Climate change

This page is about the current warming of the Earth's climate system. "Climate change" can also refer to climate trends at any point in Earth's history. For other uses see Global warming (disambiguation).

Global mean surface-temperature change from 1880 to 2018, relative to the 1951–1980 mean. The 1951–1980 mean is 14.19 °C (57.54 °F). The black line is the global annual mean, and the red line is the five-year local regression line. The blue uncertainty bars show a 95% confidence interval.

Average global temperatures from 2014 to 2018 compared to a baseline average from 1951 to 1980, according to NASA's Goddard Institute for Space Studies.

Global warming is a long-term rise in the average temperature of the Earth's climate system, an aspect of climate change shown by temperature measurements and by multiple effects of the warming. Though earlier geological periods also experienced episodes of warming, the term commonly refers to the observed and continuing increase in average air and ocean temperatures since 1900 caused mainly by emissions of greenhouse gasses in the modern industrial economy. In the modern context the terms global warming and climate change are commonly used interchangeably, but climate change includes both global warming and its effects, such as changes to precipitation and impacts that differ by region. Many of the observed changes in climate since the 1950s are unprecedented in the instrumental temperature record, and in historical and paleoclimate proxy records of climate change over thousands to millions of years.

In 2013, the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report concluded, "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century." The largest human influence has been the emission of greenhouse gases such as carbon dioxide, methane, and nitrous oxide. Climate model projections summarized in the report indicated 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) to 2.6 to 4.8 °C (4.7 to 8.6 °F) depending on the rate of greenhouse gas emissions and on climate feedback effects. These findings have been recognized by the national science academies of the major industrialized nations and are not disputed by any scientific body of national or international standing.

Effects of global warming include rising sea levels, regional changes in precipitation, more frequent extreme weather events such as heat waves, and expansion of deserts. Surface temperature increases are greatest in the Arctic, with the continuing retreat of glaciers, permafrost, and sea ice. Overall, higher temperatures bring more rain and snowfall, but for some regions droughts and wildfires increase instead. Effects directly significant to humans are predicted to include the threat to food security from decreasing crop yields, and the abandonment of populated areas due to rising sea levels. Environmental impacts appear likely to include the extinction or relocation of ecosystems as they adapt to climate change, with coral reefs, mountain ecosystems, and Arctic ecosystems most immediately threatened. Because the climate system has a large "inertia" and greenhouse gases will remain in the atmosphere for a long time, climatic changes and their effects will continue to become more pronounced for many centuries even if further increases to greenhouse gases stop.

Globally, a majority of people consider global warming a serious or very serious issue. Possible societal responses to global warming include mitigation by emissions reduction, adaptation to its effects, and possible future climate engineering. Every country in the world is a party to the United Nations Framework Convention on Climate Change (UNFCCC), whose ultimate objective is to prevent dangerous anthropogenic climate change. Parties to the UNFCCC have agreed that deep cuts in emissions are required and that global warming should be limited to well below 2.0 °C (3.6 °F) compared to pre-industrial levels, with efforts made to limit warming to 1.5 °C (2.7 °F). Some scientists call into question climate adaptation feasibility, with higher emissions scenarios, or the two degree temperature target.

Observed temperature changes

Main article: Instrumental temperature record

Annual (thin lines) and five-year lowess smooth (thick lines) for the temperature anomalies averaged over the Earth's land area (red line) and sea surface temperature anomalies (blue line) averaged over the part of the ocean that is free of ice at all times (open ocean).

Two millennia of mean surface temperatures according to different reconstructions from climate proxies, each smoothed on a decadal scale, with the instrumental temperature record overlaid in black.

Multiple independently produced datasets confirm that between 1880 and 2012, the global average (land and ocean) surface temperature increased by 0.85 [0.65 to 1.06] °C. Since 1979 the rate of warming has approximately doubled (0.13±0.03 °C per decade, against 0.07±0.02 °C per decade). Since 1950, the number of cold days and nights have decreased, and the number of warm days and night have increased.

Although the increase of the average near-surface atmospheric temperature is commonly used to track global warming, over 90% of the additional energy stored in the climate system over the last 50 years is in warmer ocean water. The rest has melted ice and warmed the continents and the atmosphere.

Climate proxies show the temperature to have been relatively stable over the two thousand years before 1850, with only regional changes such as the Medieval Warm Period and the Little Ice Age.

The warming evident in the instrumental temperature record is consistent with a wide range of observations, documented by many independent scientific groups, for example in most continental regions the frequency and intensity of heavy precipitation has increased. Further examples include sea level rise, widespread melting of snow and land ice, increased heat content of the oceans, increased humidity, and the earlier timing of spring events, such as the flowering of plants.

Regional trends

See also: Regional effects of global warming and Polar amplification

Global warming refers to global averages, with the amount of warming varying by region. Since 1979, global average land temperatures have increased about twice as fast as global average ocean temperatures. This is due to the larger heat capacity of the oceans and because oceans lose more heat by evaporation. Where greenhouse gas emissions occur does not impact the location of warming because the major greenhouse gases persist long enough to diffuse across the planet, although localized black carbon deposits on snow and ice do contribute to Arctic warming.

The Northern Hemisphere and North Pole have heated much faster than the South Pole and Southern Hemisphere. The Northern Hemisphere not only has much more land, its arrangement around the Arctic Ocean has resulted in the maximum surface area flipping from reflective snow and ice cover to ocean and land surfaces that absorb more sunlight. 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. As the temperature difference between the Arctic and the equator decreases ocean currents that are driven by that temperature difference, like the Gulf Stream, are weakening.

Short-term slowdowns and surges

Because the climate system has large thermal inertia, it can take centuries for the climate to fully adjust. While record-breaking years attract considerable public interest, individual years are less significant than the overall trend. Global surface temperature is subject to short-term fluctuations that overlay long-term trends, and can temporarily mask or magnify them.

An example of such an episode is the slower rate of surface temperature increase from 1998 to 2012, which was dubbed the global warming hiatus by the media and some scientists. 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 heating and cooling in the Pacific Ocean from natural variability such as El Niño and La Nina events, reduced solar activity, and a number of volcanic eruptions that inserted sunlight-reflecting particles into the atmosphere.

Physical drivers of climate change

Contribution of natural factors and human activities to radiative forcing of climate change. Radiative forcing values are for the year 2005, relative to the pre-industrial era (1750). The contribution of solar irradiance to radiative forcing is 5% of the value of the combined radiative forcing due to increases in the atmospheric concentrations of carbon dioxide, methane and nitrous oxide.

Further information: Attribution of recent climate change

By itself, the climate system may generate changes in global temperatures for years (such as the El Niño–Southern Oscillation) to decades or centuries at a time. Other changes emanate from so-called external forcings. These forcings are "external" to the climate system, but not necessarily external to Earth. 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 Earth's orbit around the Sun.

Attributing detected temperature changes and extreme events to manmade increases in greenhouse gases requires scientists to rule out known internal climate variabilities and natural external forcings. So a key approach is to use physical models of the climate system to determine unique fingerprints for all potential external forcings. By comparing these fingerprints with the observed pattern and evolution of the climate change, and the observed evolution of the forcings, the causes of the observed changes can be determined.

Greenhouse gases

Greenhouse effect schematic showing energy flows between space, the atmosphere, and Earth's surface. Energy exchanges are expressed in watts per square meter (W/m²).

CO₂ concentrations over the last 800,000 years as measured from ice cores (blue/green) and directly (black).

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

Greenhouse gases trap heat radiating from Earth to space. This heat, in the form of infrared radiation, gets absorbed and emitted by these gases in the planet's atmosphere thus warming the lower atmosphere and the surface. On Earth, an atmosphere containing naturally occurring amounts of greenhouse gases causes air temperature near the surface to be warmer by about 33 °C (59 °F) than it would be in their absence. Without the Earth's atmosphere, the Earth's average temperature would be well below the freezing temperature of water. The major greenhouse gases are water vapour, which causes about 36–70% of the greenhouse effect; carbon dioxide (CO₂), which causes 9–26%; methane (CH₄), which causes 4–9%; and ozone (O₃), which causes 3–7%.

Human activity since the Industrial Revolution has increased the amount of greenhouse gases in the atmosphere, leading to increased radiative forcing from CO₂, methane, tropospheric ozone, CFCs, and nitrous oxide. In 2011, the concentrations of CO₂ and methane had increased by about 40% and 150% respectively since pre-industrial times, with CO₂ readings taken at the world's primary benchmark site in Mauna Loa surpassing 400 ppm in 2013 for the first time. These levels are much higher than at any time during the last 800,000 years, the period for which reliable data has been extracted from ice cores. Less direct geological evidence indicates that CO₂ values haven't been this high for millions of years.

Fossil fuel burning and industry produced about three-quarters of the increase in greenhouse gases between 1970 and 2010. Emissions from land use change, particularly deforestation did not increase over the last ten years of that period. Estimates of global CO₂ emissions in 2011 from fossil fuel combustion, including cement production and gas flaring, was 34.8 billion tonnes (9.5 ± 0.5 PgC), an increase of 54% above emissions in 1990. Coal burning was responsible for 43% of the total emissions, oil 34%, gas 18%, cement 4.9% and gas flaring 0.7%. Carbon emission rates plateaued from 2014 to 2016, rose by 1.6% in 2017, then rose again by 2.7% in 2018.

Aerosols and soot

Ship tracks can be seen as lines in these clouds over the Atlantic Ocean on the East Coast of the United States, an example of the Twomey effect.

Solid and liquid particles known as aerosols; from volcanoes, plankton and human-made pollutants; reflect incoming sunlight so would cool the climate. 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, typically attributed to aerosols from biofuel and fossil fuel burning. 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. Global aerosols have been declining since 1990, removing some of the masking of global warming that aerosols had been providing.

In addition to their direct effect by 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, a phenomenon known as the Twomey effect. This effect also causes droplets to be of more uniform size, which reduces growth of raindrops and makes the cloud more reflective to incoming sunlight, known as the Albrecht effect. Indirect effects are most noticeable in marine stratiform clouds, and have very little radiative effect on convective clouds. Indirect effects of aerosols are the largest uncertainty in radiative forcing.

While aerosols typically limit global warming by reflecting sunlight, if black carbon in soot falls on snow or ice it can also increase global warming. Not only does it increase the absorption of sunlight, it also directly exacerbates melting and sea level rise. Limiting new black carbon deposits in the Arctic could reduce global warming by 0.2 degrees Celsius by 2050. When soot is suspended in the atmosphere it directly absorbs solar radiation, heating the atmosphere and cooling the surface. In areas with high soot production, such as rural India, as much as 50% of surface warming due to greenhouse gases may be masked by atmospheric brown clouds. The influences of atmospheric particles, including black carbon, are most pronounced in the tropics and northern mid-latitudes, with the effects of greenhouse gases dominant in the other parts of the world.

Incoming sunlight

Further information: Solar activity and climate

As the Sun is Earth's primary energy source changes in incoming sunlight directly impact the climate system. Solar irradiance has been measured directly by satellites since 1978, but indirect measurements are available since the early 1600s. 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. Physical climate models are also unable to reproduce the rapid warming observed in recent decades when only taking into account variations in solar output and volcanic activity.

Another line of evidence for the warming not being due to the Sun is the differing temperature changes at different levels in the Earth's atmosphere. According to basic physical principles, the greenhouse effect produces warming of the lower atmosphere (the troposphere), but cooling of the upper atmosphere (the stratosphere). 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.

While variations in solar activity have not produced recent global warming, variations in solar output over geologic time (millions to billions of years ago) are believed to have caused major changes in the earth's climate. The 11 year solar cycle of sunspot activity also introduces climate changes that have a small cyclical effect on annual global temperatures. The tilt of the Earth's axis and the shape of its orbit around the Sun vary slowly over tens of thousands of years in a phenomenon known as the Milankovitch cycles. This changes climate by changing the seasonal and latitudinal distribution of incoming solar energy at the Earth's surface. It has been found that periodic glacial and interglacial periods over last few million years have been driven by this process. During the last few thousand years, this phenomenon contributed to a slow cooling trend at high latitudes of the Northern Hemisphere during summer, a trend that was reversed by greenhouse-gas-induced warming during the 20th century. Orbital cycles favorable for glaciation are not expected within the next 50,000 years.

Climate change feedback

Main articles: Climate change feedback and Climate sensitivity

The dark ocean surface reflects only 6 percent of incoming solar radiation, whereas sea ice reflects 50 to 70 percent.

The response of the climate system to an initial forcing is increased by positive feedbacks and reduced by negative feedbacks. The main negative feedback to global temperature change is radiative cooling to space as infrared radiation, which increases strongly with increasing temperature. The main positive feedbacks are the water vapour feedback, the ice–albedo feedback, and probably the net effect of clouds. Uncertainty over feedbacks is the major reason why different climate models project different magnitudes of warming for a given amount of emissions.

As air gets warmer, 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. The reduction of snow cover and sea ice in the Arctic reduces the albedo (reflectivity) of the Earth's surface. More of the sun's energy is now absorbed in these regions, contributing to Arctic amplification, which has caused Arctic temperatures to increase at almost twice the rate of the rest of the world. Arctic amplification also causes methane to be released as permafrost melts, which is expected to surpass land use changes as the second strongest anthropogenic source of greenhouse gases by the end of the century.

Cloud cover may change in the future. To date, cloud changes have had a cooling effect, with NASA estimating that aerosols produced by the burning of hydrocarbons have limited warming by half from 1850 to 2010. An analysis of satellite data between 1983 and 2009 reveals that cloud tops are reaching higher into the atmosphere and that cloudy storm tracks are shifting toward Earth's poles, suggesting clouds will be a positive feedback in the future. A 2019 study indicates that if greenhouse gases reach three times the current level of atmospheric carbon dioxide that stratocumulus clouds could abruptly disperse, contributing up to 8 degrees Celsius of warming.

Carbon dioxide stimulates plant growth so the carbon cycle has been a negative feedback so far: roughly half of each year's CO₂ emissions have been absorbed by plants on land and in oceans, with an estimated 30% increase in plant growth from 2000 to 2017. The limits and reversal point for this feedback are an area of uncertainty. As more CO₂ and heat are absorbed by the ocean it is acidifying and ocean circulation can change, changing the rate at which the ocean can absorb atmospheric carbon. On land, greater plant growth will be constrained by nitrogen levels and can be reversed by plant heat stress, desertification, and the release of carbon from soil as the ground warms.

A concern is that positive feedbacks will lead to a tipping point, where global temperatures transition to a hothouse climate state even if greenhouse gas emissions are reduced or eliminated. A 2018 study tried to identify such a planetary threshold for self-reinforcing feedbacks and found that even a 2 °C (3.6 °F) increase in temperature over pre-industrial levels, may be enough to trigger such a hothouse Earth scenario.

Climate models

Future CO₂ projections, including all forcing agents' atmospheric CO₂-equivalent concentrations (in parts-per-million-by-volume (ppmv)) according to four RCPs (Representative Concentration Pathways).

Projected change in annual mean surface air temperature from the late 20th century to the middle 21st century, based on a medium emissions scenario (SRES A1B). This scenario assumes that no future policies are adopted to limit greenhouse gas emissions. Image credit: NOAA GFDL.

Main article: Global climate model

A climate model is a representation of the physical, chemical and biological processes that affect the climate system. Computer models are run on supercomputers 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. There are more than two dozen scientific institutions that develop climate models. Model forecasts vary due to different greenhouse gas inputs and different assumptions about the impact of different feedbacks on climate sensitivity.

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, scientists can produce scenarios of how greenhouse gas emissions may vary in the future. Scientists can then run these scenarios through physical climate models to generate climate change projection.

Climate models include different external forcings for their models. For different greenhouse gas inputs four RCPs (Representative Concentration Pathways) are used: "a stringent mitigation scenario (RCP2.6), two intermediate scenarios (RCP4.5 and RCP6.0) and one scenario with very high GHG emissions (RCP8.5). Models also include changes in the Earth's orbit, historical changes in the sun's activity and volcanic forcing. RCPs only look at concentrations of greenhouse gases, factoring out uncertainty as to whether the carbon cycle will continue to remove about half of the carbon dioxide from the atmosphere each year.

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

Effects

Main article: Effects of global warming

Historical sea level reconstruction and projections up to 2100 published in January 2017 by the U.S. Global Change Research Program for the Fourth National Climate Assessment.

Map of the Earth with a six-meter sea level rise represented in red.

Physical environmental

Main article: Physical impacts of climate change

The environmental effects of global warming are broad and far-reaching. They include the following diverse effects:

• Arctic sea ice decline, sea level rise, retreat of glaciers: global warming has led to decades of shrinking and thinning of the Arctic sea ice, making it vulnerable to atmospheric anomalies. Projections of declines in Arctic sea ice vary. Recent projections suggest that Arctic summers could be ice-free (defined as an ice extent of less than 1 million square km) as early as 2025–2030. Since 1993, sea level has on average risen with 3.1 ± 0.3 mm per year. Additionally, sea level rise has accelerated from 1993 to 2017. Over the 21st century, the IPCC projects (for a high emissions scenario) that global mean sea level could rise by 52–98 cm. The rate of ice loss from glaciers and ice sheets in the Antarctic is a key area of uncertainty since Antarctica contains 90% of potential sea level rise. Polar amplification and increased ocean warmth are undermining and threatening to unplug Antarctic glacier outlets, potentially resulting in more rapid sea level rise.
• Extreme weather, extreme events, tropical cyclones: Data analysis of extreme events from 1960 until 2010 suggests that droughts and heat waves appear simultaneously with increased frequency. Extremely wet or dry events within the monsoon period have increased since 1980. Projections suggest a probable increase in the frequency and severity of some extreme weather events, such as heat waves. Studies have also linked the rapidly warming Arctic to extreme weather in mid-latitudes as the jet stream becomes more erratic. Maximum rainfall and wind speed from hurricanes and typhoons are likely increasing.
• Changes in ocean properties: increases in atmospheric CO₂ concentrations have led to an increase in dissolved CO₂ and as a consequence ocean acidity. Furthermore, oxygen levels decrease because oxygen is less soluble in warmer water, an effect known as ocean deoxygenation.
• Long-term effects of global warming: On the timescale of centuries to millennia, the magnitude of global warming will be determined primarily by anthropogenic CO₂ emissions. This is due to carbon dioxide's very long lifetime in the atmosphere. Long-term effects also include a response from the Earth's crust, due to ice melting and deglaciation, in a process called post-glacial rebound, when land masses are no longer depressed by the weight of ice. This could lead to landslides and increased seismic and volcanic activities. Tsunamis could be generated by submarine landslides caused by warmer ocean water thawing ocean-floor permafrost or releasing gas hydrates. Sea level rise will continue over many centuries.
• Abrupt climate change, tipping points in the climate system: Climate change could result in global, large-scale changes. Some large-scale changes could occur abruptly, i.e. over a short time period, and might also be irreversible. Examples of abrupt climate change are the rapid release of methane and carbon dioxide from permafrost, which would lead to amplified global warming. Another example is the possibility for the Atlantic Meridional Overturning Circulation to slow or to shut down (see also shutdown of thermohaline circulation). This could trigger cooling in the North Atlantic, Europe, and North America. It would particularly affect areas such as the British Isles, France and the Nordic countries, which are warmed by the North Atlantic drift.

Biosphere

As the climate change melts sea ice, the U.S. Geological Survey projects that two-thirds of polar bears will disappear by 2050.

Main article: Climate change and ecosystems

Ecosystem changes: In terrestrial ecosystems, the earlier timing of spring events, as well as poleward and upward shifts in plant and animal ranges, have been linked with high confidence to recent warming. It is expected that most ecosystems will be affected by higher atmospheric CO₂ levels, combined with higher global temperatures. Expansion of deserts in the subtropics is probably linked to global warming. Ocean acidification threatens damage to coral reefs, fisheries, protected species, and other natural resources of value to society. Without substantial actions to reduce the rate of global warming, land-based ecosystems are at risk of major ecological shifts, transforming composition and structure.

Overall, it is expected that climate change will result in the extinction of many species and reduced diversity of ecosystems. Rising temperatures have been found to push bees to their physiological limits, and could cause the extinction of bee populations. Continued ocean uptake of CO₂ may affect the brains and central nervous system of certain fish species, and that this impacts their ability to hear, smell, and evade predators.

Impacts on humans

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

A helicopter drops water on the wildfire in California. Drought and higher temperatures linked to climate change are driving a trend towards larger fires.

The effects of climate change on human systems, mostly due to warming or shifts in precipitation patterns, or both, have been detected worldwide. The future social impacts of climate change will be uneven across the world. All regions are at risk of experiencing negative impacts, with low-latitude, less developed areas facing the greatest risk. Global warming has likely already increased global economic inequality, and is projected to do so in the future.

Regional impacts of climate change are now observable on all continents and across ocean regions. The Arctic, Africa, small islands and Asian megadeltas are regions that are likely to be especially affected by future climate change. Many risks are expected to increase with higher magnitudes of global warming.

Food and water

Crop production will probably be negatively affected in low latitude countries, while effects at northern latitudes may be positive or negative. Global warming of around 4 °C relative to late 20th century levels could pose a large risk to global and regional food security. The impact of climate change on crop productivity for the four major crops was negative for wheat and maize, and neutral for soy and rice, in the years 1960–2013. Climate variability and change is projected to severely compromise agricultural production, including access to food, across Africa.

By 2050, between 350 million and 600 million people are projected to experience increased water stress due to climate change in Africa. Water availability will also become more limited in regions dependent on glacier water, regions with reductions in rainfall and small islands.

Health and security

Aerial view over southern Bangladesh after the passage of cyclone Cyclone Sidr. A combination of sea level rise and increased rainfall from cyclones makes countries more vulnerable to floods, impacting people's livelihoods and health

Generally impacts on public health will be more negative than positive. Impacts include the direct effects of extreme weather, leading to injury and loss of life; and indirect effects, such as undernutrition brought on by crop failures. There has been a shift from cold- to heat-related mortality in some regions as a result of warming. Temperature rise has been connected to increased numbers of suicides.

Climate change has been linked to an increase in violent conflict by amplifying poverty and economic shocks, which are well-documented drivers of these conflicts. Links have been made between a wide range of violent behaviors including fist fights, violent crimes, civil unrest, or wars. The violent herder–farmer conflicts in Nigeria, Sudan and other countries in the Sahel region have been exacerbated by climate change.

Livelihoods, industry and infrastructure

In small islands and mega deltas, inundation as a result of sea level rise is expected to threaten vital infrastructure and human settlements. This could lead to issues of homelessness in countries with low-lying areas such as Bangladesh, as well as statelessness for populations in island nations, such as the Maldives and Tuvalu. Climate change can be an important driver of migration, both within and between countries.

Africa is one of the most vulnerable continents to climate variability and change because of multiple existing stresses and low adaptive capacity. Existing stresses include poverty, political conflicts, and ecosystem degradation. Regions may even become uninhabitable, with humidity and temperature reaching levels too high for humans to survive.

Responses

Mitigation of and adaptation to climate change are two complementary responses to global warming. Successful adaptation is easier in the case of substantial emission reduction. Many of the countries that contributed least to global greenhouse gas emissions are most vulnerable to climate change, which raises questions about justice and fairness with regard to mitigation and adaptation.

Mitigation

Main article: Climate change mitigation

The graph on the right shows three "pathways" to meet the UNFCCC's 2 °C target, labelled "global technology", "decentralized solutions", and "consumption change". Each pathway shows how various measures (e.g., improved energy efficiency, increased use of renewable energy) could contribute to emissions reductions. Image credit: PBL Netherlands Environmental Assessment Agency.

Annual greenhouse gas emissions attributed to different sectors as of the year 2010. Emissions are given as a percentage share of total emissions, measured in carbon dioxide-equivalents, using global warming potentials from the IPCC fifth Assessment Report.

Mitigation of climate change is the reduction of greenhouse gas emissions, or the enhancement of the capacity of carbon sinks to absorb greenhouse gases from the atmosphere. There is a large potential for future reductions in emissions by a combination of activities, including energy conservation and increased energy efficiency; the use of low-carbon energy technologies, such as renewable energy, nuclear energy, and carbon capture and storage; decarbonizing buildings and transport; and enhancing carbon sinks through, for example, reforestation and preventing deforestation. A 2015 report by Citibank concluded that transitioning to a low carbon economy would yield positive return on investments.

Global carbon dioxide emissions by country in 2015

Over the last three decades of the twentieth century, gross domestic product per capita and population growth were the main drivers of increases in greenhouse gas emissions. CO₂ emissions are continuing to rise due to the burning of fossil fuels and land-use change. Emissions can be attributed to different regions. Attribution of emissions due to land-use change are subject to considerable uncertainty.

Emissions scenarios, estimates of changes in future emission levels of greenhouse gases, have been projected that depend upon uncertain economic, sociological, technological, and natural developments. In most scenarios, emissions continue to rise over the century, while in a few, emissions are reduced. Fossil fuel reserves are abundant, and will not limit carbon emissions in the 21st century. Emission scenarios, combined with modelling of the carbon cycle, have been used to produce estimates of how atmospheric concentrations of greenhouse gases might change in the future. Using the six IPCC SRES "marker" scenarios, models suggest that by the year 2100, the atmospheric concentration of CO₂ could range between 541 and 970 ppm.

Near- and long-term trends in the global energy system are inconsistent with limiting global warming at below 1.5 or 2 °C, relative to pre-industrial levels. Current pledges made as part of the Paris Agreement would lead to about 3.0 °C of warming at the end of the 21st century, relative to pre-industrial levels. In limiting warming at below 2 °C, more stringent emission reductions in the near-term would allow for less rapid reductions after 2030, and be cheaper overall. Many integrated models are unable to meet the 2 °C target if pessimistic assumptions are made about the availability of mitigation technologies.

Co-benefits of climate change mitigation may help society and individuals more quickly. For example cycling reduces greenhouse gas emissions while reducing the effects of a sedentary lifestyle at the same time The development and scaling-up of clean technology, such as cement that produces less CO₂. is critical to achieve sufficient emission reductions for the Paris agreement goals.

Mitigation at an individual level is also possible. The most significant action individuals could make to mitigate their own carbon footprint is to have fewer children, followed by living car-free, forgoing air travel, and adopting a plant-based diet. Some argue that allocating emissions to one's ancestors is a category mistake and that children "embody a profound hope for the future", and that we should focus more on overconsumption, lifestyle choices of the world's wealthy, fossil fuel companies and government inaction.

Adaptation

Main article: Climate change adaptation

Climate change adaptation is the process of adjusting to actual or expected climate change and its effects. Humans can strive to moderate or avoid harm due to climate change and exploit opportunities. Examples of adaptation are improved coast-line protection, better disaster management and the development of crops that are more resistant. The adaptation may be planned, either in reaction to or anticipation of global warming, or spontaneous, i.e., without government intervention.

The public section, private sector and communities are all gaining experience with adaptation and adaptation is becoming embedded within certain planning processes. While some adaptation responses call for trade-offs, others bring synergies and co-benefits. Environmental organizations and public figures have emphasized changes in the climate and the risks they entail, while promoting adaptation to changes in infrastructural needs and emissions reductions.

Adaptation is especially important in developing countries since those countries are predicted to bear the brunt of the effects of global warming. That is, 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 capacity to adapt.

Climate engineering

Main article: Climate engineering

Climate engineering (sometimes called geoengineering or climate intervention) is the deliberate modification of the climate. It has been investigated as a possible response to global warming, e.g. by NASA and the Royal Society. Techniques under research fall generally into the categories solar radiation management and carbon dioxide removal, although various other schemes have been suggested. A study from 2014 investigated the most common climate engineering methods and concluded they are either ineffective or have potentially severe side effects and cannot be stopped without causing rapid climate change.

Society and culture

Political response

Main article: Politics of global warming

Article 2 of the UN Framework Convention refers explicitly to "stabilization of greenhouse gas concentrations". To stabilize the atmospheric concentration of CO
₂, emissions worldwide would need to be dramatically reduced from their present level.

As of 2019 all countries in the world are parties to the United Nations Framework Convention on Climate Change (UNFCCC), but 12 countries have not ratified it, which means they are not legally bound by the agreement. The ultimate objective of the Convention is to prevent dangerous human interference to the climate system. 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. The Framework Convention was agreed on in 1992, but global emissions have risen since then. Its yearly conferences are the stage of global negotiations.

During these negotiations, the G77 (a lobbying group in the United Nations representing developing countries) pushed for a mandate requiring developed countries to "[take] the lead" in reducing their emissions. This was justified on the basis that the developed countries' emissions had contributed most to the accumulation of greenhouse gases in the atmosphere, per-capita emissions (i.e., emissions per head of population) were still relatively low in developing countries, and the emissions of developing countries would grow to meet their development needs.

This mandate was sustained in the 2005 Kyoto Protocol to the Framework Convention. In ratifying the Kyoto Protocol, most developed countries accepted legally binding commitments to limit their emissions. These first-round commitments expired in 2012. United States President George W. Bush rejected the treaty on the basis that "it exempts 80% of the world, including major population centres such as China and India, from compliance, and would cause serious harm to the US economy".

In 2009 several UNFCCC Parties produced the Copenhagen Accord, which has been widely portrayed as disappointing because of its low goals, leading poor nations to reject it. Parties associated with the Accord aim to limit the future increase in global mean temperature to below 2 °C. In 2015 all UN countries negotiated the Paris Agreement, which aims to keep climate change well below 2 °C. The agreement replaced the Kyoto protocol. Unlike Kyoto, no binding emission targets are 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. The Paris agreement reiterated that developing countries must be financially supported.

Scientific discussion

Main article: Scientific opinion on climate change

Scientific discussion takes place in articles that are peer-reviewed and assessed by scientists who work in the relevant fields and participate in the Intergovernmental Panel on Climate Change. The scientific consensus as of 2013 stated in the IPCC Fifth Assessment Report is that it "is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century".

A 2008 report by the U.S. National Academy of Sciences stated that most scientists by then agreed that observed warming in recent decades was primarily caused by human activities increasing the amount of greenhouse gases in the atmosphere. In 2005 the Royal Society stated that while the overwhelming majority of scientists were in agreement on the main points, some individuals and organizations opposed to the consensus on urgent action needed to reduce greenhouse gas emissions had tried to undermine the science and work of the IPCC. National science academies have called on world leaders for policies to cut global emissions.

In 2018 the IPCC published a Special Report on Global Warming of 1.5 °C which warned that, if the current rate of greenhouse gas emissions is not mitigated, global warming is likely to reach 1.5°C (2.7 °F) between 2030 and 2052 causing major crises. The report said that preventing such crises will require a swift transformation of the global economy that has "no documented historic precedent".

In the scientific literature, there is an overwhelming consensus that global surface temperatures have increased in recent decades and that the trend is caused mainly by human-induced emissions of greenhouse gases. No scientific body of national or international standing disagrees with this view. In November 2017, a second warning to humanity signed by 15,364 scientists from 184 countries stated that "the current trajectory of potentially catastrophic climate change due to rising greenhouse gases from burning fossil fuels, deforestation, and agricultural production – particularly from farming ruminants for meat consumption" is "especially troubling".

Position of oil and gas companies

See also: Fossil fuels lobby

In the 20th century and early 2000s some companies, such as ExxonMobil, challenged IPCC climate change scenarios, funded scientists who disagreed with the scientific consensus, and provided their own projections of the economic cost of stricter controls. In general, since the 2010s, global oil companies do not dispute that climate change exists and is caused by the burning of fossil fuels. As of 2019, however, some are lobbying against a carbon tax and plan to increase production of oil and gas but others are in favor of a carbon tax in exchange for immunity from lawsuits which seek climate change compensation.

Public opinion and disputes

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

Global warming was the cover story in this 2007 issue of Ms. magazine

The global warming problem came to international public attention in the late 1980s and polling groups began to track opinions on the subject. The longest consistent polling, by Gallup in the US, found relatively small deviations of 10% or so from 1998 to 2015 in opinion on the seriousness of global warming, but with increasing polarization between those concerned and those unconcerned. By 2010 in the US, just a little over half the population (53%) viewed it as a serious concern for either themselves or their families. Latin America and developed Asia saw themselves most at risk at 73% and 74%. In the assessed 111 countries, people were more likely to attribute global warming to human activities than to natural causes, except in the US where nearly half (47%) of the population attributed global warming to natural causes. Public reactions to global warming and concern about its effects have been increasing, while many perceiving is as the worst global threat. A 2015 global survey showed that a median of 54% of respondents consider it "a very serious problem", with significant regional differences: Americans and Chinese (whose economies are responsible for the greatest annual CO₂ emissions) are among the least concerned.

From about 1990 onward, American conservative think tanks had begun challenging the legitimacy of global warming as a social problem. They challenged the scientific evidence, argued that global warming would have benefits, and asserted that proposed solutions would do more harm than good. Organizations such as the libertarian Competitive Enterprise Institute, conservative commentators have challenged IPCC climate change scenarios, funded scientists who disagree with the scientific consensus, and provided their own projections of the economic cost of stricter controls.

Global warming has been the subject of controversy, substantially more pronounced in the popular media than in the scientific literature, with disputes regarding the nature, causes, and consequences of global warming. The disputed issues include the causes of increased global average air temperature, especially since the mid-20th century, whether this warming trend is unprecedented or within normal climatic variations, whether humankind has contributed significantly to it, and whether the increase is completely or partially an artifact of poor measurements. Additional disputes concern estimates of climate sensitivity, predictions of additional warming, what the consequences of global warming will be, and what to do about it.

Due to confusing media coverage in the early 1990s, issues such as ozone depletion and climate change were often mixed up, affecting public understanding of these issues. According to a 2010 survey of Americans, a majority thought that the ozone layer and spray cans contribute to global warming. Although there are a few areas of linkage, the relationship between the two is not strong. Reduced stratospheric ozone has had a slight cooling influence on surface temperatures, while increased tropospheric ozone has had a somewhat larger warming effect. However, chemicals causing ozone depletion are also powerful greenhouse gases, and as such the Montreal protocol against their emissions may have done more than any other measure to mitigate climate change.

In a response to perceived inaction on climate change, a climate movement is protesting in various ways, such as fossil fuel divestment, worldwide demonstrations and the school strike for climate.

History

Main article: History of climate change science

The history of climate change science began in the early 19th century when ice ages and other natural changes in paleoclimate were first suspected and the natural greenhouse effect first identified. In the late 19th century, scientists first argued that human emissions of greenhouse gases could change the climate. In the 1960s, the warming effect of carbon dioxide gas became increasingly convincing. By the 1990s, as a result of improving fidelity of computer models and observational work confirming the Milankovitch theory of the ice ages, a consensus position formed: greenhouse gases were deeply involved in most climate changes, and human-caused emissions were bringing discernible global warming. Since the 1990s, scientific research on climate change has included multiple disciplines and has expanded. Research during this period is summarized in the Assessment Reports by the Intergovernmental Panel on Climate Change.

The greenhouse effect was proposed by Joseph Fourier in 1824, discovered in 1856 by Eunice Newton Foote, expanded upon by John Tyndall, investigated quantitatively by Svante Arrhenius in 1896, and the hypothesis was reported in the popular press as early as 1912. The scientific description of global warming was further developed in the 1930s through the 1960s by Guy Stewart Callendar.

Terminology

Research in the 1950s suggested increasing temperatures, and a 1952 newspaper reported "climate change". This phrase next appeared in a November 1957 report in The Hammond Times which described Roger Revelle's research into the effects of increasing human-caused CO₂ emissions on the greenhouse effect "a large scale global warming, with radical climate changes may result". Both phrases were used only occasionally until 1975, when Wallace Smith Broecker published a scientific paper on the topic, "Climatic Change: Are We on the Brink of a Pronounced Global Warming?" The phrase began to come into common use, and in 1976 Mikhail Budyko's statement that "a global warming up has started" was widely reported. Other studies, such as a 1971 MIT report, referred to the human impact as "inadvertent climate modification", but an influential 1979 National Academy of Sciences study headed by Jule Charney followed Broecker in using global warming to refer to rising surface temperatures, while describing the wider effects of increased CO₂ as climate change.

In 1986 and November 1987, NASA climate scientist James Hansen gave testimony to Congress on global warming. There were increasing heatwaves and drought problems in the summer of 1988, and when Hansen testified in the Senate on 23 June he sparked worldwide interest. He said "global warming has reached a level such that we can ascribe with a high degree of confidence a cause and effect relationship between the greenhouse effect and the observed warming". Public attention increased over the summer, and global warming became the dominant popular term, commonly used both by the press and in public discourse.

Since 2018 terms such as the climate crisis are increasingly being used.

See also

• Anthropocene – proposed geological time interval for the period where humans have significant impact on geological processes
• Geologic temperature record
• Global cooling – minority view held by scientists in the 1970 that imminent cooling of the Earth would take place
• Holocene extinction
• Planetary boundaries – climate change/global warming is one of them

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External links

Research

• NASA Goddard Institute for Space Studies – Global change research
• NOAA State of the Climate Report – U.S. and global monthly state of the climate reports
• Climate Change at the National Academies – repository for reports
• Met Office: Climate Guide – UK National Weather Service
• Educational Global Climate Modelling (EdGCM) – research-quality climate change simulator

Educational

• Climate Science Special Report United States 2017
• NASA: Climate change: How do we know?
• Global Climate Change: NASA's Eyes on the Earth – NASA, JPL, Caltech
• Global Climate Change Indicators – NOAA
• NOAA Climate Services – NOAA
• Skeptical Science: Getting skeptical about global warming skepticism
• Global Carbon Dioxide Circulation (NASA; 13 December 2016)
• The World Bank – Climate Change – A 4 Degree Warmer World – We must and can avoid it
• Climate change tutorial by Prof. Myles Allen (Oxford), March 2018: Parts 1, 2, 3, 4, 5 (45 min. total); background & slide deck
• Experts Discuss Recent Heat Waves and Atmospheric Changes (July 2018)