice cores
provide climate proxies from their ice, and atmospheric samples from
included bubbles of air. Because water containing heavier isotopes has
a higher heat of evaporation, its proportion decreases with colder
conditions. This allows a temperature record to be constructed.
However, this evidence can be confounded by other factors recorded by
The paleontological evidence consists of changes in the geographical
distribution of fossils. During a glacial period cold-adapted
organisms spread into lower latitudes, and organisms that prefer
warmer conditions become extinct or are squeezed into lower latitudes.
This evidence is also difficult to interpret because it requires (1)
sequences of sediments covering a long period of time, over a wide
range of latitudes and which are easily correlated; (2) ancient
organisms which survive for several million years without change and
whose temperature preferences are easily diagnosed; and (3) the
finding of the relevant fossils.
Despite the difficulties, analysis of ice core and ocean sediment
cores has shown periods of glacials and interglacials over the
past few million years. These also confirm the linkage between ice
ages and continental crust phenomena such as glacial moraines,
drumlins, and glacial erratics. Hence the continental crust phenomena
are accepted as good evidence of earlier ice ages when they are found
in layers created much earlier than the time range for which ice cores
and ocean sediment cores are available.
Major ice ages
Timeline of glaciations, shown in blue.
There have been at least five major ice ages in the Earth's history
(the Huronian, Cryogenian, Andean-Saharan, Karoo Ice Age, and the
current Quaternary glaciation). Outside these ages, the
Earth seems to
have been ice free even in high latitudes.
Ice age map of northern Germany and its northern neighbours. Red:
maximum limit of
Weichselian glacial; yellow: Saale glacial at maximum
(Drenthe stage); blue: Elster glacial maximum glaciation.
Rocks from the earliest well established ice age, called the Huronian,
formed around 2.4 to 2.1 Ga (billion years) ago during the early
Proterozoic Eon. Several hundreds of km of the
Huronian Supergroup are
exposed 10–100 km north of the north shore of Lake Huron
extending from near Sault Ste. Marie to Sudbury, northeast of Lake
Huron, with giant layers of now-lithified till beds, dropstones,
varves, outwash, and scoured basement rocks. Correlative Huronian
deposits have been found near Marquette, Michigan, and correlation has
been made with
Paleoproterozoic glacial deposits from Western
Huronian ice age was caused by the elimination of
atmospheric methane, a greenhouse gas, during the Great Oxygenation
The next well-documented ice age, and probably the most severe of the
last billion years, occurred from 850 to 630 million years ago (the
Cryogenian period) and may have produced a Snowball
Earth in which
glacial ice sheets reached the equator, possibly being ended by
the accumulation of greenhouse gases such as CO2 produced by
volcanoes. "The presence of ice on the continents and pack ice on the
oceans would inhibit both silicate weathering and photosynthesis,
which are the two major sinks for CO2 at present." It has been
suggested that the end of this ice age was responsible for the
Ediacaran and Cambrian explosion, though this model is
recent and controversial.
Andean-Saharan occurred from 460 to 420 million years ago, during
Late Ordovician and the
Sediment records showing the fluctuating sequences of glacials and
interglacials during the last several million years.
The evolution of land plants at the onset of the
caused a long term increase in planetary oxygen levels and reduction
of CO2 levels, which resulted in the Karoo Ice Age. It is named after
the glacial tills found in the Karoo region of South Africa, where
evidence for this ice age was first clearly identified. There were
extensive polar ice caps at intervals from 360 to 260 million years
ago in South Africa during the
Carboniferous and early Permian
Periods. Correlatives are known from Argentina, also in the center of
the ancient supercontinent Gondwanaland.
The current ice age, the Pliocene-Quaternary glaciation, started about
2.58 million years ago during the late Pliocene, when the spread of
ice sheets in the
Northern Hemisphere began. Since then, the world has
seen cycles of glaciation with ice sheets advancing and retreating on
40,000- and 100,000-year time scales called glacial periods, glacials
or glacial advances, and interglacial periods, interglacials or
glacial retreats. The earth is currently in an interglacial, and the
last glacial period ended about 10,000 years ago. All that remains of
the continental ice sheets are the
Greenland and Antarctic ice sheets
and smaller glaciers such as on Baffin Island.
Ice ages can be further divided by location and time; for example, the
names Riss (180,000–130,000 years bp) and Würm (70,000–10,000
years bp) refer specifically to glaciation in the Alpine region. The
maximum extent of the ice is not maintained for the full interval. The
scouring action of each glaciation tends to remove most of the
evidence of prior ice sheets almost completely, except in regions
where the later sheet does not achieve full coverage.
Evidence of a recent, extreme ice age on
Mars was published by the
journal Science in 2016. Just 370,000 years ago, the planet would have
appeared more white than red.
Glacials and interglacials
Glacial period and Interglacial
Shows the pattern of temperature and ice volume changes associated
with recent glacials and interglacials
Minimum and maximum glaciation
Minimum (interglacial, black) and maximum (glacial, grey) glaciation
of the northern hemisphere
Minimum (interglacial, black) and maximum (glacial, grey) glaciation
of the southern hemisphere
Within the ice ages (or at least within the current one), more
temperate and more severe periods occur. The colder periods are called
glacial periods, the warmer periods interglacials, such as the Eemian
Glacials are characterized by cooler and drier climates over most of
the earth and large land and sea ice masses extending outward from the
poles. Mountain glaciers in otherwise unglaciated areas extend to
lower elevations due to a lower snow line. Sea levels drop due to the
removal of large volumes of water above sea level in the icecaps.
There is evidence that ocean circulation patterns are disrupted by
glaciations. Since the earth has significant continental glaciation in
Arctic and Antarctic, we are currently in a glacial minimum of a
glaciation. Such a period between glacial maxima is known as an
interglacial. The glacials and interglacials also coincided with
changes in Earth's orbit called Milankovitch cycles.
The earth has been in an interglacial period known as the
around 11,700 years, and an article in Nature in 2004 argues that
it might be most analogous to a previous interglacial that lasted
28,000 years. Predicted changes in orbital forcing suggest that
the next glacial period would begin at least 50,000 years from now,
even in absence of human-made global warming (see Milankovitch
cycles). Moreover, anthropogenic forcing from increased greenhouse
gases might outweigh orbital forcing for as long as intensive use of
fossil fuels continues.
Positive and negative feedback in glacial periods
Each glacial period is subject to positive feedback which makes it
more severe, and negative feedback which mitigates and (in all cases
so far) eventually ends it.
Positive feedback processes
Ice and snow increase Earth's albedo, i.e. they make it reflect more
of the sun's energy and absorb less. Hence, when the air temperature
decreases, ice and snow fields grow, and this continues until
competition with a negative feedback mechanism forces the system to an
equilibrium. Also, the reduction in forests caused by the ice's
expansion increases albedo.
Another theory proposed by Ewing and Donn in 1956 hypothesized
that an ice-free
Arctic Ocean leads to increased snowfall at high
latitudes. When low-temperature ice covers the
Arctic Ocean there is
little evaporation or sublimation and the polar regions are quite dry
in terms of precipitation, comparable to the amount found in
mid-latitude deserts. This low precipitation allows high-latitude
snowfalls to melt during the summer. An ice-free
Arctic Ocean absorbs
solar radiation during the long summer days, and evaporates more water
Arctic atmosphere. With higher precipitation, portions of
this snow may not melt during the summer and so glacial ice can form
at lower altitudes and more southerly latitudes, reducing the
temperatures over land by increased albedo as noted above.
Furthermore, under this hypothesis the lack of oceanic pack ice allows
increased exchange of waters between the
Arctic and the North Atlantic
Oceans, warming the
Arctic and cooling the North Atlantic. (Current
projected consequences of global warming include a largely ice-free
Arctic Ocean within 5–20 years, see
Arctic shrinkage.) Additional
fresh water flowing into the North Atlantic during a warming cycle may
also reduce the global ocean water circulation. Such a reduction (by
reducing the effects of the Gulf Stream) would have a cooling effect
on northern Europe, which in turn would lead to increased low-latitude
snow retention during the summer. It has also been suggested that
during an extensive glacial, glaciers may move through the Gulf of
Saint Lawrence, extending into the North Atlantic Ocean far enough to
block the Gulf Stream.
Negative feedback processes
Ice sheets that form during glaciations cause erosion of the land
beneath them. After some time, this will reduce land above sea level
and thus diminish the amount of space on which ice sheets can form.
This mitigates the albedo feedback, as does the lowering in sea level
that accompanies the formation of ice sheets.
Another factor is the increased aridity occurring with glacial maxima,
which reduces the precipitation available to maintain glaciation. The
glacial retreat induced by this or any other process can be amplified
by similar inverse positive feedbacks as for glacial advances.
According to research published in Nature Geoscience, human emissions
of carbon dioxide (CO2) will defer the next ice age. Researchers used
data on Earth's orbit to find the historical warm interglacial period
that looks most like the current one and from this have predicted that
the next ice age would usually begin within 1,500 years. They go on to
say that emissions have been so high that it will not.
Causes of ice ages
The causes of ice ages are not fully understood for either the
large-scale ice age periods or the smaller ebb and flow of
glacial–interglacial periods within an ice age. The consensus is
that several factors are important: atmospheric composition, such as
the concentrations of carbon dioxide and methane (the specific levels
of the previously mentioned gases are now able to be seen with the new
ice core samples from EPICA Dome C in
Antarctica over the past 800,000
years) changes in the earth's orbit around the
Sun known as
Milankovitch cycles, the motion of tectonic plates resulting in
changes in the relative location and amount of continental and oceanic
crust on the earth's surface, which affect wind and ocean currents,
variations in solar output, the orbital dynamics of the Earth–Moon
system, the impact of relatively large meteorites and volcanism
including eruptions of supervolcanoes.
Some of these factors influence each other. For example, changes in
Earth's atmospheric composition (especially the concentrations of
greenhouse gases) may alter the climate, while climate change itself
can change the atmospheric composition (for example by changing the
rate at which weathering removes CO2).
William Ruddiman and others propose that the Tibetan
and Colorado Plateaus are immense CO2 "scrubbers" with a capacity to
remove enough CO2 from the global atmosphere to be a significant
causal factor of the 40 million year Cenozoic Cooling trend. They
further claim that approximately half of their uplift (and CO2
"scrubbing" capacity) occurred in the past 10 million years.
Changes in Earth's atmosphere
There is evidence that greenhouse gas levels fell at the start of ice
ages and rose during the retreat of the ice sheets, but it is
difficult to establish cause and effect (see the notes above on the
role of weathering).
Greenhouse gas levels may also have been affected
by other factors which have been proposed as causes of ice ages, such
as the movement of continents and volcanism.
Earth hypothesis maintains that the severe freezing in
Proterozoic was ended by an increase in CO2 levels in the
atmosphere, mainly from volcanoes, and some supporters of Snowball
Earth argue that it was caused in the first place by a reduction in
atmospheric CO2. The hypothesis also warns of future Snowball Earths.
In 2009, further evidence was provided that changes in solar
insolation provide the initial trigger for the earth to warm after an
Ice Age, with secondary factors like increases in greenhouse gases
accounting for the magnitude of the change.
There is considerable evidence that over the very recent period of the
last 100–1000 years, the sharp increases in human activity,
especially the burning of fossil fuels, has caused the parallel sharp
and accelerating increase in atmospheric greenhouse gases which trap
the sun's heat. The consensus theory of the scientific community is
that the resulting greenhouse effect is a principal cause of the
increase in global warming which has occurred over the same period,
and a chief contributor to the accelerated melting of the remaining
glaciers and polar ice. A 2012 investigation finds that dinosaurs
released methane through digestion in a similar amount to humanity's
current methane release, which "could have been a key factor" to the
very warm climate 150 million years ago.
William Ruddiman has proposed the early anthropocene hypothesis,
according to which the anthropocene era, as some people call the most
recent period in the earth's history when the activities of the human
species first began to have a significant global impact on the earth's
climate and ecosystems, did not begin in the 18th century with the
advent of the Industrial Era, but dates back to 8,000 years ago, due
to intense farming activities of our early agrarian ancestors. It was
at that time that atmospheric greenhouse gas concentrations stopped
following the periodic pattern of the Milankovitch cycles. In his
overdue-glaciation hypothesis Ruddiman states that an incipient
glacial would probably have begun several thousand years ago, but the
arrival of that scheduled glacial was forestalled by the activities of
At a meeting of the
American Geophysical Union
American Geophysical Union (December 17, 2008),
scientists detailed evidence in support of the controversial idea that
the introduction of large-scale rice agriculture in Asia, coupled with
extensive deforestation in Europe began to alter world climate by
pumping significant amounts of greenhouse gases into the atmosphere
over the last 1,000 years. In turn, a warmer atmosphere heated the
oceans making them much less efficient storehouses of carbon dioxide
and reinforcing global warming, possibly forestalling the onset of a
new glacial age.
Position of the continents
The geological record appears to show that ice ages start when the
continents are in positions which block or reduce the flow of warm
water from the equator to the poles and thus allow ice sheets to form.
The ice sheets increase Earth's reflectivity and thus reduce the
absorption of solar radiation. With less radiation absorbed the
atmosphere cools; the cooling allows the ice sheets to grow, which
further increases reflectivity in a positive feedback loop. The ice
age continues until the reduction in weathering causes an increase in
the greenhouse effect.
There are three main contributors from the layout of the continents
that obstruct the movement of warm water to the poles:[citation
A continent sits on top of a pole, as
Antarctica does today.
A polar sea is almost land-locked, as the
Arctic Ocean is today.
A supercontinent covers most of the equator, as
Rodinia did during the
Earth has a continent over the South Pole and an almost
land-locked ocean over the North Pole, geologists believe that Earth
will continue to experience glacial periods in the geologically near
Some scientists believe that the
Himalayas are a major factor in the
current ice age, because these mountains have increased Earth's total
rainfall and therefore the rate at which carbon dioxide is washed out
of the atmosphere, decreasing the greenhouse effect. The
Himalayas' formation started about 70 million years ago when the
Indo-Australian Plate collided with the Eurasian Plate, and the
Himalayas are still rising by about 5 mm per year because the
Indo-Australian plate is still moving at 67 mm/year. The history
Himalayas broadly fits the long-term decrease in Earth's
average temperature since the mid-Eocene, 40 million years ago.
Fluctuations in ocean currents
Another important contribution to ancient climate regimes is the
variation of ocean currents, which are modified by continent position,
sea levels and salinity, as well as other factors. They have the
ability to cool (e.g. aiding the creation of Antarctic ice) and the
ability to warm (e.g. giving the British Isles a temperate as opposed
to a boreal climate). The closing of the
Isthmus of Panama
Isthmus of Panama about 3
million years ago may have ushered in the present period of strong
glaciation over North America by ending the exchange of water between
the tropical Atlantic and Pacific Oceans.
Analyses suggest that ocean current fluctuations can adequately
account for recent glacial oscillations. During the last glacial
period the sea-level has fluctuated 20–30 m as water was
sequestered, primarily in the
Northern Hemisphere ice sheets. When ice
collected and the sea level dropped sufficiently, flow through the
Bering Strait (the narrow strait between Siberia and Alaska is about
50 m deep today) was reduced, resulting in increased flow from
the North Atlantic. This realigned the thermohaline circulation in the
Atlantic, increasing heat transport into the Arctic, which melted the
polar ice accumulation and reduced other continental ice sheets. The
release of water raised sea levels again, restoring the ingress of
colder water from the Pacific with an accompanying shift to northern
hemisphere ice accumulation.
Uplift of the Tibetan plateau and surrounding mountain areas above the
Matthias Kuhle's geological theory of Ice Age development was
suggested by the existence of an ice sheet covering the Tibetan
Plateau during the Ice Ages (Last Glacial Maximum?). According to
Kuhle, the plate-tectonic uplift of Tibet past the snow-line has led
to a surface of c. 2,400,000 square kilometres (930,000 sq mi)
changing from bare land to ice with a 70% greater albedo. The
reflection of energy into space resulted in a global cooling,
Pleistocene Ice Age. Because this highland is at a
subtropical latitude, with 4 to 5 times the insolation of
high-latitude areas, what would be Earth's strongest heating surface
has turned into a cooling surface.
Kuhle explains the interglacial periods by the 100,000-year cycle of
radiation changes due to variations in Earth's orbit. This
comparatively insignificant warming, when combined with the lowering
of the Nordic inland ice areas and Tibet due to the weight of the
superimposed ice-load, has led to the repeated complete thawing of the
inland ice areas.
Variations in Earth's orbit (Milankovitch cycles)
Milankovitch cycles are a set of cyclic variations in
characteristics of the Earth's orbit around the Sun. Each cycle has a
different length, so at some times their effects reinforce each other
and at other times they (partially) cancel each other.
Past and future of daily average insolation at top of the atmosphere
on the day of the summer solstice, at 65 N latitude.
There is strong evidence that the
Milankovitch cycles affect the
occurrence of glacial and interglacial periods within an ice age. The
present ice age is the most studied and best understood, particularly
the last 400,000 years, since this is the period covered by ice cores
that record atmospheric composition and proxies for temperature and
ice volume. Within this period, the match of glacial/interglacial
frequencies to the Milanković orbital forcing periods is so close
that orbital forcing is generally accepted. The combined effects of
the changing distance to the Sun, the precession of the Earth's axis,
and the changing tilt of the Earth's axis redistribute the sunlight
received by the Earth. Of particular importance are changes in the
tilt of the Earth's axis, which affect the intensity of seasons. For
example, the amount of solar influx in July at 65 degrees north
latitude varies by as much as 22% (from 450 W/m² to 550 W/m²). It is
widely believed that ice sheets advance when summers become too cool
to melt all of the accumulated snowfall from the previous winter. Some
believe that the strength of the orbital forcing is too small to
trigger glaciations, but feedback mechanisms like CO2 may explain this
While Milankovitch forcing predicts that cyclic changes in the Earth's
orbital elements can be expressed in the glaciation record, additional
explanations are necessary to explain which cycles are observed to be
most important in the timing of glacial–interglacial periods. In
particular, during the last 800,000 years, the dominant period of
glacial–interglacial oscillation has been 100,000 years, which
corresponds to changes in Earth's orbital eccentricity and orbital
inclination. Yet this is by far the weakest of the three frequencies
predicted by Milankovitch. During the period 3.0–0.8 million years
ago, the dominant pattern of glaciation corresponded to the
41,000-year period of changes in Earth's obliquity (tilt of the axis).
The reasons for dominance of one frequency versus another are poorly
understood and an active area of current research, but the answer
probably relates to some form of resonance in the Earth's climate
system. Recent work suggests that the 100K year cycle dominates due to
increased southern-pole sea-ice increasing total solar
The "traditional" Milankovitch explanation struggles to explain the
dominance of the 100,000-year cycle over the last 8 cycles. Richard A.
Muller, Gordon J. F. MacDonald, and others have pointed
out that those calculations are for a two-dimensional orbit of Earth
but the three-dimensional orbit also has a 100,000-year cycle of
orbital inclination. They proposed that these variations in orbital
inclination lead to variations in insolation, as the
Earth moves in
and out of known dust bands in the solar system. Although this is a
different mechanism to the traditional view, the "predicted" periods
over the last 400,000 years are nearly the same. The Muller and
MacDonald theory, in turn, has been challenged by Jose Antonio
Another worker, William Ruddiman, has suggested a model that explains
the 100,000-year cycle by the modulating effect of eccentricity (weak
100,000-year cycle) on precession (26,000-year cycle) combined with
greenhouse gas feedbacks in the 41,000- and 26,000-year cycles. Yet
another theory has been advanced by
Peter Huybers who argued that the
41,000-year cycle has always been dominant, but that the
entered a mode of climate behavior where only the second or third
cycle triggers an ice age. This would imply that the 100,000-year
periodicity is really an illusion created by averaging together cycles
lasting 80,000 and 120,000 years. This theory is consistent with a
simple empirical multi-state model proposed by Didier Paillard.
Paillard suggests that the late
Pleistocene glacial cycles can be seen
as jumps between three quasi-stable climate states. The jumps are
induced by the orbital forcing, while in the early
41,000-year glacial cycles resulted from jumps between only two
climate states. A dynamical model explaining this behavior was
proposed by Peter Ditlevsen. This is in support of the suggestion
that the late
Pleistocene glacial cycles are not due to the weak
100,000-year eccentricity cycle, but a non-linear response to mainly
the 41,000-year obliquity cycle.
Variations in the Sun's energy output
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There are at least two types of variation in the Sun's energy output
In the very long term, astrophysicists believe that the Sun's output
increases by about 7% every one billion (109) years.
Shorter-term variations such as sunspot cycles, and longer episodes
such as the Maunder Minimum, which occurred during the coldest part of
the Little Ice Age.
The long-term increase in the Sun's output cannot be a cause of ice
Volcanic eruptions may have contributed to the inception and/or the
end of ice age periods. At times during the paleoclimate, carbon
dioxide levels were two or three times greater than today. Volcanoes
and movements in continental plates contributed to high amounts of CO2
in the atmosphere.
Carbon dioxide from volcanoes probably contributed
to periods with highest overall temperatures. One suggested
explanation of the Paleocene-
Eocene Thermal Maximum is that undersea
volcanoes released methane from clathrates and thus caused a large and
rapid increase in the greenhouse effect. There appears to be no
geological evidence for such eruptions at the right time, but this
does not prove they did not happen.
Recent glacial and interglacial phases
Northern hemisphere glaciation during the last ice ages. The setup of
3 to 4 kilometer thick ice sheets caused a sea level lowering of about
Main article: Timeline of glaciation
This section needs expansion with: Recent glacial and interglacial
phases in other areas outside North America. You can help by adding to
it. (March 2008)
Glacial stages in North America
The major glacial stages of the current ice age in North America are
Eemian and Wisconsin glaciation. The use of the
Nebraskan, Afton, Kansan, and Yarmouthian stages to subdivide the ice
age in North America has been discontinued by Quaternary geologists
and geomorphologists. These stages have all been merged into the
Pre-Illinoian in the 1980s.
During the most recent North American glaciation, during the latter
part of the
Last Glacial Maximum
Last Glacial Maximum (26,000 to 13,300 years ago), ice
sheets extended to about 45th parallel north. These sheets were 3 to 4
kilometres (1.9 to 2.5 mi) thick.
Wisconsin glaciation left widespread impacts on the North
American landscape. The
Great Lakes and the
Finger Lakes were carved
by ice deepening old valleys. Most of the lakes in
Wisconsin were gouged out by glaciers and later filled with glacial
meltwaters. The old
Teays River drainage system was radically altered
and largely reshaped into the
Ohio River drainage system. Other rivers
were dammed and diverted to new channels, such as Niagara Falls, which
formed a dramatic waterfall and gorge, when the waterflow encountered
a limestone escarpment. Another similar waterfall, at the present
Clark Reservation State Park
Clark Reservation State Park near Syracuse, New York, is now dry.
The area from
Long Island to
Nantucket, Massachusetts was formed from
glacial till, and the plethora of lakes on the
Canadian Shield in
northern Canada can be almost entirely attributed to the action of the
ice. As the ice retreated and the rock dust dried, winds carried the
material hundreds of miles, forming beds of loess many dozens of feet
thick in the Missouri Valley.
Post-glacial rebound continues to
Great Lakes and other areas formerly under the weight of
the ice sheets.
The Driftless Area, a portion of western and southwestern Wisconsin
along with parts of adjacent Minnesota, Iowa, and Illinois, was not
covered by glaciers.
See also: Glacial history of Minnesota
Last Glacial Period in the semiarid Andes around Aconcagua and
A specially interesting climatic change during glacial times has taken
place in the semi-arid Andes. Beside the expected cooling down in
comparison with the current climate, a significant precipitation
change happened here. So, researches in the presently semiarid
subtropic Aconcagua-massif (6,962 m) have shown an unexpectedly
extensive glacial glaciation of the type "ice stream
network". The connected valley glaciers exceeding
100 km in length, flowed down on the East-side of this section of
the Andes at 32–34°S and 69–71°W as far as a height of
2,060 m and on the western luff-side still clearly
deeper. Where current glaciers scarcely reach 10 km in
length, the snowline (ELA) runs at a height of 4,600 m and at
that time was lowered to 3,200 m asl, i.e. about 1,400 m.
From this follows that—beside of an annual depression of temperature
about c. 8.4 °C— here was an increase in precipitation.
Accordingly, at glacial times the humid climatic belt that today is
situated several latitude degrees further to the S, was shifted much
further to the N.
Effects of glaciation
Scandinavia exhibits some of the typical effects of ice age glaciation
such as fjords and lakes.
See also: Glacial landform
Although the last glacial period ended more than 8,000 years ago, its
effects can still be felt today. For example, the moving ice carved
out the landscape in Canada (See Canadian
Greenland, northern Eurasia and Antarctica. The erratic boulders,
till, drumlins, eskers, fjords, kettle lakes, moraines, cirques,
horns, etc., are typical features left behind by the glaciers.
The weight of the ice sheets was so great that they deformed the
Earth's crust and mantle. After the ice sheets melted, the ice-covered
land rebounded. Due to the high viscosity of the Earth's mantle, the
flow of mantle rocks which controls the rebound process is very
slow—at a rate of about 1 cm/year near the center of rebound
During glaciation, water was taken from the oceans to form the ice at
high latitudes, thus global sea level dropped by about 110 meters,
exposing the continental shelves and forming land-bridges between
land-masses for animals to migrate. During deglaciation, the melted
ice-water returned to the oceans, causing sea level to rise. This
process can cause sudden shifts in coastlines and hydration systems
resulting in newly submerged lands, emerging lands, collapsed ice dams
resulting in salination of lakes, new ice dams creating vast areas of
freshwater, and a general alteration in regional weather patterns on a
large but temporary scale. It can even cause temporary reglaciation.
This type of chaotic pattern of rapidly changing land, ice, saltwater
and freshwater has been proposed as the likely model for the Baltic
and Scandinavian regions, as well as much of central North America at
the end of the last glacial maximum, with the present-day coastlines
only being achieved in the last few millennia of prehistory. Also, the
effect of elevation on
Scandinavia submerged a vast continental plain
that had existed under much of what is now the North Sea, connecting
the British Isles to Continental Europe.
The redistribution of ice-water on the surface of the
Earth and the
flow of mantle rocks causes changes in the gravitational field as well
as changes to the distribution of the moment of inertia of the Earth.
These changes to the moment of inertia result in a change in the
angular velocity, axis, and wobble of the Earth's rotation.
The weight of the redistributed surface mass loaded the lithosphere,
caused it to flex and also induced stress within the Earth. The
presence of the glaciers generally suppressed the movement of faults
below. However, during deglaciation, the faults experience
accelerated slip triggering earthquakes. Earthquakes triggered near
the ice margin may in turn accelerate ice calving and may account for
the Heinrich events. As more ice is removed near the ice margin,
more intraplate earthquakes are induced and this positive feedback may
explain the fast collapse of ice sheets.
In Europe, glacial erosion and isostatic sinking from weight of ice
made the Baltic Sea, which before the Ice Age was all land drained by
the Eridanos River.
International Union for Quaternary Research
Irish Sea Glacier
Late Glacial Maximum
Little Ice Age
Timeline of glaciation
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The Wikibook Historical Geology has a page on the topic of: Ice ages
Wikimedia Commons has media related to Ice age.
Wikisource has the text of The New Student's Reference Work article
about Ice age.
Cracking the Ice Age from PBS
Montgomery, Keith (2010). "Development of the glacial theory,
1800–1870". Historical Simulation
Raymo, M. (July 2011). "Overview of the Uplift-
Archived from the original on 2008-10-22.
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Lake Baikal during the Last Glacial Maximum.
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BBC News: Science and Environment.
Last glacial period
1st: Würm, Wisconsin, Weichselian, Devensian/Midlandian,
Pinedale/Fraser, Greenland, Merida, Llanquihue
2nd: Riss, Illinoian, Saale, Wolstonian, Santa María
3rd–6th: Mindel, Pre-Illinoian, Elster, Anglian, Rio Llico
7th–8th: Günz, Pre-Illinoian, Elbe or Menapian, Beestonian, Caracol
Karoo (360 Mya to 260 Mya)
Andean-Saharan (460 Mya to 430 Mya)
Gaskiers (579.63 to 579.63 Mya)
Baykonurian (547 to 541.5 Mya)
Sturtian (717 to 660 Mya); Marinoan (650 to 635 Mya)
Huronian (2.4 to 2.1 Gya)
Pongola (2.9 to 2.78 Gya)
Greenhouse and icehouse Earth
Great Oxygenation Event
Timeline of glaciation
Greenhouse and Icehouse Earth
Tropical temperatures may reach poles
Global climate during an ice age
Earth's surface entirely or nearly frozen over
Stadials and Interstadials
Glacial history of Minnesota
Last Glacial Maximum
Laurentide Ice Sheet
List of prehistoric lakes
Timeline of glaciation
Giant current ripples
Arrowhead Provincial Park, Ontario
Big Rock (glacial erratic), Alberta
Cypress Hills (Canada), Saskatchewan
Eramosa River, Ontario
Eskers Provincial Park, British Columbia
Foothills Erratics Train, Alberta
Lion's Head Provincial Park, Ontario
Origin of the Oak Ridges Moraine, Ontario
Ovayok Territorial Park, Nunavut
Moraine State Recreation Area, Wisconsin
Coteau des Prairies, South Dakota
Devil's Lake State Park, Wisconsin
Glacial Lake Wisconsin, Wisconsin
Glacial Lakes State Park, Minnesota
Horicon Marsh State Wildlife Area, Wisconsin
Ice Age Floods National Geologic Trail, Idaho, Oregon & Washington
Ice Age National Scientific Reserve, Wisconsin
Ice Age Trail, Wisconsin
Interstate State Park,
Minnesota & Wisconsin
Kelleys Island, Ohio
Moraine State Forest, Wisconsin
Lake Bonneville, Utah
Lake Lahontan, Nevada
Lake Missoula, Montana
Mill Bluff State Park, Wisconsin
Oneida Lake, New York
Two Creeks Buried Forest State Natural Area, Wisconsin
Moraine and Jameson Lake
Drumlin Field, Washington
Yosemite National Park, California
Ross Ice Shelf
Last glacial period
Little Ice Age