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Global Warming

Global Warming








Understanding the causes of and responses to global warming requires interdisciplinary cooperation between social and natural scientists. The theory behind global warming has been understood by climatologists since at least the 1980s, but only in the new millennium, with an apparent tipping point in 2005, has the mounting empirical evidence convinced most doubters, politicians, and the general public as well as growing sections of business that global warming caused by human action is occurring.


Global warming is understood to result from an overall, long-term increase in the retention of the suns heat around Earth due to blanketing by greenhouse gases, especially CO2 and methane. Emissions of CO2 have been rising at a speed unprecedented in human history, due to accelerating fossil fuel burning that began in the Industrial Revolution.

The effects of the resulting climate change are uneven and can even produce localized cooling (if warm currents change direction). The climate change may also initiate positive feedback in which the initial impact is further enhanced by its own effects, for example if melting ice reduces the reflective properties of white surfaces (the albedo effect) or if melting tundra releases frozen methane, leading to further warming. Debate continues about which manifestations are due to long-term climate change and which to normal climate variability.


Global warming involves an unprecedented speeding up of the rate of change in natural processes, which now converges with the (previously much faster) rate of change in human societies, leading to a crisis of adaptation. Most authoritative scientific bodies predict that on present trends a point of no return could come within ten years, and that the world needs to cut emissions by 50 percent by mid twenty-first century.

It was natural scientists who first discovered and raised global warming as a political problem. This makes many of the global warming concerns unique. Science becomes the author of issues that dominate the political agenda and become the sources of political conflict (Stehr 2001, p. 85). Perhaps for this reason, many social scientists, particularly sociologists, wary of trusting the truth claims of natural science but knowing themselves lacking the expertise to judge their validity, have avoided saying much about global warming and its possible consequences. Even sociologists such as Ulrich Beck and Anthony Giddens, who see risk as a key attribute of advanced modernity, have said little about climate change.

For practical purposes, it can no longer be assumed that nature is a stable, well understood, background constant and thus social scientists do not need direct knowledge about its changes. Any discussion of likely social, economic, and political futures will have to heed what natural scientists say about the likely impacts of climate change.


While originally eccentric, global warming was placed firmly on the agenda in 1985, at a conference in Austria of eighty-nine climate researchers participating as individuals from twenty-three countries. The researchers forecast substantial warming, unambiguously attributable to human activities.

Since that conference the researchers position has guided targeted empirical research, leading to supporting (and increasingly dire) evidence, resolving anomalies and winning near unanimous peer endorsement. Skeptics have been confounded and reduced to a handful, some discredited by revelations of dubious funding from fossil fuel industries.

Just before the end of the twentieth century, American researchers released ice-thickness data, gathered by nuclear submarines. The data showed that over the previous forty years the ice depth in all regions of the Arctic Ocean had declined by approximately 40 percent.

Five yearly aerial photographs show the ice cover on the Arctic Ocean at a record low, with a loss of 50 cubic kilometers annually and glacier retreat doubling to 12 kilometers a year. In September 2005 the National Aeronautics and Space Administration (NASA) doubled its estimates of the volume of melted fresh water flowing into the North Atlantic, reducing salinity and thus potentially threatening the conveyor that drives the Gulf Stream. Temperate mussels have been found in Arctic waters, and news broadcasts in 2005 and 2006 have repeatedly shown scenes of Inuit and polar bears (recently listed as endangered) cut off from their hunting grounds as the ice bridges melt.

In 2001 the Intergovernmental Panel on Climate Change (IPCC), the United Nations scientific panel on climate change, had predicted that Antarctica would not contribute significantly to sea level rise this century. The massive west Antarctic ice sheet was assumed to be stable. However, in June 2005 a British Antarctic survey reported measurements of the glaciers on this ice sheet shrinking. In October 2005 glaciologists reported that the edges of the Antarctic ice sheets were crumbling at an unprecedented rate and, in one area, glaciers were discharging ice three times faster than a decade earlier.

In 2005 an eight-year European study drilling Antarctic ice cores to measure the past composition of the atmosphere reported that CO2 levels were at least 30 percent higher than at any time in the last 65,000 years. The speed of the rise in CO2 was unprecedented, from 280 parts per million (ppm) before the Industrial Revolution to 388 ppm in 2006. Early in 2007 the Norwegian Polar Institute reported acceleration to a new level of 390 ppm. In January 2006 a British Antarctic survey, analyzing CO2 in crevasse ice in the Antarctic Peninsula, found levels of CO2 higher than at any time in the previous 800,000 years.

In April 2005 a NASA Goddard Institute oceanic study reported that the earth was holding on to more solar energy than it was emitting into space. The Institutes director said: This energy imbalance is the smoking gun that we have been looking for (Columbia 2005).

The second IPCC report in 1996 had predicted a maximum temperature rise of 3.5 degrees Fahrenheit by the end of the twenty-first century. The third report, in 2001, predicted a maximum rise of 5.8 degrees Fahrenheit by the end of the twenty-first century. In October 2006 Austrian glaciologists reported in Geophysical Research Letters (Kaser et al.) that almost all the worlds glaciers had been shrinking since the 1940s, and the shrinking rate had increased since 2001. None of the glaciers (contrary to skeptics) was growing. Melting glaciers could pose threats to the water supply of major South American cities and is already manifest in the appearance of many new lakes in Bhutan.

In January 2007 global average land and sea temperatures were the highest ever recorded for this month; in February 2007 the IPCC Fourth Report, expressing greater certainty and worse fears than the previous one, made headlines around the world. In 1995 few scientists believed the effects of global warming were already manifest, but by 2005 few scientists doubted it and in 2007 few politicians were willing to appear skeptical.

Although rising temperatures; melting tundra, ice and glaciers; droughts; extreme storms; stressed coral reefs; changing geographical range of plants, animals, and diseases; and sinking atolls may conceivably all be results of many temporary climate variations, their cumulative impact is hard to refute.


The science of global warming has progressed through tackling anomalies cited by skeptics. Critics of global warming made attempts to discredit the methodology of climatologist Michael Manns famous Hockey stick graph (first published in Nature in 1998). Manns graph showed average global temperatures over the last 1,000 years, with little variation for the first 900 and a sharp rise in the last century. After more than a dozen replication studies, some using different statistical techniques and different combinations of proxy records (indirect measures of past temperatures such as ice cores or tree rings), Manns results were vindicated. A report in 2006 by the U.S. National Academy of Sciences, National Research Council, supported much of Manns image of global warming history. There is sufficient evidence from the tree rings, boreholes, retreating glaciers and other proxies of past surface temperatures to say with a high level of confidence that the last few decades of the twentieth century were warmer than any comparable period for the last 400 years. For periods before 1600, the 2006 report found there was not enough reliable data to be sure but the committee found the Mann teams conclusion that warming in the last few decades of the twentieth century was unprecedented over the last 1,000 years to be plausible (National Academy of Science press release 2006).

Measurements from satellites and balloons in the lower troposphere have until recently indicated cooling, which contradicted measurements from the surface and the upper troposphere. In August 2005 a publication in Science of the findings of three independent studies described their measurements as nails in the coffin of the skeptics case. These showed that faulty data, which failed to allow for satellite drift, lay behind the apparent anomaly.

Another anomaly was that observed temperature rises were in fact less than the modelling of CO2 impacts predicted. This is now explained by evidence on the temporary masking properties of aerosols, from rising pollution and a cyclical upward swing of volcanic eruptions since 1960.

Critics of global warming have been disarmed and discredited. Media investigations and social research have increasingly highlighted the industry funding of skeptics and their think tanks, and the political pressures on government scientists to keep silent. Estimates of the catastrophic costs of action on emissions have also been contradicted most dramatically by the British Stern Report in October 2006. Many companies have been abandoning the skeptical business coalitions. The Australian Business Round Table on Climate Change estimated in 2005 that the cost to gross domestic product of strong early action would be minimal and would create jobs.


In May 2001 sixteen of the worlds national academies of science issued a statement, confirming that the IPCC should be seen as the worlds most reliable source of scientific information on climate change, endorsing its conclusions and stating that doubts about the conclusions were not justified.

In July 2005 the heads of eleven influential national science academies (from Brazil, Canada, China, France, Germany, India, Italy, Japan, Russia, the United Kingdom, and the United States) wrote to the G8 leaders warning that global climate change was a clear and increasing threat and that they must act immediately. They outlined strong and long-term evidence from direct measurements of rising surface air temperatures and subsurface ocean temperatures and from phenomena such as increases in average global sea levels, retreating glaciers and changes to many physical and biological systems (Joint Science Academies Statement 2005).

There are many unknowns regarding global warming, particularly those dependent on human choices; yet the consequences for society of either inadequate action or of any effective responses (through reduced consumption or enforced and subsidized technological change) will be huge. It is, for example, unlikely that the practices and values of free markets, individualism, diversity, and choice will not be significantly modified either by economic and political breakdowns or alternatively by the radical measures needed to preempt them.


Kyoto targets are at best a useful first step. However, even these targets, which seek to peg back emissions to 1990 levels by 2010, are unlikely to be met. World CO2 emissions in 2004 continued to rise in all regions of the world, by another 4.5 percent, to a level 26 percent higher than in 1990. A rise of over 2 degrees is considered inevitable if CO2 concentrations pass 400 ppm. At current growing emission rates, the concentration would reach 700 ppm by the end of the twenty-first century. The continuing industrialization of China, recently joined by India, points to the possibility of even faster rises than these projections indicate.

If unpredictable, amplifying feedback loops are triggered, improbable catastrophes become more likely. The Gulf Stream flow could be halted, freezing Britain and Northern Europe. Droughts could wipe out the agriculture of Africa and Australia, as well as Asia, where millions depend on Himalayan melt water and monsoon rains. If the ice caps melt completely over the next centuries, seas could rise by 7 meters, devastating all coastal cities. Will the human response to widespread ecological disasters give rise to solidarity and collective action, such as the aid that came after the 2004 Asian Tsunami or to social breakdowns, as seen in New Orleans after 2005s Hurricane Katrina and in the Rwandan genocide?

Social and technical changes with the scale and speed required are not unprecedented. The displacement of horsepower by automobiles, for example, was meteoric. Production of vehicles in the United States increased from 8,000 in 1900 to nearly a million by 1912. Substantial regulation or differential taxation and subsidies would be indispensable to overcome short term profit motives and free riding dilemmas (where some evade their share of the cost of collective goods from which they benefit). Gains in auto efficiency in the 1980s, for example, were rapidly reversed by a new fashion for sport utility vehicles.

The debates that have emerged in the early twenty-first century have been related to responses, with different winners and losers, costs, benefits, dangers, and time scales for each response. Advocates of reduced energy consumption or increased efficiency, or energy generation by solar, wind, tidal, hydro, biomass, geothermal, nuclear, or clean coal and geo-sequestration, argue often cacophonously. Yet it seems probable that all these options are needed.

It will be essential for social and natural scientists to learn to cooperate in understanding and preempting the potentially catastrophic collision of nature and society. In order to accomplish this, market mechanisms; technological innovation; international, national, and local regulations; and cultural change will all be needed. Agents of change include governments, nongovernmental organizations, and public opinion, but the most likely front-runner might be sectors of capital seeking profit by retooling the energy and transport systems, while able to mobilize political enforcement.

SEE ALSO Disaster Management; Greenhouse Effects; Science


Columbia University Earth Institute. 2005. Press release 28, April.

Cooper, Richard N., and Richard Layard. 2002. What the Future Holds: Insights from Social Science. Cambridge MA: MIT Press.

Diamond, Jared. 2005. Collapse: How Societies Choose to Fail or Survive. Camberwell, U.K.: Penguin, Allen Lane.

Dunlap, Riley H., Frederick H. Buttel, Peter H. Dickens, and August Gijswijt, eds. 2002, Sociological Theory and the Environment: Classical Foundations, Contemporary Insights, Lanham, MD: Rowman and Littlefield.

Flannery, Tim. 2006. The Weather Makers. Berkeley, CA: Grove Atlantic.

Kaser, G., et al. Mass Balance of Glaciers and Ice Caps: Consensus Estimates for 19612004. Geophysical Research Letters, Vol. 33. 2006.

Legget, Jeremy. 2000. The Carbon War: Global Warming and the End of the Oil Era. New York: Routledge.

Leggett, Jeremy. 2005. Half Gone: Oil, Gas, Hot Air and the Global Energy Crisis. London: Portobello.

Monbiot, George. 2006. Heat: How to Stop the Planet Burning. London: Allen Lane.

National Academy of Sciences. 2006. Press release 22, June.

Stehr, Nico. 2001. Economy and Ecology in an Era of Knowledge-Base Economies. Current Sociology 49(1) January: 6790.

Zillman, John W. 2005. Uncertainty in the Science of Climate Change. In Uncertainty and Climate Change: The Challenge for Policy, Policy Paper 3. Canberra: Academy of the Social Sciences in Australia.

Constance Lever-Tracy

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Global Warming


GLOBAL WARMING. 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."


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.


See alsoClimate ; Conservation .

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Global Warming

Global Warming

Global warming is the gradual rise of the earth's near-surface temperature over approximately the last hundred years. The best available scientific evidencebased on continuous satellite monitoring and data from about 2,000 meteorological stations around the worldindicates that globally averaged surface temperatures have warmed by about 0.3 to 0.6°C since the late nineteenth century. Different regions have warmedsome have even cooledby different amounts. Generally, the Northern Hemisphere has warmed to a greater extent than the Southern Hemisphere, and mid to high latitudes have generally warmed more than the tropics.

Since the advent of satellites, it has become possible for scientists to thoroughly monitor the earth's climate on a global scale. To examine the historical climate record, however, scientists have to use earlier, sparser forms of measurement, such as long-standing temperature records and less exact "proxy" data, such as the growth of coral, tree rings, as well as information from ice cores, which contain trapped gas bubbles and dust grains representative of the climate in which they were deposited. The bubbles in these cores contain oxygen, particularly oxygen isotopes 180 to 160, which are sensitive to variations in temperature. From the ratio between these isotopes at varying ice depths scientists can reconstruct a picture of the temperature variations over time in specific locations. Greater measurement uncertainty surrounds the earlier parts of this record because of sparse coverage (especially in ocean regions). Despite this uncertainty, the balance of scientific evidence confirms that there has been a discernable warming over the last century.


Gases such as water vapor, methane, and carbon dioxide allow short-wave radiation from the sun to pass through to the surface of the earth, but do not allow long-wave radiation reflected from the earth to travel back out into space. This naturally occurring insulation processdubbed the greenhouse

rank state use (million tons)
source: Adapted from U.S. Department of Energy. Electric Power Annual 2000 , vol. 1. Available from
1 texas 99.7
2 indiana 59.5
3 ohio 55.9
4 pennsylvania 52.1
5 illinois 46.6
6 kentucky 40.2
7 missouri 37.3
8 west virginia 37.0
9 alabama 35.6
10 michigan 33.7
11 georgia 33.5
12 north carolina 29.9
13 florida 29.9
14 wyoming 26.5
15 tennessee 26.1
16 north dakota 25.1
  other states 322.4
  total 990.966

effectkeeps the earth warm: In its absence, the earth would be about 33°C cooler than it is now. However, as the concentration of greenhouse gases increases (due largely to human activities), most scientists agree that the effect is expected to intensify, raising average global temperatures.

However, the earth's climate is known to vary on long timescales. The existence of naturally occurring ice ages and warm periods in the distant past demonstrates that natural factors such as solar variability, volcanic activity, and fluctuations in greenhouse gases play important roles in regulating the earth's climate. A minority of scientists believe that purely natural variations in these factors can account for the observed global warming.

Climate in the Twenty-first Century

Climate forecasts are inherently imprecise largely because of two different sorts of uncertainty: incomplete knowledge about how the system worksunderstandable for a system governed by processes the spatial scales of which range from the molecular to the global and uncertainty about how important climate factors will evolve in the future. A variety of factors affect temperature near the surface of the earth, including variability in solar output, volcanic activity, and dust and other aerosols, in addition to concentrations of greenhouse gases.

However, this uncertainty does not stop one from making some broad statements about (1) the likelihood of the sources of observed global warming and (2) the likely effects of continued warming. In the first case, attempts by climate modelers to reproduce the observed global near-surface temperature record using only natural variability in climate models have proved inadequate. The Third Assessment Report (2001) of the Intergovernmental Panel on Climate Change (IPCC) attributes some 80 percent of recent rises in global temperature to human activities, with other important contributions coming from volcanic and solar sources. Over the coming century, likely effects of continued warming include higher daily maximum and minimum temperatures, more hot days over most land areas, fewer frosts in winter, fewer cold days over most land areas, a reduced daily range of temperatures, more extreme precipitation events (all very likely), increased risk of drought, increases in cyclone peak wind, and precipitation intensity (likely). Other effects, such as the disintegration of Antarctic ice sheets, carry potentially enormous implications, but are considered very unlikely.

Responses to Climate Change

These effects are likely to be beneficial in some places, but disruptive in most, and as a consequence, governments around the world have begun planning responses to climate change. These fall into two categories: mitigation, which involves taking action to prevent climate change (usually by cutting greenhouse gas emissions) and adaptation, which involves adapting to the effects as and after they happen. For example, if sea levels rise in the next century due to thermal expansion of the oceans, low lying areas such as the Netherlands and Bangladesh may be flooded. A mitigation strategy would involve trying to cut emissions to forestall the heat-driven sea level rise, whereas an adaptation strategy might be to build large barriers to prevent the sea level rise from flooding these countries. In wealthy countries such as the Netherlands this is perhaps a viable option. It is not so clear that Bangladeshone of the world's poorest countrieswill be in a position to implement this sort of strategy.

Because of the potentially serious ramifications of continued global warming, the World Meteorological Organisation and United Nations Environment Programme jointly established the IPCC in 1988. It assesses scientific and socioeconomic information on climate change and related impacts, and provides advice on the options for either mitigating climate change by limiting the emissions of greenhouse gases, or adapting to expected changes through developments such as building higher flood defenses.

In the wake of the general increase in the awareness of environmental issues in the Western world since the 1970s, global warming has become an important political issue in the last decade. Following the successful implementation of the Montréal Protocol (1987) that prohibited the production of ozone-depleting gases (i.e., chlorofluorocarbons [CFCs], halons, and carbon tetrachloride) starting in 2000, the international community sought to address the problem of global warming in the Kyoto Protocol (1992). This involves industrialized countries taking the lead on cutting greenhouse gas emissions. The protocol requires them to decrease their emissions to 90 percent of their 1990 levels. The Kyoto Protocol comes into effect if fifty-five parties to the convention ratify the protocol, with "annex 1" (or industrialized) parties accounting for 55 percent of that group's carbon dioxide emissions in 1990.

This approach has proved controversial for a variety of reasons: (1) It applies primarily to industrialized countries, freeing some of the world's worst polluters, such as China and Saudi Arabia, from having to comply; (2) the reductions are arbitrarily fixed at 10 percent of a country's 1990 level, irrespective of whether that country is a big polluter, like the United States, or a relatively small polluter, like Sweden; (3) disagreements about whether the cuts imposed by the treaty will actually be worth the economic costs; (4) the treaty targets only gross emissions rather than net emissionsduring the negotiations key differences emerged between a group of nations that favored the use of man-made forests as "carbon sinks" planted to soak up carbon emissions, and countries that believed this to be an inadequate response.

Although the Kyoto Protocol has been enthusiastically backed by European countries, various wealthy countries remain outside the treaty, most notably Australia and the United States. The U.S. decision to not sign the Kyoto Protocol has proved particularly controversial, as the United States emits some 23 percent of global greenhouse emissions, while only containing 5 percent of the global population. The current Bush administration does not intend to ratify the agreement on the grounds "that the protocol is not sound policy," according to U.S. Undersecretary of State Paula Dobriansky.

see also Carbon Dioxide; CFCs (Chlorofluorocarbons); Greenhouse Gases; Halon; Methane (CH4); NOx (Nitrogen Oxides); Ozone; Treaties and Conferences.


Burroughs, William James. Climate Change: A Multidisciplinary Approach. Cambridge University Press, 2001.

Climate Change 2001: The Scientific Basis, The Intergovernmental Panel on Climate Change Third Assessment Report. Cambridge University Press, 2001.

Harvey, L.D. Danny. Global Warming: The Hard Science. Prentice Hall, 1999.

Internet Resources

Intergovernmental Panel on Climate Change Web site. Available at

Intergovernmental Panel on Climate Change. "Climate Change 2001: IPCC Third Assessment Report." Available at

David Frame

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Global Warming and the Ocean

Global Warming and the Ocean

The Earth's climate seems stable in respect to humankind's limited length of historical knowledge, but in reality, it is an ever-changing system. Climate change has been occurring since the Earth began, passing through long periods of fluctuating temperatures.

Climatologists refer to the historical record, which goes back to the mid-nineteenth century, to study recent shifts in climate. This record of temperature measurements indicates that since 1860, the mean (average) annual surface temperature of the Earth has risen by about 0.5 Celsius degrees (0.9 Fahrenheit degrees). This finding supports the theory that the Earth is presently in a period of global warming. The questions important to scientists and policymakers are the extent, period, and cause of the warming.

Factors in Global Warming

One major factor in global warming is a solar heating process termed the greenhouse effect . The glass structure of a greenhouse allows most of the Sun's light inside, but stops a good share of the heat from escaping. This causes the temperature inside the greenhouse to be warmer than the outside air.

The Earth's atmosphere, along with certain greenhouse gases , acts much like a greenhouse, absorbing the infrared energy emitted by the Earth and warming the atmosphere. Without the presence of a greenhouse effect, the temperature of the Earth would be about 18°C (0.4°F) instead of its present 15°C (59°F).

The most abundant greenhouse gas is water vapor, followed closely by carbon dioxide (CO2). There also are trace gases including methane (CH4), nitrous oxide (N2O), tropospheric ozone (O3), and human-made chlorofluorocarbons (CFCs). These trace compounds, though in very low concentrations, are important because they absorb far more radiation, molecule per molecule, than does carbon dioxide. The estimated percent contributions of these greenhouse gases to increased greenhouse effect based on their present concentration in parts per billion by volume (ppbv) are as follows.

Gas ppbv %
CO2 353,000 60
CH4 1700 15
N2O 310 5
O3 1050 8
CFC-11 0.28 4
CFC-12 0.48 8

Carbon Dioxide.

The carbon dioxide content of the atmosphere varies over time. Carbon dioxide is both natural and human-made, and has increased by 25 percent in the last 125 years. Human industrial activities, especially since the Industrial Revolution , have increased the CO2 content of the atmosphere. The increase is evident in the following figure, which shows atmospheric CO2 in parts per million (ppm) at three locations: South Pole (red circle); Siple, Antarctica (blue square); and Mauna Loa, Hawaii (green square).

The burning of fossil fuels , such as oil, coal and natural gases, are sources of energy that release carbon dioxide. Carbon dioxide uptake by plants during photosynthesis, and release by animals during respiration also influences the amount of atmospheric CO2.

There are more land plants in the Northern Hemisphere than in the Southern Hemisphere simply because there is more land north of the equator. Each year during Northern summers, plants absorb more carbon dioxide than is produced. When the growing season ends in the Northern Hemisphere, the carbon dioxide content of the atmosphere resumes the increase that results from the burning of fossil fuels. The seasonal influence of land plants is obvious in the following diagram, which shows atmospheric CO2 in parts per million (ppm) for Mauna Loa, Hawaii.

Because carbon dioxide is 30 times more soluble in water than are most common gases, the ocean contains most of the carbon dioxide in the oceanatmosphere system. The phytoplankton living in the surface layers of the world's oceans convert CO2 into plant tissue, and in some cases use CO2 to build calcium carbonate (CaCO3) shells. As organisms die, their remains deposit on the ocean floor, along with other debris, burying calcium carbonate and organic carbon in sea-floor sediments. The ocean therefore performs as a giant sink for carbon dioxide, absorbing the gas and removing it from the atmosphere while depositing much if it as marine sediments.

Ocean Water and Temperature.

The Earth seems to have had a relatively constant temperature over long periods of geologic time. It is reradiating energy back to space at a rate approximately equal to the rate it receives energy. Most of the energy the Earth receives from the Sun lies within the ultraviolet and visible light spectra. The atmosphere is transparent to most of this radiation, but the oceans and the continents absorb about half of it.

Because of the high heat capacity of water, the oceans can absorb and hold much more solar energy than the air or the continents. When the oceans reradiate this stored energy back toward space, it is changed to infrared energy. The greenhouse gases in the atmosphere absorb some of this infrared radiation, which warms the atmosphere.


To understand how the present-day global climate compares to past climates, scientists have had to look beyond the limited 140 years of weather data and examine the Earth's paleoclimate. Paleoclimate is a term used to describe the ancient climate long before instruments were developed. Instead of instrumental measurements of weather and climate, paleoclimatologists use natural environmental (proxy) records to estimate past climate conditions.

Research methods involve analyzing sediment core samples from the ocean floor and ice cores from the polar ice packs.* Some of the things being sought are fossil plankton , plant pollen, and preserved insects that are locked in ocean sediments, and chemical and isotopic data from sediments and polar ice. By dating the samples and identifying species and abundance, researchers can reconstruct the general climate of a region during its geologic past. For example, globally averaged temperatures and the atmospheric concentration of CO2 in parts per million (ppm) over the past 160,000 years have been estimated as follows.

The paleoclimatic record not only allows scientists to examine global temperature fluctuations over the last several centuries, but it also reveals past climate change even farther back in time. This perspective is an important tool used to help understand the possible causes of the present-day global warming.

The Effects of Global Warming

In 1988, the United Nations established the Intergovernmental Panel on Climate Change (IPCC) to evaluate information on climate change. In a 2001 report, the IPCC concluded that 1) global warming will occur if greenhouse gas concentrations increase, and 2) the concentration of greenhouse gases in the atmosphere is increasing. It can thus be inferred that global warming is occurring.

Each year, human activities inject 6 billion tons (6 gigatons) of CO2 into the atmosphere. Three gigatons remain there, 1.5 gigatons go into the ocean, and the fate of the remaining 1.5 gigatons is unknown. Pre-industrial levels of carbon dioxide were about 280 parts per million by volume (ppmv), and current levels are about 370 ppmv. The concentration of carbon dioxide in the atmosphere today has not been exceeded in the last 420,000 years. By the end of the twenty-first century, some scientists expect to see carbon dioxide concentrations of anywhere from 490 to 1260 ppmv, which is 75 percent to 350 percent above the estimated pre-industrial concentration.

The projected temperature change of 1.5°C to 4°C (3°F to 7°F) by the year 2100 would be unprecedented in comparison with the best available records from the last several thousand years. This could cause higher sea-surface temperatures, intense tropical storms, longer and more intense heat waves, and melting of ice in glaciers and ice shelves.

Warming is expected to be more pronounced in high northern latitudes than in high southern latitudes. An increase in temperature accompanied by an increase in rainfall could decrease the density of the surface sea water that now sinks to the ocean floor forming the North Atlantic Deep Water. In that case, the thermohaline circulation of the ocean would be altered, and could further accelerate global warming. Computer models of climate change are undergoing continual refinement in an effort to decrease the uncertainty of these predictions.

see also Algal Blooms in the Ocean; Carbon Dioxide in the Ocean and Atmosphere; Climate and the Ocean; El NiÑo and La NiÑa; Glaciers, Ice Sheets, and Climate Change; Global Warming and Glaciers; Global Warming and the Hydrologic Cycle; Global Warming: Policy-making; Ice at Sea; Ice Cores and Ancient Climatic Conditions; Ocean Biogeochemistry; Ocean Currents; Ocean-Floor Sediments; Oceans, Polar; Sea Level; Sea Water, Physics and Chemistry Of.

Ron Crouse


Intergovernmental Panel on Climate Change. Climate Change 2001: The Scientific Basis. Geneva, Switzerland: World Meteorological Organization and UN Environment Programme, 2001. Available online at <>.

Philander, S. George. Is the Temperature Rising? The Uncertain Science of Global Warming. Princeton, NJ: Princeton University Press, 1998.

Thurman, Harold V., and Elizabeth A. Burton. Introductory Oceanography, 9th ed. Upper Saddle River, NJ: Prentice Hall, 2001.

Internet Resources

Climate Monitoring and Diagnostic Laboratory. National Oceanic and Atmospheric Administration. <>.

Intergovernmental Panel on Climate Change. <>.

* See "Ocean-Floor Sediments" for a photograph of a sediment core, and "Ice Cores and Ancient Climatic Conditions" for a photograph of an ice core.

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Global Warming

Global Warming

Global warming is an example of global climatic change. To understand the concept of global warming and make decisions about how to respond to the seemingly contradictory information received from various sources, it is important to distinguish between climate and weather. Weather applies to short-term changes in properties of the lower atmosphere such as temperature, relative humidity, precipitation, cloud cover, barometric pressure, and wind speed. Climate is the general pattern of weather conditions, seasonal variation, and weather extremes over a long timeat least thirty years. A summer with record high temperatures is not a signal that global warming is occurring. A winter with record cold is not proof that global warming is not occurring. Climate change, especially global climate change, must be determined from global averages of weather conditions collected, averaged, and compared over decades.

Climate Change

Earth's climate has changed dramatically many times in the past and will almost certainly change many times in the future. Twenty thousand years ago, the places where Minneapolis, Milwaukee, Chicago, and Detroit now stand were covered with ice. Scientists do not know what caused the ice to spread or what caused it to retreat. Once the ice began to retreat, it did so very rapidly, completely disappearing in a few thousand years. Only a few remnants, such as the Greenland ice sheet, still exist. If the Greenland ice sheet were to melt, global sea levels would rise by 8 to 10 meters (26 to 33 feet), and many major seaports and coastlines would be flooded. If the Antarctic ice sheet melted, Earth's oceans would rise by 100 meters (330 feet).

Humans would survive climate changes of this magnitude, but social and political organizations probably would not. Scientists know this because past civilizations have not survived similar climate changes. Around 1000 C. E. , a well-established Norse colony thrived in what is now southern Greenland. The colony had been established during a relatively warm period when the temperatures in the area were 2 to 4°C (4 to 7°F) above average. It vanished almost without trace as the climate returned to normal, an ice sheet moved back over pastures, and the advancing sea ice cut off communications. That small temperature change made the difference between a thriving colony and disaster.

Earth's climate is still changing. Research strongly indicates that Earth is gradually warming up. According to the United States Environmental Protection Agency, the best estimates are that Earth's temperature has increased by 0.5°C (1.0°F) in the last century, precipitation has increased by 1 percent, and sea level has risen by 2 to 5 centimeters (1.0 to 2.0 inches). This is strong evidence for a small but significant increase in global average temperature. Almost all scientists agree with these facts. However, scientists cannot agree on what causes global warming. Many researchers are convinced the data show unequivocally that global warming is directly related to the increase in greenhouse gases such as carbon dioxide. Others feel the data simply indicate a short-term climatic phenomenon.

The "greenhouse" effect is somewhat misnamed. A greenhouse gets warm on a sunny winter day because the sunlight passes through the glass, warming the plants and other surfaces in the greenhouse. The plants warm the air, but the warm air cannot escape, so the temperature in the greenhouse rises. The planetary greenhouse effect operates a little differently. Infrared radiation from the sun passes through the atmosphere and warms the surface of Earth. As the surface warms, it also radiates infrared. However, since the temperature of Earth is much lower than the temperature of the surface of the sun, the infrared radiation emitted by the ground, building, rocks, and plants has a much longer wavelength. Radiation of this longer wavelength cannot pass through the atmosphere, and is absorbed by the air or reflected back to the ground.

A little greenhouse effect is a good thing. If it were not for the greenhouse effect, Earth's average surface temperature would be well below the freezing point of water and life could not exist. The question is, can we have too much of a good thing? Is it possible that rising temperatures on Earth are due to increased levels of greenhouse gases in the atmosphere?

Greenhouse Gases

There are several greenhouse gases. Naturally occurring greenhouse gases include water vapor, carbon dioxide, methane (from plant decay and other sources), nitrous oxide (from volcanoes), and ozone. All these gases can also result from human activity. Carbon dioxide is released when fossil fuels are burned. Methane is emitted from livestock operations and the decomposition of organic waste. Nitrous oxide is emitted by internal combustion engines and by the burning of solid waste. Several synthetic materials are powerful greenhouse gases, including hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride.

Of all the greenhouse gases, carbon dioxide causes the most concern and is therefore closely monitored. Scientists know that carbon dioxide levels in the atmosphere have increased steadily since the beginning of the Industrial Revolution. Most scientists also agree that the average surface temperature of Earth has increased by about 0.5°C (1°F) over the last 100 years. In addition, most scientists now think there is a direct correlation between the increase of carbon dioxide in the atmosphere and the increase in the global average temperature. What remains uncertain is what will happen in the future and what should be done about it. Although the consensus among scientists is that Earth's temperature will continue to increase over the next 100 years, there is no consensus on the size of the increase. Estimates range from 1°C (2°F) to over 5°C (9°F). A 10°C rise will have little effect and is no cause for alarm. However, a 5°C rise could have disastrous consequences. Sea level could rise by 100 meters (330 feet), deserts could expand dramatically, and precipitation patterns would change in unpredictable ways.

Controversy Over Global Warming

Discussions about global warming have become intensely political, with "conservatives" and "liberals" taking contradictory positions. Two questions related to global warming should be discussed and debated. The first question is whether global warming is occurring and whether humans are causing it. The second question is thisif global warming is occurring and humans are causing it, what should be done about it? This second question is clearly a matter of public policy and political process. Public media, Congress, and other public forums are the appropriate arenas for the debate about this question.

Many national governments and international organizations continue to raise concerns about global warming and the possible link to carbon dioxide emissions. Most countries are firmly committed to strengthening international response to risks of adverse climate change. Since gases emitted into the atmosphere do not recognize political boundaries, this is a legitimate question of international concern. The United Nations Framework Convention on Climate Change currently provides a vehicle for discussion and continuing scientific research into this difficult problem.

see also Fossil Fuels.

Elliot Richmond


Barnes-Svarney, Patricia. New York Public Library Desk Reference. New York: Macmillan, 1995.

Berger, John J. Beating the Heat: Why & How We Must Combat Global Warming. Berkeley, CA: Berkeley Hills Books, 2000.

Gelbspan, Ross. The Heat Is On: The Climate Crisis, the Cover-Up, the Prescription. Cambridge, MA: Perseus Publishing Company, Inc., 1998.

Goodstein, Eban. The Trade-Off Myth: Fact and Fiction About Jobs and the Environment. Washington, D.C.: Island Press, 1999.

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Global Warming and the Hydrologic Cycle

Global Warming and the Hydrologic Cycle

To maintain the global water balance, evaporation from oceans worldwide must be balanced by precipitation into the oceans plus runoff from the continents. Earth's atmosphere contains only 0.001 percent of the Earth's water, yet it is an essential component of the global hydrologic cycle: currents of air carry water vapor over land, and the resulting precipitation enables life on land.

Increasing atmospheric concentrations of greenhouse gases , mainly carbon dioxide, have led to a warming at the surface, by nearly 0.6°C (1.0°F) during the twentieth century, and it is widely believed that this trend will continue in the twenty-first century, leading to a higher sea-surface temperature, among other factors.

One intuitive consequence of a warmer ocean surface is a larger vapor-pressure difference between the sea surface and the adjacent atmosphere. This would enhance the evaporation rate, and hence increase the other components of the hydrologic cycle.

Numerous empirical observations and models of the global climate confirm the hypothesis that global warming enhances the global hydrologic cycle. For instance, a global warming by 4°C (7.2°F) is expected to increase global precipitation by about 10 percent. Models suggest that the increase is more likely to come as heavier rainfall, rather than as more frequent rainfalls or falls of longer duration.

Evidence of Hydrologic Changes

The following seven arguments suggest that the hydrologic cycle already has measurably intensified. First, observed global warming is almost entirely due to an increase in nighttime temperature. Daily minimum temperatures have increased at twice the rate of daytime temperatures since 1950 (roughly 1.0°C versus 0.5°C). This suggests increased cloudiness and/or humidity at night, and increased evaporative cooling during the daytime. (This cooling is analogous to body heat evaporating rubbing alcohol from one's skin, leaving one's body somewhat cooled in the process.)

Second, radiosonde (balloon-borne instrumentation) and satellite data suggest that the mean (average) atmospheric water-vapor concentration has increased. This enables storms to generate more precipitation.

Third, precipitation amounts have changed in different ways in various regions during the last 80 years, but they generally have increased in the middle and high latitudes, often in excess of 10 percent. In the United States, annual rainfall has increased by about 10 percent during the twentieth century, on average. The largest increases in precipitation are expected to occur near polar regions, for two reasons. One, observations and climate models indicate that the warming rate has been and will continue to be the highest there, and warmer air can hold more water vapor. Two, the warming will reduce the extent of sea ice , thereby allowing more evaporation from open water.

It should be noted that decreases in precipitation have been observed in some regions. In the Northern Hemisphere tropics, especially in Africa, a significant decrease in rainfall has occurred since 1950. Intense drought plagues the African Sahel region more than ever before. Yet in the tropical Pacific, the sea-surface temperature, evaporation rate, and rainfall amounts all have increased.

Fourth, the observed increase in precipitation in the last few decades has been due in large part to a disproportionate increase in heavy and extreme precipitation rates. This is consistent with climate model predictions.

Fifth, an increased intensity of storms associated with atmospheric fronts in the Northern Hemisphere has been observed over the past few decades.

Sixth, on a longer timescale, ice cores drilled on the ice sheets of Greenland and Antarctica show that during the last glacial maximum, 20,000 years before the present (BP), there was more dust in the air. There is other evidence that suggests that global precipitation amounts were lower during the Ice Age. Apparently the hydrologic cycle intensified after the Ice Age, and more aerosols were washed out before they could settle on the ice sheets, explaining the cleaner layers of ice in the ice cores after 20,000 BP.

Finally, more rain tends to fall if the monthly mean (average) temperature is above normal, as shown by observations in the United States and in Australia. This is consistent with model predictions.

An enhancing hydrologic cycle, in turn, may enhance global warming, through several mechanisms. One mechanism is the water-vapor feedback, because water vapor is a key greenhouse gas. Also, increased cloudiness heats the planet, at least if the clouds are high or deep, as is the case for most storms. This is because high or deep clouds reduce the outgoing long-wave radiation more than the net incoming short-wave radiation.

Consequences of Global Warming

Although a larger global mean annual rainfall and a smaller number of frost days may have beneficial effects, especially for agriculture, the various factors listed above also have the following undesirable consequences.

  • Rising nighttime temperatures exacerbate heat waves and reduce the beneficial effects of frost in killing pests or insect-borne diseases.
  • The positive water-vapor feedback loop may increase the rate of global warming. This response is one of the major uncertainties in climate-change prediction.
  • Higher temperatures and heavier precipitation in winter imply more runoff and smaller mountain snowpacks in places such as the northwestern United States. In other words, both winter floods and summer water shortages are expected to become more frequent and intense. More surface reservoirs will be needed to handle both pressures, yet the building or even the maintenance of dams is facing increasing opposition, often from environmental groups.
  • Higher rainfall intensities imply more frequent floods, and require more expensive flood-control measures.
  • An increase in rainfall intensity also may be detrimental to agriculture, because it causes more soil erosion and implies relatively less soil infiltration.
  • More inclement storms, mainly in winter, increases the risk of hazards along shorelines, especially as more people live near the coast.

see also Coastal Waters Management; Floodplain Management; Glaciers, Ice Sheets, and Climate Change; Global Warming and Glaciers; Global Warming and the Ocean; Hydrologic Cycle; Ice at Sea; Sea LEvel.

Bart Geerts


Bates, J. J., X. Wu, and D. L. Jackson. "Interannual Variability of Upper-Tropospheric Water Vapor Band Brightness Temperature." Journal of Climate 9 (1996): 427438.

Fowler, A. M., and K. T. Hennessy. "Potential Impacts of Global Warming on the Frequency and Magnitude of Heavy Precipitation." Natural Hazards 11 (1995): 283303.

Groisman, P. et al. "Changes in the Probability of Heavy Precipitation: Important Indicators of Climatic Change." Climatic Change 42 (1999):243283.

Huang, J., and H. M. van den Dool. "Monthly Precipitation-Temperature Relations and Temperature Prediction over the United States." Journal of Climate 6 (1993): 11111132.

Hulme M. "Estimating Global Changes in Precipitation." Weather 50 (1995):3645.

Karl, T. et al. "A New Perspective on Recent Global Warming: Asymmetric Trends of Daily Maximum and Minimum Temperature." Bulletin of the American Meteorological Society 74 (1993):10071024.

Morrissey, M. L., and N. E. Graham. "Recent Trends in Rain Gage Measurements from the Tropical Pacific: Evidence for an Enhanced Hydrologic Cycle." Bulletin of the American Meteorological Society 77 (1996):12071219.

Yung, Y. L. et al. "Dust: A Diagnostic of the Hydrologic Cycle During the Last Glacial Maximum." Science 271 (1996):962963.

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Global Warming

Global Warming

The term "global warming" refers to an increase in Earth's mean global temperature because a part of Earth's outgoing infrared radiation is retained by several trace gases in the atmosphere whose concentrations have been increasing because of human industrial, commercial, and agricultural activities. These gases have the ability to absorb radiation, leading to the tendency of the atmosphere to create warmer climates than would otherwise be the case. The most important naturally occurring trace gases that have the ability to absorb infrared radiation are water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone (O3). In addition, there are some industrial gases that are extremely effective absorbers of the radiation. Important among these are chlorofluorocarbons (CFCs) , perfluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons (HFCs), and sulfur hexafluoride (SF6). These gases are analogous to the glass in a greenhouse, which also leads to the net trapping of infrared radiation, hence the terms "greenhouse gases" (GHGs) and "greenhouse effect ." These GHGs act as a partial blanket for the thermal radiation from the surface and make the atmosphere warmer than it would otherwise be. Before human intervention, Earth's radiation balance was in equilibrium , resulting in a mean average temperature of Earth at 15°C (59°F). Without the presence of naturally occurring GHGs, Earth's average surface temperature would have been 18°C (0.4°F). This difference of 33°C (91.4°F) is due to the natural greenhouse effect, which has made Earth a habitable planet. Although the level of naturally occurring GHGs may change naturally over time, their concentration has steadily increased since the Industrial Revolution that began around 1750, because of fossil fuel combustion , deforestation, biomass burning, drainage of wetlands, conversion of natural into agricultural ecosystems, and plowing/cultivation of soil.

The concentration of CO2 has increased from about 280 parts per million by volume (ppmv) in the preindustrial era to about 370 ppm in 2000, and is increasing at the rate of about 1.5 ppm/yr (0.4 percent/yr). The atmospheric concentration of CH4 has increased from 700 parts per billion by volume (ppbv) to 1,720 ppbv, and is increasing at the rate of 10 ppbv/yr (0.6 percent/yr). Similarly, the concentration of N2O has increased from 275 ppbv to 312 ppbv and is increasing at the rate of about 0.8 ppbv/yr (0.25 percent/yr) (Bruce, Lee, and Haites 1996). Because of the successful implementation of the Montreal Protocol in 1987, the concentration of industrial gases has been decreasing.

There are three anthropogenic human-derived sources of atmospheric enrichment of CO2: (1) fossil fuel combustion; (2) cement manufacturing; and (3) land use change involving deforestation, biomass burning, and cultivation. Fossil fuel combustion annually emits 6.22 Pg C (1 Pg = 1 billion metric tons) as CO2, 46 to 155 Tg of C (1 Tg = 1 million metric tons) as CH4, and 0.7 to 1.8 Tg N as N2O and NOx. The fossil fuel emission has steadily increased over the last 150 years. The CO2-C emission from fossil fuel combustion was negligible in 1850, 1,850 Tg/yr in 1900, 1.7 Pg/yr in 1950, and 6.2 Pg/yr in 1995 (Hansen, et al. 1998, pp. 1275312758). Cement manufacturing emits 0.2 Pg C/yr as CO2. Tropical deforestation and soil cultivation annually emit 0.6 to 2.6 Pg C as CO2, 160 to 460 Tg of C as CH4, and 2.2 to 6.8 Tg N as NOx (Harvey 2000, pp. 1620). From 1850 to 1998, approximately 270 (±30) Pg C has been emitted as CO2 by fossil fuel combustion and cement production. During the same time, about 136 (±55) Pg has been emitted as a result of deforestation and land use change, of which 78 (±17 Pg) is due to depletion of the soil organic carbon pool (Watson, et al. 2000, p. 4; Lal 1999, p. 317).

The alteration in Earth's radiation budget because of an increase in atmospheric concentration of GHGs is referred to as "radiative forcing," and is measured in w/m2. The radiative forcing of three gases (CO2, CH4, and N2O) since the preindustrial era is 2.45 w/m2, due to the accelerated greenhouse effect or global warming. The GHGs differ with regard to their radiative forcing and their life span, or residence time in the atmosphere. This relative ability of GHGs is called the global warming potential or GWP. The GWP is computed relative to CO2, and is 1 for CO2, 21 for CH4, 210 for N2O, 1,800 for O3, and 4,000 to 12,000 for CFCs. It is estimated that the mean global temperature has increased by about 0.5°C (32.9°F) since the preindustrial era. With business as usual, the radiation budget of Earth may change within a short span of several decades to a century, with an attendant increase in Earth's mean temperature of 1 to 4°C (33.8 to 39.2°F). The projected increase will be less in the tropics than in the boreal, temperate, and cold regions. The greenhouse effect is tolerable (i.e., the biomes or ecological communities comprising plants and animals can adapt) if the rate of increase in Earth's mean temperature is about 0.1°C (32.18°F) per decade.

World soils constitute the third largest global C pool (after oceanic and geologic), and comprise 1,550 Pg of soil organic carbon (SOC) and 750 Pg of soil inorganic carbon (SIC). Thus, the soil C pool is 3.2 times the atmospheric pool (720 Pg), and 4.1 times the biotic pool (560 Pg). The C depleted from the SOC pool can be resequestered through adoption of appropriate land use and soil/crop/vegetation management practices (Lal and Bruce 1999, p. 182; Lal 2001a, pp. 171172). Restoration of degraded soils and ecosystems and desertification control have a potential to sequester C in soil and the biota and to decrease the rate of enrichment of GHGs in the atmosphere (Lal 2001b, p. 52; 2001c, p. 23).

There are short-term and long-term strategies of mitigating the accelerated greenhouse effect. In the short term, it is important to improve energy use efficiency and to identify strategies of CO2 sequestration. In the long term, it is important to develop noncarbon fuel sources. Carbon sequestration in soil and vegetation through restoration of degraded soils and the ecosystem and adoption of appropriate land uses is a winning strategy.

see also Air Pollution.

Rattan Lal


Bruce, James P.; Lee, Hoesung; and Haites, Erik F., eds. (1996). Climate Change 1995:Economic and Social Dimensions of Climate Change. Cambridge, U.K.: Cambridge University Press.

Hansen, J. E.; Sato, M.; and Lacis, A.; et al. (1998). "Climate Forcing in the Industrial Era." Proceedings of the National Academy of Sciences 95:1275312758.

Harvey, Leslie Daryl Danny (2000). Global Warming: The Hard Science. Harlow, U.K.: Longman.

Lal, Rattan (1999). "Soil Management and Restoration for C Sequestration to Mitigate the Accelerated Greenhouse Effect." Progress in Environmental Science 1:307326.

Lal, Rattan (2001a). "World Cropland Soils as a Source or Sink for Atmospheric Carbon." Advances in Agronomy 71:145191.

Lal, Rattan (2001b). "Potential of Desertification Control to Sequester Soil Carbon and Mitigate the Greenhouse Effect." Climate Change 15:3572.

Lal, Rattan (2001c). "The Potential of Soils of the Tropics to Sequester Carbon and Mitigate the Greenhouse Effect." Advances in Agronomy 74:23, 155192.

Lal, Rattan, and Bruce, J. (1999). "The Potential of World Cropland to Sequester C and Mitigate the Greenhouse Effect." Environment Science & Policy 2:177185.

Watson, Tina, et al. (2000). Land Use, Land Use Change, and Forestry. Cambridge, U.K.: Cambridge University Press.

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Global Warming

Global warming

Global warming, as used in the popular context, is a scientifically controversial phenomenon that attributes an increase in the average annual surface temperature of Earth to increased atmospheric concentrations of carbon dioxide and other gases. Global warming describes only one of several components involved in climate change and specifically refers to a warming of Earth's surface outside of the range of normal fluctuations that have occurred throughout Earth's history.

Climate describes the long-term meteorological conditions or average weather for a region. Throughout Earth's history there have been dramatic and cyclic changes in climatic weather patterns corresponding to cycles of glacial advance and retreat that occur on the scale of 100,000 years. Within these larger cycles are shorter duration warming and cooling trends that last from 20,000 to 40,000 years. Scientists estimate that approximately 10,000 years have elapsed since the end of the last ice age, and examination of physical and biological processes establishes that since the end of the last ice age there have been fluctuating periods of global warming and cooling.

Measurements made of weather and climate trends during the last decades of the twentieth century raised concern that global temperatures are rising not in response to natural cyclic fluctuations, but rather in response to increasing concentrations of atmospheric gases that are critical to the natural and life-enabling greenhouse effect (infrared re-radiation, mostly from water vapor and clouds , that warms the earth's surface).

Observations collected over the last century indicate that the average land surface temperature increased by 0.81.0°F (0.450.6°C). The effects of temperature increase, however, cannot be fully isolated and many meteorological models suggest that such increases temperatures also result in increased precipitation and rising sea levels.

Measurements and estimates of global precipitation indicate that precipitation over the world's landmasses has increased by approximately 1% during the twentieth century. Further, as predicted by many global warming models, the increases in precipitation were not uniform. High latitude regions tended to experience greater increases in precipitation while precipitation declined in tropical areas.

Measurements and estimates of sea level show increases of 68 in (1520 cm) during the twentieth century. Geologists and meteorologists estimate that approximately 25% of the sea level rise resulted from the melting of mountain glaciers . The remainder of the rise can be accounted for by the expansion of ocean water in response to higher atmospheric temperatures.

Many scientists express concern that the measured increases in global temperature are not natural cyclic fluctuations, but rather reflect human alteration of the natural phenomena known as the greenhouse effect by increasing concentrations of greenhouse-related atmospheric gases. Estimates of atmospheric greenhouse gases prior to the nineteenth century (extrapolated from measurements involving ice cores) indicate that of the last few million years the concentration of greenhouse gases remained relatively unchanged prior to the European and American industrial revolutions. During the last two centuries, however, increased emissions from internal combustion engines and the use of certain chemicals have measurably increased concentrations of greenhouse gases that might result in an abnormal amount of global warming.

Although most greenhouse gases occur naturally, the evolution of an industrial civilization has significantly increased levels of these naturally occurring gases. In addition, new gases have been put into the atmosphere that potentiate (i.e., increase) the greenhouse effect. Important greenhouse gases in the modern Earth atmosphere include water vapor and carbon dioxide, methane, nitrous oxides, ozone , halogens (bromine, chlorine, and fluorine), halocarbons, and other trace gases.

The sources of the greenhouse gases are both natural and man-made. For example, ozone is a naturally occurring greenhouse gas found in the atmosphere. Ozone is constantly produced and broken down in natural atmospheric processes. In contrast, halocarbons enter the atmosphere primarily as the result of human use of products such as chlorofluorocarbons (CFCs). Water vapor and carbon dioxide are natural components of respiration, transpiration, evaporation and decay processes. Carbon dioxide is also a by-product of combustion. Although occurring at lower levels than water vapor or carbon dioxide, methane is also a potent greenhouse gas. Nitrous oxides, enhanced by the use of nitrogen fertilizers, nylon production, and the combustion of organic material, including fossil fuels have also been identified as contributing to strong greenhouse effects.

Alterations in the concentrations of greenhouse gases results in a disruption of equilibrium processes. Both increased formation and retardation of destruction cause compensatory mechanisms to fail and result in an increased or potentiated greenhouse effect. For example, the amount of water vapor released through evaporation increases directly with increases in the surface temperature of Earth. Within normal limits, increased levels of water vapor are usually controlled by increased warming and precipitation. Likewise, within normal limits, concentrations of carbon dioxide and methane are usually maintained with specified limits by a variety of physical and chemical processes.

Measurements made late in the twentieth century showed that since 1800, methane concentrations have doubled and carbon dioxide concentrations measured at he highest values estimated to have existed during the last 160,000 years. In fact, increases in carbon dioxide over the last 200 years were exponential up until 1973 (the rate of increase has since slowed).

Although the effects of these increases in global greenhouse gases are debated among scientists, the correlation of the increased levels of greenhouse gases with a measured increase in global temperature during the twentieth century, have strengthened the arguments of models that predict pronounced global warming over the next few centuries. In the alternative, some scientists remain skeptical because the earth has not actually responded to the same extent as predicted by these models. For example, where many models based upon the rate of change of greenhouse gases predicted a global warming of .8°F to 2.5°F (0.44°C to 1.39°C) over the last century, the actual measured increase is significantly less with a mean increase generally measured at .9°F (.5°C) and that this amount of global warming is within the natural variation of global temperatures.

One problem in reaching a scientific consensus regarding global warming is that the data used in many models is neither global nor a result of high-reliance systematic scientific measurement (i.e., that it generally neglects oceans and vast uninhabited areas). Other problems involve forming an accurate articulation of the interplay of global surface warming phenomena that include thermal conduction, greenhouse radiation, and convective currents. Most scientists agree, however, that an enhanced greenhouse effect will result in some degree of global warming.

See also Acid rain; Atmospheric pollution

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Global Warming and Glaciers

Global Warming and Glaciers

Evidence suggests that the Earth's average temperature is becoming warmer. Because atmospheric warming will be most readily apparent in the melting of perennial and permanent ice, and because most glaciers are small relative to vast ice sheets and expansive sea ice , glaciers are important indicators of climate change. Approximately 160,000 glaciers are found on Earth; more than 40 have been monitored since (at least) the 1980s. One of these monitoring projects is Global Land Ice Measurement from Space (GLIMS), which uses laser altimetry to help determine glacier volume. The results of such observations are clear: Earth's glaciers are disappearing. Consider the following.

  • Over the past 100 years, global mean (average) sea level has been rising at an average rate of 1 to 2 millimeters per year. A possible contributor to sea-level rise is increased meltwater from the snow and ice of glaciers, ice sheets, ice caps, icebergs, and sea ice.
  • Glaciers in Alaska and neighboring Canada, with a combined area of approximately 90,000 square kilometers (roughly 35,000 square miles), and accounting for about 13 percent of mountain glaciers on Earth, have thinned substantially. Over the last 40 years, thinning has been on the order of 50 to 100 meters (several hundred feet) at lower elevations of glacier occurrence, and about 18 meters (60 feet) at higher elevations.
  • Many glaciers in South America's Andes are melting so fast that, if the current rate continues, they could disappear by 2020. The Quelccava glacier in Peru retreated 32 times faster during the period 19832000 than in the 20 years from 1963 to 1983. In the Patagonian ice fields of Argentina, glaciers have receded 1.5 kilometers since 1990.
  • In Africa, Mount Kilamanjaro's ice fields have shrunk by a least 80 percent since 1912. Mount Kenya's ice cap has shrunk by 40 percent since the 1960s.
  • The 15,000 glaciers of the Himalayas, which collectively constitute the largest body of ice outside the polar caps, are reported to be receding faster than anywhere on Earth. Some 2,000 have melted since the 1950s. Instead of snow accumulation in winter, Himalayan glaciers are being hit by summer monsoon rains. The Dokriani Barnak glacier has receded about 0.8 kilometer since 1990. If Himalayan glaciers recede at this rapid rate, they will be gone by 2035.
  • In the European Alps, several glaciers have disappeared entirely since the 1960s.
  • Permanent sea-ice cover in the Arctic Ocean is shrinking by an area the size of the Netherlands each year. The Arctic ice cap thinned from 3 meters (10 feet) in 1970 to 2 meters (nearly 7 feet) in 2000.
  • In the Antarctic, rising temperatures have resulted in the collapse of massive ice shelves, some of which have been there for 20,000 years.*
  • In Glacier National Park in Montana, the number of glaciers has dropped from an estimated 150 in 1850 to only 50 in 2000. At this rate of decline, all of the glaciers in the park will be gone by 2030.

In addition to the loss of ice and potential threat of a rising sea level, there are other ecological and environmental concerns posed by the melting of snow and ice. For example, increased iceberg calving can bring changes to the Antarctic ecosystem by blocking sunlight needed for the growth of microscopic phytoplankton , the base of the marine food chain. In addition to the increased presence of icebergs, smaller icebergs that break off from a larger mass can form dams along the coast, preventing some of the pack ice from drifting out to sea in summer. This increased ice cover can cause a decline in plankton productivity due to reduced sunlight penetration. Large icebergs also can disrupt penguin colonies and interfere with breeding success.

Massive flooding is predicted in areas where glaciers are receding, particularly in the Himalayas. By one estimate, 20 percent of Bangledesh will be under water by 2020 if the current rate of glacier melting continues.

The absence of glaciers will have a significant effect on late summer streamflow and stream temperature in mountainous watersheds (drainage basins). Both of these properties are important in maintaining fish populations. In streams historically dependent on meltwater, sufficient quantities of meltwater must be present to ensure adequate habitats for stream biota, and to keep stream water cool. Temperatures too warm, often above only 13°C (55°F), may have an impact on the survival of young fish. Moreover, many sources of drinking water for large cities in mountainous regions would be negatively affected by the loss of glaciers because the streams that supply their water will flow at a considerably lower volume.

Attendent to global warming is a potential change in precipitation patterns. In Montana's Glacier National Park, one computer model suggests a 30-percent increase in precipitation and a small increase in average temperature. One of the likely results of this is a redistribution of types of trees. For example, cedarhemlock forests may expand in lower elevations. Sub-alpine fir trees may become nitrogen-stressed at the current treeline. Coarse woody debris accumulation may increase the frequency of large stand-replacing forest fires in other areas.

The United Nations Intergovernmental Panel on Climate Change is trying to assess the impact of humans on global climate. Scientists disagree over the actual cause of rising temperatures and to what degree greenhouse gases are responsible. With respect to the recession of glaciers, some scientists claim that the melting effects have been exaggerated and that recently observed melting events are due to natural variation in Earth's climate. Yet all seem to agree that the nature of the melting events and the underlying causes need to be better understood.

see also Glaciers and Ice Sheets; Glaciers, Ice Sheets, and Climate Change; Global Warming and the Hydrologic Cycle; Ice at Sea; Oceans, Polar.

Dennis O. Nelson


Intergovernmental Panel on Climate Change. Climate Change 2001: The Scientific Basis. Geneva, Switzerland: World Meteorological Organization and UN Environment Programme, 2001. Available online at <>.

Williams, R. S., and J. G. Ferrigno, eds. Satellite Image Atlas of Glaciers of the World. U.S. Geological Survey Professional Paper 1386-J (2003).

Internet Resources

Glacier Retreat in Glacier National Park, Montana. U.S. Geological Survey. <>.

Intergovernmental Panel on Climate Change. <>.

Sea Level and Climate. U.S. Geological Survey, Fact Sheet FS-002-00 (2000). Available online at <>.

World Data Center for Glaciology. The National Snow and Ice Data Center. <>.

* See "Glaciers, Ice Sheets, and Climate Change" for a photograph of the breakup of the Larsen B Ice Shelf in 2002.

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global warming

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. Global warming and its effects, such as more intense summer and winter storms, are also referred to as climate change.

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 smaller quantities of chlorofluorocarbons and other more potent greenhouse 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 increase in coastal flooding; disruption of drinking water supplies dependent on snow melt; profound changes in agriculture due to local climate changes; extinction of species as ecological niches disappear; more intense hurricanes and typhoons due to warmer ocean water, as well as more intense winter storms; and an increased incidence of tropical diseases. Oceans are expected to become more acidic, to the detriment of sea life. The effect of such changes on people and communities would not only be locally disruptive but could also aggravate or cause political instability and national and international conflicts.

Among factors that may be contributing to global warming are the burning of coal, petroleum, and other fossil fuels (sources of carbon dioxide, methane, nitrous oxide, and ozone); deforestation, which increases the amount of carbon dioxide in the atmosphere; and methane gas released in animal waste. The thawing of the tundra as the climate warms may lead to additional increases in methane release.

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. Some 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. Most scientists do agree that it is difficult in general to tie the effects of human activity to specific unusual or extreme weather events, but a number of studies released in 2014 agreed that the 2013–14 extreme heat waves in Australia would not have been as severe without the effects of emissions resulting from human activity.

Despite the political controversies over global warming, for some time there has been general scientific agreement that at least part of the observed warming is the result of human activity, and that the question of addressing the problem deserves serious consideration. Some climate scientists have proposed the use of geoengineering, such as introducing sulfur compounds into the atmosphere to produce global cooling (as volcanic eruptions do); this approach is not without risk, and has been rejected by most environmentalists. 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 (IPCC), 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 were taken to cut down on the production of greenhouse gases; a more recent study by the same panel estimated a rise of 3 to 7.5°F (1.8 to 4°C). Even if action is taken, the already released gases will persist in the atmosphere, and a rise of from 1 to 3.6°F (0.5–2°C) is expected to occur. A 2007 IPCC report, based on a three-year study, termed global warming "unequivocal" and said that most of the change was most likely due to human activities, and its report five years later restated those findings even more strongly.

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, known as the Kyoto Protocol. In 2001 the G. W. Bush administration announced it would abandon the accord; 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.

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 created (2005) an agreement outside the Kyoto Protocol that proposed to reduce emissions through the development and implementation of new technologies. The Asia-Pacific Partnership on Clean Development and Climate established 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 U.S. President 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. The 15th UN Conference on Climate Change, held in Copenhagen, Denmark, in Dec., 2009, failed to lead to a legally binding treaty on reducing global greenhouse-gas emissions. It had been hoped that the meeting would result in a new protocol that would replace that agreed to at Kyoto. The 17th conference, which met in 2012 in Durban, South Africa, agreed to extend the accord (which was extended to 2020 later in 2012) and also agreed to work toward an unspecified new accord; at the same time, however, Canada became the first ratifying nation to withdraw from the Kyoto Protocol. The present rate of carbon dioxide emissions has been increasing since 1970, and measures adopted so far have not slowed the increase in emissions.

See G. E. Christianson, Greenhouse (1999); T. Flannery, The Weather Makers (2006); E. Kolbert, Field Notes from a Catastrophe (2006); E. Linden, The Winds of Change (2006); P. Conkling et al., The Fate of Greenland: Lessons from Abrupt Climate Change (2011); B. McKibben, ed., The Global Warming Reader (2012); W. D. Nordhaus, The Climate Casino (2013).

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Global Warming

Global Warming

Three important indicators suggest that the Earth's climate is going through a period of global warming: (1) an increase in atmospheric temperatures near the Earth's surface; (2) an increase in the surface temperature of the Earth's oceans; and (3) an increase in sea levels. Since global weather patterns are extraordinarily complex, with different systems influencing one another, the effects of global warming will vary from region to region. For instance, as global warming continues, some regions should have dramatic increases in annual precipitation levels, whereas other regions should have dramatic decreaseseven desertification. Within the science and religion literature, discussions of global warming occur most frequently within ecological ethics. Ethicists draw from the climatology sciences to inform their reflection and analysis.

See also Ecology; Ecology, Ethics of; Ecology, Religious and Philosophical Aspects; Ecology, Science of

richard o. randolph

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Global Warming

Global Warming

The term global warming simply means that the global climate is warming. Humans are popularly assumed to be the cause of global warming. Further, global warming is usually assumed to be harmful to humans and to plant and animal life. Global warming is a commonly discussed and debated scientific topic both in the media and in the scientific community.

Scientific Debate: Existence, Extent, Causes, and Pace

Nearly all of the scientific community agrees that based on surface temperature observations, the global climate warmed by about 0.5° C in the twentieth century. Satellite observations of global temperatures show warming trends between 1970 and 1990 similar to those found in surface observations. Decreases in sea ice cover and global glacier retreat provide corroborating evidence of global warming. A few scientists believe that the warming at weather stations is due to the development of cities around weather stations, but analysis in the late 1990s has shown that warming is similar at urban and rural areas. Different areas of the world and different seasons have warmed more than others. Due to global atmospheric circulation patterns that transport heat from the tropics to the poles, warming has been greatest in high latitudes. In some areas of Alaska and Asia, average temperatures have warmed by over 4°C. Warming has also been greatest in spring months, particularly March, and in nighttime minimum temperatures much more than in daytime maximum temperatures.

While the presence of global warming is not seriously debated, its causes are. Greenhouse gases, including carbon dioxide, trap heat radiated from Earth and reemit some of it back to the ground. Without greenhouse gases, our planet would be uninhabitably cold. As shown by research on air bubbles trapped in ice cores, high temperatures have been associated with high levels of greenhouse gases during the geologic past. Most human-made carbon dioxide is produced by cars and industrial activity. As a result of fossil fuel burning, carbon dioxide levels have increased from 280 parts per million in preindustrial times to more than 360 parts per million by the late 1990s. Scientists believe that the pace of greenhouse gas emissions in the late twentieth century is partially responsible for the recorded temperature increases. This is called the enhanced greenhouse effect. A minority of scientists believe that natural processes, such as increased sun spot activity, account for the observed warming. However, statistical analysis shows that compared with the enhanced greenhouse effect, these other processes are highly unlikely causes of the observed warming.

Computer models called global circulation models (GCMs) predict that warming will continue to increase. Due to processes called negative feedbacks that reduce global warming, there is some uncertainty over how rapidly warming will occur. Clouds, for example, are a result of evaporation. In a warmer climate, more evaporation will occur, leading to more clouds. Most types of clouds reflect solar radiation; this would act to cool temperatures. Other processes can act as positive feedbacks that increase global warming. Sulfur particles emitted from factories block radiation and thus cool temperatures. In an ironic twist, making factories cleaner could actually increase global warming. Accurately considering the complex network of feedbacks is a critical field of global warming research. In spite of these uncertainties, GCMs consistently predict further global warming.

The pace at which record high temperatures are being broken is increasing. The years 1998, 1997, 1995, and 1990 were the warmest since at least 1400 C. E. The twentieth century was also the warmest century since at least 1000 C. E. The bulk of observational evidence shows that not only is warming occurring, but that it is occurring at a progressively more rapid pace. Scientists researching past climates have shown that during glacial and interglacial periods, global temperatures (and carbon dioxide levels) have changed by more than the 0.5 degree (and 25 percent) currently seen. If plants and animals have survived these past changes, why should we be concerned about present changes? The answer is that the rate of greenhouse gas and temperature increase appears to be unprecedented. There is no guarantee that ecosystems and the people that depend on them will be able to adjust to the predicted levels of global warming.

Plant-Atmosphere Interactions

Global warming effects on plants depend on the existing climate and vegetation, but in general, there are three main categories of potential effects. First, for plants existing in climatic extremes, global warming may have drastic impacts. For example, plants requiring cold temperatures may be forced off of mountain peak habitats. If this is their only habitat, extinction will occur. Second, global warming is likely to cause large shifts in biome distribution. Coniferous forests will shift farther north and grasslands and deserts will expand. Third, global warming can alter how plants function in their existing environment. For many areas, global warming is likely to lengthen the growing season, causing an increase in photosynthesis. Multiple observations show that the growing season has already significantly lengthened, especially in northern latitudes. For most plants, alteration in plant function without causing their extinction or displacement is the most likely consequence of global warming.

While people usually think about the effects of global warming on plants, plants can also moderate the effects of global warming. Only about half of the carbon dioxide put in the atmosphere remains there; the other half is taken up by Earth. Some of it is dissolved in the ocean, but plants take up some of it too. Plants are therefore acting to slow the pace of the enhanced greenhouse effect. Plants and especially trees are storing some of the carbon dioxide in wood. Unfortunately, this process is unlikely to continue forever. Eventually, when trees and shrubs die, the carbon stored in wood will enter the soil and will begin to decompose. Increased temperatures will cause high rates of decomposition, leading to an accelerated release of carbon dioxide from the soil. Consequently, it is likely that at a global level, ecosystems will begin to release carbon dioxide. Only by reducing fossil fuel emissions, a very difficult task both politically and economically, will we reduce greenhouse gasses.

see also Atmosphere and Plants; Carbon Cycle; Human Impacts.

Michael A. White


Houghton, J. Global Warming: The Complete Briefing, 2nd ed. New York: Cambridge University Press, 1997.

Intergovernmental Panel on Climate Change. Climate Change 1995: The Science of Climate Change. New York: Cambridge University Press, 1996.

Mann, M. E., R. S. Bradley, and M. K. Hughes. "Global-scale Temperature Patterns and Climate Forcing Over the Past Six Centuries." Nature 392 (1998): 779-87.

Philander, G. S. Is the Temperature Rising?: The Uncertain Science of Global Warming. Princeton, NJ: Princeton University Press, 1998.

Shugart, H. H. Terrestrial Ecosystems in Changing Environments. New York: Cambridge University Press, 1998.

Woodwell, G. M., and F. T. Mackenzie, eds. Biotic Feedbacks in the Global Climate System: Will the Warming Feed the Warming? New York: Oxford University Press, 1995.

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global warming

global warming Trend towards higher average temperatures on Earth's surface. During the last few million years, there have been several periods when surface temperatures have been significantly higher or lower than at present. During cold periods (Ice Ages), glaciers covered much of the land. The Earth is currently in the middle of a warm period (inter-glacial), which began c.10,000 years ago. Since the 1960s, some scientists have called attention to signs that the Earth is becoming unnaturally warmer as the result of an increased greenhouse effect caused by human activity.

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global warming

global warming The modification of climates that would result from the retention of an increased proportion of terrestrial radiation by certain atmospheric gases emitted mainly as by-products of human activities (i.e. an anthropogenically induced ‘greenhouse effect’). Computer models of the general circulation predict that the warming would occur mainly in middle to high latitudes, but its precise extent and consequences (e.g. in terms of changes in sea level) are uncertain.

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global warming potential

global warming potential (GWP) A ranking of the absorptive capacity of the principal greenhouse gases expressed as the atmospheric warming effect of each compared with that of carbon dioxide, which is given a value of 1. The values take account of the wavelengths at which each gas absorbs radiation and its atmospheric residence time. On this scale, the GWP of methane is 11, nitrous oxide 270, CFC-11 4000, and CFC-12 8500.

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global warming

glob·al warm·ing • n. the gradual increase in the overall temperature of the earth's atmosphere due to the greenhouse effect caused by increased levels of carbon dioxide, chlorofluorocarbons, and other pollutants. See climate change.

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global warming

global warming the gradual increase in the overall temperature of the earth's atmosphere due to the greenhouse effect caused by increased levels of carbon dioxide, CFCs, and other pollutants.

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global warming

global warming See greenhouse effect.

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