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global warming

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Dictionary: global warming
 
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n.

An increase in the average temperature of the earth's atmosphere, especially a sustained increase sufficient to cause climatic change.


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Geography Dictionary: global warming
 
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The increase in global temperatures brought about by the increased emission of greenhouse gases into the atmosphere. There is no doubt that concentrations of, for example, atmospheric carbon dioxide have risen since the 1950s; what is less certain is the extent to which this has altered the earth's climates, or the extent to which climates will change in the future. see greenhouse effect.

 
Political Dictionary: global warming
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A phenomenon (otherwise known as ‘climate change’ or ‘the greenhouse effect’) whereby solar radiation that has reflected back off the surface of the earth remains trapped at atmospheric levels, due to the build-up of CO2 and other greenhouse gases, rather than being emitted back into space. The effect of this is a warming of the global atmosphere.

Climate change is a long-standing phenomenon, as the mix of the various gases that make up the earth's atmosphere have changed over long periods of time, so average global temperatures have fluctuated. What is alleged to be different about the current spell of global warming is that it is taken to be (1) caused by human action and (2) occurring at an unprecedented rate. The consequences of global warming remain uncertain, but climate change models predict deforestation, desertification, a poleward shift of vegetation and animal populations, rising sea levels, and decreased precipitation.

Global warming has received increasing political attention over the past thirty years, having constituted one of the key themes in the rise of green politics over the same period. This increasing political salience resulted in an intergovernmental meeting in Kyoto in 1997, at which 38 industrialized countries signed up to the Kyoto Protocol. The terms of this agreement were that these nations would reduce their atmospheric emissions of CO2 by an average of 5.2 per cent from 1990 levels by 2012. This is well below the 60 per cent target that scientists working on climate change claim is necessary to present further global warming, but the agreement was seen by many campaigners as a useful first step that established the framework necessary for further cuts in the future. The Kyoto Protocol will not, however, become effective until it has been ratified by 55 per cent of the signatory nations, and only then if these nations contribute 55 per cent or more of global carbon emissions.

There have been three crucial intergovernmental meetings in the attempt to transform the original protocol into a ratified treaty with legal powers of enforcement. The first of these was at The Hague in November 2000. This meeting broke down over disagreements between the European Union (EU) and the United States—in particular over American proposals to count forests and other vegetation as ‘carbon sinks’, against which their fossil fuel emissions could be set. The EU feared that this would create significant loopholes in the agreement, as the carbon storage capacity of vegetation is uncertain, temporary, and unstable. Following the election of George W. Bush the United States unilaterally withdrew from the Kyoto Protocol, claiming that it would inflict disproportionate damage on the US economy. Given that the US produces 24 per cent of global CO2 emissions, its non-participation in any binding agreement remains a serious handicap.

Further climate change negotiations took place in Bonn in July 2001, involving 186 nations, where the Kyoto protocols were successfully translated into an international treaty. In order to achieve agreement the EU nations had to make concessions to Canada, Australia, Japan, and Russia over the extent to which forests could count as ‘carbon sinks’, and over the mechanisms by which any agreement could be enforced. By some estimates this cut the effective size of emission reductions from the proposed 5.2 per cent on 1990 levels to between 1.8 and 3 per cent.

— Mathew Humphrey

 
Britannica Concise Encyclopedia: global warming
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Increase in the global average surface temperature resulting from enhancement of the greenhouse effect, primarily by air pollution. In 2007 the UN Intergovernmental Panel on Climate Change forecasted that by 2100 global average surface temperatures would increase 3.2 – 7.2 °F (1.8 – 4.0 °C), depending on a range of scenarios for greenhouse gas emissions, and stated that it was now 90 percent certain that most of the warming observed over the previous half century could be attributed to greenhouse gas emissions produced by human activities (i.e., industrial processes and transportation). Many scientists predict that such an increase in temperature would cause polar ice caps and mountain glaciers to melt rapidly, significantly raising the levels of coastal waters, and would produce new patterns and extremes of drought and rainfall, seriously disrupting food production in certain regions. Other scientists maintain that such predictions are overstated. The 1992 Earth Summit and the 1997 Kyoto Protocol to the United Nations Framework Convention on Climate Change attempted to address the issue of global warming, but in both cases the efforts were hindered by conflicting national economic agendas and disputes between developed and developing nations over the cost and consequences of reducing emissions of greenhouse gases.

For more information on global warming, visit Britannica.com.

 
US History Encyclopedia: Global Warming
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Gases created through human industrial and agricultural practices (primarily carbon dioxide from burning fossil fuels and wood, as well as methane, nitrous oxide, and chlorofluorocarbons) increase the heat-reflecting potential of the atmosphere, thereby raising the planet's average temperature.

Early Scientific Work

Since the late nineteenth century, atmospheric scientists in the United States and overseas have known that significant changes in the chemical composition of atmospheric gases might cause climate change on a global scale. In 1824, the French scientist Jean-Baptiste Fourier described how the earth's atmosphere functioned like the glass of a greenhouse, trapping heat and maintaining the stable climate that sustained life. By the 1890s, some scientists, including the Swedish chemist Svante Arrhenius and the American geologist Thomas Chamberlain, had discerned that carbon dioxide had played a central role historically in regulating global temperatures.

In 1896, Arrhenius provided the first quantitative analysis of how changes in atmospheric carbon dioxide could alter surface temperatures and ultimately lead to climatic change on a scale comparable with the ice ages. In 1899, Chamberlain similarly linked glacial periods to changes in atmospheric carbon dioxide and posited that water vapor might provide crucial positive feedback to changes in carbon dioxide. In the first decade of the twentieth century, Arrhenius further noted that industrial combustion of coal and other fossil fuels could introduce enough carbon dioxide into the atmosphere to change the temperature of the planet over the course of a few centuries. However, he predicted that warming would be delayed because the oceans would absorb most of the carbon dioxide. Arrhenius further posited various societal benefits from this planetary warming.

Developing Scientific Consensus

Over the course of the twentieth century, scientists con-firmed these early predictions as they probed further into the functioning of the earth's atmospheric system. Early in the century, dozens of scientists around the world contributed to an internationally burgeoning understanding of atmospheric science. By the century's close, thousands of scientists collaborated to refine global models of climate change and regional analyses of how rising temperatures might alter weather patterns, ecosystem dynamics, agriculture, oceans and ice cover, and human health and disease.

While no one scientific breakthrough revolutionized climate change science or popular understanding of the phenomenon, several key events stand out to chart developing scientific understanding of global warming. In 1938, Guy S. Callendar provided an early calculation of warming due to human-introduced carbon dioxide and contended that this warming was evident already in the temperature record. Obscured by the onset of World War II and by a short-term cooling trend that began in the 1940s, Callendar's analysis received short shrift. Interest in global warming increased in the 1950s with new techniques for studying climate, including analysis of ancient pollens, ocean shells, and new computer models. Using computer models, in 1956, Gilbert N. Plass attracted greater attention to the carbon dioxide theory of climate change. The following year, Roger Revelle and Hans Suess showed that oceanic absorption of atmospheric carbon dioxide would not be sufficient to delay global warming. They stressed the magnitude of the phenomenon:

Human beings are now carrying out a large scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future. Within a few centuries we are returning to the atmosphere and oceans the concentrated organic carbon stored in sedimentary rocks over hundreds of millions of years. (Cristianson, Greenhouse, pp. 155–156)

At the same time, Charles Keeling began to measure the precise year-by-year rise in atmospheric carbon dioxide from the Mauna Loa Observatory in Hawaii. In 1965, the President's Scientific Advisory Committee issued the first U.S. government report that summarized recent climate research and outlined potential future changes resulting from increased atmospheric carbon dioxide, including the melting of the Antarctic ice cap, the rise of sea level, and the warming of oceans.

By the late 1970s, atmospheric scientists had grown increasingly confident that the buildup of carbon dioxide, methane, chlorofluorocarbons, and related gases in the atmosphere would have a significant, lasting impact on global climate. Several jointly written government reports issued during President Jimmy Carter's administration presented early consensus estimates of global climate change. These estimates would prove consistent with more sophisticated models refined in the two decades following. A 1979 National Research Council report by Jule G. Charney, Carbon Dioxide and Climate: A Scientific Assessment, declared that "we now have incontrovertible evidence that the atmosphere is indeed changing and that we ourselves contribute to that change. Atmospheric concentrations of carbon dioxide are steadily increasing, and these changes are linked with man's use of fossil fuels and exploitation of the land" (p. vii). The Charney report estimated a doubling of atmospheric carbon dioxide concentrations would probably result in a roughly 3-degree Celsius rise in temperature, plus or minus 1.5 degrees.

Global Warming Politics

As climate science grew more conclusive, global warming became an increasingly challenging political problem. In January 1981, in the closing days of the Carter administration, the Council on Environmental Quality (CEQ) published Global Energy Futures and the Carbon Dioxide Problem. The CEQ report described climate change as the "ultimate environmental dilemma," which required collective judgments to be made, either by decision or default, "largely on the basis of scientific models that have severe limitations and that few can understand." The report reviewed available climate models and predicted that carbon dioxide–related global warming "should be observable now or sometime within the next two decades"

(p. v). With atmospheric carbon dioxide increasing rapidly, the CEQ report noted that the world was already "performing a great planetary experiment" (p. 52).

By the early 1980s, the scientific models of global warming had established the basic contours of this atmospheric phenomenon. Federal environmental agencies and scientific advisory boards had urged action to curb carbon dioxide emissions dramatically, yet little state, federal, or international policymaking ensued. Decades-old federal and state subsidies for fossil fuel production and consumption remained firmly in place. The federal government lessened its active public support for energy efficiency initiatives and alternative energy development. Falling oil and natural gas prices throughout the decade further undermined political support for a national energy policy that would address the problem of global warming.

A complicated intersection of climate science and policy further hindered effective lawmaking. Scientists urged political action, but spoke in a measured language that emphasized probability and uncertainty. Many scientists resisted entering the political arena, and expressed skepticism about their colleagues who did. This skepticism came to a head in reaction to the government scientist James Hansen's efforts to focus national attention on global warming during the drought-filled summer of 1988. As more than 400,000 acres of Yellowstone National Park burned in a raging fire, Hansen testified to Congress that he was 99 percent certain that the earth was getting warmer because of the greenhouse effect. While the testimony brought significant new political attention in the United States to the global warming problem, many of Hansen's scientific colleagues were dismayed by his definitive assertions. Meanwhile, a small number of skeptical scientists who emphasized the un-certainty of global warming and the need to delay policy initiatives fueled opposition to political action.

In 1988, delegates from nearly fifty nations met in Toronto and Geneva to address the climate change problem. The delegates formed the Intergovernmental Panel on Climate Change (IPCC), consisting of more than two thousand scientists from around the world, to assess systematically global warming science and policy options. The IPCC issued its first report in 1990, followed by second and third assessments in 1995 and 2001. Each IPCC report provided increasingly precise predictions of future warming and the regional impacts of climate change. Meanwhile, books like Bill McKibben's The End of Nature (1989) and Senator Albert Gore Jr.'s Earth in the Balance (1992) focused popular attention in the United States on global warming.

Yet these developments did not prompt U.S. government action. With its major industries highly dependent on fossil fuel consumption, the United States instead helped block steps to combat climate change at several international conferences in the late 1980s and 1990s. At the United Nations Conference on Environment and Development in Rio de Janeiro in 1992, U.S. negotiators successfully thwarted a treaty with mandatory limits on greenhouse gas emissions. As a result, the Rio conference adopted only voluntary limits. In 1993, the new administration of Bill Clinton and Albert Gore Jr. committed itself to returning United States emissions to 1990 levels by the year 2000. The administration also attempted to adjust incentives for energy consumption in its 1993 energy tax bill. Defeated on the tax bill and cowed when Republicans gained control of Congress in 1994, however, the Clinton administration backed away from significant new energy and climate initiatives.

At the highly charged 1997 United Nations Conference on Climate Change in Kyoto, Japan, more than 160 countries approved a protocol that would reduce emissions of carbon dioxide, methane, nitrous oxide, and three chlorofluorocarbon substitutes. In the United States, powerful industry opponents to the Kyoto Protocol, represented by the Global Climate Coalition (an industry association including Exxon, Mobil, Shell Oil, Ford, and General Motors, as well as other automobile, mining, steel, and chemical companies), denounced the protocol's "unrealistic targets and timetables" and argued instead for voluntary action and further research. Along with other opponents, the coalition spent millions of dollars on television ads criticizing the agreement, focusing on possible emissions exemptions for developing nations. Although the Clinton administration signed the Kyoto Protocol, strong Senate opposition to the agreement prevented ratification. In 2001, President George W. Bush withdrew his executive support for the protocol.

Growing Signals of Global Warming

By the end of the 1990s, climate science had grown increasingly precise and achieved virtual worldwide scientific consensus on climate change. The 2001 report of the Intergovernmental Panel on Climate Change concluded that global average surface temperature had increased by 0.6 degrees Celsius during the twentieth century, largely due to greenhouse gas emissions. Carbon dioxide concentrations in the atmosphere had increased by approximately 30 percent since the late nineteenth century, rising from 280 parts per million (ppm) by volume to 367 ppm in 1998.

By 2001, signs of global warming were increasingly widespread. With glaciers around the world melting, average sea levels rising, and average precipitation increasing, the 1990s registered as the hottest decade on record in the past thousand years. Regional models predicted widespread shifting of ecosystems in the United States, with alpine ecosystems expected largely to disappear in the lower forty-eight states while savannas or grasslands replace desert ecosystems in the Southwest. The IPCC 2001 report estimated an increase of between 1.4 and 5.8 degrees Celsius by 2100, a projected increase in global temperature very likely "without precedent during at least the last 10,000 years."

Bibliography

Christianson, Gale E. Greenhouse: The 200-Year Story of Global Warming. New York: Walker, 1999.

Council on Environmental Quality. Global Energy Futures and the Carbon Dioxide Problem. Washington, D.C.: Government Printing Office, 1981.

Handel, Mark David, and James S. Risbey. An Annotated Bibliography on Greenhouse Effect Change. Cambridge, Mass.: Massachusetts Institute of Technology, Center for Global Change Science, 1992.

Intergovernmental Panel on Climate Change. Climate Change 2001: Impacts, Adaptations, and Vulnerability. Edited by James J. McCarthy et al. Cambridge, U.K.: Cambridge University Press, 2001.

———. Climate Change 2001: Mitigation. Edited by Bert Metz et al. Cambridge, U.K.: Cambridge University Press, 2001.

———. Climate Change 2001: The Scientific Basis. Edited by J. T. Houghton et al. Cambridge, U.K.: Cambridge University Press, 2001.

McKibben, Bill. The End of Nature. 10th anniv. ed. New York: Anchor, 1999.

National Research Council. Carbon Dioxide and Climate: A Scientific Assessment. Washington, D.C.: National Academy of Sciences, 1979.

—Paul Sabin

 
Columbia Encyclopedia: global warming
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global warming, the gradual increase of the temperature of the earth's lower atmosphere as a result of the increase in greenhouse gases since the Industrial Revolution.

The temperature of the atmosphere near the earth's surface is warmed through a natural process called the greenhouse effect. Visible, shortwave light comes from the sun to the earth, passing unimpeded through a blanket of thermal, or greenhouse, gases composed largely of water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Infrared radiation reflects off the planet's surface toward space but does not easily pass through the thermal blanket. Some of it is trapped and reflected downward, keeping the planet at an average temperature suitable to life, about 60°F (16°C).

Growth in industry, agriculture, and transportation since the Industrial Revolution has produced additional quantities of the natural greenhouse gases plus chlorofluorocarbons and other gases, augmenting the thermal blanket. It is generally accepted that this increase in the quantity of greenhouse gases is trapping more heat and increasing global temperatures, making a process that has been beneficial to life potentially disruptive and harmful. During the 20th cent., the atmospheric temperature rose 1.1°F (0.6°C), and sea level rose several inches. Some projected, longer-term results of global warming include melting of polar ice, with a resulting rise in sea level and coastal flooding; disruption of drinking water supplies dependent on snow melts; profound changes in agriculture due to climate change; extinction of species as ecological niches disappear; more frequent tropical storms; and an increased incidence of tropical diseases.

Among factors that may be contributing to global warming are the burning of coal and petroleum products (sources of carbon dioxide, methane, nitrous oxide, ozone); deforestation, which increases the amount of carbon dioxide in the atmosphere; methane gas released in animal waste; and increased cattle production, which contributes to deforestation, methane production, and use of fossil fuels.

Much of the debate surrounding global warming has centered on the accuracy of scientific predictions concerning future warming. To predict global climatic trends, climatologists accumulate large historical databases and use them to create computerized models that simulate the earth's climate. The validity of these models has been a subject of controversy. Skeptics say that the climate is too complicated to be accurately modeled, and that there are too many unknowns. Some also question whether the observed climate changes might simply represent normal fluctuations in global temperature. Nonetheless, for some time there has been general agreement that at least part of the observed warming is the result of human activity, and that the problem needs to be addressed. In 1992, at the United Nations Conference on Environment and Development, over 150 nations signed a binding declaration on the need to reduce global warming.

In 1994, however, a UN scientific advisory panel, the Intergovernmental Panel on Climate Change, concluded that reductions beyond those envisioned by the treaty would be needed to avoid global warming. The following year, the advisory panel forecast a rise in global temperature of from 1.44 to 6.3°F (0.8–3.5°C) by 2100 if no action is taken to cut down on the production of greenhouse gases, and a rise of from 1 to 3.6°F (0.5–2°C) even if action is taken (because of already released gases that will persist in the atmosphere). A 2007 report by the Intergovernmental Panel on Climate Change, based on a three-year study, termed global warming “unequivocal” and said that most of the change was most likely due to human activities.

A UN Conference on Climate Change, held in Kyoto, Japan, in 1997 resulted in an international agreement to fight global warming, which called for reductions in emissions of greenhouse gases by industrialized nations. Not all industrial countries, however, immediately signed or ratified the accord. In 2001 the G. W. Bush administration announced it would abandon the Kyoto Protocol; because the United States produces about one quarter of the world's greenhouse gases, this was regarded as a severe blow to the effort to slow global warming. Despite the American move, most other nations agreed later in the year (in Bonn, Germany, and in Marrakech, Morocco) on the details necessary to convert the agreement into a binding international treaty, which came into force in 2005 after ratification by more than 125 nations.

Improved automobile mileage, reforestation projects, energy efficiency in construction, and national support for mass transit are among relatively simpler adjustments that could significantly lower U.S. production of greenhouse gases. More aggressive adjustments include a gradual worldwide shift away from the use of fossil fuels, the elimination of chlorofluorocarbons, and the slowing of deforestation by restructuring the economies of developing nations. In 2002 the Bush administration proposed several voluntary measures for slowing the increase in, instead of reducing, emissions of greenhouses gases. The United States, Australia, China, India, Japan, and South Korea established (2005) an agreement outside the Kyoto Protocal that proposed to reduce emissions through the development and implementation of new technologies. The Asia-Pacific Partnership on Clean Development and Climate, as it is called, involves no commitments on the part of its members; it held its first meeting in 2006. Also in 2006, California enacted legislation that called for cutting carbon dioxide emissions by 25% by 2020; the state is responsible for nearly 7% of all such emissions in the United States. In 2007 President George W. Bush called for the world's major polluting nations to set global and national goals for the reduction of greenhouse gas emissions, but the nonbinding nature of the proposed goals provoked skepticism from nations that favored stronger measures.

Bibliography

See P. Brown, Global Warming: Can Civilization Survive? (1997); T. G. Moore, Climate of Fear: Why We Shouldn't Worry about Global Warming (1998); S. G. Philander, Is the Temperature Rising?: The Uncertain Science of Global Warming (1998); K. E. Ready, GAIA Weeps: The Crisis of Global Warming (1998); G. E. Christianson, Greenhouse: The 200-Year Story of Global Warming (1999); T. Flannery, The Weather Makers: How Man Is Changing the Climate and What It Means for Life on Earth (2006); E. Kolbert, Field Notes from a Catastrophe (2006); E. Linden, The Winds of Change (2006).

 
Science Dictionary: global warming
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The term attached to the notion that the Earth's temperature is increasing due to the greenhouse effect.

  • Whether global warming is actually happening is a subject of scientific debate.

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    Wikipedia: Global warming
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    For past climate change, see paleoclimatology and geologic temperature record.
    Global mean surface temperature anomaly relative to 1961–1990
    Global mean surface temperature anomaly relative to 1961–1990
     
    Mean surface temperature anomalies during the period 1999 to 2008 with respect to the average temperatures from 1940 to 1980
    Mean surface temperature anomalies during the period 1999 to 2008 with respect to the average temperatures from 1940 to 1980

    Global warming is the increase in the average temperature of the Earth's near-surface air and oceans since the mid-20th century and its projected continuation. Global surface temperature increased 0.74 ± 0.18 °C (1.33 ± 0.32 °F)during the last century.[1][A] The Intergovernmental Panel on Climate Change (IPCC) concludes that increasing greenhouse gas concentrations resulting from human activity such as fossil fuel burning and deforestation are responsible for most of the observed temperature increase since the middle of the 20th century.[1] The IPCC also concludes that natural phenomena such as solar variation and volcanoes produced most of the warming from pre-industrial times to 1950 and had a small cooling effect afterward.[2][3] These basic conclusions have been endorsed by more than 45 scientific societies and academies of science,[B] including all of the national academies of science of the major industrialized countries.[4]

    Climate model projections summarized in the latest IPCC report indicate that the global surface temperature will probably rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the twenty-first century.[1] The uncertainty in this estimate arises from the use of models with differing sensitivity to greenhouse gas concentrations and the use of differing estimates of future greenhouse gas emissions. Some other uncertainties include how warming and related changes will vary from region to region around the globe. Most studies focus on the period up to the year 2100. However, warming is expected to continue beyond 2100 even if emissions stop, because of the large heat capacity of the oceans and the long lifetime of carbon dioxide in the atmosphere.[5][6]

    Increasing global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, probably including expansion of subtropical deserts.[7] The continuing retreat of glaciers, permafrost and sea ice is expected, with the Arctic region being particularly affected. Other likely effects include shrinkage of the Amazon rainforest and Boreal forests, increases in the intensity of extreme weather events, species extinctions, and changes in agricultural yields.

    Political and public debate continues regarding climate change, and what actions (if any) to take in response to global warming. The available options are mitigation to reduce further emissions; adaptation to reduce the damage caused by warming; and, more speculatively, geoengineering to reverse global warming. Most national governments have signed and ratified the Kyoto Protocol aimed at reducing greenhouse gas emissions.

    Contents

    Temperature changes

    Main article: Temperature record
    Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference.

    The most commonly cited indication 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°C ±0.18°C over the period 1906-2005. The rate of warming over the last 50 years of that period was almost double that for the period as a whole (0.13°C ±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.[8] Temperatures in the lower troposphere have increased between 0.12 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 or the Little Ice Age.

    Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[9] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[10][11] Temperatures in 1998 were unusually warm because the strongest El Niño in the past century occurred during that year.[12]

    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).[13] 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.[14] 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 the 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.[15]

    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.[16]

    Radiative forcing

    Main article: Radiative forcing

    External forcing is a term used in climate science for processes external to the climate system (though not necessarily external to Earth). Climate responds to several types of external forcing, such as changes in greenhouse gas concentrations, changes in solar luminosity, volcanic eruptions, and variations in Earth's orbit around the Sun.[2] 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 gas and Greenhouse effect
    Recent increases in atmospheric carbon dioxide (CO2). Monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere.

    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 discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896.[17] Existence of the greenhouse effect as such is not disputed, even by those who do not agree that the recent temperature increase is attributable to human activity. The question is instead how the strength of the 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).[18][C] The major greenhouse gases are water vapor (not including clouds), 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[not in citation given]; and ozone (O3), which causes 3–7 percent.[19][20]

    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 the mid-1700s.[21] These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores.[22] Less direct geological evidence indicates that CO2 values this high were last seen approximately 20 million years ago.[23] 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.[24]

    CO2 concentrations are continuing to rise due to burning of fossil fuels and land-use change. The future rate of rise will depend on uncertain economic, sociological, technological, and natural developments. Accordingly, the IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[25] Fossil fuel reserves are sufficient to reach these levels and continue emissions past 2100 if coal, tar sands or methane clathrates are extensively exploited.[26]

    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, but substantial ozone depletion did not occur until the late 1970s.[27] Tropospheric ozone contributes to surface warming.[28]

    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.[29] The main cause of this dimming is aerosols produced by volcanic activity and emissions of pollutants such as sulfur dioxide. These aerosols exert a cooling effect by increasing the reflection of incoming sunlight. James Hansen and colleagues have proposed that 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 driven mainly by non-CO2 greenhouse gases.[30]

    In addition to their direct effect by scattering and absorbing solar radiation, aerosols have indirect effects on the radiation budget.[31] 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.[32] This effect also causes droplets to be of more uniform size, which reduces growth of raindrops by collision-coalescence. Clouds modified by pollution have been shown to produce less drizzle, making the cloud brighter and more reflective to incoming sunlight, especially in the near-infrared part of the spectrum.[33]

    Soot may cool or warm, depending on whether it is airborne or deposited. Atmospheric soot aerosols directly absorb solar radiation, which heats the atmosphere and cools the surface. Regionally (but not globally), as much as 50% of surface warming due to greenhouse gases may be masked by atmospheric brown clouds.[34] When deposited, especially on glaciers or on ice in arctic regions, the lower surface albedo can also directly heat the surface.[35] 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.[36]

    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.[37] Although solar forcing is generally thought to be too small to account for a significant part of global warming in recent decades,[38][39] a few studies disagree, such as a recent phenomenological analysis that indicates the contribution of solar forcing may be underestimated.[40]

    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.[2] Observations show that temperatures in the stratosphere have been steady or 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.[41]

    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.[42] Other research has found no relation between warming in recent decades and cosmic rays.[43][44] 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.[45]

    Feedback

    Main article: Effects of global warming

    A positive feedback is a process that amplifies some change. Thus, when a warming trend results in effects that induce further warming, the result is a positive feedback; when the warming results in effects that reduce the original warming, the result is a negative feedback. The main positive feedback in global warming involves the tendency of warming to increase the amount of water vapor in the atmosphere. The main negative feedback in global warming is the effect of temperature on emission of infrared radiation: as the temperature of a body increases, the emitted radiation increases with the fourth power of its absolute temperature.

    Water vapor feedback 
    If the atmosphere is warmed, the saturation vapor pressure increases, and the amount of water vapor in the atmosphere will tend to increase. Since water vapor is a greenhouse gas, the increase in water vapor content makes the atmosphere warm further; this warming causes the atmosphere to hold still more water vapor (a positive feedback), and so on until other processes stop the feedback loop. The result is a much larger greenhouse effect than that due to CO2 alone. Although this feedback process causes an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[46]
    Cloud feedback 
    Warming is expected to change the distribution and type of clouds. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud, details that are difficult to represent in climate models.[46]
    Lapse rate 
    The atmosphere's temperature decreases with height in the troposphere. Since emission of infrared radiation varies with the fourth power of temperature, longwave radiation escaping to space from the relatively cold upper atmosphere is less than that emitted toward the ground from the lower atmosphere. Thus, the strength of the greenhouse effect depends on the atmosphere's rate of temperature decrease with height. Both theory and climate models indicate that global warming will reduce the rate of temperature decrease with height, producing a negative lapse rate feedback that weakens the greenhouse effect. Measurements of the rate of temperature change with height are very sensitive to small errors in observations, making it difficult to establish whether the models agree with observations.[47]
    Ice-albedo feedback 
    Aerial photograph showing a section of sea ice. The lighter blue areas are melt ponds and the darkest areas are open water, both have a lower albedo than the white sea ice. The melting ice contributes to the ice-albedo feedback.
    When ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.[48]
    Arctic methane release 
    Warming is also the triggering variable for the release of methane from sources both on land and on the deep ocean floor, making both of these possible feedback effects. Thawing permafrost, such as the frozen peat bogs in Siberia, creates a positive feedback due to the release of CO2 and methane.[49]
    Reduced absorption of CO2 by the oceans 
    Ocean ecosystems' ability to sequester carbon is expected to decline as the oceans warm. This is because warming reduces the nutrient levels of the mesopelagic zone (about 200 to 1000 m deep), which limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[50]

    Climate models

    Main article: Global climate model
    Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions and regionally divided economic development.
    Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions and regionally divided economic development.
     
    The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).
    The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

    The main tools for projecting future climate changes are computer models of the climate. These models are based on physical principles including fluid dynamics 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 include an atmospheric model that is coupled to an ocean model and models for ice cover on land and sea. Some models also include treatments of chemical and biological processes.[51] These models project a warmer climate due to increasing levels of greenhouse gases.[52] 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.[53]

    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.[54][55][56]

    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.[1] A 2008 paper predicts that the global temperature may not increase during the next decade because short-term natural fluctuations may temporarily outweigh greenhouse gas-induced warming.[57]

    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 1975 is dominated by man-made greenhouse gas emissions.

    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.[24] The physical realism of models is tested by examining their ability to simulate current or past climates.[58] While a 2007 study by David Douglass and colleagues found that the models did not accurately predict observed changes in the tropical troposphere,[59] a 2008 paper published by a 17-member team led by Ben Santer noted errors in the Douglass study, and found instead that the models and observations were not statistically different.[60] 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.[61]

    Attributed and expected effects

    Environmental

    Main article: Effects of global warming
    See also: Ocean acidification and Regional effects of global warming
    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.

    It usually is impossible to connect specific weather events to global warming. Instead, global warming is expected to cause changes in the overall distribution and intensity of events, such as changes to the frequency and intensity of heavy precipitation. Broader effects are expected to include glacial retreat, Arctic shrinkage, and worldwide sea level rise. Other effects may include changes in crop yields, addition of new trade routes,[62] species extinctions,[63] and changes in the range of disease vectors.

    Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events are attributable in part to global warming.[64] Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.[65]

    Social and economic effects of global warming may be exacerbated by growing population densities in affected areas. Temperate regions are projected to experience some benefits, such as fewer cold-related deaths.[66] A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[64] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature (see Atlantic Multidecadal Oscillation), but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.[1]

    Additional anticipated effects include sea level rise of 0.18 to 0.59 meters (0.59 to 1.9 ft) in 2090-2100 relative to 1980-1999, [1] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increasingly intense (but less frequent)[67] hurricanes and extreme weather events, lowering of ocean pH, oxygen depletion in the oceans,[68] and the spread of diseases such as malaria and dengue fever,[69][70] as well as Lyme disease, hantavirus infections, bubonic plague, and cholera.[71] One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections.[72] However, few mechanistic studies have documented extinctions due to recent climate change[73] and one study suggests that projected rates of extinction are uncertain.[74]

    Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[75] CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in ocean acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,[76] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][77] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.[78] Heat and carbon dioxide trapped in the oceans may still take hundreds years to be re-emitted, even after greenhouse gas emissions are eventually reduced.[6]

    Economic

    Main articles: Economics of global warming and Low-carbon economy
    The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4.

    Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far yielded no conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide).[79]

    One widely publicized report on potential economic impact is the Stern Review. It suggests that extreme weather might reduce global gross domestic product by up to one percent, and that in a worst-case scenario global per capita consumption could fall 20 percent.[80] The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios.[81] Others have supported the general attempt to quantify economic risk, even if not the specific numbers.[82][83]

    Preliminary studies suggest that costs and benefits of mitigating global warming are broadly comparable in magnitude.[84]

    According to United Nations Environment Programme (UNEP), economic sectors likely to face difficulties related to climate change include banks, agriculture, transport and others.[85] Developing countries dependent upon agriculture will be particularly harmed by global warming.[86]

    Responses to global warming

    The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement responses. These responses to global warming can be divided into mitigation of the causes and effects of global warming, adaptation to the changing global environment, and geoengineering to reverse global warming.

    Mitigation

    Main article: Mitigation of global warming

    Mitigation of global warming is accomplished through reductions in the rate of anthropogenic greenhouse gas release. Models suggest that mitigation can quickly begin to slow global warming, but that temperatures will appreciably decrease only after several centuries.[87] The world's primary international agreement on reducing greenhouse gas emissions is the Kyoto Protocol, an amendment to the UNFCCC negotiated in 1997. The Protocol now covers more than 160 countries and over 55 percent of global greenhouse gas emissions.[88] As of June 2009, only the United States, historically the world's largest emitter of greenhouse gases, has refused to ratify the treaty. The treaty expires in 2012. International talks began in May 2007 on a future treaty to succeed the current one.[89] UN negotiations are now gathering pace in advance of a meeting in Copenhagen in December 2009.[90]

    Many environmental groups encourage individual action against global warming, as well as community and regional actions. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions.[91][92]

    There has also been business action on climate change, including efforts to improve energy efficiency and limited moves towards use of alternative fuels. In January 2005 the European Union introduced its European Union Emission Trading Scheme, through which companies in conjunction with government agree to cap their emissions or to purchase credits from those below their allowances. Australia announced its Carbon Pollution Reduction Scheme in 2008. United States President Barack Obama has announced plans to introduce an economy-wide cap and trade scheme.[93]

    The IPCC's Working Group III is responsible for crafting reports on mitigation of global warming and the costs and benefits of different approaches. The 2007 IPCC Fourth Assessment Report concludes that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6 percent increase and three percent decrease in global gross domestic product.[79]

    Adaptation

    Main article: Adaptation to global warming

    A wide variety of measures have been suggested for adaptation to global warming. These range from the trivial, such as the installation of air-conditioning equipment, up to major infrastructure projects, such as abandonment of settlements threatened by sea level rise. Measures including water conservation,[94] water rationing, adaptive agricultural practices,[95] construction of flood defences,[96] changes to medical care,[97] and interventions to protect threatened species[98] have all been suggested. A wide-ranging study of the possible opportunities for adaptation of infrastructure has been published by the Institute of Mechanical Engineers.[99]

    Geoengineering

    Main article: Geoengineering

    Geoengineering is the deliberate modification of Earth's natural environment on a large scale to suit human needs.[100] An example is greenhouse gas remediation, which removes greenhouse gases from the atmosphere, usually through carbon sequestration techniques such as carbon dioxide air capture.[101] Solar radiation management reduces insolation, such as by the addition of stratospheric sulfur aerosols.[102] No large-scale geoengineering projects have yet been undertaken.

    Economic and political debate

    Main articles: Global warming controversy, Politics of global warming, and Economics of global warming
    See also: Scientific opinion on climate change, Climate change denial, List of countries by greenhouse gas emissions per capita, List of countries by carbon dioxide emissions per capita, List of countries by carbon dioxide emissions, and List of countries by ratio of GDP to carbon dioxide emissions
    Per capita greenhouse gas emissions in 2000, including land-use change.
    Per capita greenhouse gas emissions in 2000, including land-use change.
     
    Per country greenhouse gas emissions in 2000, including land-use change.
    Per country greenhouse gas emissions in 2000, including land-use change.

    Increased publicity of the scientific findings surrounding global warming has resulted in political and economic debate.[103] Poor regions, particularly Africa, appear at greatest risk from the projected effects of global warming, while their emissions have been small compared to the developed world.[104] At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and used as part of a rationale for continued non-ratification by the U.S.[105]

    In the Western world, the idea of human influence on climate has gained wider public acceptance in Europe than in the United States.[106][107] One 2006 poll of Americans by the Pew Research Center reported that 41% of respondents believed in anthropogenic global warming, 21% stated that they believed global warming was caused by "natural patterns in the earth's environment", and 20% did not believe that there was sufficient evidence for anthropogenic global warming. In the poll, Republicans were more likely than Democrats to disagree with the anthropogenic global warming model.[107]

    The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail.[79] Using economic incentives, alternative and renewable energy have been promoted to reduce emissions while building infrastructure.[108][109] Business-centered organizations, conservative commentators, and companies such as the Competitive Enterprise Institute and ExxonMobil have downplayed IPCC climate change scenarios, funded scientists who disagree with the scientific consensus, and provided their own projections of the economic cost of stricter controls.[110][111][112][113] 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.[114] Some fossil fuel companies have scaled back their efforts in recent years,[115] or called for policies to reduce global warming.[116]

    Another point of contention is the degree to which emerging economies such as India and China should be expected to constrain their emissions.[117] According to recent reports, China's gross national CO2 emissions may now exceed those of the U.S.[118][119][120] China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.[121] India, also exempt from Kyoto restrictions and another source of emissions, has made similar assertions.[122] The U.S. contends that if it must bear the cost of reducing emissions, then China should do the same.[123][124]

    See also

    Global warming portal

    Notes

    1. ^ Increase is for years 1905 to 2005. Global surface temperature is defined in the IPCC Fourth Assessment Report as the average of near-surface air temperature over land and sea surface temperature. These error bounds are constructed with a 90% uncertainty interval.
    2. ^ The 2001 joint statement was signed by the scientific academies of Australia, Belgium, Brazil, Canada, the Caribbean, China, France, Germany, India, Indonesia, Ireland, Italy, Malaysia, New Zealand, Sweden, and the UK. The 2005 statement added Japan, Russia, and the U.S. The 2007 statement added Mexico and South Africa. Polish Academy of Sciences has issued the separate statement. Professional societies include American Association for the Advancement of Science, American Astronomical Society, American Chemical Society, American Geophysical Union, American Institute of Physics, American Meteorological Society, American Physical Society, American Quaternary Association, Australian Meteorological and Oceanographic Society, Canadian Foundation for Climate and Atmospheric Sciences, Canadian Meteorological and Oceanographic Society, European Academy of Sciences and Arts, European Geosciences Union, European Science Foundation, Geological Society of America, Geological Society of London-Stratigraphy Commission, InterAcademy Council, International Council of Academies of Engineering and Technological Sciences, International Union of Geodesy and Geophysics, International Union for Quaternary Research, National Research Council, Network of African Science Academies, Royal Meteorological Society and World Meteorological Organization.
    3. ^ Note that the greenhouse effect produces an average worldwide temperature increase of about 33 °C (59 °F) compared to black body predictions without the greenhouse effect, not an average surface temperature of 33 °C (91 °F). The average worldwide surface temperature is about 14 °C (57 °F).

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