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Contemporary climate change includes both the global warming caused by humans, and its impacts on Earth's weather patterns. There have been previous periods of climate change, but the current changes are more rapid than any known events in Earth's history. The main cause is the
emission of greenhouse gases Greenhouse gas emissions are emissions of greenhouse gases created from a range of human activities that cause climate change Climate change includes both global warming driven by human emissions of greenhouse gases and the resulting ...
, mostly
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is ...

carbon dioxide
() and
methane Methane (, ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Earth ...
. Burning
fossil fuel A fossil fuel is a -containing material formed underground from the remains of dead plants and animals that humans extract and to release for use. The main fossil s are , and , which humans extract through and . Fossil fuels may be burnt ...
s for
energy use Energy consumption is the amount of energy In , energy is the that must be to a or to perform on the body, or to it. Energy is a ; the law of states that energy can be in form, but not created or destroyed. The unit of measurement ...

energy use
creates most of these emissions. Agriculture,
steelmaking Steelmaking is the process of producing steel Steel is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared ...
, cement production, and forest loss are additional sources. Temperature rise is also affected by climate feedbacks such as the loss of
sunlight-reflecting snow cover
sunlight-reflecting snow cover
, and the release of carbon dioxide from drought-stricken forests. Collectively, these amplify global warming. On land, temperatures have risen about twice as fast as the global average. Deserts are expanding, while
heat wave A heat wave, or heatwave, is a period of excessively hot weather, which may be accompanied by high humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to th ...

heat wave
s and
wildfire A wildfire, bushfire, wildland fire or rural fire is an unplanned, unwanted, uncontrolled fire BBQ. Fire is the rapid oxidation of a material in the exothermic chemical process of combustion, releasing heat, light, and various reaction ...

wildfire
s are becoming more common.
Increased warming in the Arctic
Increased warming in the Arctic
has contributed to melting
permafrost Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the ocean The ocean (also the sea The sea, connected as the world ocean or simply the ocean The oc ...

permafrost
,
glacial retreat Image:Glacial lakes, Bhutan.jpg, 300px, Termini of the glaciers in the Bhutan-Himalaya. Glacial lakes have been rapidly forming on the surface of the debris-covered glaciers in this region during the last few decades. USGS researchers have found ...
and sea ice loss. Higher temperatures are also causing more intense storms and other
weather extremes
weather extremes
. In places such as
coral reefs ''Coral Reefs'' is a quarterly peer-reviewed scientific journal dedicated to the study of coral reefs. It was established in 1982 and is published by Springer Science+Business Media on behalf of the International Society for Reef Studies, of which ...
, mountains, and the Arctic, many species are forced to relocate or become extinct, as their environment changes. Climate change threatens people with
food Food is any substance consumed to provide Nutrient, nutritional support for an organism. Food is usually of plant, animal or Fungus, fungal origin, and contains essential nutrients, such as carbohydrates, fats, protein (nutrient), proteins, vi ...
and
water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , even though it provide ...
scarcity, increased flooding, extreme heat, more disease, and economic loss. It can also drive
human migration Human migration involves the movement of people from one place to another with intentions of settling, permanently or temporarily, at a new location (geographic region). The movement often occurs over long distances and from one country to anoth ...
.; . The World Health Organization calls climate change the greatest threat to global health in the 21st century. Even if efforts to minimise future warming are successful, some
effects Effect may refer to: * A result A result (also called upshot) is the final consequence of a sequence of actions or events expressed qualitatively or quantitatively. Possible results include wikt:advantage, advantage, disadvantage, Gain (accounting ...
will continue for centuries. These include
sea level rise Tide gauge measurements show that the current global sea level rise began at the start of the 20th century. Between 1900 and 2017, the globally averaged sea level Mean sea level (MSL) (often shortened to sea level) is an average In colloqui ...
, and warmer, more acidic oceans. Many of these impacts are already felt at the current level of warming, which is about 1.2 °C (2 °F). The
Intergovernmental Panel on Climate Change The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations The United Nations (UN) is an intergovernmental organization that aims to maintain international peace and international security, s ...
(IPCC) projects even greater impacts as warming continues to 1.5 °C and beyond. Additional warming also increases the risk of triggering tipping points, such as the melting of the
Greenland ice sheet The Greenland ice sheet ( da, Grønlands indlandsis, kl, Sermersuaq) is a vast body of ice Ice is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of wate ...

Greenland ice sheet
. Responding to these changes involves taking actions to limit the amount of warming, and adapting to them. Future warming can be reduced (mitigated) by lowering greenhouse gas emissions and removing them from the atmosphere. This will involve using more wind and solar energy, , and increasing
energy efficiency Energy efficiency may refer to: * Energy efficiency (physics), the ratio between the useful output and input of an energy conversion process ** Electrical efficiency, useful power output per electrical power consumed ** Mechanical efficiency, a rat ...
. Switching to
electric vehicles An electric vehicle (EV) is a vehicle that uses one or more traction motor, electric motors for propulsion. It can be powered by a Current collector, collector system, with electricity from extravehicular sources, or it can be powered autonomous ...
, and to
heat pump A heat pump is a system used to heat or cool an enclosed space or domestic water by transferring thermal energy from a cooler space to a warmer space using the Heat pump and refrigeration cycle, refrigeration cycle, moving heat in the opposite ...

heat pump
s for homes and commercial buildings, will further limit emissions.
Prevention Prevention may refer to: Health and medicine * Preventive healthcare Preventive healthcare, or prophylaxis, consists of measures taken for disease prevention.Hugh R. Leavell and E. Gurney Clark as "the science and art of preventing disease, ...
of
deforestation deforestation in 1750-2004 (net loss) showing anthropogenic modification of remaining forest. File:MODIS (2020-08-01).jpg, 300px, Dry seasons, exacerbated by climate change, and the use of slash-and-burn methods for clearing tropical forest ...

deforestation
and enhancing forests can help absorb . Some communities may adapt to climate change through better coastline protection,
disaster management Emergency management is the organization and management of the resources and responsibilities for dealing with all humanitarian Humanitarianism is an active belief in the value of human life, whereby human Humans (''Homo sapiens'') are ...
, and development of more resistant crops. By themselves, these efforts to adapt cannot avert the risk of severe, widespread and permanent impacts. Under the 2015
Paris Agreement The Paris Agreement (french: l'accord de Paris) is an agreement within the United Nations Framework Convention on Climate Change (UNFCCC), on climate change mitigation, Climate change adaptation, adaptation, and Climate finance, finance, signed i ...
, nations collectively agreed to keep warming "well under 2 °C" through mitigation efforts. However, with pledges made under the Agreement, global warming would still reach about 2.7 °C by the end of the century. Limiting warming to 1.5 °C would require halving emissions by 2030 and achieving net-zero emissions by 2050.


Terminology

Before the 1980s, it was unclear whether warming by greenhouse gases would dominate aerosol-induced cooling. Scientists then often used the term ''inadvertent climate modification'' to refer to the human impact on the climate. In the 1980s, the terms ''global warming'' and ''climate change'' were popularised. The former refers only to increased surface warming, the latter describes the full effect of greenhouse gases on the climate.. ''Global warming'' became the most popular term after NASA climate scientist
James Hansen James Edward Hansen (born March 29, 1941) is an American adjunct professor directing the Program on Climate Science, Awareness and Solutions of the Earth Institute at Columbia University Columbia University (also known as Columbia, and ...

James Hansen
used it in his 1988 testimony in the
U.S. Senate The United States Senate is the Upper house, upper chamber of the United States Congress, with the United States House of Representatives, House of Representatives being the Lower house, lower chamber. Together they compose the national Bica ...
. In the 2000s, the term ''climate change'' increased in popularity. ''Global warming'' usually refers to human-induced warming of the Earth system, whereas ''climate change'' can refer to natural or anthropogenic change. The two terms are often used interchangeably. Various scientists, politicians and media figures have adopted the terms ''
climate crisis Climate crisis is a term describing global warming and climate change, and their consequences. The term has been used to describe the threat of global warming to the planet, and to urge aggressive climate change mitigation. For example, a Janu ...
'' or '' climate emergency'' to talk about climate change, and ''global heating'' instead of ''global warming''. The policy editor-in-chief of ''
The Guardian ''The Guardian'' is a British daily newspaper. It was founded in 1821 as ''The Manchester Guardian'', and changed its name in 1959. Along with its sister papers ''The Observer ''The Observer'' is a British newspaper published on Sun ...

The Guardian
'' said they included this language in their editorial guidelines "to ensure that we are being scientifically precise, while also communicating clearly with readers on this very important issue". In 2019, Oxford Languages chose ''climate emergency'' as its word of the year, defining it as "a situation in which urgent action is required to reduce or halt climate change and avoid potentially irreversible environmental damage resulting from it".


Observed temperature rise

Multiple independent instrumental datasets show that the climate system is warming. The 2011–2020 decade warmed to an average 1.09 °C .95–1.20 °Ccompared to the pre-industrial baseline (1850–1900). Surface temperatures are rising by about 0.2 °C per decade, with 2020 reaching a temperature of 1.2 °C above the pre-industrial era. Since 1950, the number of cold days and nights has decreased, and the number of warm days and nights has increased. There was little net warming between the 18th century and the mid-19th century. Climate information for that period comes from climate proxies, such as trees and
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, and an ice core contains ice form ...
s. They show that natural variations offset the early effects of the
Industrial Revolution The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe Continental Europe or mainland Europe is the contiguous continent A continent is any of several large landmasse ...
.; Thermometer records began to provide global coverage around 1850. Historical patterns of warming and cooling, like the Medieval Climate Anomaly and the
Little Ice Age The Little Ice Age (LIA) was a period of cooling that occurred after the Medieval Warm Period The Medieval Warm Period (MWP) also known as the Medieval Climate Optimum, or Medieval Climatic Anomaly was a time of warm climate Climate is the ...
, did not occur at the same time across different regions. Temperatures may have reached as high as those of the late-20th century in a limited set of regions. There have been prehistorical episodes of global warming, such as the
Paleocene–Eocene Thermal Maximum The Paleocene–Eocene Thermal Maximum (PETM), alternatively (ETM1), and formerly known as the "Initial Eocene" or "", was a time period with a more than 5–8 °C global average temperature rise across the event. This climate event o ...
. However, the modern observed rise in temperature and concentrations has been so rapid that even abrupt geophysical events in Earth's history do not approach current rates. Evidence of warming from air temperature measurements are reinforced with a wide range of other observations. There has been an increase in the frequency and intensity of heavy precipitation, melting of snow and land ice, and increased atmospheric humidity. Flora and fauna are also behaving in a manner consistent with warming; for instance, plants are
flowering A flower, sometimes known as a bloom or blossom, is the reproductive Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents ...

flowering
earlier in spring. Another key indicator is the cooling of the upper atmosphere, which demonstrates that greenhouse gases are trapping heat near the Earth's surface and preventing it from radiating into space. Regions warm at various rates. The pattern is independent of where greenhouse gases are emitted, because the gases persist long enough to diffuse across the planet. Since the pre-industrial period, the average surface temperature over land regions has increased almost twice as fast as the global-average surface temperature. This is because of the larger
heat capacity Heat capacity or thermal capacity is a physical property A physical property is any property Property is a system of rights that gives people legal control of valuable things, and also refers to the valuable things themselves. Depending on t ...
of oceans, and because oceans lose more heat by
evaporation Evaporation is a type of that occurs on the of a as it changes into the gas phase. The surrounding gas must not be saturated with the evaporating substance. When the molecules of the liquid collide, they transfer energy to each other bas ...

evaporation
. The thermal energy in the global climate system has grown with only brief pauses since at least 1970, and over 90% of this extra energy has been stored in the ocean. The rest has heated the
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

atmosphere
, melted ice, and warmed the continents. The Northern Hemisphere and the North Pole have warmed much faster than the South Pole and Southern Hemisphere. The Northern Hemisphere not only has much more land, but also more seasonal snow cover and
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 ...

sea ice
. As these surfaces flip from reflecting a lot of light to being dark after the ice has melted, they start . Local black carbon deposits on snow and ice also contribute to Arctic warming. Arctic temperatures are increasing at over . Melting of glaciers and ice sheets in the Arctic disrupts ocean circulation, including a weakened
Gulf Stream The Gulf Stream, together with its northern extension the North Atlantic Drift The North Atlantic Current (NAC), also known as North Atlantic Drift and North Atlantic Sea Movement, is a powerful warm western boundary current Boundary curren ...
, further changing the climate.


Drivers of recent temperature rise

The
climate system Earth's climate arises from the interaction of five major climate system components: the Atmosphere of Earth, atmosphere (air), the hydrosphere (water), the cryosphere (ice and permafrost), the lithosphere (earth's upper rocky layer) and the biosp ...
experiences various cycles on its own which can last for years (such as the
El Niño–Southern Oscillation El Niño–Southern Oscillation (ENSO) is an irregular periodic variation in winds and sea surface temperatures over the Tropics, tropical eastern Pacific Ocean, affecting the climate of much of the tropics and subtropics. The warming phase of th ...
), decades or even centuries. Other changes are caused by an imbalance of energy that is "external" to the climate system, but not always external to the Earth. Examples of external forcings include changes in the concentrations of
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 greenhou ...
es,
solar luminosity The solar luminosity, , is a unit of radiant flux (Power (physics), power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, galaxy, galaxies and other celestial objects in terms of the output of ...
,
volcanic A volcano is a rupture in the crust of a planetary-mass object A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet ar ...

volcanic
eruptions, and variations in the Earth's orbit around the Sun. To determine the human contribution to climate change, known internal
climate variability Climate variability includes all the variations in the climate that last longer than individual weather events, whereas the term climate change only refers to those variations that persist for a longer period of time, typically decades or more. ...
and natural external forcings need to be ruled out. A key approach is to determine unique "fingerprints" for all potential causes, then compare these fingerprints with observed patterns of climate change. For example, solar forcing can be ruled out as a major cause. Its fingerprint would be warming in the entire atmosphere. Yet, only the lower atmosphere has warmed, consistent with greenhouse gas forcing.
Attribution of recent climate change Attribution of recent climate change is the effort to scientifically ascertain mechanisms responsible for recent global warming and related climate changes on Earth. The effort has focused on changes observed during the period of instrumental t ...
shows that the main driver is elevated greenhouse gases, but that aerosols also have a strong effect.


Greenhouse gases

The Earth absorbs
sunlight Sunlight is a portion of the given off by the , in particular , , and light. On , sunlight is and through , and is obvious as when the Sun is above the . When direct is not blocked by s, it is experienced as sunshine, a combination of b ...

sunlight
, then radiates it as heat. Greenhouse gases in the atmosphere absorb and reemit
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior ...

infrared
radiation, slowing the rate at which it can pass through the atmosphere and escape into space. Before the Industrial Revolution, naturally-occurring amounts of greenhouse gases caused the air near the surface to be about 33 °C warmer than it would have been in their absence. While
water vapour (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous p ...

water vapour
(~50%) and clouds (~25%) are the biggest contributors to the greenhouse effect, they increase as a function of temperature and are therefore
feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain Image:Kettenvergleich.jpg, Roller chains A chain is a wikt:series#Noun, serial assembly of connected pieces, called links, typically made of metal, with ...

feedback
s. On the other hand, concentrations of gases such as (~20%),
tropospheric ozone Ground level ozone (O3) also tropospheric ozone, is a trace gas#REDIRECT Trace gas Trace gases are those gases in the atmosphere other than nitrogen Nitrogen is the chemical element with the Symbol (chemistry), symbol N and atomic number 7. ...
,
CFCs Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly halogenated Alkane, paraffin hydrocarbons that contain only carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F), produced as volatility (chemistry), volatile ...
and
nitrous oxide Nitrous oxide, commonly known as laughing gas, nitrous, or nos, is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is an ...
are not temperature-dependent, and are therefore external forcings. Human activity since the Industrial Revolution, mainly extracting and burning fossil fuels (
coal Coal is a combustible , Germany ) , image_map = , map_caption = , map_width = 250px , capital = Berlin , coordinates = , largest_city = capital , languages_type = Official language , languages = German language, German , ...

coal
,
oil An oil is any nonpolar In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound ...

oil
, and
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon gas mixture consisting of methane and commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carbon dioxid ...

natural gas
), has increased the amount of greenhouse gases in the atmosphere, resulting in a radiative imbalance. In 2019, the
concentrations In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: #Mass concentration, mass concentration, #Molar concentration, molar concentrat ...
of and methane had increased by about 48% and 160%, respectively, since 1750. These levels are higher than they have been at any time during the last 2 million years. Concentrations of methane are far higher than they were over the last 800,000 years. Global anthropogenic
greenhouse gas emissions Greenhouse gas emissions from human activities strengthen the greenhouse effect The greenhouse effect is the process by which radiation from a planet's atmosphere warms the planet's surface to a temperature above what it would be without ...
in 2018, excluding those from land use change, were 52 billion tonnes of . Of these emissions, 72% was , 19% was
methane Methane (, ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Earth ...
, 6% was nitrous oxide, and 3% was
fluorinated gasesFluorinated gases (F-gases) are man-made gases that can stay in the atmosphere for centuries and contribute to a global greenhouse effect The greenhouse effect is the process by which radiation from a planet's atmosphere warms the planet's sur ...
. emissions primarily come from burning
fossil fuel A fossil fuel is a -containing material formed underground from the remains of dead plants and animals that humans extract and to release for use. The main fossil s are , and , which humans extract through and . Fossil fuels may be burnt ...
s to provide energy for
transport Transport (in British English British English (BrE) is the standard dialect A standard language (also standard variety, standard dialect, and standard) is a language variety that has undergone substantial codification of grammar and ...

transport
, manufacturing,
heating File:Pelletkessel in Wohnhaus.JPG, upHot water central heating unit, using wood as fuel A central heating system provides warmth to the number of spaces within a building and optionally also able to heat water heating, domestic hot water from one ...
, and electricity. Additional emissions come from
deforestation deforestation in 1750-2004 (net loss) showing anthropogenic modification of remaining forest. File:MODIS (2020-08-01).jpg, 300px, Dry seasons, exacerbated by climate change, and the use of slash-and-burn methods for clearing tropical forest ...
and
industrial processes Industrial processes are procedures involving chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its consti ...
, which include the released by the chemical reactions for ,
steel Steel is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appe ...
,
aluminum Aluminium (aluminum in American American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the United States The United States of America (USA), commonly known as the Uni ...
, and
fertiliser A fertilizer (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American En ...

fertiliser
. Methane emissions come from livestock, manure,
rice cultivation Rice is the seed A seed is an embryonic plant enclosed in a protective outer covering. The formation of the seed is part of the process of reproduction Reproduction (or procreation or breeding) is the biological process by which n ...
, landfills, wastewater, and
coal mining Coal mining is the process of extracting coal Coal is a combustible black or brownish-black sedimentary rock, formed as stratum, rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, ele ...
, as well as fugitive gas emissions, oil and gas extraction. Nitrous oxide emissions largely come from the microbial decomposition of fertilizer, fertiliser. From a Greenhouse gas emissions accounting#Measuring GHG emissions, production standpoint, the primary sources of global greenhouse gas emissions are estimated as: electricity and heat (25%), agriculture and forestry (24%), industry and manufacturing (21%), transport (14%), and buildings (6%). Despite the contribution of deforestation to greenhouse gas emissions, the Earth's land surface, particularly its forests, remain a significant carbon sink for . Natural processes, such as carbon fixation in the soil and photosynthesis, more than offset the greenhouse gas contributions from deforestation. The land-surface sink is estimated to remove about 29% of annual global emissions. The ocean also serves as a significant carbon sink via a two-step process. First, dissolves in the surface water. Afterwards, the ocean's Thermohaline circulation, overturning circulation distributes it deep into the ocean's interior, where it accumulates over time as part of the carbon cycle. Over the last two decades, the world's oceans have absorbed 20 to 30% of emitted .


Aerosols and clouds

Air pollution, in the form of aerosols, not only puts a large burden on human health, but also Particulates#Climate effects, affects the climate on a large scale. From 1961 to 1990, a gradual reduction in the amount of irradiance, sunlight reaching the Earth's surface was observed, a phenomenon popularly known as ''global dimming'', typically attributed to aerosols from biofuel and fossil fuel burning. Globally, aerosols have been declining since 1990, meaning that they no longer mask greenhouse gas warming as much. Aerosols scatter and absorb solar radiation. They also have indirect effects on the Earth's energy budget, Earth's radiation budget. Sulfate aerosols act as cloud condensation nuclei and lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets. They also reduce the Cloud physics#Collision-coalescence, growth of raindrops, which makes clouds more reflective to incoming sunlight. Indirect effects of aerosols are the largest uncertainty in radiative forcing. While aerosols typically limit global warming by reflecting sunlight, black carbon in soot that falls on snow or ice can contribute to global warming. Not only does this increase the absorption of sunlight, it also increases melting and sea-level rise. Limiting new black carbon deposits in the Arctic could reduce global warming by 0.2 °C by 2050.


Changes of the land surface

Humans change the Earth's surface mainly to create more agricultural land. Today, agriculture takes up 34% of Earth's land area, while 26% is forests, and 30% is uninhabitable (glaciers, deserts, etc.). The amount of forested land continues to decrease, largely due to conversion to cropland in the tropics. This
deforestation deforestation in 1750-2004 (net loss) showing anthropogenic modification of remaining forest. File:MODIS (2020-08-01).jpg, 300px, Dry seasons, exacerbated by climate change, and the use of slash-and-burn methods for clearing tropical forest ...

deforestation
is the most significant aspect of land surface change affecting global warming. The main causes of deforestation are: permanent land-use change from forest to agricultural land producing products such as beef and palm oil (27%), logging to produce forestry/forest products (26%), short term shifting cultivation (24%), and wildfires (23%). Land use changes not only affect greenhouse gas emissions. The type of vegetation in a region affects the local temperature. It impacts how much of the sunlight gets reflected back into space (albedo), and how much evaporative cooling, heat is lost by evaporation. For instance, the change from a dark forest to grassland makes the surface lighter, causing it to reflect more sunlight. Deforestation can also affect temperatures by modifying the release of chemical compounds that influence clouds, and by changing wind patterns. In tropic and temperate areas the net effect is to produce significant warming, while at latitudes closer to the poles a gain of albedo (as forest is replaced by snow cover) leads to a cooling effect. Globally, these effects are estimated to have led to a slight cooling, dominated by an increase in surface albedo.


Solar and volcanic activity

Physical climate models are unable to reproduce the rapid warming observed in recent decades when taking into account only variations in solar output and volcanic activity. As the Sun is the Earth's primary energy source, changes in incoming sunlight directly affect the climate system.. solar variation, Solar irradiance has been measured directly by satellites, and indirect measurements are available from the early 1600s onwards. There has been no upward trend in the amount of the Sun's energy reaching the Earth. Further evidence for greenhouse gases causing global warming comes from measurements that show a warming of the lower atmosphere (the troposphere), coupled with a cooling of the upper atmosphere (the stratosphere). If solar variations were responsible for the observed warming, the troposphere and stratosphere would both warm.. Types of volcanic eruptions#Plinian, Explosive volcanic eruptions represent the largest natural forcing over the industrial era. When the eruption is sufficiently strong (with sulfur dioxide reaching the stratosphere), sunlight can be partially blocked for a couple of years. The temperature signal lasts about twice as long. In the industrial era, volcanic activity has had negligible impacts on global temperature trends. Present-day Volcanic gas, volcanic CO2 emissions are equivalent to less than 1% of current anthropogenic CO2 emissions.


Climate change feedback

The response of the climate system to an initial forcing is modified by feedbacks: increased by self-reinforcing feedbacks and reduced by balancing feedbacks. The main reinforcing feedbacks are the Water vapour feedback, water-vapour feedback, the ice–albedo feedback, and probably the net effect of clouds. The primary balancing mechanism is radiative cooling, as Earth's surface gives off more Infrared, heat to space in response to rising temperature. In addition to temperature feedbacks, there are feedbacks in the carbon cycle, such as the fertilizing effect of on plant growth. Uncertainty over feedbacks is the major reason why different climate models project different magnitudes of warming for a given amount of emissions. As air gets warmer, Relative humidity, it can hold more moisture. After initial warming due to emissions of greenhouse gases, the atmosphere will hold more water. Water vapour is a potent greenhouse gas, so this further heats the atmosphere. If cloud cover increases, more sunlight will be reflected back into space, cooling the planet. If clouds become higher and thinner, they act as an insulator, reflecting heat from below back downwards and warming the planet. Overall, the net cloud feedback over the industrial era has probably exacerbated temperature rise. The reduction of snow cover and sea ice in the Arctic reduces the albedo of the Earth's surface. More of the Sun's energy is now absorbed in these regions, contributing to polar amplification, amplification of Arctic temperature changes. Arctic amplification is also melting
permafrost Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the ocean The ocean (also the sea The sea, connected as the world ocean or simply the ocean The oc ...

permafrost
, which releases methane and into the atmosphere. Around half of human-caused emissions have been absorbed by land plants and by the oceans. On land, elevated and an extended growing season have stimulated plant growth. Climate change increases droughts and heat waves that inhibit plant growth, which makes it uncertain whether this carbon sink will continue to grow in the future. Soils contain large quantities of carbon and Soil carbon feedback, may release some when they heat up. As more and heat are absorbed by the ocean, it acidifies, its circulation changes and phytoplankton takes up less carbon, decreasing the rate at which the ocean absorbs atmospheric carbon. Climate change can increase methane emissions from wetlands, marine and freshwater systems, and permafrost.


Future warming and the carbon budget

Future warming depends on the Climate sensitivity, strengths of climate feedbacks and on emissions of greenhouse gases. The former are often estimated using climate models, developed by multiple scientific institutions. A climate model is a representation of the physical, chemical, and biological processes that affect the climate system. Models include changes in the Earth's orbit, historical changes in the Sun's activity, and volcanic forcing. Computer models attempt to reproduce and predict the circulation of the oceans, the annual cycle of the seasons, and the flows of carbon between the land surface and the atmosphere. Models project different future temperature rises for given emissions of greenhouse gases; they do not fully agree on the strength of different #Feedback, feedbacks on climate sensitivity and magnitude of Climate inertia, inertia of the climate system. The physical realism of models is tested by examining their ability to simulate contemporary or past climates. Past models have underestimated the rate of Arctic shrinkage and underestimated the rate of precipitation increase. Sea level rise since 1990 was underestimated in older models, but more recent models agree well with observations. The 2017 United States-published National Climate Assessment notes that "climate models may still be underestimating or missing relevant feedback processes". A Integrated assessment modelling, subset of climate models add societal factors to a simple physical climate model. These models simulate how population, economic growth, and energy use affectand interact withthe physical climate. With this information, these models can produce scenarios of future greenhouse gas emissions. This is then used as input for physical climate models to generate climate change projections. In some scenarios emissions continue to rise over the century, while others have reduced emissions. Fossil fuel resources are too abundant for shortages to be relied on to limit carbon emissions in the 21st century. Emissions scenarios can be combined with modelling of the carbon cycle to predict how atmospheric concentrations of greenhouse gases might change in the future. According to these combined models, by 2100 the atmospheric concentration of CO2 could be as low as 380 or as high as 1400 ppm, depending on the Shared Socioeconomic Pathways, socioeconomic scenario and the mitigation scenario. The IPCC Sixth Assessment Report projects that global warming is very likely to reach 1.0 °C to 1.8 °C by the late 21st century under the Shared Socioeconomic Pathways#SSP1-1.9: Sustainability (Taking the Green Road), very low GHG emissions scenario. In an Shared Socioeconomic Pathways#SSP2-4.5: Regional rivalry (A Rocky Road), intermediate scenario global warming would reach 2.1 °C to 3.5 °C, and 3.3 °C to 5.7 °C under the Shared Socioeconomic Pathways#SSP5-8.5: Fossil-Fueled Development (Taking the Highway), very high GHG emissions scenario. These projections are based on climate models in combination with observations. The remaining carbon budget is determined by modelling the carbon cycle and the climate sensitivity to greenhouse gases. According to the IPCC, global warming can be kept below 1.5 °C with a two-thirds chance if emissions after 2018 do not exceed 420 or 570 gigatonnes of . This corresponds to 10 to 13 years of current emissions. There are high uncertainties about the budget. For instance, it may be 100 gigatonnes of smaller due to methane release from permafrost and wetlands.


Impacts


Physical environment

The environmental effects of climate change are broad and far-reaching, affecting oceans, ice, and weather. Changes may occur gradually or rapidly. Evidence for these effects comes from studying climate change in the past, from modelling, and from modern observations. Since the 1950s, droughts and
heat wave A heat wave, or heatwave, is a period of excessively hot weather, which may be accompanied by high humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to th ...

heat wave
s have appeared simultaneously with increasing frequency. Extremely wet or dry events within the monsoon period have increased in India and East Asia. The rainfall rate and intensity of Tropical cyclones and climate change, hurricanes and typhoons is likely increasing.. Frequency of tropical cyclones has not increased as a result of climate change. Sea level rise, Global sea level is rising as a consequence of Retreat of glaciers since 1850, glacial melt, melt of the ice sheets in Greenland ice sheet, Greenland and Antarctic ice sheet, Antarctica, and thermal expansion. Between 1993 and 2020, the rise increased over time, averaging 3.3 ± 0.3 mm per year. Over the 21st century, the IPCC projects that in a very high emissions scenario the sea level could rise by 61–110 cm. Increased ocean warmth is undermining and threatening to unplug Antarctic glacier outlets, risking a large melt of the ice sheet and the possibility of a 2-meter sea level rise by 2100 under high emissions. Climate change has led to decades of Arctic sea ice decline, shrinking and thinning of the Arctic sea ice. While ice-free summers are expected to be rare at 1.5 °C degrees of warming, they are set to occur once every three to ten years at a warming level of 2 °C. Higher atmospheric concentrations have led to changes in ocean chemistry. An increase in dissolved is causing ocean acidification, oceans to acidify. In addition, ocean deoxygenation, oxygen levels are decreasing as oxygen is less soluble in warmer water. Dead zone (ecology), Dead zones in the ocean, regions with very little oxygen, are expanding too.


Tipping points and long-term impacts

The greater the amount of global warming, the greater the risk of passing through ‘ tipping points’, thresholds beyond which certain impacts can no longer be avoided even if temperatures are reduced. An example is the collapse of Collapse of the West Antarctic Ice Sheet, West Antarctic and Greenland ice sheets, where a temperature rise of 1.5 to 2 °C may commit the ice sheets to melt, although the time scale of melt is uncertain and depends on future warming. Some large-scale changes could occur abrupt climate change, over a short time period, such as a Shutdown of thermohaline circulation, collapse of the Atlantic Meridional Overturning Circulation,. which would trigger major climate changes in the North Atlantic, Europe, and North America. The long-term effects of climate change include further ice melt, ocean warming, sea level rise, and ocean acidification. On the timescale of centuries to millennia, the magnitude of climate change will be determined primarily by anthropogenic emissions. This is due to 's long atmospheric lifetime. Oceanic uptake is slow enough that ocean acidification will continue for hundreds to thousands of years. These emissions are estimated to have prolonged the current interglacial period by at least 100,000 years. Sea level rise will continue over many centuries, with an estimated rise of after 2000 years.


Nature and wildlife

Recent warming has driven many terrestrial and freshwater species poleward and towards higher altitudes. Higher atmospheric levels and an extended growing season have resulted in global greening. However, heatwaves and drought have reduced ecosystem productivity in some regions. The future balance of these opposing effects is unclear. Climate change has contributed to the expansion of drier climate zones, such as the Desertification, expansion of deserts in the subtropics. The size and speed of global warming is making Ecological threshold, abrupt changes in ecosystems more likely. Overall, it is expected that climate change will result in the extinction of many species. The oceans have heated more slowly than the land, but plants and animals in the ocean have migrated towards the colder poles faster than species on land. Just as on land, heat waves in the ocean occur more frequently due to climate change, harming a wide range of organisms such as corals, kelp, and seabirds. Ocean acidification makes it harder for organisms such as mussels, barnacles and corals to Biomineralization, produce shells and skeletons; and heatwaves have Coral bleaching, bleached coral reefs. Harmful algal blooms enhanced by climate change and eutrophication lower oxygen levels, disrupt food webs and cause great loss of marine life. Coastal ecosystems are under particular stress. Almost half of global wetlands have disappeared due to climate change and other human impacts.


Humans

The effects of climate change on humans, mostly due to warming and shifts in precipitation, have been detected worldwide. Impacts are now observable Regional effects of climate change, on all continents and across ocean regions, with low-latitude, Developing countries, less developed areas facing the greatest risk. Continued warming has potentially “severe, pervasive and irreversible impacts” for people and ecosystems. The risks are unevenly distributed, but are generally greater for disadvantaged people in developing and developed countries.


Food and health

Health impacts include both the direct effects of extreme weather, leading to injury and loss of life, and indirect effects such as undernutrition brought on by crop failures. Various infectious diseases are more easily transmitted in a warmer climate, such as dengue fever, which affects children most severely, and malaria. Young children are the most vulnerable to food shortages, and together with older people, to extreme heat. The World Health Organization (WHO) has estimated that between 2030 and 2050, climate change is expected to cause around 250,000 additional deaths per year from heat exposure in elderly people, increases in diarrheal disease, malaria, dengue, coastal flooding, and childhood undernutrition. Over 500,000 additional adult deaths are projected yearly by 2050 due to reductions in food availability and quality. Other major health risks associated with climate change include air and water quality. The WHO has classified human impacts from climate change as the greatest threat to global health in the 21st century. Climate change is affecting food security. It has caused reduction in global mean yields of maize, wheat, and soybeans between 1981 and 2010. Future warming could further reduce global yields of major crops. crop productivity, Crop production will probably be negatively affected in low-latitude countries, while effects at northern latitudes may be positive or negative. Up to an additional 183 million people worldwide, particularly those with lower incomes, are at risk of hunger as a consequence of these impacts. The effects of warming on the oceans impact fish stocks, with a global decline in the maximum catch potential. Only polar stocks are showing an increased potential. Regions dependent on glacier water, regions that are already dry, and small islands are at increased risk of water stress due to climate change.


Livelihoods

Economic damages due to climate change may be severe and there is a probability of disastrous Black swan theory, tail-risk events. Climate change has likely already increased global economic inequality, and this trend is projected to continue. Most of the severe impacts are expected Climate change in Africa, in sub-Saharan Africa and South-East Asia, where existing poverty is already exacerbated. The World Bank estimates that climate change could drive over 120 million people into poverty by 2030. Current inequalities between men and women, between rich and poor, and between different ethnicities have been observed to worsen as a consequence of climate variability and climate change. An expert elicitation concluded that the role of climate change in armed conflict has been small compared to factors such as socio-economic inequality and state capabilities, but that future warming will bring increasing risks. Low-lying islands and coastal communities are threatened through hazards posed by sea level rise, such as flooding and permanent submergence. This could lead to statelessness for populations in island nations, such as the Maldives and Tuvalu. In some regions, rise in temperature and humidity may be too severe for humans to adapt to. With worst-case climate change, models project that almost one-third of humanity might live in extremely hot and uninhabitable climates, similar to the current climate found in the Sahara. These factors, plus weather extremes, can drive Environmental migrant, environmental migration, both within and between countries. Displacement of people is expected to increase as a consequence of more frequent extreme weather, sea level rise, and conflict arising from increased competition over natural resources. Climate change may also increase vulnerability, leading to "trapped populations" who are not able to move due to a lack of resources.


Responses


Mitigation

Climate change can be mitigated by reducing greenhouse gas emissions and by enhancing Carbon sink, sinks that absorb greenhouse gases from the atmosphere. In order to limit global warming to less than 1.5 °C with a high likelihood of success, global greenhouse gas emissions needs to be Carbon neutrality, net-zero by 2050, or by 2070 with a 2 °C target. This requires far-reaching, systemic changes on an unprecedented scale in energy, land, cities, transport, buildings, and industry. The United Nations Environment Programme estimates that countries need to triple their Nationally Determined Contributions, pledges under the Paris Agreement within the next decade to limit global warming to 2 °C. An even greater level of reduction is required to meet the 1.5 °C goal. With pledges made under the Agreement as of October 2021, global warming would still have a 66% chance of reaching about 2.7 °C (range: 2.2–3.2 °C) by the end of the century. Although there is no single pathway to limit global warming to 1.5 or 2 °C, most scenarios and strategies see a major increase in the use of renewable energy in combination with increased energy efficiency measures to generate the needed greenhouse gas reductions. To reduce pressures on ecosystems and enhance their carbon sequestration capabilities, changes would also be necessary in agriculture and forestry, such as preventing
deforestation deforestation in 1750-2004 (net loss) showing anthropogenic modification of remaining forest. File:MODIS (2020-08-01).jpg, 300px, Dry seasons, exacerbated by climate change, and the use of slash-and-burn methods for clearing tropical forest ...

deforestation
and restoring natural ecosystems by reforestation. Other approaches to mitigating climate change have a higher level of risk. Scenarios that limit global warming to 1.5 °C typically project the large-scale use of carbon dioxide removal, carbon dioxide removal methods over the 21st century. There are concerns, though, about over-reliance on these technologies, and environmental impacts. Solar radiation management (SRM) is also a possible supplement to deep reductions in emissions. However, SRM would raise significant ethical and legal issues, and the risks are poorly understood.


Clean energy

Renewable energy is key to limiting climate change. Fossil fuels accounted for 80% of the world's energy in 2018. The remaining share was split between nuclear power and renewables (including Solar energy, solar and wind power, Biomass, bioenergy, geothermal energy, and hydropower). That mix is projected to change significantly over the next 30 years.. Solar and wind have seen substantial growth and progress over the last few years. Photovoltaic system, Solar panels and Wind power, onshore wind are the cheapest forms of adding new power generation capacity in most countries. Renewables represented 75% of all new electricity generation installed in 2019, nearly all solar and wind. Meanwhile, nuclear power share remains the same but costs are increasing. Nuclear power generation is now several times more expensive per megawatt-hour than wind and solar. To achieve carbon neutrality by 2050, renewable energy would become the dominant form of electricity generation, rising to 85% or more by 2050 in some scenarios. The use of electricity for heating and transport, would rise to the point where electricity becomes the largest form of energy. Investment in coal would be eliminated and coal use nearly phased out by 2050. In transport, scenarios envision sharp increases in the market share of electric vehicles, of public transport and of a switch to low-carbon fuel for other transportation modes like shipping. Heating would be increasingly decarbonised with the use of technologies like
heat pump A heat pump is a system used to heat or cool an enclosed space or domestic water by transferring thermal energy from a cooler space to a warmer space using the Heat pump and refrigeration cycle, refrigeration cycle, moving heat in the opposite ...

heat pump
s. There are obstacles to the continued rapid growth of renewables. For solar and wind power, a key challenge is their Variable renewable energy, intermittency and seasonal variability. Traditionally, Pumped-storage hydroelectricity, hydro dams with reservoirs and conventional power plants have been used when variable energy production is low. Intermittency is further countered by expanding Battery storage power station, battery storage and Demand response, matching energy demand and supply. Long-distance Electric power transmission, transmission can smooth variability of renewable output across wider geographic areas.; ; There can be environmental and land use concerns with large solar and wind projects, while bioenergy is often not carbon-neutral and may have negative consequences for food security. Hydropower growth has been slowing and is set to decline further due to concerns about social and environmental impacts. Life-cycle greenhouse gas emissions of energy sources, Low-carbon energy improves human health by minimising climate change and has the near-term benefit of reducing air pollution deaths, which were estimated at 7 million annually in 2016. Meeting the Paris Agreement goals that limit warming to a 2 °C increase could save about a million of those lives per year by 2050, whereas limiting global warming to 1.5 °C could save millions and simultaneously increase energy security and reduce poverty.


Energy efficiency

Reducing energy demand is another major aspect of reducing emissions. If less energy is needed, there is more flexibility for clean energy development. It also makes it easier to manage the electricity grid, and minimises emission intensity, carbon-intensive infrastructure development. Major increases in energy efficiency investment will be required to achieve climate goals, comparable to the level of investment in renewable energy. Several COVID-19 related changes in energy use patterns, energy efficiency investments, and funding have made forecasts for this decade more difficult and uncertain. Strategies to reduce energy demand vary by sector. In transport, passengers and freight can switch to more efficient travel modes, such as buses and trains, or use electric vehicles. Industrial strategies to reduce energy demand include increasing the energy efficiency of heating systems and motors, designing less energy-intensive products, and increasing product lifetimes. In the building sector the focus is on better design of new buildings, and aiming for higher levels of energy efficiency in retrofitting. The use of technologies like
heat pump A heat pump is a system used to heat or cool an enclosed space or domestic water by transferring thermal energy from a cooler space to a warmer space using the Heat pump and refrigeration cycle, refrigeration cycle, moving heat in the opposite ...

heat pump
s can also increase building energy efficiency.


Agriculture and industry

Agriculture and forestry face a triple challenge of limiting greenhouse gas emissions, preventing the further conversion of forests to agricultural land, and meeting increases in world food demand. A set of actions could reduce agriculture and forestry-based emissions by two thirds from 2010 levels. These include reducing growth in demand for food and other agricultural products, increasing land productivity, protecting and restoring forests, and reducing greenhouse gas emissions from agricultural production. Steel and cement production, responsible for about 13% of industrial emissions, present particular challenges. In these industries, carbon-intensive materials such as coke and lime play an integral role in the production, so that reducing emissions requires research into alternative chemistries.


Carbon sequestration

Natural carbon sinks can be enhanced to sequester significantly larger amounts of beyond naturally occurring levels. Reforestation and Afforestation, tree planting on non-forest lands are among the most mature sequestration techniques, although the latter raises food security concerns. Regenerative agriculture, Soil carbon sequestration and blue carbon, coastal carbon sequestration are less understood options. The feasibility of land-based negative emissions methods for mitigation are uncertain; the IPCC has described mitigation strategies based on them as risky. Where energy production or -intensive heavy industry, heavy industries continue to produce waste , the gas can be captured and stored instead of released to the atmosphere. Although its current use is limited in scale and expensive, carbon capture and storage (CCS) may be able to play a significant role in limiting emissions by mid-century. This technique, in combination with bio-energy (BECCS) can result in net negative emissions: is drawn from the atmosphere. It remains highly uncertain whether carbon dioxide removal techniques, such as BECCS, will be able to play a large role in limiting warming to 1.5 °C. Policy decisions that rely on carbon dioxide removal increase the risk of global warming rising beyond international goals.


Adaptation

Adaptation is "the process of adjustment to current or expected changes in climate and its effects". Without additional mitigation, adaptation cannot avert the risk of "severe, widespread and irreversible" impacts. More severe climate change requires more transformative adaptation, which can be prohibitively expensive. The Adaptive capacity, capacity and potential for humans to adapt is unevenly distributed across different regions and populations, and developing countries generally have less. The first two decades of the 21st century saw an increase in adaptive capacity in most low- and middle-income countries with improved access to basic sanitation and electricity, but progress is slow. Many countries have implemented adaptation policies. However, there is a considerable gap between necessary and available finance. Adaptation to sea level rise consists of avoiding at-risk areas, learning to live with increased flooding and protection. If that fails, managed retreat may be needed. There are economic barriers for tackling dangerous heat impact. Avoiding strenuous work or having air conditioning is not possible for everybody. In agriculture, adaptation options include a switch to more sustainable diets, diversification, erosion control and genetic improvements for increased tolerance to a changing climate. Insurance allows for risk-sharing, but is often difficult to get for people on lower incomes. Education, migration and early warning systems can reduce climate vulnerability. Climate change and ecosystems, Ecosystems adapt to climate change, a process that can be supported by human intervention. By increasing connectivity between ecosystems, species can migrate to more favourable climate conditions. Species can also be directly moved. Protection and restoration of natural and semi-natural areas helps build resilience, making it easier for ecosystems to adapt. Many of the actions that promote adaptation in ecosystems, also help humans adapt via ecosystem-based adaptation. For instance, restoration of Fire regime, natural fire regimes makes catastrophic fires less likely, and reduces human exposure. Giving rivers more space allows for more water storage in the natural system, reducing flood risk. Restored forest acts as a carbon sink, but planting trees in unsuitable regions can exacerbate climate impacts. There are Synergy, synergies and trade-offs between adaptation and mitigation. Adaptation often offer short-term benefits, whereas mitigation has longer-term benefits. Increased use of air conditioning allows people to better cope with heat, but increases energy demand. Compact Urban planning, urban development may lead to reduced emissions from transport and construction. At the same time, it may increase the urban heat island effect, leading to higher temperatures and increased exposure. Increased food productivity has large benefits for both adaptation and mitigation.


Policies and politics

Countries that are most Climate change vulnerability, vulnerable to climate change have typically been responsible for a small share of global emissions. This raises questions about justice and fairness. Climate change is strongly linked to sustainable development. Limiting global warming makes it easier to achieve sustainable development goals, such as eradicating poverty and reducing inequalities. The connection is recognised in Sustainable Development Goal 13 which is to "[t]ake urgent action to combat climate change and its impacts". The goals on food, clean water and ecosystem protection have synergies with climate mitigation. The geopolitics of climate change is complex. It has often been framed as a free-rider problem, in which all countries benefit from mitigation done by other countries, but individual countries would lose from switching to a low-carbon economy themselves. This framing has been challenged. For instance, the benefits of a coal phase out, coal phase-out to public health and local environments exceed the costs in almost all regions. Furthermore, net importers of fossil fuels win economically from switching to clean energy, causing net exporters to face stranded assets: fossil fuels they cannot sell.


Policy options

A wide range of Policy, policies, regulations, and laws are being used to reduce emissions. As of 2019, carbon pricing covers about 20% of global greenhouse gas emissions. Carbon can be priced with carbon taxes and Carbon emission trading, emissions trading systems. Direct global fossil fuel subsidies reached $319 billion in 2017, and $5.2 trillion when indirect costs such as air pollution are priced in. Ending these can cause a 28% reduction in global carbon emissions and a 46% reduction in air pollution deaths. Subsidies could be used to support the Renewable energy transition, transition to clean energy instead. More direct methods to reduce greenhouse gases include vehicle efficiency standards, renewable fuel standards, and air pollution regulations on heavy industry. Several countries Renewable portfolio standard, require utilities to increase the share of renewables in power production. Policy designed through the lens of climate justice tries to address human rights issues and social inequality. For instance, wealthy nations responsible for the largest share of emissions would have to pay poorer countries to adapt. As the use of fossil fuels is reduced, jobs in the sector are being lost. To achieve a just transition, these people would need to be retrained for other jobs. Communities with many fossil fuel workers would need additional investments.


International climate agreements

Nearly all countries in the world are parties to the 1994 United Nations Framework Convention on Climate Change (UNFCCC). The goal of the UNFCCC is to prevent dangerous human interference with the climate system. As stated in the convention, this requires that greenhouse gas concentrations are stabilised in the atmosphere at a level where ecosystems can adapt naturally to climate change, food production is not threatened, and Economics of climate change, economic development can be sustained. The UNFCCC does not itself restrict emissions but rather provides a framework for protocols that do. Global emissions have risen since the UNFCCC was signed.. United Nations Climate Change conference, Its yearly conferences are the stage of global negotiations. The 1997 Kyoto Protocol extended the UNFCCC and included legally binding commitments for most developed countries to limit their emissions. During the negotiations, the Group of 77, G77 (representing Developing country, developing countries) pushed for a mandate requiring Developed country, developed countries to "[take] the lead" in reducing their emissions, since developed countries contributed most to the Greenhouse gas#Cumulative and historical emissions, accumulation of greenhouse gases in the atmosphere. Greenhouse gas#Annual emissions, Per-capita emissions were also still relatively low in developing countries and developing countries would need to emit more to meet their development needs. The 2009 Copenhagen Accord has been widely portrayed as disappointing because of its low goals, and was rejected by poorer nations including the G77. Associated parties aimed to limit the global temperature rise to below 2 °C. The Accord set the goal of sending $100 billion per year to developing countries for mitigation and adaptation by 2020, and proposed the founding of the Green Climate Fund. , the fund has failed to reach its expected target, and risks a shrinkage in its funding. In 2015 all UN countries negotiated the
Paris Agreement The Paris Agreement (french: l'accord de Paris) is an agreement within the United Nations Framework Convention on Climate Change (UNFCCC), on climate change mitigation, Climate change adaptation, adaptation, and Climate finance, finance, signed i ...
, which aims to keep global warming well below 2.0 °C and contains an aspirational goal of keeping warming under . The agreement replaced the Kyoto Protocol. Unlike Kyoto, no binding emission targets were set in the Paris Agreement. Instead, a set of procedures was made binding. Countries have to regularly set ever more ambitious goals and reevaluate these goals every five years. The Paris Agreement restated that developing countries must be financially supported. , 194 states and the European Union have signed the treaty and 191 states and the EU have Ratification, ratified or acceded to the agreement. The 1987 Montreal Protocol, an international agreement to stop emitting ozone-depleting gases, may have been more effective at curbing greenhouse gas emissions than the Kyoto Protocol specifically designed to do so. The 2016 Kigali Amendment to the Montreal Protocol aims to reduce the emissions of hydrofluorocarbons, a group of powerful greenhouse gases which served as a replacement for banned ozone-depleting gases. This made the Montreal Protocol a stronger agreement against climate change.


National responses

In 2019, the Parliament of the United Kingdom, United Kingdom parliament became the first national government to declare a climate emergency. Other countries and jurisdictions followed suit. That same year, the European Parliament declared a "climate and environmental emergency". The European Commission presented its European Green Deal with the goal of making the EU carbon-neutral by 2050. Major countries in Asia have made similar pledges: South Korea and Japan have committed to become carbon-neutral by 2050, and China by 2060. In 2021, the European Commission released its “Fit for 55” legislation package, which contains guidelines for the automotive industry, car industry; all new cars on the European market must be Zero-emissions vehicle, zero-emission vehicles from 2035. While India has strong incentives for renewables, it also plans a significant expansion of coal in the country. As of 2021, based on information from 48 Nationally Determined Contributions, national climate plans, which represent 40% of the parties to the Paris Agreement, estimated total greenhouse gas emissions will be 0.5% lower compared to 2010 levels, below the 45% or 25% reduction goals to limit global warming to 1.5 °C or 2 °C, respectively.


Scientific consensus and society


Scientific consensus

There is a near-complete scientific consensus that the climate is warming and that this is caused by human activities. Agreement in recent literature reached over 99%. Older surveys found 90% to 100% of climate scientists agreeing on humanity's role in causing climate change, based on the exact question and who responded.; No scientific body of national or international standing Scientific consensus on climate change#Opposing, disagrees with this view. Consensus has further developed that some form of action should be taken to protect people against the impacts of climate change. National science academies have called on world leaders to cut global emissions. Scientific discussion takes place in Scientific journal, journal articles that are Peer review, peer-reviewed. Scientists assess these every few years in the Intergovernmental Panel on Climate Change reports. The 2021 IPCC Assessment Report stated that it is "unequivocal" that climate change is caused by humans.


Denial and misinformation

Public debate about climate change has been strongly affected by climate change denial and misinformation, which originated in the United States and has since spread to other countries, particularly Canada and Australia. The actors behind climate change denial form a well-funded and relatively coordinated coalition of fossil fuel companies, industry groups, conservative think tanks, and contrarian scientists. Tobacco industry playbook, Like the tobacco industry, the main strategy of these groups has been to manufacture doubt about scientific data and results. Many who deny, dismiss, or hold unwarranted doubt about the scientific consensus on anthropogenic climate change are labelled as "climate change skeptics", which several scientists have noted is a misnomer. There are different variants of climate denial: some deny that warming takes place at all, some acknowledge warming but attribute it to natural influences, and some minimise the negative impacts of climate change. Manufacturing uncertainty about the science later developed into a manufactured controversy: creating the belief that there is significant uncertainty about climate change within the scientific community in order to delay policy changes. Strategies to promote these ideas include criticism of scientific institutions, and questioning the motives of individual scientists. An echo chamber (media), echo chamber of climate-denying blogs and media has further fomented misunderstanding of climate change.


Public awareness and opinion

Climate change came to international public attention in the late 1980s. Due to confusing media coverage in the early 1990s, people often confounded climate change with other environmental issues like ozone depletion.; Climate change in popular culture, In popular culture, the first movie to reach a mass public on the topic was ''The Day After Tomorrow'' in 2004, followed a few years later by the Al Gore documentary ''An Inconvenient Truth''. Books, stories and films about climate change fall under the genre of climate fiction. Significant regional, gender, age and political differences exist in both public concern for, and understanding of, climate change. More highly educated people, and in some countries, women and younger people, were more likely to see climate change as a serious threat. Partisan gaps also exist in many countries,. and countries with high emissions tend to be less concerned. Views on causes of climate change vary widely between countries. Concern has increased over time, to the point where a majority of citizens in many countries now express a high level of worry about climate change, or view it as a global emergency. Higher levels of worry are associated with stronger public support for policies that address climate change.


Protests and lawsuits

Climate protests have risen in popularity in the 2010s. These protests demand that political leaders take action to prevent climate change. They can take the form of public demonstrations, fossil fuel divestment, lawsuits and other activities. Prominent demonstrations include the School Strike for Climate. In this initiative, young people across the globe have been protesting since 2018 by skipping school on Fridays, inspired by Swedish teenager Greta Thunberg. Mass civil disobedience actions by groups like Extinction Rebellion have protested by disrupting roads and public transport. Climate change litigation, Litigation is increasingly used as a tool to strengthen climate action from public institutions and companies. Activists also initiate lawsuits which target governments and demand that they take ambitious action or enforce existing laws on climate change. Lawsuits against fossil-fuel companies generally seek compensation for loss and damage.


Discovery

In the 1820s, Joseph Fourier proposed the greenhouse effect to explain why Earth's temperature was higher than the sun's energy alone could explain. Earth's atmosphere is transparent to sunlight, so sunlight reaches the surface where it is converted to heat. However, the atmosphere is not transparent to infrared, heat radiating from the surface, and captures some of that heat which warms the planet. In 1856 Eunice Newton Foote demonstrated that the warming effect of the sun is greater for air with water vapour than for dry air, and the effect is even greater with carbon dioxide. She concluded that "An atmosphere of that gas would give to our earth a high temperature..." Starting in 1859, John Tyndall established that nitrogen and oxygen—together totalling 99% of dry air—are transparent to infrared, radiated heat. However, water vapour and some gases (in particular methane and carbon dioxide) absorb radiated heat and re-radiate that heat within the atmosphere. Tyndall proposed that changes in the concentrations of these gases may have caused climatic changes in the past, including ice ages. Svante Arrhenius noted that water vapour in air continuously varied, but the concentration in air was influenced by long-term geological processes. At the end of an ice age, warming from increased levels would increase the amount of water vapour, amplifying warming in a feedback loop. In 1896, he published the first climate model of its kind, showing that halving of levels could have produced the drop in temperature initiating the ice age. Arrhenius calculated the temperature increase expected from doubling to be around 5–6 °C. Other scientists were initially sceptical and believed the greenhouse effect to be saturated so that adding more would make no difference. They thought climate would be self-regulating.; From 1938 onwards Guy Stewart Callendar published evidence that climate was warming and levels rising, but his calculations met the same objections. In the 1950s, Gilbert Plass created a detailed computer model that included different atmospheric layers and the infrared spectrum. This model predicted that increasing levels would cause warming. Around the same time, Hans Suess found evidence that levels had been rising, and Roger Revelle showed that the oceans would not absorb the increase. The two scientists subsequently helped Charles David Keeling, Charles Keeling to begin a record of continued increase, which has been termed the "Keeling Curve". Scientists alerted the public, and the dangers were highlighted at James Hansen's 1988 Congressional testimony.
"News reporters gave only a little attention ..."
The
Intergovernmental Panel on Climate Change The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations The United Nations (UN) is an intergovernmental organization that aims to maintain international peace and international security, s ...
, set up in 1988 to provide formal advice to the world's governments, spurred Interdisciplinarity, interdisciplinary research..


See also

* 2020s in environmental history * Anthropocene – proposed new geological time interval in which humans are having significant geological impact * Global cooling – minority view held by scientists in the 1970s that imminent cooling of the Earth would take place


References


Explanatory notes


Notes


Sources


IPCC reports


= Fourth Assessment Report

= * ** ** ** * ** ** * **


=Fifth Assessment report

= *
AR5 Climate Change 2013: The Physical Science Basis — IPCC
** ** ** ** ** ** * . Chapters 1–20, SPM, and Technical Summary. ** ** ** ** ** ** * . Chapters 21–30, Annexes, and Index. ** * ** ** * ** **


=Special Report: Global Warming of 1.5 °C

= *
Global Warming of 1.5 ºC —
** ** ** ** ** **


=Special Report: Climate change and Land

= * ** ** **


=Special Report: The Ocean and Cryosphere in a Changing Climate

= * ** ** ** **


Sixth Assessment Report

* ** **


Other peer-reviewed sources

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Books, reports and legal documents

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Non-technical sources

* ''American Institute of Physics'' ** ** *** *** **** *** **** * ''Associated Press'' ** * ''BBC'' ** ** * ''Bulletin of the Atomic Scientists'' ** * ''Carbon Brief'' ** ** ** ** ** * ''Deutsche Welle'' ** * ''EPA'' ** ** ** * ''EUobserver'' ** * ''European Parliament'' ** * ''
The Guardian ''The Guardian'' is a British daily newspaper. It was founded in 1821 as ''The Manchester Guardian'', and changed its name in 1959. Along with its sister papers ''The Observer ''The Observer'' is a British newspaper published on Sun ...

The Guardian
'' ** ** ** ** Too risky ** ** ** * ''NASA'' ** ** ** ** ** ** ** ** * ''National Conference of State Legislators'' ** * ''National Geographic'' ** * ''National Science Digital Library'' ** * ''Natural Resources Defense Council'' ** * ''Nature (journal), Nature'' ** * ''The New York Times'' ** * ''NOAA'' ** ** ** * ''Our World in Data'' ** ** ** * Oxford Languages ** * ''Pew Research Center'' ** * ''Politico'' ** * ''RIVM'' ** * ''Salon (website), Salon'' ** * ''ScienceBlogs'' ** * ''Scientific American'' ** * ''Smithsonian'' ** * ''The Sustainability Consortium'' ** * ''UN Environment'' ** * ''UNFCCC'' ** ** * ''Union of Concerned Scientists'' ** * ''USA Today'' ** * ''Vice (website), Vice'' ** * ''The Verge'' ** * ''Vox (website), Vox'' ** * ''World Health Organization'' ** * ''World Resources Institute'' ** ● ''Mongabay'' graphing WRI data from ** ** * ''Yale Climate Connections'' **


External links


Met Office: Climate Guide
– UK National Weather Service
Global Climate Change Indicators
– NOAA
Result of total melting of Polar regions on World
– National Geographic
Up-to-the-second assessment of human-induced global warming since the second half of the 19th century
– Oxford University {{Authority control Anthropocene Climate change, History of climate variability and change Global environmental issues Articles containing video clips Human impact on the environment