Climate change

Average global temperatures from 2010 to 2019 compared to a baseline average from 1951 to 1978 (Source: NASA)

Climate change includes both the global warming driven by human emissions of greenhouse gases, and the resulting large-scale shifts in weather patterns.[1] Though there have been previous periods of climatic change, since the mid-20th century the rate of human impact on Earth's climate system and its global scale have been unprecedented.[2]

That human activity has caused climate change is not disputed by any scientific body of national or international standing.[3] The largest driver has been the emission of greenhouse gases, of which more than 90% are carbon dioxide (CO
) and methane.[4] Fossil fuel burning for energy consumption is the main source of these emissions, with additional contributions from agriculture, deforestation, and industrial processes.[5] Temperature rise is accelerated or tempered by climate feedbacks, such as loss of sunlight-reflecting snow and ice cover, increased water vapour (a greenhouse gas itself), and changes to land and ocean carbon sinks.

Because land surfaces heat faster than ocean surfaces, deserts are expanding and heat waves and wildfires are more common.[7] Surface temperature rise is greatest in the Arctic, where it has contributed to melting permafrost, the retreat of glaciers and sea ice.[8] Increasing atmospheric energy and rates of evaporation cause more intense storms and weather extremes, which damage infrastructure and agriculture.[9] Rising temperatures are limiting ocean productivity and harming fish stocks in most parts of the globe.[10] Current and anticipated effects from undernutrition, heat stress and disease have led the World Health Organization to declare climate change the greatest threat to global health in the 21st century.[11] Environmental effects include the extinction or relocation of many species as their ecosystems change, most immediately in coral reefs, mountains, and the Arctic.[12] Even if efforts to minimize future warming are successful, some effects will continue for centuries, including rising sea levels, rising ocean temperatures, and ocean acidification.[13]

Many of these effects are already observed at the current level of warming, which is about 1.1 °C (2.0 °F).[15] The Intergovernmental Panel on Climate Change (IPCC) has issued a series of reports that project significant increases in these impacts as warming continues to 1.5 °C (2.7 °F) and beyond.[16] Under the Paris Agreement, nations agreed to keep warming "well under 2.0 °C (3.6 °F)" by reducing greenhouse gas emissions. However, under those pledges, global warming would reach about 2.8 °C (5.0 °F) by the end of the century, and current policies will result in about 3.0 °C (5.4 °F) of warming.[17] Limiting warming to 1.5 °C (2.7 °F) would require halving emissions by 2030, then reaching near-zero levels by 2050.[18]

Mitigation efforts include the development and deployment of low-carbon energy technologies, enhanced energy efficiency, phase-out of coal, reforestation, and forest preservation. Climate engineering techniques, most prominently solar radiation management and carbon dioxide removal, have substantial limitations and carry large uncertainties. Societies are also working to adapt to current and future global-warming effects through improved coastline protection, better disaster management, and the development of more resistant crops.

Multiple independently produced instrumental datasets show that the climate system is warming,[21] with the 2009–2018 decade being 0.93 ± 0.07 °C (1.67 ± 0.13 °F) warmer than the pre-industrial baseline (1850–1900).[22] Currently, surface temperatures are rising by about 0.2 °C (0.36 °F) per decade.[23] Since 1950, the number of cold days and nights has decreased, and the number of warm days and nights has increased.[24] Historical patterns of warming and cooling, like the Medieval Climate Anomaly and the Little Ice Age, were not as synchronous across regions as current warming, but may have reached temperatures as high as those of the late-20th century in a limited set of regions.[25] There have been prehistorical episodes of global warming, such as the Paleocene–Eocene Thermal Maximum.[26] However, the observed rise in temperature and CO
concentrations has been so rapid that even abrupt geophysical events that took place in Earth's history do not approach current rates.[27]

Climate proxy records show that natural variations offset the early effects of the Industrial Revolution, so there was little net warming between the 18th century and the mid-19th century.[28] The Intergovernmental Panel on Climate Change (IPCC) has adopted the baseline reference period 1850–1900 as an approximation of pre-industrial global mean surface temperature,[28] when thermometer records began to provide global coverage.[29]

While the common measure of global warming is near-surface atmospheric temperature changes, those measurements are reinforced with a wide range of other types of observations.[30] There has been an increase in the frequency and intensity of heavy precipitation, melting of snow and land ice, and increased atmospheric humidity.[31] Flora and fauna are also behaving in a manner consistent with warming; for instance, plants are flowering earlier in spring.[32] 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.[33]

Although record-breaking years attract considerable media attention, individual years are less significant than the longer global temperature trend.[34] An example of a shorter episode is the slower rate of surface temperature increase from 1998 to 2012, which was labeled the "global warming hiatus".[35] Throughout this period, ocean heat storage continued to progress steadily upwards, and in subsequent years, surface temperatures have spiked upwards. The slower pace of warming can be attributed to a combination of natural fluctuations, reduced solar activity, and increased reflection of sunlight by particles from volcanic eruptions.[36]

Global warming refers to global averages, with the amount of warming varying by region. Patterns of warming are independent of the locations of greenhouse gas emissions, because the gases persist long enough to diffuse across the planet; however, localized black carbon deposits on snow and ice do contribute to Arctic warming.[37]

Since the pre-industrial period, global average land temperatures have increased almost twice as fast as global average surface temperatures.[38] This is because of the larger heat capacity of oceans, and because oceans lose more heat by evaporation.[39] Over 90% of the additional energy in the climate system over the last 50 years has been stored in the ocean, warming it.[40] The remainder of the additional energy has melted ice and warmed the continents and the atmosphere.[41] The ocean heat uptake drives thermal expansion which has contributed to observed sea level rise.[42]

The Northern Hemisphere and 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 snow area and sea ice, because of how the land masses are arranged around the Arctic Ocean. As these surfaces flip from reflecting a lot of light to being dark after the ice has melted, they start absorbing more heat. The Southern Hemisphere already had little sea ice in summer before it started warming.[43] Arctic temperatures have increased and are predicted to continue to increase during this century at over twice the rate of the rest of the world.[44] Melting of glaciers and ice sheets in the Arctic disrupts ocean circulation, including a weakened Gulf Stream, causing increased warming in some areas.[45]

By itself, the climate system experiences various cycles which can last for years (such as the El Niño–Southern Oscillation) to decades or centuries.[46] Other changes are caused by an imbalance of energy that is "external" to the climate system, but not always external to the Earth.[47] Examples of external forcings include changes in the composition of the atmosphere (e.g. increased concentrations of greenhouse gases), solar luminosity, volcanic eruptions, and variations in the Earth's orbit around the Sun.[48]

Attribution of climate change is the effort to scientifically show which mechanisms are responsible for observed changes in Earth's climate. To determine anthropogenic attribution, known internal climate variability and natural external forcings need to be ruled out. Therefore, a key approach is to use computer modelling of the climate system to determine unique "fingerprints" for all potential causes. By comparing these fingerprints with observed patterns and evolution of climate change, and the observed history of the forcings, the causes of the observed changes can be determined.[49] For example, solar forcing can be ruled out as major cause because its fingerprint is warming in the entire atmosphere, and only the lower atmosphere has warmed, which is what is expected from greenhouse gases (which trap heat energy radiating from the surface).[50] Attribution of recent climate change shows that the primary cause is greenhouse gases, and secondarily land-use changes, and aerosols and soot.[51]

The Earth absorbs sunlight, then radiates it as heat. Some of this infrared radiation is absorbed by greenhouse gases in the atmosphere, and because they re-emit it in all directions part of the heat is trapped on Earth instead of escaping into space.[52] Before the Industrial Revolution, naturally-occurring amounts of greenhouse gases caused the air near the surface to be about 33 °C (59 °F) warmer than it would have been in their absence.[53] Without the Earth's atmosphere, the Earth's average temperature would be well below the freezing point of water.[54] 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 considered feedbacks. On the other hand, concentrations of gases such as CO
(~20%), ozone and nitrous oxide are not temperature-dependent, and are hence considered external forcings.[55] Ozone acts as a greenhouse gas in the lowest layer of the atmosphere, the troposphere (as opposed to the stratospheric ozone layer). Furthermore, ozone is highly reactive and interacts with other greenhouse gases and aerosols.[56]

Human activity since the Industrial Revolution, mainly extracting and burning fossil fuels (coal, oil, and natural gas),[57] has increased the amount of greenhouse gases in the atmosphere. These increases in levels of gases such as CO
, methane, tropospheric ozone, CFCs, and nitrous oxide have increased radiative forcing. In 2018, the concentrations of CO
and methane had increased by about 45% and 160%, respectively, since 1750.[58] In 2013, CO2 readings taken at the world's primary benchmark site in Mauna Loa surpassed 400 ppm for the first time (normal pre-industrial levels were ~270ppm).[59] These CO
levels are much higher than they have been at any time during the last 800,000 years, the period for which reliable data have been collected from air trapped in ice cores.[60] Less direct geological evidence indicates that CO
values have not been this high for millions of years.[61]

Global anthropogenic greenhouse gas emissions in 2018, excluding those from land use change, were equivalent to 52 billion tonnes of CO
. Of these emissions, 72% was CO
, 19% was methane, 6% was nitrous oxide, and 3% was fluorinated gases.[62] CO
emissions primarily come from burning fossil fuels to provide usable light and heat energy for transport, manufacturing, heating, and grid electricity.[63] Additional CO
emissions come from deforestation and industrial processes, which include the CO
released by the chemical reactions for making cement, steel, aluminum, and fertilizer.[64] Methane emissions come from livestock, manure, rice cultivation, landfills, wastewater, coal mining, as well as oil and gas extraction.[65] Nitrous oxide emissions largely come from the microbial decomposition of inorganic and organic fertilizer.[66]

From a consumption standpoint, the dominant sources of global 2010 emissions were: food and human waste (34%), thermal comfort, washing, and lighting (26%); freight, travel, commuting, and communication (25%); and building construction (15%). These emissions take into account the embodied fossil fuel energy in manufacturing materials including metals (e.g. steel, aluminum), concrete, glass, and plastic, which are largely used in buildings, infrastructure, and transportation.[67] From a 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%).[68]

Despite the contribution of deforestation to greenhouse gas emissions, the Earth's land surface, particularly its forests, remain a significant carbon sink for CO
. 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 11 billion tonnes of CO
annually from the atmosphere, or about 29% of global CO
emissions.[69] The ocean also serves as a significant carbon sink via a two-step process. First, CO
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 (changing the ocean's chemistry). Over the last two decades, the world's oceans have absorbed 20 to 30% of emitted CO
.[70] The strength of both the land and ocean sinks increases as CO
levels in the atmosphere rise. In this respect they act as suppressing feedbacks in global warming.[71]

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.).[72] The amount of forested land continues to decrease, largely due to conversion to cropland in the tropics.[73] This 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%).[74]

In addition to affecting greenhouse gas concentrations, land-use changes affect global warming through a variety of other chemical and physical mechanisms. Changing the type of vegetation in a region affects the local temperature, by changing 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 contribute to changing temperatures by affecting the release of aerosols and other chemical compounds that influence clouds, and by changing wind patterns (when the land surface presents different obstructions to wind).[75] In tropic and temperate areas the net effect is to produce a significant warming, while at latitudes closer to the poles a gain of albedo (as forest is replaced by snow cover) leads to an overall cooling effect.[75] Globally, these effects are estimated to have led to a slight cooling, dominated by an increase in surface albedo.[76]

Air pollution, in the form of aerosols, not only puts a large burden on human health, but also affects the climate on a large scale.[77] From 1961 to 1990, a gradual reduction in the amount of sunlight reaching the Earth's surface was observed, a phenomenon popularly known as global dimming,[78] typically attributed to aerosols from biofuel and fossil fuel burning.[79] Aerosol removal by precipitation gives tropospheric aerosols an atmospheric lifetime of only about a week, while stratospheric aerosols can remain in the atmosphere for a few years.[80] Globally, aerosols have been declining since 1990, meaning that they no longer mask global warming as much.[81]

In addition to their direct effects (scattering and absorbing solar radiation), aerosols have indirect effects on the Earth's radiation budget. Sulfate aerosols act as cloud condensation nuclei and thus lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets.[82] This effect also causes droplets to be more uniform in size, which reduces the growth of raindrops and makes clouds more reflective to incoming sunlight.[83] Indirect effects of aerosols are the largest uncertainty in radiative forcing.[84]

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.[85] Limiting new black carbon deposits in the Arctic could reduce global warming by 0.2 °C by 2050.[86]

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,[88] and indirect measurements are available from the early 1600s.[87] There has been no upward trend in the amount of the Sun's energy reaching the Earth, so it cannot be responsible for the current warming.[89] 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, with a temperature signal lasting about twice as long. In the industrial era, volcanic activity has had negligible impacts on global temperature change trends.[90] Present-day volcanic CO2 emissions during eruptions and during non-eruptive periods represent only about 1% of current anthropogenic CO2 emissions.[91]

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.[92] Further evidence for greenhouse gases being the cause of recent climate change come from measurements showing the warming of the lower atmosphere (the troposphere), coupled with the cooling of the upper atmosphere (the stratosphere).[93] If solar variations were responsible for the observed warming, warming of both the troposphere and the stratosphere would be expected, but that has not been the case.[50]

The response of the climate system to an initial forcing is modified by feedbacks: increased by self-reinforcing feedbacks and reduced by balancing feedbacks.[95] The main reinforcing feedbacks are the water-vapour feedback, the ice–albedo feedback, and probably the net effect of clouds (described below).[96] The primary balancing feedback to global temperature change is radiative cooling to space as infrared radiation in response to rising surface temperature.[97] Uncertainty over feedbacks is the major reason why different climate models project different magnitudes of warming for a given amount of emissions.[98]

As air gets warmer, it can hold more moisture. After an initial warming due to emissions of greenhouse gases, the atmosphere will hold more water. As water is a potent greenhouse gas, this further heats the climate: the water-vapour feedback.[96] If cloud cover increases, more sunlight will be reflected back into space, cooling the planet. If clouds become more high and thin, then clouds can act more as an insulator, reflecting heat from below back downwards and warming the planet.[99] Overall, the net cloud feedback over the industrial era has probably exacerbated temperature rise.[100]

The reduction of snow cover and sea ice in the Arctic reduces the albedo of the Earth's surface.[101] More of the Sun's energy is now absorbed in these regions, contributing to Arctic amplification, which has caused Arctic temperatures to increase at more than twice the rate of the rest of the world;[102] this is the ice-albedo feedback. Arctic amplification is also melting permafrost, which releases methane and CO
into the atmosphere as another positive feedback.[103]

Roughly half of each year's CO
emissions have been absorbed by plants on land and in oceans.[104] CO
and an extended growing season have stimulated plant growth, making the land carbon cycle a balancing feedback. Climate change also increases droughts and heat waves that inhibit plant growth, which makes it uncertain that this balancing feedback will persist in the future.[105] Soils contain large quantities of carbon and may release some when they heat up.[106] As more CO
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.[107] Climate change can also increase methane emissions from wetlands, marine and freshwater systems, and permafrost.[108]

CMIP5 average of climate model projections for 2081–2100 relative to 1986–2005, under low and high emission scenarios

Future warming depends on the strengths of climate feedbacks and on emissions of greenhouse gases.[109] The former are often estimated using climate models. A climate model is a representation of the physical, chemical, and biological processes that affect the climate system.[110] Models also include changes in the Earth's orbit, historical changes in the Sun's activity, and volcanic forcing.[111] 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.[112] There are more than two dozen scientific institutions that develop major climate models.[113] Models project different future temperature rises for given emissions of greenhouse gases; they also do not fully agree on the strength of different feedbacks on climate sensitivity and magnitude of inertia of the climate system.[114]

The physical realism of models is tested by examining their ability to simulate contemporary or past climates.[115] Past models have underestimated the rate of Arctic shrinkage[116] and underestimated the rate of precipitation increase.[117] Sea level rise since 1990 was underestimated in older models, but now agrees well with observations.[118] The 2017 United States-published National Climate Assessment notes that "climate models may still be underestimating or missing relevant feedback processes".[119]

Four Representative Concentration Pathways (RCPs) are used as input for climate models: "a stringent mitigation scenario (RCP2.6), two intermediate scenarios (RCP4.5 and RCP6.0) and one scenario with very high [greenhouse gas] emissions (RCP8.5)".[120] RCPs only look at concentrations of greenhouse gases, and so does not include the response of the carbon cycle.[121] Climate model projections summarized in the IPCC Fifth Assessment Report indicate that, during the 21st century, the global surface temperature is likely to rise a further 0.3 to 1.7 °C (0.5 to 3.1 °F) in a moderate scenario, or as much as 2.6 to 4.8 °C (4.7 to 8.6 °F) in an extreme scenario, depending on the rate of future greenhouse gas emissions and on climate feedback effects.[122]

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 how greenhouse gas emissions may vary in the future. This output is then used as input for physical climate models to generate climate change projections.[123] In some scenarios emissions continue to rise over the century, while others have reduced emissions.[124] Fossil fuel resources are too abundant for shortages to be relied on to limit carbon emissions in the 21st century.[125] Emissions scenarios can be combined with modelling of the carbon cycle to predict how atmospheric concentrations of greenhouse gases might change in the future.[126] 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 Pathway (SSP) and the mitigation scenario.[127]

The remaining carbon emissions budget is determined by modelling the carbon cycle and the climate sensitivity to greenhouse gases.[128] 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 CO
depending on the choice of the measure of global temperature. This amount corresponds to 10 to 13 years of current emissions. There are high uncertainties about the budget; for instance, it may be 100 gigatonnes of CO
smaller due to methane release from permafrost and wetlands.[129]

The environmental effects of climate change are broad and far-reaching, effecting 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.[131] Since the 1950s, droughts and heat waves have appeared simultaneously with increasing frequency.[132] Extremely wet or dry events within the monsoon period have increased in India and East Asia.[133] Various mechanisms have been identified that might explain extreme weather in mid-latitudes from the rapidly warming Arctic, such as the jet stream becoming more erratic.[134] The maximum rainfall and wind speed from hurricanes and typhoons is likely increasing.[135]

Climate change has led to decades of shrinking and thinning of the Arctic sea ice, making it vulnerable to atmospheric anomalies.[136] Projections of declines in Arctic sea ice vary.[137] While ice-free summers are expected to be rare at 1.5 °C (2.7 °F) degrees of warming, they are set to occur once every three to ten years at a warming level of 2.0 °C (3.6 °F),[138] increasing ice–albedo feedback.[139]

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 2017, the rise increased over time, averaging 3.1 ± 0.3 mm per year.[140] Over the 21st century, the IPCC projects that in a very high emissions scenario the sea level could rise by 61–110 cm.[141] Increased ocean warmth is undermining and threatening to unplug Antarctic glacier outlets, risking a large melt of the ice sheet[142] and the possibility of a 2-meter sea level rise by 2100 under high emissions.[143]

Higher atmospheric CO
concentrations have also led to changes in ocean chemistry. An increase in dissolved CO
is causing ocean acidification, harming corals and shellfish in particular.[144] In addition, oxygen levels are decreasing as oxygen is less soluble in warmer water,[145] with hypoxic dead zones expanding as a result of algal blooms stimulated by higher temperatures, higher CO
levels, ocean deoxygenation, and eutrophication.[146]

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.[147] An example is the collapse of West Antarctic and Greenland ice sheets, where a certain temperature rise commits an ice sheet to melt, although the time scale required is uncertain and depends on future warming.[148] Some large-scale changes could occur over a short time period, such as a collapse of the Atlantic Meridional Overturning Circulation,[149] which would trigger major climate changes in the North Atlantic, Europe, and North America.[150]

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 CO
emissions.[151] This is due to CO
's long atmospheric lifetime.[151] Oceanic CO
uptake is slow enough that ocean acidification will continue for hundreds to thousands of years.[152] These emissions are estimated to have prolonged the current interglacial period by at least 100,000 years.[153] 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.[154]

Recent warming has driven many terrestrial and freshwater species poleward and towards higher altitudes.[155] Higher atmospheric CO
levels and an extended growing season have resulted in global greening, whereas heatwaves and drought have reduced ecosystem productivity in some regions. The future balance of these opposing effects is unclear.[156] Climate change has contributed to the expansion of drier climate zones, such as the expansion of deserts in the subtropics.[157] The size and speed of global warming is making abrupt changes in ecosystems more likely.[158] Overall, it is expected that climate change will result in the extinction of many species and reduced diversity of ecosystems.[159]

The oceans have heated more slowly than the land, but plants and animals in the ocean have migrated towards the colder poles as fast as or faster than species on land.[160] Just as on land, heat waves in the ocean occur more frequently due to climate change, with harmful effects found on a wide range of organisms such as corals, kelp, and seabirds.[161] Ocean acidification threatens damage to coral reefs, fisheries, protected species, and other natural resources of value to society.[162] Harmful algae bloom enhanced by climate change and eutrophication cause anoxia, disruption of food webs and massive large-scale mortality of marine life.[163] Coastal ecosystems are under particular stress, with almost half of wetlands having disappeared as a consequence of climate change and other human impacts.[164]

The effects of climate change on humans, mostly due to warming and shifts in precipitation, have been detected worldwide. Regional impacts of climate change are now observable on all continents and across ocean regions,[169] with low-latitude, less developed areas facing the greatest risk.[170] The Arctic, Africa, small islands, and Asian megadeltas are likely to be especially affected by future climate change.[171]

Health impacts include both the direct effects of extreme weather, leading to injury and loss of life,[172] as well as indirect effects, such as undernutrition brought on by crop failures.[173] Various infectious diseases are more easily transmitted in a warmer climate, such as dengue fever, which affects children most severely, and malaria.[174] Young children are the most vulnerable to food shortages, and together with older people, to extreme heat.[175] The World Health Organization (WHO) has estimated that between 2030 and 2050, climate change is expected to cause approximately 250,000 additional deaths per year from heat exposure in elderly people, increases in diarrheal disease, malaria, dengue, coastal flooding, and childhood undernutrition.[176] Over 500,000 additional adult deaths are projected yearly by 2050 due to reductions in food availability and quality.[177] Other major health risks associated with climate change include air and water quality.[178] The WHO has classified human health impacts from climate change as the greatest threat to global health in the 21st century.[11]

Climate change is affecting food security and has caused reduction in global mean yields of maize, wheat, and soybeans between 1981 and 2010.[179] Future warming could further reduce global yields of major crops.[180][181] Crop production will probably be negatively affected in low-latitude countries, while effects at northern latitudes may be positive or negative.[182] Up to an additional 183 million people worldwide, particularly those with lower incomes, are at risk of hunger as a consequence of these impacts.[183] The effects of warming on the oceans also impact fish stocks, with decreases in the maximum catch potential, although there is significant geographic variability in this trend, with polar stocks showing an increase.[184] Regions dependent on glacier water, regions that are already dry, and small islands are also at increased risk of water stress due to climate change.[185]

Economic damages due to climate change have been underestimated, and may be severe, with the probability of disastrous tail-risk events being nontrivial.[186] Climate change has likely already increased global economic inequality, and is projected to continue doing so.[187] Most of the severe impacts are expected in sub-Saharan Africa and South-East Asia, where existing poverty is already exacerbated.[188] The World Bank estimates that climate change could drive over 120 million people into poverty by 2030. [189] 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.[190]

Low-lying islands and coastal communities are threatened through hazards posed by sea level rise, such as flooding and permanent submergence.[191] This could lead to statelessness for populations in island nations, such as the Maldives and Tuvalu.[192] In some regions, rise in temperature and humidity may also be too severe for humans to adapt to.[193] In the next 50 years, 1 to 3 billion people are projected to be left outside the historically favourable climate conditions.[194] These factors, plus weather extremes, can drive environmental migration, both within and between countries.[195] Up to 1 billion people could be displaced due to climate change by 2050, with 200 million being the most repeated prediction;[196] however, these numbers have been described as an upper bound.[197]

The two conventional responses are mitigation (preventing as much additional warming as possible by reducing greenhouse gas emissions) and adaptation (adjusting society to compensate for unavoidable warming). Many of the countries that have contributed least to global greenhouse gas emissions are among the most vulnerable to climate change, which raises questions about justice and fairness with regard to mitigation and adaptation.[203] A third option is climate engineering, which refers to direct interventions in the Earth's climate system.[204]

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

The IPCC has stressed the need to keep global warming below 1.5 °C (2.7 °F) compared to pre-industrial levels in order to avoid some irreversible impacts.[16] Climate change impacts can be mitigated by reducing greenhouse gas emissions and by enhancing the capacity of Earth's surface to absorb greenhouse gases from the atmosphere.[205] In order to limit global warming to less than 1.5 °C with a high likelihood of success, the IPCC estimates that global greenhouse gas emissions will need to be net zero by 2050,[206] or by 2070 with a 2 °C target. This will require far-reaching, systemic changes on an unprecedented scale in energy, land, cities, transport, buildings, and industry.[207] To make progress towards a goal of limiting warming to 1.5 °C, the United Nations Environment Programme estimates that, within the next decade, countries will need to triple the amount of reductions they have committed to in their current Paris Agreements.[208]

Long-term scenarios point to rapid and significant investment in renewable energy and energy efficiency as key to reducing GHG emissions.[210] Fossil fuels accounted for 80% of the world's energy in 2018, while the remaining share of power production was split between nuclear power, hydropower, and non-hydro renewables;[211] that mix is expected to change significantly over the next 30 years.[212] Renewable energy technologies include solar and wind power, bioenergy, geothermal energy, and hydropower.[213] Photovoltaic solar and wind, in particular, have seen substantial growth and progress over the last few years, such that they are currently among the cheapest sources of new power generation.[214] Renewables represented 75% of all new electricity generation installed in 2019, with solar and wind constituting nearly all of that amount.[215]

There are obstacles to the continued rapid development of renewable energy. Environmental and land use concerns are sometimes associated with large solar, wind and hydropower projects.[216] Solar and wind power also require energy storage systems and other modifications to the electricity grid to operate effectively,[210] although several storage technologies are now emerging to supplement the traditional use of pumped-storage hydropower.[217] The use of rare-earth metals and other hazardous materials has also been raised as a concern with solar power.[218] The use of bioenergy is often not carbon neutral, and may have negative consequences for food security,[219] largely due to the amount of land required compared to other renewable energy options.[220] Hydropower growth has been slowing and is set to decline further due to concerns about social and environmental impacts.[221] While not a traditional renewable, nuclear energy has continued to be a significant part of the global energy mix. However, nuclear power costs are increasing amidst stagnant power share, so that nuclear power generation is now several times more expensive per megawatt hour than wind and solar.[222]

Where energy production or CO
-intensive heavy industries continue to produce waste CO
, the gas can be captured and stored instead of being released to the atmosphere. Although costly,[223] carbon capture and storage (CCS) may be able to play a significant role in limiting CO
emissions by mid-century.[224]

Earth's natural carbon sinks can be enhanced to sequester significantly larger amounts of CO
beyond naturally occurring levels.[225] Forest preservation, reforestation and tree planting on non-forest lands are considered the most effective, although they raise food security concerns. Soil management on croplands and grasslands is another effective mitigation technique.[226] As models disagree on the feasibility of land-based negative emissions methods for mitigation, strategies based on them are risky.[227]

Scenarios of global greenhouse gas emissions. If all countries achieve their current Paris Agreement pledges, average warming by 2100 will go far beyond the target of the Paris Agreement to keep warming "well below 2°C".

Although there is no single pathway to limit global warming to 1.5 or 2 °C,[228] 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.[212] To reduce pressures on ecosystems and enhance their carbon sequestration capabilities, changes would also be necessary in forestry and agriculture.[229] Scenarios that limit global warming to 1.5 °C generally project the large scale use of CO
removal methods in addition to greenhouse gas reduction approaches.[230]

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 other needs, such as heating, would rise to the point where electricity becomes the largest form of overall energy supply by 2050.[231] Investment in coal would be eliminated and coal use nearly phased out by 2050.[232]

In transport, scenarios envision sharp increases in the market share of electric vehicles, low carbon fuel substitution for other transportation modes like shipping, and changes in transportation patterns that increase efficiency, for example increased public transport.[233] Buildings will see additional electrification with the use of technologies like heat pumps, as well as continued energy efficiency improvements achieved via low energy building codes.[234] Industrial efforts will focus on increasing the energy efficiency of production processes, such as the use of cleaner technology for cement production,[235] designing and creating less energy intensive products, increasing product lifetimes, and developing incentives to reduce product demand.[236]

The agriculture and forestry sector faces a triple challenge of limiting greenhouse gas emissions, preventing further conversion of forests to agricultural land, and meeting increases in world food demand.[237] A suite of actions could reduce agriculture/forestry based greenhouse gas emissions by 66% from 2010 levels by 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.[238]

Individuals can also take actions to reduce their carbon footprint. These include: driving an electric or other energy efficient car, reducing vehicles miles by using mass transit or cycling, adopting a plant-based diet, reducing energy use in the home, limiting consumption of goods and services, and foregoing air travel.[239]

Economic sectors with more greenhouse gas contributions have a greater stake in climate change policies.

A wide range of policies, regulations and laws are being used to reduce greenhouse gases. Carbon pricing mechanisms include carbon taxes and emissions trading systems.[240] As of 2019, carbon pricing covers about 20% of global greenhouse gas emissions.[241] Renewable portfolio standards have been enacted in several countries requiring utilities to increase the percentage of electricity they generate from renewable sources.[242] Phasing out of fossil fuel subsidies, currently estimated at $300 billion globally (about twice the level of renewable energy subsidies),[243] could reduce greenhouse gas emissions by 6%.[244] Subsidies could also be redirected to support the transition to clean energy.[245] More prescriptive methods that can reduce greenhouse gases include vehicle efficiency standards, renewable fuel standards, and air pollution regulations on heavy industry.[246]

Reducing air pollution from the burning of fossil fuels will have significant co-benefits in terms of human health.[247] For instance, the WHO estimates that ambient air pollution currently causes 4.2 million deaths per year due to stroke, heart disease, lung cancer, and respiratory diseases.[248] Meeting Paris Agreement goals could save about a million of those lives per year worldwide from reduced pollution by 2050.[249][250]

As the use of fossil fuels is reduced, there are Just Transition considerations involving the social and economic challenges that arise. An example is the employment of workers in the affected industries, along with the well-being of the broader communities involved.[251] Climate justice considerations, such as those facing indigenous populations in the Arctic,[252] are another important aspect of mitigation policies.[253]

Adaptation is "the process of adjustment to current or expected changes in climate and its effects". As climate change effects vary across regions, so do adaptation strategies.[254] While some adaptation responses call for trade-offs, others bring synergies and co-benefits.[255] Increased use of air conditioning allows people to better cope with heat, but also increases energy demand.[256] Other examples of adaptation include improved coastline protection, better disaster management, and the development of more resistant crops.[257]

Adaptation is especially important in developing countries since they are predicted to bear the brunt of the effects of climate change.[258] The capacity and potential for humans to adapt, called adaptive capacity, is unevenly distributed across different regions and populations, and developing countries generally have less.[259][260] There are limits to adaptation and more severe climate change requires more transformative adaptation, which can be prohibitively expensive.[254] The public sector, private sector, and communities are all gaining experience with adaptation, and adaptation is becoming embedded within their planning processes.[261]

Geoengineering or climate engineering is the deliberate large-scale modification of the climate, considered a potential future method for counteracting climate change.[262] Techniques fall generally into the categories of solar radiation management and carbon dioxide removal, although various other schemes have been suggested. A 2018 review paper concluded that although geoengineering is physically possible, all the techniques are in early stages of development, carry large risks and uncertainties and raise significant ethical and legal issues.[263]

The geopolitics of climate change is complex and 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 investing in a transition to a low-carbon economy themselves. However, net importers of fossil fuels win economically from transitioning, causing net exporters to face stranded assets: fossil fuels they cannot sell, if they choose not to transition.[265] Furthermore, the benefits in terms of public health and local environmental improvements of coal phase out exceed the costs in almost all regions, potentially further eliminating the free-rider problem.[266] The geopolitics are further complicated by the supply chain of rare earth metals necessary to produce many clean technologies.[267]

Nearly all countries in the world are parties to the (UNFCCC).[268] The objective of the UNFCCC is to prevent dangerous human interference with the climate system.[269] As stated in the convention, this requires that greenhouse gas concentrations are stabilized 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.[270] Global emissions have risen since signing of the UNFCCC, as it does not actually restrict emissions but rather provides a framework for protocols that do.[68] Its yearly conferences are the stage of global negotiations.[271]

The importance of the United Nations Framework Convention on Climate Change is underlined by the Sustainable Development Goal 13 which is to "Take urgent action to combat climate change and its impacts". It is one of the 17 Sustainable Development Goals (SDGs) to be achieved by 2030.[272] One of the targets of SDG 13 is for developed countries to implement the commitments of mobilizing $100 billion per year to address the needs of developing countries, and make sure the Green Climate Fund becomes operational as soon as possible.[273]

Other climate change treaties include the 1997 Kyoto Protocol, which extended UNFCCC and in which most developed countries accepted legally binding commitments to limit their emissions,[274] and the 2009 Copenhagen Accord.[275] During Kyoto Protocol negotiations, the G77 (representing developing countries) pushed for a mandate requiring developed countries to "[take] the lead" in reducing their emissions,[276] since developed countries contributed most to the accumulation of greenhouse gases in the atmosphere, and since per-capita emissions were still relatively low in developing countries. (and emissions of developing countries would grow to meet their development needs.)[277] Copenhagen Accord has been widely portrayed as disappointing because of its low goals, and has been rejected by poorer nations including the G77.[278] Nations associated with the Accord aimed to limit the future increase in global mean temperature to below 2 °C.[279]

In 2015 all UN countries negotiated the Paris Agreement, which aims to keep global warming well below 2 °C and contains an aspirational goal of keeping warming under 1.5 °C.[280] The agreement replaced the Kyoto Protocol. Unlike Kyoto, no binding emission targets were set in the Paris Agreement. Instead, the procedure of regularly setting ever more ambitious goals and reevaluating these goals every five years has been made binding.[281] The Paris Agreement reiterated that developing countries must be financially supported.[282] As of November 2019, 194 states and the European Union have signed the treaty and 186 states and the EU have ratified or acceded to the agreement.[283] In November 2020 the United States withdrew from the Paris Agreement.[284]

In 2019, the British Parliament became the first national government in the world to officially declare a climate emergency.[285] Other countries and jurisdictions followed suit.[286] In November 2019 the European Parliament declared a "climate and environmental emergency",[287] and the European Commission presented its European Green Deal with the goal of making the EU carbon-neutral by 2050.[288]

While ozone depletion and global warming are considered separate problems, the solution to the former has significantly mitigated global warming. The greenhouse gas emission mitigation of the Montreal Protocol, an international agreement to stop emitting ozone-depleting gases, is estimated to have been more effective than that of the Kyoto Protocol, which was specifically designed to curb greenhouse gas emissions.[289] It has been argued that the Montreal Protocol may have done more than any other measure, as of 2017, to mitigate global warming as [290]

There is an in recent decades and that the trend is caused mainly by human-induced emissions of greenhouse gases, with 97% or more of actively publishing climate scientists agreeing.[291][292] The consensus has grown to 100% among research scientists on anthropogenic global warming as of 2019.[293] No scientific body of national or international standing disagrees with this view.[294] Consensus has further developed that some form of action should be taken to protect people against the impacts of climate change, and national science academies have called on world leaders to cut global emissions.[295]

Scientific discussion takes place in journal articles that are peer-reviewed, which scientists subject to assessment every couple of years in the Intergovernmental Panel on Climate Change reports.[296] In 2013, the IPCC Fifth Assessment Report stated that "is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century".[297] Their 2018 report expressed the scientific consensus as: "human influence on climate has been the dominant cause of observed warming since the mid-20th century".[298] Scientists have issues two warnings to humanity, in 2017 and 2019, expressing concern about the current trajectory of potentially catastrophic climate change, and about untold human suffering as a consequence.[299]

Climate change came to international public attention in the late 1980s.[300] Due to confusing media coverage in the early 1990s, understanding was often confounded by conflation with other environmental issues like ozone depletion.[301] 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.[300]

Significant regional differences exist in both public concern for and public understanding of climate change.[302] In 2015 a median of 54% of respondents considered it "a very serious problem", but Americans and Chinese (whose economies are responsible for the greatest annual CO2 emissions) were among the least concerned.[302] A 2020 Pew research study finds that in the US the issue is also highly partisan, with belief that humans are contributing a great deal to climate change believed by 72% of Democrats and only 22% of Republicans, while belief that government should do more to reduce its effects supported by 89% of Democrats and only 35% of Republicans.[303]

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.[305] Like the tobacco industry before, the main strategy of these groups has been to manufacture doubt about scientific data and results.[306] 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.[307]

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 minimize the negative impacts of climate change.[308] Manufacturing uncertainty about the science later developed into a manufacturing controversy: creating the belief that there is significant uncertainty about climate change within the scientific community in order to delay policy changes.[309] Strategies to promote these ideas include criticism of scientific institutions,[310] and questioning the motives of individual scientists.[308] An "echo chamber" of climate-denying blogs and media has further fomented misunderstanding of climate change.[311]

Climate protests have risen in popularity in the 2010s in such forms as public demonstrations,[312] fossil fuel divestment, and lawsuits.[313] Prominent recent demonstrations include the school strike for climate, and civil disobedience. In the school strike, youth across the globe have protested by skipping school, inspired by Swedish teenager Greta Thunberg.[314] Mass civil disobedience actions by groups like Extinction Rebellion have protested by causing disruption.[315] Litigation is increasingly used as a tool to strengthen climate action, with many lawsuits targeting governments to demand that they take ambitious action or enforce existing laws regarding climate change. Lawsuits against fossil-fuel companies, from activists, shareholders and investors, generally seek compensation for loss and damage.[316]

Tyndall's sensitive ratio spectrophotometer (drawing published in 1861) measured the extent to which infrared radiation was absorbed and emitted by various gases filling its central tube.

In 1824 Joseph Fourier proposed a version of the greenhouse effect; transparent atmosphere lets through visible light, which warms the surface. The warmed surface emits infrared radiation, but the atmosphere is relatively opaque to infrared and slows the emission of energy, warming the planet.[317] Starting in 1859,[318] John Tyndall established that nitrogen and oxygen (99% of dry air) are transparent to infrared, but water vapour and traces of some gases (significantly methane and carbon dioxide) both absorb infrared and, when warmed, emit infrared radiation. Changing concentrations of these gases could have caused "all the mutations of climate which the researches of geologists reveal" including ice ages.[319]

Svante Arrhenius noted that water vapour in air continuously varied, but carbon dioxide (CO
) was determined by long term geological processes. At the end of an ice age, warming from increased CO
would increase the amount of water vapour, amplifying its effect in a feedback process. In 1896, he published the first climate model of its kind, showing that halving of CO
could have produced the drop in temperature initiating the ice age. Arrhenius calculated the temperature increase expected from doubling CO
to be around 5–6 °C (9.0–10.8 °F).[320] Other scientists were initially sceptical and believed the greenhouse effect to be saturated so that adding more CO
would make no difference. Experts thought climate would be self-regulating.[321] From 1938 Guy Stewart Callendar published evidence that climate was warming and CO
levels increasing,[322] but his calculations met the same objections.[321]

Early calculations treated the atmosphere as a single layer but in the 1950s, Gilbert Plass used digital computers to model the different layers and found added CO
would cause warming. In the same decade Hans Suess found evidence CO
levels had been rising, Roger Revelle showed the oceans would not absorb the increase, and together they helped Charles Keeling to begin a record of continued increase, the Keeling Curve.[321] Scientists alerted the public,[323] and the dangers were highlighted at James Hansen's 1988 Congressional testimony.[324] The Intergovernmental Panel on Climate Change, set up in 1988 to provide formal advice to the world's governments, spurred interdisciplinary research.[325]

Before the 1980s, when it was unclear whether warming by greenhouse gases would dominate aerosol-induced cooling, scientists often used the term inadvertent climate modification to refer to humankind's impact on the climate. In the 1980s, the terms global warming and climate change were introduced, the former referring only to increased surface warming, while the latter describes the full effect of greenhouse gases on the climate.[326] Global warming became the most popular term after NASA climate scientist James Hansen used it in his 1988 testimony in the U.S. Senate.[324] In the 2000s, the term climate change increased in popularity.[327] In lay usage, global warming usually refers to human-induced warming of the Earth system, whereas climate change can refer to natural as well as anthropogenic change.[328] The two terms are often used interchangeably.[329]

Various scientists, politicians and media figures have adopted the terms climate crisis or climate emergency to talk about climate change, while using global heating instead of global warming.[330] The policy editor-in-chief of The Guardian explained that 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".[331] Oxford Dictionary chose climate emergency as its word of the year in 2019 and defines the term as "a situation in which urgent action is required to reduce or halt climate change and avoid potentially irreversible environmental damage resulting from it".[332]