Climate change

"Global warming" redirects here. For other uses, see Climate change (disambiguation) and Global warming (disambiguation). This article is about contemporary climate change. For historical climate trends, see Climate variability and change.

In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to Earth's climate. The current rise in global average temperature is more rapid than previous changes, and is primarily caused by humans burning fossil fuels.^[2][3] Fossil fuel use, deforestation, and some agricultural and industrial practices increase greenhouse gases, notably carbon dioxide and methane.^[4] Greenhouse gases absorb some of the heat that the Earth radiates after it warms from sunlight. Larger amounts of these gases trap more heat in Earth's lower atmosphere, causing global warming.

Average surface air temperatures from 2011 to 2021 compared to the 1956–1976 average

Change in average surface air temperature since the Industrial Revolution, plus drivers for that change. Human activity has caused increased temperatures, with natural forces adding some variability.^[1]

Due to climate change, deserts are expanding, while heat waves and wildfires are becoming more common.^[5] Increased warming in the Arctic has contributed to melting permafrost, glacial retreat and sea ice loss.^[6] Higher temperatures are also causing more intense storms, droughts, and other weather extremes.^[7] Rapid environmental change in mountains, coral reefs, and the Arctic is forcing many species to relocate or become extinct.^[8] Even if efforts to minimise future warming are successful, some effects will continue for centuries. These include ocean heating, ocean acidification and sea level rise.^[9]

Climate change threatens people with food and water scarcity, increased flooding, extreme heat, more disease, and economic loss. Human migration and conflict can also be a result.^[10] The World Health Organization (WHO) calls climate change the greatest threat to global health in the 21st century.^[11] Communities may adapt to climate change through efforts like coastline protection or expanding access to air conditioning, but some impacts are unavoidable. Poorer countries are responsible for a small share of global emissions, yet they have the least ability to adapt and are most vulnerable to climate change.

Many climate change impacts are already felt at the current 1.2 °C (2.2 °F) level of warming. Additional warming will increase these impacts and can trigger tipping points, such as the melting of the Greenland ice sheet.^[12] Under the 2015 Paris Agreement, nations collectively agreed to keep warming "well under 2 °C". However, with pledges made under the Agreement, global warming would still reach about 2.7 °C (4.9 °F) by the end of the century.^[13] Limiting warming to 1.5 °C will require halving emissions by 2030 and achieving net-zero emissions by 2050.^[14]

Some effects of climate change, clockwise from top left: Wildfire intensified by heat and drought, worsening droughts compromising water supplies, and bleaching of coral caused by marine heatwaves.

Reducing emissions requires generating electricity from low-carbon sources rather than burning fossil fuels. This change includes phasing out coal and natural gas fired power plants, vastly increasing use of wind, solar, and other types of renewable energy, and reducing energy use. Electricity generated from non-carbon-emitting sources will need to replace fossil fuels for powering transportation, heating buildings, and operating industrial facilities.^[16][17] Carbon can also be removed from the atmosphere, for instance by increasing forest cover and by farming with methods that capture carbon in soil.^[18]

Terminology

Before the 1980s, when it was unclear whether the warming effect of increased greenhouse gases were stronger than the cooling effect of airborne particulates in air pollution, scientists used the term inadvertent climate modification to refer to human impacts on the climate.^[19]

In the 1980s, the terms global warming and climate change became more common. Though the two terms are sometimes used interchangeably,^[20] scientifically, global warming refers only to increased surface warming, while climate change describes the totality of changes to Earth's climate system.^[19] Global warming—used as early as 1975^[21]—became the more popular term after NASA climate scientist James Hansen used it in his 1988 testimony in the U.S. Senate.^[22] Since the 2000s, climate change has increased in usage.^[23] Climate change can also refer more broadly to both human-caused changes or natural changes throughout Earth's history.^[24]

Various scientists, politicians and media now use the terms climate crisis or climate emergency to talk about climate change, and global heating instead of global warming.^[25]

Observed temperature rise

Main articles: Temperature record of the last 2,000 years and Instrumental temperature record

 

Global surface temperature reconstruction over the last 2000 years using proxy data from tree rings, corals, and ice cores in blue.^[26] Directly observed data is in red.^[27]

Multiple independent instrumental datasets show that the climate system is warming.^[28] The 2011–2020 decade warmed to an average 1.09 °C [0.95–1.20 °C] compared to the pre-industrial baseline (1850–1900).^[29] Surface temperatures are rising by about 0.2 °C per decade,^[30] with 2020 reaching a temperature of 1.2 °C above the pre-industrial era.^[31] Since 1950, the number of cold days and nights has decreased, and the number of warm days and nights has increased.^[32]

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 cores.^[33] Thermometer records began to provide global coverage around 1850.^[34] Historical patterns of warming and cooling, like the Medieval Climate Anomaly and the Little Ice Age, 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.^[35] There have been prehistorical episodes of global warming, such as the Paleocene–Eocene Thermal Maximum.^[36] However, the modern observed rise in temperature and CO₂ concentrations has been so rapid that even abrupt geophysical events in Earth's history do not approach current rates.^[37]

Evidence of warming from air temperature measurements are reinforced with a wide range of other observations.^[38][39] For example, changes to the natural water cycle have been predicted and observed, such as an increase in the frequency and intensity of heavy precipitation, melting of snow and land ice, and increased atmospheric humidity.^[40] Flora and fauna are also behaving in a manner consistent with warming; for instance, plants are flowering earlier in spring.^[41] 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.^[42]

Regional aspects to temperature rises

See also: Climate variability and change § Variability between regions

Regions of the world warm at differing 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.^[43] This is because of the larger heat capacity of oceans, and because oceans lose more heat by evaporation.^[44] 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.^[45][46] The rest has heated the atmosphere, melted ice, and warmed the continents.^[47]

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. As these surfaces flip from reflecting a lot of light to being dark after the ice has melted, they start absorbing more heat.^[48] Local black carbon deposits on snow and ice also contribute to Arctic warming.^[49] Arctic temperatures are increasing at over twice the rate of the rest of the world.^[50] Melting of glaciers and ice sheets in the Arctic disrupts ocean circulation, including a weakened Gulf Stream, further changing the climate.^[51]

Attribution of recent temperature rise

Main article: Attribution of recent climate change

 

Drivers of climate change from 1850–1900 to 2010–2019. There was no significant contribution from internal variability or solar and volcanic drivers.

The climate system experiences various cycles on its own which can last for years (such as the El Niño–Southern Oscillation (ENSO)), decades or even centuries.^[52] Other changes are caused by an imbalance of energy that is "external" to the climate system, but not always external to the Earth.^[53] Examples of external forcings include changes in the concentrations of greenhouse gases, solar luminosity, volcanic eruptions, and variations in the Earth's orbit around the Sun.^[54]

To determine the human contribution to climate change, known internal climate variability 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.^[55] 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.^[56] Attribution of recent climate change shows that the main driver is elevated greenhouse gases, with aerosols having a dampening effect.^[57]

Greenhouse gases

Main articles: Greenhouse gas, Greenhouse gas emissions, Greenhouse effect, and Carbon dioxide in Earth's atmosphere

 

CO₂ concentrations over the last 800,000 years as measured from ice cores^{[58][59][60][61]} (blue/green) and directly^[62] (black)

Greenhouse gases are transparent to sunlight, and thus allow it to pass through the atmosphere to heat the Earth's surface. The Earth radiates it as heat, and greenhouse gases absorb a portion of it. This absorption slows the rate at which heat escapes into space, trapping heat near the Earth's surface and warming it over time.^[63] 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.^[64][65] While water vapour (~50%) and clouds (~25%) are the biggest contributors to the greenhouse effect, they increase as a function of temperature and are therefore feedbacks. On the other hand, concentrations of gases such as CO₂ (~20%), tropospheric ozone,^[66] CFCs and nitrous oxide are not temperature-dependent, and are therefore external forcings.^[67]

Human activity since the Industrial Revolution, mainly extracting and burning fossil fuels (coal, oil, and natural gas),^[68] has increased the amount of greenhouse gases in the atmosphere, resulting in a radiative imbalance. In 2019, the concentrations of CO₂ and methane had increased by about 48% and 160%, respectively, since 1750.^[69] These CO₂ 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.^[70]

 

The Global Carbon Project shows how additions to CO₂ since 1880 have been caused by different sources ramping up one after another.

Global anthropogenic greenhouse gas emissions in 2019 were equivalent to 59 billion tonnes of CO₂. Of these emissions, 75% was CO₂, 18% was methane, 4% was nitrous oxide, and 2% was fluorinated gases.^[71] CO₂ emissions primarily come from burning fossil fuels to provide energy for transport, manufacturing, heating, and electricity.^[4] Additional CO₂ emissions come from deforestation and industrial processes, which include the CO₂ released by the chemical reactions for making cement, steel, aluminum, and fertiliser.^[72] Methane emissions come from livestock, manure, rice cultivation, landfills, wastewater, and coal mining, as well as oil and gas extraction.^[73] Nitrous oxide emissions largely come from the microbial decomposition of fertiliser.^[74]

Despite the contribution of deforestation to greenhouse gas emissions, the Earth's land surface, particularly its forests, remain a significant carbon sink for CO₂. Land-surface sink processes, such as carbon fixation in the soil and photosynthesis, remove about 29% of annual global CO₂ emissions.^[75] The ocean also serves as a significant carbon sink via a two-step process. First, CO₂ dissolves in the surface water. Afterwards, the ocean's 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 CO₂.^[76]

Aerosols and clouds

Air pollution, in the form of aerosols, affects the climate on a large scale.^[77] Aerosols scatter and absorb solar radiation. From 1961 to 1990, a gradual reduction in the amount of sunlight reaching the Earth's surface was observed. This phenomenon is popularly known as global dimming,^[78] and is attributed to aerosols produced by dust, pollution and combustion of biofuels and fossil fuels.^{[79] [80] [81] [82][83]} Globally, aerosols have been declining since 1990 due to pollution controls, meaning that they no longer mask greenhouse gas warming as much.^[84]

Aerosols also have indirect effects on the 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.^[85] They also reduce the growth of raindrops, which makes clouds more reflective to incoming sunlight.^[86] Indirect effects of aerosols are the largest uncertainty in radiative forcing.^[87]

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.^[88] Limiting new black carbon deposits in the Arctic could reduce global warming by 0.2 °C by 2050.^[89]

Land surface changes

 

The rate of global tree cover loss has approximately doubled since 2001, to an annual loss approaching an area the size of Italy.^[90]

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.).^[91] The amount of forested land continues to decrease, which is the main land use change that causes global warming.^[92] Deforestation releases CO₂ contained in trees when they are destroyed, plus it prevents those trees from absorbing more CO₂ in the future.^[18] 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%).^[93]

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 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.^[94] 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.^[94] Globally, these effects are estimated to have led to a slight cooling, dominated by an increase in surface albedo.^[95]

Solar and volcanic activity

Further information: Solar activity and climate

As the Sun is the Earth's primary energy source, changes in incoming sunlight directly affect the climate system.^[87] Solar irradiance has been measured directly by satellites,^[96] and indirect measurements are available from the early 1600s onwards.^[87] There has been no upward trend in the amount of the Sun's energy reaching the Earth.^[97]

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.^[98] Present-day volcanic CO₂ emissions are equivalent to less than 1% of current anthropogenic CO₂ emissions.^[99]

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.^[100] 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).^[101] If solar variations were responsible for the observed warming, the troposphere and stratosphere would both warm.^[56]

Climate change feedback

Main articles: Climate change feedback and Climate sensitivity

 

Sea ice reflects 50% to 70% of incoming sunlight, while the ocean, being darker, reflects only 6%. As an area of sea ice melts and exposes more ocean, more heat is absorbed by the ocean, raising temperatures that melt still more ice. This process is a positive feedback.^[102]

The response of the climate system to an initial forcing is modified by feedbacks: increased by "self-reinforcing" or "positive" feedbacks and reduced by "balancing" or "negative" feedbacks.^[103] The main reinforcing feedbacks are the water-vapour feedback, the ice–albedo feedback, and the net effect of clouds.^[104][105] The primary balancing mechanism is radiative cooling, as Earth's surface gives off more heat to space in response to rising temperature.^[106] In addition to temperature feedbacks, there are feedbacks in the carbon cycle, such as the fertilizing effect of CO₂ on plant growth.^[107] Uncertainty over feedbacks is the major reason why different climate models project different magnitudes of warming for a given amount of emissions.^[108]

As air warms, it can hold more moisture. Water vapour, as a potent greenhouse gas, holds heat in the atmosphere.^[104] 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.^[109] The effect of clouds is the largest source of feedback uncertainty.^[110]

Another major feedback is the reduction of snow cover and sea ice in the Arctic, which reduces the reflectivity of the Earth's surface.^[111] More of the Sun's energy is now absorbed in these regions, contributing to amplification of Arctic temperature changes.^[112] Arctic amplification is also melting permafrost, which releases methane and CO₂ into the atmosphere.^[113] Climate change can also cause methane releases from wetlands, marine systems, and freshwater systems.^[114] Overall, climate feedbacks are expected to become increasingly positive.^[115]

Around half of human-caused CO₂ emissions have been absorbed by land plants and by the oceans.^[116] On land, elevated CO₂ 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.^[117] Soils contain large quantities of carbon and may release some when they heat up.^[118] As more CO₂ 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.^[119] Overall, at higher CO₂ concentrations the Earth will absorb a reduced fraction of our emissions.^[120]

Modelling

Further information: Carbon budget, Climate model, and Climate change scenario

 

Projected global surface temperature changes relative to 1850–1900, based on CMIP6 multi-model mean changes

A climate model is a representation of the physical, chemical, and biological processes that affect the climate system.^[121] Models also include natural processes like changes in the Earth's orbit, historical changes in the Sun's activity, and volcanic forcing.^[122] Models are used to calculate the degree of warming future emissions will cause when accounting for the strength of climate feedbacks, ^[123][124] or 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.^[125]

The physical realism of models is tested by examining their ability to simulate contemporary or past climates.^[126] Past models have underestimated the rate of Arctic shrinkage^[127] and underestimated the rate of precipitation increase.^[128] Sea level rise since 1990 was underestimated in older models, but more recent models agree well with observations.^[129] The 2017 United States-published National Climate Assessment notes that "climate models may still be underestimating or missing relevant feedback processes".^[130]

A subset of climate models add societal factors to a simple physical climate model. These models simulate how population, economic growth, and energy use affect – and interact with – the 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 and carbon cycle models to predict how atmospheric concentrations of greenhouse gases might change in the future.^[131][132] Depending on the socioeconomic scenario and the mitigation scenario, models produce atmospheric CO₂ concentrations that range widely between 380 and 1400 ppm.^[133]

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 very low GHG emissions scenario. In an intermediate scenario global warming would reach 2.1 °C to 3.5 °C, and 3.3 °C to 5.7 °C under the very high GHG emissions scenario.^[134] These projections are based on climate models in combination with observations.^[135]

The remaining carbon budget is determined by modelling the carbon cycle and the climate sensitivity to greenhouse gases.^[136] 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 CO₂. 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 CO₂ smaller due to methane release from permafrost and wetlands.^[137] However, it is clear that fossil fuel resources are too abundant for shortages to be relied on to limit carbon emissions in the 21st century.^[138]

Impacts

Main article: Effects of climate change

 

The sixth IPCC Assessment Report projects changes in average soil moisture that can disrupt agriculture and ecosystems. A reduction in soil moisture by one standard deviation means that average soil moisture will approximately match the ninth driest year between 1850 and 1900 at that location.

Environmental effects

Further information: Effects of climate change on oceans and Effects of climate change on the water cycle

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.^[139] Since the 1950s, droughts and heat waves have appeared simultaneously with increasing frequency.^[140] Extremely wet or dry events within the monsoon period have increased in India and East Asia.^[141] The rainfall rate and intensity of hurricanes and typhoons is likely increasing,^[142] and the geographic range likely expanding poleward in response to climate warming.^[143] Frequency of tropical cyclones has not increased as a result of climate change.^[144]

 

Historical sea level reconstruction and projections up to 2100 published in 2017 by the U.S. Global Change Research Program^[145]

Global sea level is rising as a consequence of glacial melt, melt of the ice sheets in Greenland and Antarctica, and thermal expansion. Between 1993 and 2020, the rise increased over time, averaging 3.3 ± 0.3 mm per year.^[146] Over the 21st century, the IPCC projects that in a very high emissions scenario the sea level could rise by 61–110 cm.^[147] Increased ocean warmth is undermining and threatening to unplug Antarctic glacier outlets, risking a large melt of the ice sheet^[148] and the possibility of a 2-meter sea level rise by 2100 under high emissions.^[149]

Climate change has led to decades of shrinking and thinning of the Arctic sea ice.^[150] 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.^[151] Higher atmospheric CO₂ concentrations have led to changes in ocean chemistry. An increase in dissolved CO₂ is causing oceans to acidify.^[152] In addition, oxygen levels are decreasing as oxygen is less soluble in warmer water.^[153] Dead zones in the ocean, regions with very little oxygen, are expanding too.^[154]

Tipping points and long-term impacts

Greater degrees of global warming increase the risk of passing through ‘tipping points’—thresholds beyond which certain impacts can no longer be avoided even if temperatures are reduced.^[155][156] An example is the collapse of 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.^[157][158] Some large-scale changes could occur over a short time period, such as a shutdown of certain ocean currents like the Atlantic Meridional Overturning Circulation (AMOC).^[159] Tipping points can also include irreversible damage to ecosystems like the Amazon rainforest and coral reefs.^[160]

The long-term effects of climate change on oceans include further ice melt, ocean warming, sea level rise, and ocean acidification.^[161] On the timescale of centuries to millennia, the magnitude of climate change will be determined primarily by anthropogenic CO₂ emissions. This is due to CO₂'s long atmospheric lifetime.^[162] Oceanic CO₂ uptake is slow enough that ocean acidification will continue for hundreds to thousands of years.^[163] These emissions are estimated to have prolonged the current interglacial period by at least 100,000 years.^[164] Sea level rise will continue over many centuries, with an estimated rise of 2.3 metres per degree Celsius (4.2 ft/°F) after 2000 years.^[165]

Nature and wildlife

Further information: Effects of climate change on oceans and Climate change and ecosystems

Recent warming has driven many terrestrial and freshwater species poleward and towards higher altitudes.^[166] Higher atmospheric CO₂ 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.^[167] Climate change has contributed to the expansion of drier climate zones, such as the expansion of deserts in the subtropics.^[168] The size and speed of global warming is making abrupt changes in ecosystems more likely.^[169] Overall, it is expected that climate change will result in the extinction of many species.^[170]

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.^[171] 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.^[172] Ocean acidification makes it harder for marine calcifying organisms such as mussels, barnacles and corals to produce shells and skeletons; and heatwaves have bleached coral reefs.^[173] Harmful algal blooms enhanced by climate change and eutrophication lower oxygen levels, disrupt food webs and cause great loss of marine life.^[174] Coastal ecosystems are under particular stress. Almost half of global wetlands have disappeared due to climate change and other human impacts.^[175]

Climate change impacts on the environment

Ecological collapse. Coral bleaching from thermal stress has damaged the Great Barrier Reef and threatens coral reefs worldwide.^[176]   Extreme weather. Drought and high temperatures worsened the 2020 bushfires in Australia.^[177]   Arctic warming. Permafrost thaws undermine infrastructure and release methane, a greenhouse gas.^[113]   Habitat destruction. Many arctic animals rely on sea ice, which has been disappearing in a warming Arctic.^[178]   Pest propagation. Mild winters allow more pine beetles to survive to kill large swaths of forest.^[179]

Humans

Main article: Effects of climate change

Further information: Effects of climate change on human health, Climate security, Economics of climate change, and Effects of climate change on agriculture

 

Extreme weather will be progressively more common as the Earth warms.^[180]

The effects of climate change are impacting humans everywhere in the world. Impacts can now be observed on all continents and ocean regions,^[181] with low-latitude, less developed areas facing the greatest risk.^[182] Continued warming has potentially “severe, pervasive and irreversible impacts” for people and ecosystems.^[183] The risks are unevenly distributed, but are generally greater for disadvantaged people in developing and developed countries.^[184]

Food and health

The WHO has classified climate change as the greatest threat to global health in the 21st century.^[185] Extreme weather leads to injury and loss of life,^[186] and crop failures to undernutrition.^[187] Various infectious diseases are more easily transmitted in a warmer climate, such as dengue fever and malaria.^[188] Young children are the most vulnerable to food shortages. Both children and older people are vulnerable to extreme heat.^[189] The World Health Organization (WHO) has estimated that between 2030 and 2050, climate change would cause around 250,000 additional deaths per year. They assessed deaths from heat exposure in elderly people, increases in diarrhea, malaria, dengue, coastal flooding, and childhood undernutrition.^[190] Over 500,000 more adult deaths are projected yearly by 2050 due to reductions in food availability and quality.^[191] By 2100, 50% to 75% of the global population may face climate conditions that are life-threatening due to combined effects of extreme heat and humidity.^[192]

Climate change is affecting food security. It has caused reduction in global yields of maize, wheat, and soybeans between 1981 and 2010.^[193] Future warming could further reduce global yields of major crops.^[194] Crop production will probably be negatively affected in low-latitude countries, while effects at northern latitudes may be positive or negative.^[195] Up to an additional 183 million people worldwide, particularly those with lower incomes, are at risk of hunger as a consequence of these impacts.^[196] Climate change also impacts fish populations. Globally, less will be available to be fished.^[197] Regions dependent on glacier water, regions that are already dry, and small islands have a higher risk of water stress due to climate change.^[198]

Livelihoods

Economic damages due to climate change may be severe and there is a chance of disastrous consequences.^[199] Climate change has likely already increased global economic inequality, and this trend is projected to continue.^[200] Most of the severe impacts are expected in sub-Saharan Africa, where most of the local inhabitants are dependent upon natural and agricultural resources^[201], and South-East Asia.^[202] The World Bank estimates that climate change could drive over 120 million people into poverty by 2030.^[203]

Current inequalities based on wealth and social status have worsened due to climate change.^[204] Major difficulties in mitigating, adapting, and recovering to climate shocks are faced by marginalized people who have less control over resources.^[205][201] Indigenous people, who are subsistent on their land and ecosystems, will face endangerment to their wellness and lifestyles due to climate change.^[206] 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.^[207]

Low-lying islands and coastal communities are threatened by sea level rise, which makes flooding more common. Sometimes, land is permanently lost to the sea.^[208] This could lead to statelessness for people in island nations, such as the Maldives and Tuvalu.^[209] In some regions, the rise in temperature and humidity may be too severe for humans to adapt to.^[210] 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.^[211] These factors can drive environmental migration, both within and between countries.^[10] More people are expected to be displaced because of sea level rise, extreme weather and conflict 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.^[212]

Climate change impacts on people

Environmental migration. Sparser rainfall leads to desertification that harms agriculture and can displace populations. Shown: Telly, Mali (2008).^[213]   Agricultural changes. Droughts, rising temperatures, and extreme weather negatively impact agriculture. Shown: Texas, US (2013).^[214]   Tidal flooding. Sea-level rise increases flooding in low-lying coastal regions. Shown: Venice, Italy (2004).^[215]   Storm intensification. Bangladesh after Cyclone Sidr (2007) is an example of catastrophic flooding from increased rainfall.^[216]   Heat wave intensification. Events like the June 2019 European heat wave are becoming more common.^[217]

Reducing and recapturing emissions

Main article: Climate change mitigation

 

Global greenhouse gas emission scenarios, based on policies and pledges as of 11/21

Climate change can be mitigated by reducing greenhouse gas emissions and enhancing sinks that absorb greenhouse gases from the atmosphere.^[218] In order to limit global warming to less than 1.5 °C global greenhouse gas emissions needs to be net-zero by 2050, or by 2070 with a 2 °C target.^[137] This requires far-reaching, systemic changes on an unprecedented scale in energy, land, cities, transport, buildings, and industry.^[219] The United Nations Environment Programme estimates that countries need to triple their 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.^[220] 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.^[13] Globally, limiting warming to 2 °C may result in higher benefits than costs.^[221]

Although there is no single pathway to limit global warming to 1.5 or 2 °C,^[222] 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.^[223] To reduce pressures on ecosystems and enhance their carbon sequestration capabilities, changes would also be necessary in agriculture and forestry,^[224] such as preventing deforestation and restoring natural ecosystems by reforestation.^[225]

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 methods over the 21st century.^[226] There are concerns, though, about over-reliance on these technologies, and environmental impacts.^[227] 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.^[228]

Clean energy

Main articles: Sustainable energy and Sustainable transport

 

Coal, oil, and natural gas remain the primary global energy sources even as renewables have begun rapidly increasing.^[229]

 

Wind and solar power, Germany

Renewable energy is key to limiting climate change.^[230] Fossil fuels accounted for 80% of the world's energy in 2018. The remaining share was split between nuclear power and renewables (including hydropower, bioenergy, wind and solar power and geothermal energy).^[231] That mix is projected to change significantly over the next 30 years.^[223] Solar panels and onshore wind are now among the cheapest forms of adding new power generation capacity in many locations.^[232] Renewables represented 75% of all new electricity generation installed in 2019, nearly all solar and wind.^[233] Other forms of clean energy, such as nuclear and hydropower, currently have a larger share of the energy supply. However, their future growth forecasts appear limited in comparison.^[234]

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. Investment in coal would be eliminated and coal use nearly phased out by 2050.^[235][236]

Electricity generated from renewable sources would also need to become the main energy source for heating and transport.^[237] Transport can switch away from internal combustion engine vehicles and towards electric vehicles, public transit, and active transport (cycling and walking).^[238][239] For shipping and flying, low-carbon fuels would reduce emissions.^[238] Heating could be increasingly decarbonised with technologies like heat pumps.^[240]

There are obstacles to the continued rapid growth of clean energy, including renewables. For wind and solar, there are environmental and land use concerns for new projects.^[241] Wind and solar also produce energy intermittently and with seasonal variability. Traditionally, hydro dams with reservoirs and conventional power plants have been used when variable energy production is low. Going forward, battery storage can be expanded, energy demand and supply can be matched, and long-distance transmission can smooth variability of renewable outputs.^[230] Bioenergy is often not carbon-neutral and may have negative consequences for food security.^[242] The growth of nuclear power is constrained by controversy around nuclear waste, nuclear weapon proliferation, and accidents.^[243][244] Hydropower growth is limited by the fact that the best sites have been developed, and new projects are confronting increased social and environmental concerns.^[245]

Low-carbon energy improves human health by minimising climate change. It also has the near-term benefit of reducing air pollution deaths,^[246] which were estimated at 7 million annually in 2016.^[247] 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.^[248]

Energy conservation

Main articles: Efficient energy use and Energy conservation

Reducing energy demand is another major aspect of reducing emissions.^[249] 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 carbon-intensive infrastructure development.^[250] Major increases in energy efficiency investment will be required to achieve climate goals, comparable to the level of investment in renewable energy.^[251] Several COVID-19 related changes in energy use patterns, energy efficiency investments, and funding have made forecasts for this decade more difficult and uncertain.^[252]

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.^[253] Industrial strategies to reduce energy demand include improving heating systems and motors, designing less energy-intensive products, and increasing product lifetimes.^[254] In the building sector the focus is on better design of new buildings, and higher levels of energy efficiency in retrofitting.^[255] The use of technologies like heat pumps can also increase building energy efficiency.^[256]

Agriculture and industry

See also: Sustainable agriculture and Green industrial policy

 

Taking into account direct and indirect emissions, industry is the sector with the highest share of global emissions.

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.^[257] 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.^[258]

On the demand side, a key component of reducing emissions is shifting people towards plant-based diets.^[259] Eliminating the production of livestock for meat and dairy would eliminate about 3/4ths of all emissions from agriculture and other land use.^[260] Livestock also occupy 37% of ice-free land area on Earth and consume feed from the 12% of land area used for crops, driving deforestation and land degradation.^[261]

Steel and cement production are responsible for about 13% of industrial CO₂ emissions. In these industries, carbon-intensive materials such as coke and lime play an integral role in the production, so that reducing CO₂ emissions requires research into alternative chemistries.^[262]

Carbon sequestration

Main articles: Carbon dioxide removal and Carbon sequestration

 

Most CO₂ emissions have been absorbed by carbon sinks, including plant growth, soil uptake, and ocean uptake (2020 Global Carbon Budget).

Natural carbon sinks can be enhanced to sequester significantly larger amounts of CO₂ beyond naturally occurring levels.^[263] Reforestation and tree planting on non-forest lands are among the most mature sequestration techniques, although the latter raises food security concerns.^[264] Farmers can promote sequestration of carbon in soils through practices such as use of winter cover crops, reducing the intensity and frequency of tillage, and using compost and manure as soil amendments.^[265] Restoration/recreation of coastal wetlands and seagrass meadows increases the uptake of carbon into organic matter (blue carbon).^[266] When carbon is sequestered in soils and in organic matter such as trees, there is a risk of the carbon being re-released into the atmosphere later through changes in land use, fire, or other changes in ecosystems.^[267]

Where energy production or CO₂-intensive heavy industries continue to produce waste CO₂, the gas can be captured and stored instead of released to the atmosphere. Although its current use is limited in scale and expensive,^[268] carbon capture and storage (CCS) may be able to play a significant role in limiting CO₂ emissions by mid-century.^[269] This technique, in combination with bio-energy (BECCS) can result in net negative emissions: CO₂ is drawn from the atmosphere.^[270] 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.^[271]

Adapting to a changing climate

Main article: Climate change adaptation

Adaptation is "the process of adjustment to current or expected changes in climate and its effects".^[272] Without additional mitigation, adaptation cannot avert the risk of "severe, widespread and irreversible" impacts.^[273] More severe climate change requires more transformative adaptation, which can be prohibitively expensive.^[272] The capacity and potential for humans to adapt is unevenly distributed across different regions and populations, and developing countries generally have less.^[274] 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.^[275]

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.^[276] There are economic barriers for tackling dangerous heat impact. Avoiding strenuous work or having air conditioning is not possible for everybody.^[277] In agriculture, adaptation options include a switch to more sustainable diets, diversification, erosion control and genetic improvements for increased tolerance to a changing climate.^[278] Insurance allows for risk-sharing, but is often difficult to get for people on lower incomes.^[279] Education, migration and early warning systems can reduce climate vulnerability.^[280]

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 introduced to areas acquiring a favorable climate. 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 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.^[281]

There are synergies and trade-offs between adaptation and mitigation. Adaptation often offer short-term benefits, whereas mitigation has longer-term benefits.^[282] Increased use of air conditioning allows people to better cope with heat, but increases energy demand. Compact 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.^[283] Increased food productivity has large benefits for both adaptation and mitigation.^[284]

Policies and politics

Main article: Politics of climate change

 

The Climate Change Performance Index ranks countries by greenhouse gas emissions (40% of score), renewable energy (20%), energy use (20%), and climate policy (20%).

High

Medium

Low

Very Low

Countries that are most vulnerable to climate change have typically been responsible for a small share of global emissions. This raises questions about justice and fairness.^[285] 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 "take urgent action to combat climate change and its impacts".^[286] The goals on food, clean water and ecosystem protection have synergies with climate mitigation.^[287]

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 to public health and local environments exceed the costs in almost all regions.^[288] 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.^[289]

Policy options

A wide range of policies, regulations, and laws are being used to reduce emissions. As of 2019, carbon pricing covers about 20% of global greenhouse gas emissions.^[290] Carbon can be priced with carbon taxes and emissions trading systems.^[291] Direct global fossil fuel subsidies reached $319 billion in 2017, and $5.2 trillion when indirect costs such as air pollution are priced in.^[292] Ending these can cause a 28% reduction in global carbon emissions and a 46% reduction in air pollution deaths.^[293] Money saved on fossil subsidies could be used to support the transition to clean energy instead.^[294] More direct methods to reduce greenhouse gases include vehicle efficiency standards, renewable fuel standards, and air pollution regulations on heavy industry.^[295] Several countries require utilities to increase the share of renewables in power production.^[296]

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.^[297] 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.^[298]

International climate agreements

Further information: United Nations Framework Convention on Climate Change

 

Since 2000, rising CO₂ emissions in China and the rest of world have surpassed the output of the United States and Europe.^[299]

 

Per person, the United States generates CO₂ at a far faster rate than other primary regions.^[299]

Nearly all countries in the world are parties to the 1994 United Nations Framework Convention on Climate Change (UNFCCC).^[300] The goal of the UNFCCC is to prevent dangerous human interference with the climate system.^[301] 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 economic development can be sustained.^[302] 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.^[303] Its yearly conferences are the stage of global negotiations.^[304]

The 1997 Kyoto Protocol extended the UNFCCC and included legally binding commitments for most developed countries to limit their emissions.^[305] During the negotiations, the G77 (representing developing countries) pushed for a mandate requiring developed countries to "[take] the lead" in reducing their emissions,^[306] since developed countries contributed most to the accumulation of greenhouse gases in the atmosphere. Per-capita emissions were also still relatively low in developing countries and developing countries would need to emit more to meet their development needs.^[307]

The 2009 Copenhagen Accord has been widely portrayed as disappointing because of its low goals, and was rejected by poorer nations including the G77.^[308] Associated parties aimed to limit the global temperature rise to below 2 °C.^[309] 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.^[310] As of 2020^[update], the fund has failed to reach its expected target, and risks a shrinkage in its funding.^[311]

In 2015 all UN countries negotiated the Paris Agreement, which aims to keep global warming well below 2.0 °C and contains an aspirational goal of keeping warming under 1.5 °C.^[312] 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.^[313] The Paris Agreement restated that developing countries must be financially supported.^[314] As of October 2021^[update], 194 states and the European Union have signed the treaty and 191 states and the EU have ratified or acceded to the agreement.^[315]

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.^[316] 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.^[317]

National responses

In 2019, the United Kingdom parliament became the first national government to declare a climate emergency.^[318] Other countries and jurisdictions followed suit.^[319] That same year, the European Parliament declared a "climate and environmental emergency".^[320] The European Commission presented its European Green Deal with the goal of making the EU carbon-neutral by 2050.^[321] Major countries in Asia have made similar pledges: South Korea and Japan have committed to become carbon-neutral by 2050, and China by 2060.^[322] In 2021, the European Commission released its “Fit for 55” legislation package, which contains guidelines for the car industry; all new cars on the European market must be zero-emission vehicles from 2035.^[323] While India has strong incentives for renewables, it also plans a significant expansion of coal in the country.^[324]

As of 2021, based on information from 48 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.^[325]

Society

Denial and misinformation

Further information: Global warming controversy, Fossil fuels lobby, Climate change denial, and Global warming conspiracy theory

 

Data has been cherry picked from short periods to falsely assert that global temperatures are not rising. Blue trendlines show short periods that mask longer-term warming trends (red trendlines). Blue dots show the so-called global warming hiatus.^[326]

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.^[327] Like the tobacco industry, the main strategy of these groups has been to manufacture doubt about scientific data and results.^[328] 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.^[329]

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.^[330] 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.^[331] Strategies to promote these ideas include criticism of scientific institutions,^[332] and questioning the motives of individual scientists.^[330] An echo chamber of climate-denying blogs and media has further fomented misunderstanding of climate change.^[333]

Public awareness and opinion

Further information: Climate communication, Media coverage of climate change, and Public opinion on climate change

 

The public substantially underestimates the degree of scientific consensus that humans are causing climate change.^[334] Studies from 2019 to 2021^{[335][3][336]} found scientific consensus to range from 98.7 to 100%.

Climate change came to international public attention in the late 1980s.^[337] Due to media coverage in the early 1990s, people often confused climate change with other environmental issues like ozone depletion.^[338] In popular culture, the climate fiction movie The Day After Tomorrow (2004) and the Al Gore documentary An Inconvenient Truth (2006) focused on climate change.^[337]

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.^[339] Partisan gaps also exist in many countries,^[340] and countries with high CO₂ emissions tend to be less concerned.^[341] Views on causes of climate change vary widely between countries.^[342] Concern has increased over time,^[340] to the point where in 2021 a majority of citizens in many countries express a high level of worry about climate change, or view it as a global emergency.^[343] Higher levels of worry are associated with stronger public support for policies that address climate change.^[344]

Climate movement

Main articles: Climate movement and Climate change litigation

Climate 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.^[345] 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.^[346] Mass civil disobedience actions by groups like Extinction Rebellion have protested by disrupting roads and public transport.^[347] 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.^[348] Lawsuits against fossil-fuel companies generally seek compensation for loss and damage.^[349]

History

For broader coverage of this topic, see History of climate change science.

Early discoveries

 

This 1912 article succinctly describes the greenhouse effect, how burning coal creates carbon dioxide to cause global warming and climate change.^[350]

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 heat radiating from the surface, and captures some of that heat, which in turn warms the planet.^[351]

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 that the effect is even greater with carbon dioxide (CO₂). She concluded that "An atmosphere of that gas would give to our earth a high temperature..."^[352][353]

Starting in 1859,^[354] John Tyndall established that nitrogen and oxygen—together totaling 99% of dry air—are transparent to radiated heat. However, water vapour and gases such as methane and carbon dioxide absorb radiated heat and re-radiate that heat into the atmosphere. Tyndall proposed that changes in the concentrations of these gases may have caused climatic changes in the past, including ice ages.^[355]

Svante Arrhenius noted that water vapour in air continuously varied, but the CO₂ concentration in air was influenced by long-term geological processes. Warming from increased CO₂ levels would increase the amount of water vapour, amplifying warming in a positive feedback loop. In 1896, he published the first climate model of its kind, projecting that halving CO₂ levels could have produced a drop in temperature initiating an ice age. Arrhenius calculated the temperature increase expected from doubling CO₂ to be around 5–6 °C.^[356] Other scientists were initially skeptical and believed that the greenhouse effect was saturated so that adding more CO₂ would make no difference, and that the climate would be self-regulating.^[357] Beginning in 1938, Guy Stewart Callendar published evidence that climate was warming and CO₂ levels were rising,^[358] but his calculations met the same objections.^[357]

Development of a scientific consensus

See also: Scientific consensus on climate change

In the 1950s, Gilbert Plass created a detailed computer model that included different atmospheric layers and the infrared spectrum. This model predicted that increasing CO₂ levels would cause warming. Around the same time, Hans Suess found evidence that CO₂ levels had been rising, and Roger Revelle showed that the oceans would not absorb the increase. The two scientists subsequently helped Charles Keeling to begin a record of continued increase, which has been termed the "Keeling Curve".^[357] Scientists alerted the public,^[359] and the dangers were highlighted at James Hansen's 1988 Congressional testimony.^[22] The Intergovernmental Panel on Climate Change (IPCC), set up in 1988 to provide formal advice to the world's governments, spurred interdisciplinary research.^[360] As part of the IPCC reports, scientists assess the scientific discussion that takes place in peer-reviewed journal articles.^[361]

There is a near-complete scientific consensus that the climate is warming and that this is caused by human activities. As of 2019, agreement in recent literature reached over 99%.^[362][363] No scientific body of national or international standing disagrees with this view.^[364] 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.^[365] The 2021 IPCC Assessment Report stated that it is "unequivocal" that climate change is caused by humans.^[363]

See also

•  Climate change portal
•  Environment portal
•  Science portal
•  World portal

• Anthropocene – proposed new geological time interval in which humans are having significant geological impact
• List of climate scientists

References

1. IPCC AR6 WG1 2021, SPM-7
2. IPCC SR15 Ch1 2018, p. 54: Since 1970 the global average temperature has been rising at a rate of 1.7°C per century, compared to a long-term decline over the past 7,000 years at a baseline rate of 0.01°C per century (NOAA, 2016; Marcott et al., 2013). These global-level rates of human-driven change far exceed the rates of change driven by geophysical or biosphere forces that have altered the Earth System trajectory in the past (e.g., Summerhayes, 2015; Foster et al., 2017); even abrupt geophysical events do not approach current rates of human-driven change.
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4. Our World in Data, 18 September 2020
5. IPCC SRCCL 2019, p. 7: Since the pre-industrial period, the land surface air temperature has risen nearly twice as much as the global average temperature (high confidence). Climate change... contributed to desertification and land degradation in many regions (high confidence).; IPCC SRCCL 2019, p. 45: Climate change is playing an increasing role in determining wildfire regimes alongside human activity (medium confidence), with future climate variability expected to enhance the risk and severity of wildfires in many biomes such as tropical rainforests (high confidence).
6. IPCC SROCC 2019, p. 16: Over the last decades, global warming has led to widespread shrinking of the cryosphere, with mass loss from ice sheets and glaciers (very high confidence), reductions in snow cover (high confidence) and Arctic sea ice extent and thickness (very high confidence), and increased permafrost temperature (very high confidence).
7. IPCC AR6 WG1 Ch11 2021, p. 1517
8. EPA (19 January 2017). "Climate Impacts on Ecosystems". from the original on 27 January 2018. Retrieved 5 February 2019. Mountain and arctic ecosystems and species are particularly sensitive to climate change... As ocean temperatures warm and the acidity of the ocean increases, bleaching and coral die-offs are likely to become more frequent.
9. IPCC SR15 Ch1 2018, p. 64: Sustained net zero anthropogenic emissions of CO₂ and declining net anthropogenic non-CO₂ radiative forcing over a multi-decade period would halt anthropogenic global warming over that period, although it would not halt sea level rise or many other aspects of climate system adjustment.
10. Cattaneo et al. 2019; UN Environment, 25 October 2018.
11. IPCC AR5 SYR 2014, pp. 13–16; WHO, Nov 2015: "Climate change is the greatest threat to global health in the 21st century. Health professionals have a duty of care to current and future generations. You are on the front line in protecting people from climate impacts – from more heat-waves and other extreme weather events; from outbreaks of infectious diseases such as malaria, dengue and cholera; from the effects of malnutrition; as well as treating people that are affected by cancer, respiratory, cardiovascular and other non-communicable diseases caused by environmental pollution."
12. IPCC AR6 WG1 Technical Summary 2021, p. 71
13. United Nations Environment Programme 2021, p. 36: "A continuation of the effort implied by the latest unconditional NDCs and announced pledges is at present estimated to result in warming of about 2.7 °C (range: 2.2–3.2 °C) with a 66 per cent chance."
14. IPCC SR15 Ch2 2018, pp. 95–96: In model pathways with no or limited overshoot of 1.5 °C, global net anthropogenic CO₂ emissions decline by about 45% from 2010 levels by 2030 (40–60% interquartile range), reaching net zero around 2050 (2045–2055 interquartile range); IPCC SR15 2018, p. 17, SPM C.3:All pathways that limit global warming to 1.5 °C with limited or no overshoot project the use of carbon dioxide removal (CDR) on the order of 100–1000 GtCO2 over the 21st century. CDR would be used to compensate for residual emissions and, in most cases, achieve net negative emissions to return global warming to 1.5 °C following a peak (high confidence). CDR deployment of several hundreds of GtCO2 is subject to multiple feasibility and sustainability constraints (high confidence).; Rogelj et al. 2015; Hilaire et al. 2019
15. Ivanova, Irina (2 June 2022). "California is rationing water amid its worst drought in 1,200 years". CBS News.
16. United Nations Environment Programme 2019, p. xxiii, Table ES.3; Teske, ed. 2019, p. xxvii, Fig.5.
17. United Nations Environment Programme 2019, Table ES.3 & p. 49; NREL 2017, pp. vi, 12
18. IPCC SRCCL Summary for Policymakers 2019, p. 18
19. NASA, 5 December 2008.
20. NASA, 7 July 2020; Shaftel 2016: " 'Climate change' and 'global warming' are often used interchangeably but have distinct meanings. ... Global warming refers to the upward temperature trend across the entire Earth since the early 20th century ... Climate change refers to a broad range of global phenomena ...[which] include the increased temperature trends described by global warming."; Associated Press, 22 September 2015: "The terms global warming and climate change can be used interchangeably. Climate change is more accurate scientifically to describe the various effects of greenhouse gases on the world because it includes extreme weather, storms and changes in rainfall patterns, ocean acidification and sea level.".
21. Broeker, Wallace S. (8 August 1975). "Climatic Change: Are We on the Brink of a Pronounced Global Warming?". Science. 189 (4201): 460–463. Bibcode:1975Sci...189..460B. doi:10.1126/science.189.4201.460. JSTOR 1740491. PMID 17781884. S2CID 16702835.
22. Weart "The Public and Climate Change: The Summer of 1988", "News reporters gave only a little attention ...".
23. Joo et al. 2015.
24. IPCC AR5 SYR Glossary 2014, p. 120: "Climate change refers to a change in the state of the climate that can be identified (e.g., by using statistical tests) by changes in the mean and/or the variability of its properties and that persists for an extended period, typically decades or longer. Climate change may be due to natural internal processes or external forcings such as modulations of the solar cycles, volcanic eruptions and persistent anthropogenic changes in the composition of the atmosphere or in land use."
25. Hodder & Martin 2009; BBC Science Focus Magazine, 3 February 2020
26. Neukom et al. 2019b.
27. "Global Annual Mean Surface Air Temperature Change". NASA. Retrieved 23 February 2020.
28. EPA 2016: The U.S. Global Change Research Program, the National Academy of Sciences, and the Intergovernmental Panel on Climate Change (IPCC) have each independently concluded that warming of the climate system in recent decades is "unequivocal". This conclusion is not drawn from any one source of data but is based on multiple lines of evidence, including three worldwide temperature datasets showing nearly identical warming trends as well as numerous other independent indicators of global warming (e.g. rising sea levels, shrinking Arctic sea ice).
29. IPCC AR6 WG1 Summary for Policymakers 2021, p. SPM-5
30. IPCC SR15 Ch1 2018, p. 81.
31. WMO 2021, p. 6.
32. IPCC AR5 WG1 Ch2 2013, p. 162.
33. IPCC SR15 Ch1 2018, p. 57: This report adopts the 51-year reference period, 1850–1900 inclusive, assessed as an approximation of pre-industrial levels in AR5 ... Temperatures rose by 0.0 °C–0.2 °C from 1720–1800 to 1850–1900; Hawkins et al. 2017, p. 1844
34. IPCC AR5 WG1 Summary for Policymakers 2013, pp. 4–5: "Global-scale observations from the instrumental era began in the mid-19th century for temperature and other variables ... the period 1880 to 2012 ... multiple independently produced datasets exist."
35. IPCC AR5 WG1 Ch5 2013, p. 386; Neukom et al. 2019a
36. IPCC AR5 WG1 Ch5 2013, pp. 389, 399–400: "The PETM [around 55.5–55.3 million years ago] was marked by ... global warming of 4 °C to 7 °C ... Deglacial global warming occurred in two main steps from 17.5 to 14.5 ka [thousand years ago] and 13.0 to 10.0 ka."
37. IPCC SR15 Ch1 2018, p. 54.
38. Kennedy et al. 2010, p. S26. Figure 2.5.
39. Loeb et al. 2021.
40. Kennedy et al. 2010, pp. S26, S59–S60; USGCRP Chapter 1 2017, p. 35.
41. IPCC AR4 WG2 Ch1 2007, p. 99, Sec. 1.3.5.1
42. "Global Warming". NASA JPL. 3 June 2010. Retrieved 11 September 2020. Satellite measurements show warming in the troposphere but cooling in the stratosphere. This vertical pattern is consistent with global warming due to increasing greenhouse gases but inconsistent with warming from natural causes.
43. IPCC SRCCL Summary for Policymakers 2019, p. 7
44. Sutton, Dong & Gregory 2007.
45. "Climate Change: Ocean Heat Content". Noaa Climate.gov. NOAA. 2018. from the original on 12 February 2019. Retrieved 20 February 2019.
46. IPCC AR5 WG1 Ch3 2013, p. 257: "Ocean warming dominates the global energy change inventory. Warming of the ocean accounts for about 93% of the increase in the Earth's energy inventory between 1971 and 2010 (high confidence), with warming of the upper (0 to 700 m) ocean accounting for about 64% of the total.
47. von Schuckman, K.; Cheng, L.; Palmer, M. D.; Hansen, J.; et al. (7 September 2020). "Heat stored in the Earth system: where does the energy go?". Earth System Science Data. 12 (3): 2013–2041. Bibcode:2020ESSD...12.2013V. doi:10.5194/essd-12-2013-2020.
48. NOAA, 10 July 2011.
49. United States Environmental Protection Agency 2016, p. 5: "Black carbon that is deposited on snow and ice darkens those surfaces and decreases their reflectivity (albedo). This is known as the snow/ice albedo effect. This effect results in the increased absorption of radiation that accelerates melting."
50. IPCC AR5 WG1 Ch12 2013, p. 1062; IPCC SROCC Ch3 2019, p. 212.
51. NASA, 12 September 2018.
52. Delworth & Zeng 2012, p. 5; Franzke et al. 2020
53. National Research Council 2012, p. 9
54. IPCC AR5 WG1 Ch10 2013, p. 916.
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Special Report: Climate change and Land

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Sixth Assessment Report

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