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Temperature changes

Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale, with the actual recorded temperatures overlaid in black.

Evidence for warming of the climate system includes observed increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.^[13]^[14]^[15]^[16]^[17] The most common measure of global warming is the trend in globally averaged temperature near the Earth's surface. Expressed as a linear trend, this temperature rose by 0.74 ± 0.18 °C over the period 1906–2005. The rate of warming over the last half of that period was almost double that for the period as a whole (0.13 ± 0.03 °C per decade, versus 0.07 °C ± 0.02 °C per decade). The urban heat island effect is estimated to account for about 0.002 °C of warming per decade since 1900.^[18] Temperatures in the lower troposphere have increased between 0.13 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with regionally varying fluctuations such as the Medieval Warm Period and the Little Ice Age.^[19]
Estimates by NASA's Goddard Institute for Space Studies (GISS) and the National Climatic Data Center show that 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 19th century, exceeding the previous record set in 1998 by a few hundredths of a degree.^[20]^[21] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit show 2005 as the second warmest year, behind 1998.^[22]^[23] Temperatures in 1998 were unusually warm because the strongest El Niño in the past century occurred during that year.^[24] Global temperature is subject to short-term fluctuations that overlay long term trends and can temporarily mask them. The relative stability in temperature from 2002 to 2009 is consistent with such an episode.^[25]^[26]
Temperature changes vary over the globe. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).^[27] Ocean temperatures increase more slowly than land temperatures because of the larger effective heat capacity of the oceans and because the ocean loses more heat by evaporation.^[28] The Northern Hemisphere warms faster than the Southern Hemisphere because it has more land and because it has extensive areas of seasonal snow and sea-ice cover subject to ice-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.^[29]
The thermal inertia of the oceans and slow responses of other indirect effects mean that climate can take centuries or longer to adjust to changes in forcing. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.^[30]

External forcings

External forcing refers to processes external to the climate system (though not necessarily external to Earth) that influence climate. Climate responds to several types of external forcing, such as radiative forcing due to changes in atmospheric composition (mainly greenhouse gas concentrations), changes in solar luminosity, volcanic eruptions, and variations in Earth's orbit around the Sun.^[31] Attribution of recent climate change focuses on the first three types of forcing. Orbital cycles vary slowly over tens of thousands of years and thus are too gradual to have caused the temperature changes observed in the past century.

Greenhouse gases

Main articles: Greenhouse effect, Radiative forcing, and Atmospheric CO2

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).

Recent atmospheric carbon dioxide (CO₂) increases. Monthly CO₂ measurements display seasonal oscillations in overall yearly uptrend; each year's maximum occurs during the Northern Hemisphere's late spring, and declines during its growing season as plants remove some atmospheric CO₂.

The greenhouse effect is the process by which absorption and emission of infrared radiation by gases in the atmosphere warm a planet's lower atmosphere and surface. It was proposed by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896.^[32] The question in terms of global warming is how the strength of the presumed greenhouse effect changes when human activity increases the concentrations of greenhouse gases in the atmosphere.
Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F).^[33]^[C] The major greenhouse gases are water vapor, which causes about 36–70 percent of the greenhouse effect; carbon dioxide (CO₂), which causes 9–26 percent; methane (CH₄), which causes 4–9 percent; and ozone (O₃), which causes 3–7 percent.^[34]^[35]^[36] Clouds also affect the radiation balance, but they are composed of liquid water or ice and so have different effects on radiation from water vapor.
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. The concentrations of CO₂ and methane have increased by 36% and 148% respectively since 1750.^[37] These levels are much higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores.^[38]^[39]^[40] Less direct geological evidence indicates that CO₂ values higher than this were last seen about 20 million years ago.^[41] Fossil fuel burning has produced about three-quarters of the increase in CO₂ from human activity over the past 20 years. Most of the rest is due to land-use change, particularly deforestation.^[42]
Over the last three decades of the 20th century, GDP per capita and population growth were the main drivers of increases in greenhouse gas emissions.^[43] CO₂ emissions are continuing to rise due to the burning of fossil fuels and land-use change.^[44]^[45]^:71 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.^[46] In most scenarios, emissions continue to rise over the century, while in a few, emissions are reduced.^[47]^[48] These emission scenarios, combined with carbon cycle modelling, have been used to produce estimates of how atmospheric concentrations of greenhouse gases will 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.^[49] This is an increase of 90-250% above the concentration in the year 1750. Fossil fuel reserves are sufficient to reach these levels and continue emissions past 2100 if coal, tar sands or methane clathrates are extensively exploited.^[50]
The destruction of stratospheric ozone by chlorofluorocarbons is sometimes mentioned in relation to global warming. Although there are a few areas of linkage, the relationship between the two is not strong. Reduction of stratospheric ozone has a cooling influence on the entire troposphere, but a warming influence on the surface.^[51] Substantial ozone depletion did not occur until the late 1970s.^[52] Ozone in the troposphere (the lowest part of the Earth's atmosphere) does contribute to surface warming.^[53]

Aerosols and soot

Ship tracks over the Atlantic Ocean on the east coast of the United States. The climatic impacts from aerosol forcing could have a large effect on climate through the indirect effect.

Global dimming, a gradual reduction in the amount of global direct irradiance at the Earth's surface, has partially counteracted global warming from 1960 to the present.^[54] The main cause of this dimming is aerosols produced by volcanoes and pollutants. These aerosols exert a cooling effect by increasing the reflection of incoming sunlight. The effects of the products of fossil fuel combustion—CO₂ and aerosols—have largely offset one another in recent decades, so that net warming has been due to the increase in non-CO₂ greenhouse gases such as methane.^[55] Radiative forcing due to aerosols is temporally limited due to wet deposition which causes aerosols to have an atmospheric lifetime of one week. Carbon dioxide has a lifetime of a century or more, and as such, changes in aerosol concentrations will only delay climate changes due to carbon dioxide.^[56]
In addition to their direct effect by scattering and absorbing solar radiation, aerosols have indirect effects on the radiation budget.^[57] 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.^[58] 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.^[59] Indirect effects are most noticeable in marine stratiform clouds, and have very little radiative effect on convective clouds. Aerosols, particularly indirect effects, represent the largest uncertainty in radiative forcing.^[60]
Soot may cool or warm the surface, depending on whether it is airborne or deposited. Atmospheric soot aerosols directly absorb solar radiation, which heats the atmosphere and cools the surface. In isolated 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.^[61] Atmospheric soot always contributes additional warming to the climate system. When deposited, especially on glaciers or on ice in arctic regions, the lower surface albedo can also directly heat the surface.^[62] The influences of aerosols, including black carbon, are most pronounced in the tropics and sub-tropics, particularly in Asia, while the effects of greenhouse gases are dominant in the extratropics and southern hemisphere.^[63]

Solar variation

Main article: Solar variation

Solar variation over the last thirty years.

Variations in solar output have been the cause of past climate changes.^[64] The consensus among climate scientists is that changes in solar forcing probably had a slight cooling effect in recent decades. This result is less certain than some others, with a few papers suggesting a warming effect.^[31]^[65]^[66]^[67]
Greenhouse gases and solar forcing affect temperatures in different ways. While both increased solar activity and increased greenhouse gases are expected to warm the troposphere, an increase in solar activity should warm the stratosphere while an increase in greenhouse gases should cool the stratosphere.^[31] Observations show that temperatures in the stratosphere have been cooling since 1979, when satellite measurements became available. Radiosonde (weather balloon) data from the pre-satellite era show cooling since 1958, though there is greater uncertainty in the early radiosonde record.^[68]
A related hypothesis, proposed by Henrik Svensmark, is that magnetic activity of the sun deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.^[69] Other research has found no relation between warming in recent decades and cosmic rays.^[70]^[71] The influence of cosmic rays on cloud cover is about a factor of 100 lower than needed to explain the observed changes in clouds or to be a significant contributor to present-day climate change.^[72]

Feedback

Main article: Climate change feedback

Feedback is a process in which changing one quantity changes a second quantity, and the change in the second quantity in turn changes the first. Positive feedback amplifies the change in the first quantity while negative feedback reduces it. Feedback is important in the study of global warming because it may amplify or diminish the effect of a particular process. The main positive feedback in global warming is the tendency of warming to increase the amount of water vapor in the atmosphere, a significant greenhouse gas. The main negative feedback is radiative cooling, which increases as the fourth power of temperature; the amount of heat radiated from the Earth into space increases with the temperature of Earth's surface and atmosphere. Imperfect understanding of feedbacks is a major cause of uncertainty and concern about global warming.

Climate models

Main article: Global climate model

The main tools for projecting future climate changes are mathematical models based on physical principles including fluid dynamics, thermodynamics and radiative transfer. Although they attempt to include as many processes as possible, simplifications of the actual climate system are inevitable because of the constraints of available computer power and limitations in knowledge of the climate system. All modern climate models are in fact combinations of models for different parts of the Earth. These include an atmospheric model for air movement, temperature, clouds, and other atmospheric properties; an ocean model that predicts temperature, salt content, and circulation of ocean waters; models for ice cover on land and sea; and a model of heat and moisture transfer from soil and vegetation to the atmosphere. Some models also include treatments of chemical and biological processes.^[73] Warming due to increasing levels of greenhouse gases is not an assumption of the models; rather, it is an end result from the interaction of greenhouse gases with radiative transfer and other physical processes.^[74] Although much of the variation in model outcomes depends on the greenhouse gas emissions used as inputs, the temperature effect of a specific greenhouse gas concentration (climate sensitivity) varies depending on the model used. The representation of clouds is one of the main sources of uncertainty in present-generation models.^[75]
Global climate model projections of future climate most often have used estimates of greenhouse gas emissions from the IPCC Special Report on Emissions Scenarios (SRES). In addition to human-caused emissions, some models also include a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain. Some observational studies also show a positive feedback.^[76]^[77]^[78] Including uncertainties in future greenhouse gas concentrations and climate sensitivity, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.^[2]
Models are also used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes. Although these models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects, they do indicate that the warming since 1970 is dominated by man-made greenhouse gas emissions.^[31]
The physical realism of models is tested by examining their ability to simulate current or past climates.^[79] Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate.^[42] Not all effects of global warming are accurately predicted by the climate models used by the IPCC. For example, observed Arctic shrinkage has been faster than that predicted.^[80]

Attributed and expected effects

Main articles: Effects of global warming and Regional effects of global warming

Global warming may be detected in natural, ecological or social systems as a change having statistical significance.^[81] Attribution of these changes e.g., to natural or human activities, is the next step following detection.^[82]

Natural systems

Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC.

Global warming has been detected in a number of systems. Some of these changes, e.g., based on the instrumental temperature record, have been described in the section on temperature changes. Rising sea levels and observed decreases in snow and ice extent are consistent with warming.^[17] Most of the increase in global average temperature since the mid-20th century is, with high probability,^[D] atttributable to human-induced changes in greenhouse gas concentrations.^[83]
Even with current policies to reduce emissions, global emissions are still expected to continue to grow over the coming decades.^[84] Over the course of the 21st century, increases in emissions at or above their current rate would very likely induce changes in the climate system larger than those observed in the 20th century.
In the IPCC Fourth Assessment Report, across a range of future emission scenarios, model-based estimates of sea level rise for the end of the 21st century (the year 2090-2099, relative to 1980-1999) range from 0.18 to 0.59 m. These estimates, however, were not given a likelihood due to a lack of scientific understanding, nor was an upper bound given for sea level rise. Over the course of centuries to millennia, the melting of ice sheets could result in sea level rise of 4–6 m or more.^[85]
Changes in regional climate are expected to include greater warming over land, with most warming at high northern latitudes, and least warming over the Southern Ocean and parts of the North Atlantic Ocean.^[84] Snow cover area and sea ice extent are expected to decrease. The frequency of hot extremes, heat waves, and heavy precipitation will very likely increase.

Ecological systems

In terrestrial ecosystems, the earlier timing of spring events, and poleward and upward shifts in plant and animal ranges, have been linked with high confidence to recent warming.^[17] Future climate change is expected to particularly affect certain ecosystems, including tundra, mangroves, and coral reefs.^[84] It is expected that most ecosystems will be affected by higher atmospheric CO₂ levels, combined with higher global temperatures.^[86] Overall, it is expected that climate change will result in the extinction of many species and reduced diversity of ecosystems.^[87]

Social systems

There is some evidence of regional climate change affecting systems related to human activities, including agricultural and forestry management activities at higher latitudes in the Northern Hemisphere.^[17] Future climate change is expected to particularly affect some sectors and systems related to human activities.^[84] Water resources may be stressed in some dry regions at mid-latitudes, the dry tropics, and areas that depend on snow and ice melt. Reduced water availability may affect agriculture in low latitudes. Low-lying coastal systems are vulnerable to sea level rise and storm surge. Human health in populations with limited capacity to adapt to climate change. It is expected that some regions will be particularly affected by climate change, including the Arctic, Africa, small islands, and Asian and African megadeltas. Some people, such as the poor, young children, and the elderly, are particularly at risk, even in high-income areas.

Responses to global warming

Carbon capture and storage (CCS) is an approach to mitigation. Emissions may be sequestered from fossil fuel power plants, or removed during processing in hydrogen production. When used on plants, it is known as bio-energy with carbon capture and storage.

Mitigation

Main article: Global warming mitigation

Reducing the amount of future climate change is called mitigation of climate change. The IPCC defines mitigation as activities that reduce greenhouse gas (GHG) emissions, or enhance the capacity of carbon sinks to absorb GHGs from the atmosphere.^[88] Many countries, both developing and developed, are aiming to use cleaner, less polluting, technologies.^[45]^:192 Use of these technologies aids mitigation and could result in substantial reductions in CO₂ emissions. Policies include targets for emissions reductions, increased use of renewable energy, and increased energy efficiency. Studies indicate substantial potential for future reductions in emissions.^[89] Since even in the most optimistic scenario, fossil fuels are going to be used for years to come, mitigation may also involve carbon capture and storage, a process that traps CO₂ produced by factories and gas or coal power stations and then stores it, usually underground.^[90]

Adaptation

Main article: Adaptation to global warming

Other policy responses include adaptation to climate change. Adaptation to climate change may be planned, e.g., by local or national government, or spontaneous, i.e., done privately without government intervention.^[91] The ability to adapt is closely linked to social and economic development.^[89] Even societies with high capacities to adapt are still vulnerable to climate change. Planned adaptation is already occurring on a limited basis. The barriers, limits, and costs of future adaptation are not fully understood.
Another policy response is engineering of the climate (geoengineering). This policy response is sometimes grouped together with mitigation.^[92] Geoengineering is largely unproven, and reliable cost estimates for it have not yet been published.^[93]

UNFCCC

Most countries are Parties to the United Nations Framework Convention on Climate Change (UNFCCC).^[94] The ultimate objective of the Convention is to prevent "dangerous" human interference of the climate system.^[95] As is stated in the Convention, this requires that GHGs 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 proceed in a sustainable fashion.
The UNFCCC recognizes differences among countries in their responsibility to act on climate change.^[96] In the Kyoto Protocol to the UNFCCC, most developed countries (listed in Annex I of the treaty) took on legally binding commitments to reduce their emissions.^[97] Policy measures taken in response to these commitments have reduced emissions.^[98] For many developing (non-Annex I) countries, reducing poverty is their overriding aim.^[99]
At the 15^th UNFCCC Conference of the Parties, held in 2009 at Copenhagen, several UNFCCC Parties produced the Copenhagen Accord.^[100] Parties agreeing with the Accord aim to limit the future increase in global mean temperature to below 2 °C.^[101]

Views on global warming

Main articles: Global warming controversy and Politics of global warming

Per capita greenhouse gas emissions in 2000, including land-use change.

Total greenhouse gas emissions in 2000, including land-use change.

There are different views over what the appropriate policy response to climate change should be.^[102]^[103] These competing views weigh the benefits of limiting emissions of greenhouse gases against the costs. In general, it seems likely that climate change will impose greater damages and risks in poorer regions.^[104]

Politics

Developing and developed countries have made different arguments over who should bear the burden of costs for cutting emissions. Developing countries often concentrate on per capita emissions, that is, the total emissions of a country divided by its population.^[105] Per capita emissions in the industrialized countries are typically as much as ten times the average in developing countries.^[106] This is used to make the argument that the real problem of climate change is due to the profligate and unsustainable lifestyles of those living in rich countries.^[105] On the other hand, commentators from developed countries more often point out that it is total emissions that matter.^[105] In 2008, developing countries made up around half of the world's total emissions of CO₂ from cement production and fossil fuel use.^[107]
The Kyoto Protocol, which came into force in 2005, sets legally binding emission limitations for most developed countries.^[97] Developing countries are not subject to limitations. This exemption led the U.S. and Australia to decide not to ratify the treaty.^[108]^[109] At the time, almost all world leaders expressed their disappointment over President Bush's decision.^[109] Australia has since ratified the Kyoto protocol.^[110]

Public opinion

In 2007–2008 Gallup Polls surveyed 127 countries. Over a third of the world's population was unaware of global warming, with people in developing countries less aware than those in developed, and those in Africa the least aware. Of those aware, Latin America leads in belief that temperature changes are a result of human activities while Africa, parts of Asia and the Middle East, and a few countries from the Former Soviet Union lead in the opposite belief.^[111] In the Western world, opinions over the concept and the appropriate responses are divided. Nick Pidgeon of Cardiff University finds that "results show the different stages of engagement^{[clarification needed]} about global warming on each side of the Atlantic"; where Europe debates the appropriate responses while the United States debates whether climate change is happening.^[112]^[113]^[vague]^{[dubious – discuss]}

Other views

Most scientists accept that humans are contributing to observed climate change.^[44]^[114] National science academies have called on world leaders for policies to cut global emissions.^[115] There are, however, some scientists and non-scientists who question aspects of climate change science.^[116]^[117]
Organizations such as the libertarian Competitive Enterprise Institute, conservative commentators, and companies such as ExxonMobil 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.^[118]^[119]^[120]^[121] Environmental organizations and public figures have emphasized changes in the current climate and the risks they entail, while promoting adaptation to changes in infrastructural needs and emissions reductions.^[122] Some fossil fuel companies have scaled back their efforts in recent years,^[123] or called for policies to reduce global warming.^[124]

Etymology

The term global warming was probably first used in its modern sense on 8 August 1975 in a science paper by Wally Broecker in the journal Science called "Are we on the brink of a pronounced global warming?".^[125]^[126]^[127] Broecker's choice of words was new and represented a significant recognition that the climate was warming, previously the phrasing used by scientists was "inadvertent climate modification," because while it was recognized humans could change the climate, no one was sure which direction it was going.^[128] The National Academy of Sciences first used global warming in a 1979 paper called the Charney Report, it said: "if carbon dioxide continues to increase, [we find] no reason to doubt that climate changes will result and no reason to believe that these changes will be negligible."^[129] The report made a distinction between referring to surface temperature changes as global warming, while referring to other changes caused by increased CO2 as climate change.^[128] This distinction is still often used in science reports, with global warming meaning surface temperatures, and climate change meaning other changes (increased storms, etc..)^[128]
Global warming became more widely popular after 1988 when NASA scientist James E. Hansen used the term in a testimony to Congress.^[128] 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."^[130] His testimony was widely reported and afterward global warming was commonly used by the press and in public discourse.^[128]

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