Global Warming

DEFINITION OF GLOBAL WARMING

SPEEDING UP THE PROCESS

GROWING EVIDENCE OF GLOBAL WARMING

ANOMALIES AND REFUTATIONS

SCIENTIFIC CONSENSUS

INADEQUATE ACTION AND NEEDED TRANSFORMATIONS

BIBLIOGRAPHY

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.

DEFINITION OF GLOBAL WARMING

Global warming is understood to result from an overall, long-term increase in the retention of the sun’s 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.

SPEEDING UP THE PROCESS

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.

GROWING EVIDENCE OF GLOBAL WARMING

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 Nation’s 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 Institute’s 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 world’s 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.

ANOMALIES AND REFUTATIONS

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 Mann’s famous “Hockey stick” graph (first published in Nature in 1998). Mann’s 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), Mann’s results were vindicated. A report in 2006 by the U.S. National Academy of Sciences, National Research Council, supported much of Mann’s 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 team’s 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.

SCIENTIFIC CONSENSUS

In May 2001 sixteen of the world’s national academies of science issued a statement, confirming that the IPCC should be seen as the world’s 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.

INADEQUATE ACTION AND NEEDED TRANSFORMATIONS

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 2005’s 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

BIBLIOGRAPHY

Columbia University Earth Institute. 2005. Press release 28, April. http://www.earthinstitute.columbia.edu/news/2005/story04-28-05.html.

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 1961–2004. 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. http://www8.nationalacademies.org/onpinews/newsitem.aspx?RecordID=11676.

Stehr, Nico. 2001. Economy and Ecology in an Era of Knowledge-Base Economies. Current Sociology 49(1) January: 67–90.

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. http://www.assa.edu.au/publications/op/op22005.pdf.

Constance Lever-Tracy

Global Warming

Introduction

Global warming is a long-term increase in Earth's average surface temperature. Because global warming does not cause uniform warming in all locations and because many other changes in climate are occurring, scientists often prefer to speak of “global climate change” rather than of global warming when referring to the whole complex of changes being caused by human activities. In such contexts, the phrase “global warming” is reserved for the warming trend as such, apart from other effects. However, in general usage it still refers to the sum of all changes occurring in Earth's climate as a result of human activities. A major effect of global warming is redistribution of climatic zones defined by temperature, precipitation, and associated ecosystems.

Global climate changes, including episodes of global cooling and warming, have occurred many times throughout Earth's history as a result of natural variations in solar radiation, atmospheric chemistry, oceanic and atmospheric circulations, volcanic eruptions, and other causes. Global warming in the last few decades, however, is primarily caused by human activities that started during the Industrial Revolution, when human burning of fossil fuels began to increase drastically, releasing large amounts of carbon dioxide (CO₂). Atmospheric concentrations of carbon CO₂, methane (CH₄), nitrous oxide (N₂O), and artificial chemicals called halocarbons have long been increasing as a result of emissions from fossil-fuel burning and other activities. Increased atmospheric concentrations of these gases are causing Earth to warm.

As of 2005, atmospheric levels of CO₂ and CH₄ were far higher than at any time in at least 650,000 years. Global average surface temperature has increased by about 0.23°F (0.13°C) per decade for the last 50 years, and has increased more rapidly in recent years: the two warmest years since instrumental records began in the 1800s were 1998 and 2005.

Although a few scientists continue to dispute that recent climate warming is caused by human activities, this is no longer doubted by the great majority of climatologists, meteorologists, and geophysicists. The 2007 Assessment Report of the United Nations' Intergovernmental Panel on Climate Change (IPCC) pronounced that global warming is now “unequivocal” and that it is more than 90% likely that most of the global warming observed since the mid-twentieth century is caused by anthropogenic (human-caused) releases of greenhouse gases.

Historical Background and Scientific Foundations

Solar radiation is the major source of energy arriving at Earth's surface (a small fraction, about 1/7500, comes from Earth's interior). Much of this incoming energy is at short wavelengths (ultraviolet and visible light). These forms of light are absorbed by Earth's surface, where their energy drives atmospheric and oceanic circulation and fuels biological processes like photosynthesis. The surface then re-radiates most of this energy as longer-wavelength light, variously termed heat radiation, infrared light, or infrared radiation. If Earth's atmosphere were completely transparent to infrared radiation, this re-radiated energy would simply shine out into space and the planet would have an average surface temperature of about 0°F (–18°C).

However, certain gases in the atmosphere, mostly notably CO₂, CH₄, and water vapor (H₂O), absorb some of the infrared energy and are warmed, warming the air and, by conduction, the land and sea. These gases are termed greenhouse gases because, like the glass windows of a greenhouse, they allow solar energy to enter Earth's atmosphere but impede the loss of heat. Earth's atmosphere naturally contains certain levels of these greenhouse gases. The resulting natural greenhouse effect maintains the planet's average temperature at a livable 59°F (15°C). This natural greenhouse effect has been crucial to the evolution and survival of life on Earth. A surface temperature of 59°F is sufficient to maintain most of Earth's water in liquid form, whereas 0°F (18°C) is too cold for most organisms to live or for ecological processes to function.

Although atmospheric levels of CO₂ and CH₄ have varied greatly over deep geologic time (hundreds of millions of years), by the time of the beginning of the Industrial Revolution around 1750, their concentrations had been fairly stable for hundreds of thousands of years. The pre-industrial concentration of CO₂ was about 280 ppm (parts per million by volume), of CH₄ about 0.7 ppm, and of NO₂ about 0.285 ppm. (The special case of water vapor is discussed below.) As of 2005, the atmospheric concentration of CO₂ had increased to about 379 ppm, a 36% increase. The concentration of CH₄ has gone up to 1.8 ppm and that of N₂O to 0.319 ppm. Concentrations of chlorofluorocarbons (CFCs) and other completely human-made greenhouse gases have increased from zero in pre-industrial times to about 0.7 ppb (parts per billion by volume).

Atmospheric concentrations of greenhouse gases have increased more quickly since the middle of the twentieth century, coinciding with rapid human population growth and global industrialization. The largest single cause of greenhouse gas releases has been the burning of fossil fuels, including oil, natural gas, and coal. Fossil fuels contain carbon which, in burning, combines with oxygen to create CO₂.

Deforestation is the second-largest emitter of greenhouse gases. As they grow, trees, like all plants, take in CO₂, incorporate the carbon in their structure, and release O₂ into the atmosphere. Deforestation therefore destroys a carbon sink that tends to lower the atmospheric concentration of CO₂. Deforestation also releases CO₂ and CH₄ through the burning and decay of trees and soil carbon in deforested areas.

Agriculture is a primary cause of deforestation. It also releases large quantities of CH₄ through decomposition of organic materials in livestock digestive systems, livestock waste-treatment facilities, and flooded rice fields. Agricultural fertilizers and combustion of fossil fuels and solid wastes are the primary sources of increased N₂O emissions.

Industrial processes emit a variety of powerful synthetic greenhouse gases, including chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF₆). Many of these chemicals also attack the ozone layer, and their manufacture and use has been limited by the Montreal Protocol treaty of 1987.

Greenhouse gases vary greatly in their ability to absorb infrared radiation. On a per-molecule basis, methane is about 21–40 times more absorptive than carbon dioxide, nitrous oxide is 200–270 times more absorptive, and CFCs are 3–15 thousand times more absorptive. CO₂, however, is present in the atmosphere at much higher concentrations than the more-powerful greenhouse gases and has experienced the greatest increases due to human activity. CO₂ is responsible for about 60% of the human contribution to increased atmospheric heat retention, termed radiative forcing.

Water vapor is responsible for more of the greenhouse effect than any other single gas, but is not a primary driver of global warming. This apparent contradiction is explained by the fact that the residence time of water vapor in the atmosphere is short. Increased water vapor concentrations, left to themselves, would disappear in a matter of weeks as rain or snow fall from clouds. The other greenhouse gases, however, linger for decades: these gases force the atmosphere to warm, increasing its ability to hold water vapor. This increased level of water vapor acts to warm the world further. Therefore, water vapor acts as a feedback factor in global warming.

WORDS TO KNOW

ANTHROPOGENIC: Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.

CARBON SINK: Any process or collection of processes that is removing more carbon from the atmosphere than it is emitting. A forest, for example, is a carbon sink if more carbon is accumulating in its soil, wood, and other biomass than is being released by fire, forestry, and decay. The opposite of a carbon sink is a carbon source.

INDUSTRIAL REVOLUTION: The period, beginning about the middle of the eighteenth century, during which humans began to use steam engines as a major source of power.

INFRARED RADIATION: Electromagnetic radiation of a wavelength shorter than radio waves but longer than visible light that takes the form of heat.

KYOTO PROTOCOL: Extension in 1997 of the 1992 United Nations Framework Convention on Climate Change (UNFCCC), an international treaty signed by almost all countries with the goal of mitigating climate change. The United States, as of early 2008, was the only industrialized country to have not ratified the Kyoto Protocol, which is due to be replaced by an improved and updated agreement starting in 2012.

PHOTOSYNTHESIS: The process by which green plants use light to synthesize organic compounds from carbon dioxide and water. In the process, oxygen and water are released. Increased levels of carbon dioxide can increase net photosynthesis in some plants. Plants create a very important reservoir for carbon dioxide.

RADIATIVE FORCING: A change in the balance between incoming solar radiation and outgoing infrared radiation. Without any radiative forcing, solar radiation coming to Earth would continue to be approximately equal to the infrared radiation emitted from Earth. The addition of greenhouse gases traps an increased fraction of the infrared radiation, reradiating it back toward the surface and creating a warming influence (i.e., positive radiative forcing because incoming solar radiation will exceed outgoing infrared radiation).

RESIDENCE TIME: For a greenhouse gas, the average amount of time a given amount of the gas stays in the atmosphere before being absorbed or chemically altered. The residence time of a greenhouse gas is relevant to policy decisions about emitting the gas: emissions of a gas with a long residence time create global warming over decades, centuries, or millennia, until natural processes remove the emitted quantities of gas.

Impacts and Issues

Most scientists today are convinced that an increase in greenhouse gases is resulting in an intensification of Earth's greenhouse effect, with resulting global warming and climate change. The exact climatic response to increased concentrations of greenhouse gases and its potential effects on humans are difficult to predict, but can be foretold in outline. Moreover, computer models of climate are constantly improving, allowing more realistic predictions to be made about future climate change. If global climate change proceeds as recent scientific studies forecast, it will likely have substantial negative ecological consequences.

Earth's surface temperature is variable from place to place and over time. Furthermore, the systems that interact to maintain the planet's temperature and climate are complex; cause-and-effect relationships between the oceans, atmosphere, land, living things, and nonliving chemistry are numerous. In spite of these scientific challenges, there is overwhelming evidence that Earth has warmed significantly during the past 150 years or so. Climate records show that from 1906 to 2005 there was a 1.3°F (0.74°C) increase in Earth's average surface temperature, with twice as much warming as this global average occurring in the Arctic and over the West Antarctic Peninsula. The oceans have also warmed, though not as much.

Recent observations show dramatic melting of the Arctic sea ice, accelerated melting of the Greenland ice cap, and accelerated seaward flow of glaciers on the western peninsula of Antarctica. Global sea-level has risen since the mid 1800s; for 1961–2003, at about 0.07 in (1.8 mm) per year. Alterations have been seen in large-scale weather phenomena like the southeast Indian monsoon season, Atlantic hurricane season, El Niño/Southern Oscillation, and North African drought cycle. Wildlife and plants are shifting their ranges to higher latitudes (closer to the poles, to track cool weather as climate warms) and to higher altitudes in mountainous regions. Many other types of observations, some of which are reviewed elsewhere in this book, independently demonstrate the reality of global warming and the complex changes it has produced in regional climates.

The aforementioned observations agree with predictions by computerized mathematical models of global climate processes. These three-dimensional general circulation models (GCMs) simulate the complex movements of energy and mass involved in the global circulation of the atmosphere, oceans, and living things. Scientists use GCMs to predict the effects of changes in specific variables, like the atmospheric concentration of CO₂, on the rest of the global climate system. Because of the complexity of the computational problem, GCMs that attempt to predict global climate change have somewhat variable results. However, modelers now agree that increased concentrations of atmospheric greenhouse gases have resulted, and will continue to result, in global warming. For example, one GCM that doubles the present CO₂ concentration to about 700 ppm predicts a 2–6°F (1.1–3.3°C) rise in global temperature and suggests that the warming would be 2–3 times more intense at high latitudes—nearer the poles— than in the tropics. These predictions are consistent with changes seen so far, or, if anything, tend to underestimate the pace of warming.

Other predicted consequences of warming include large-scale shifts in atmospheric and oceanographic circulation patterns, partial melting of the polar ice caps, global sea-level rise, reorganization of Earth's climatic zones, and establishment of new large-scale weather patterns. Changes in the distribution of heat, precipitation, and weather phenomena like storms and floods will affect the productivity and distribution of natural and managed vegetation. Animals and microorganisms will experience dramatic changes in their habitats and face higher rates of species extinction. According to the Intergovernmental Panel on Climate Change, a rise of about 4°F (2.2°C) above the pre-industrial average will likely commit 15– 37% of all plant and animal species to extinction. Most biologists agree that global warming is a serious threat to biodiversity and to the health of ecosystems worldwide.

IN CONTEXT: GREENHOUSE-GAS EMISSION TRENDS

According to the Intergovernmental Panel on Climate Change (IPCC): “Global greenhouse gas (GHG) emissions have grown since pre-industrial times, with an increase of 70% between 1970 and 2004 (high agreement, much evidence).”

• “Since pre-industrial times, increasing emissions of GHGs due to human activities have led to a marked increase in atmospheric GHG concentrations….”
• “Between 1970 and 2004, global emissions of CO₂, CH₄, N₂O, HFCs, PFCs, and SF₆, weighted by their global warming potential (GWP), have increased by 70% (24% between 1990 and 2004)…. The emissions of these gases have increased at different rates. CO₂ emissions have grown between 1970 and 2004 by about 80% (28% between 1990 and 2004) and represented 77% of total anthropogenic GHG emissions in 2004….”

SOURCE:Metz, B., et al., eds. Climate Change 2007: Mitigation of Climate Change. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

The predicted climatic and biological changes associated with anthropogenic global warming are likely to have negative outcomes for Earth's human population, although it is likely that some regions will be more severely affected than others. As of 2000, about 400 million people lived within 65.6 ft (20 m) of sea level and within 12.4 mi (20 km) of a coast. Even small increases in global sea level, accompanied by increasingly intense coastal storms and floods, can therefore threaten the lives and property of a large number of people. Changes in regional temperature, precipitation, and extreme weather events would affect the managed agriculture, fishing, and forestry that provide food and shelter for Earth's human population.

Most scientists and many international policymakers now consider global warming to be a credible threat to Earth's natural environment and human population. However, attempts to prevent anthropogenic global warming, especially measures that require socioeconomic sacrifice or unfamiliar forms of behavior, have been controversial. The 1997 Kyoto Protocol—an extension of the 1992 United Nations Framework Convention on Climate Change (UNFCCC), an international treaty signed by most nations—acknowledges that human activities can alter global climate and commits signatory nations to reducing their greenhouse-gas emissions. As of June 2007, 172 nations had ratified the Kyoto Protocol. As of early 2008, the United States, the world's second-largest producer of greenhouse gases (edged out by China in 2007) and one of the biggest emitters on a person-by-person basis, had not ratified the treaty. Neither, up to that time, had Australia, the world's largest per capita producer of greenhouse gases as of 2005. These governments argued that the science of global warming remains inconclusive and that the economic consequences of action would be too great. Negotiations for a follow-up treaty to the Kyoto Protocol began in November 2007. However, in December of that year, Australia (with a new prime minister after a general election) announced that it was going to ratify the protocol.

Although there are ongoing research efforts to study methods of carbon capture and storage to reduce greenhouse gas emissions, most proposals to inject large quantities of CO₂ into underground reservoirs or oceans have, so far, met with economic, technological, and environmental barriers.

Increased extreme weather is a predicted consequence of global climate change. In August 2007, scientists at the World Meteorological Organization, an agency of the United Nations, announced that during the first half of 2007, Earth showed significant increases above long-term global averages in both high temperatures and frequency of extreme weather events, including heavy rainfalls, cyclones, and windstorms. Global average land temperatures for January and April of 2007 were the warmest recorded keeping for those two months since record keeping began in the 1800s.

Primary Source Connection

This article details comments about global climate change made to schoolchildren by Ban Ki-moon, Secretary-General of the United Nations. Ban states that global climate change will have major impacts on agriculture, economic activity, and migration patterns. Poor people living in developing countries will be among those who suffer the greatest impact from global climate change. Ban asserts that international cooperation will be necessary to win the “war” on global climate change.

William M. Reilly is the U.N. Correspondent for the United Press International (UPI).

THE U.N.'S WAR ON GLOBAL WARMING

UNITED NATIONS, March 5 (UPI) —U.N. Secretary-General Ban Ki-moon reached back to his past to tell the students he was addressing how as a child he first became aware of the world organization he now heads and how his experience shapes the way he equates the fight against global warming with war.

He spoke last Thursday at the U.N. International School in New York debating climate change.

Having taken over as secretary-general only 59 days earlier, from Kofi Annan, Ban said the speech was the first at the GA podium.

“A child of the Korean war, I grew up viewing the United Nations as a savior; an organization which helped my country, the Republic of Korea, recover and rebuild from a devastating conflict,” the secretary-general said, referring to the 1950–1953 war. “Because of decisions taken in this building, my country was able to grow and prosper in peace,” he said.

The prosperity helped Ban, from a farming village, rise up through his country's diplomatic ranks and become secretary-general.

But Ban said the big difference between the era in which he grew up and the world his audience would inherit was “the relative dangers we face.”

“Yet there is one crucial difference,” he said. “For my generation, coming of age at the height of the cold war, fear of a nuclear winter seemed the leading existential threat on the horizon.”

“Today, war continues to threaten countless men, women and children across the globe,” the secretary-general said. “It is the source of untold suffering and loss and the majority of the U.N.'s work still focuses on preventing and ending conflict. But the danger posed by war to all of humanity—and to our planet—is at least matched by the climate crisis and global warming.”

Said Ban, “I believe that the world has reached a critical stage in its efforts to exercise responsible environmental stewardship. Despite our best intentions and some admirable efforts to date, degradation of the global environment continues unabated, and the world's natural resource base is being used in an unsustainable manner.”

“Moreover, the effects of climate change are being felt around the world,” he said. “The latest assessment by the Intergovernmental Panel on Climate Change has established a strong link between human activity and climate change. The panel's projections suggest that all countries will feel the adverse impact.”

Not for the first time, Ban warned, “It is the poor—in Africa, small-island developing states and elsewhere— who will suffer most, even though they are the least responsible for global warming.”

He has mentioned such consequences previously, when listing climate change among his top priorities as new secretary-general.

“I am encouraged to know that, in the industrialized countries from which leadership is most needed, awareness is growing,” Ban said. “In increasing numbers, decision makers are recognizing that the cost of inaction or delayed action will far exceed the short-term investments needed to address this challenge.”

One of the issues he hoped the students would consider is “that there is an inextricable, mutually dependent relationship between environmental sustainability and economic development” around the world.

“Global warming has profound implications for jobs, growth and poverty. It affects agricultural output, the spread of disease and migration patterns,” Ban said. “It determines the ferocity and frequency of natural disasters. It can prompt water shortages, degrade land and lead to the loss of biodiversity.”

The secretary-general said in coming decades, “changes in our environment and the resulting upheavals—from droughts to inundated coastal areas to loss of arable lands—are likely to become a major driver of war and conflict.”

“These issues transcend borders. That is why protecting the world's environment is largely beyond the capacity of individual countries,” he said, arguing the need for concerted and coordinated international action will mean “the natural arena for such action is the United Nations.”

Said the secretary-general, “We are all complicit in the process of global warming. Unsustainable practices are deeply entrenched in our everyday lives. But in the absence of decisive measures, the true cost of our actions will be borne by succeeding generations, starting with yours.”

“That would be an unconscionable legacy; one which we must all join hands to avert,” he said. “As it stands, the damage already inflicted on our ecosystem will take decades, perhaps centuries, to reverse, if we act now.”

“Unfortunately, my generation has been somewhat careless in looking after our one and only planet,” Ban said. “But, I am hopeful that is finally changing and I am also hopeful that your generation will prove far better stewards of our environment.”

William M. Reilly

reilly, william m. “the u.n.'s war on global warming.” upi. march5, 2007. < http://www.upi.com/international_intelligence/analysis/2007/03/05/analysis_the_uns_war_on_global_warming/6584/> (accessed november29, 2007).

See Also Feedback Factors; Greenhouse Effect; Greenhouse Gases.

BIBLIOGRAPHY

Books

Gore, Al. An Inconvenient Truth:The Planetary Emergency of Global Warming and What We Can Do About It. Emmaus, PA: Rodale Press, 2006.

Lal, Rattan, et al. Climate Change and Global Food Security. New York: CRC, 2005.

McCaffrey, Paul. Global Climate Change. Minneapolis, MN: H. W. Wilson, 2006.

Metz, B., et al, eds. Climate Change 2007: Mitigation of Climate Change: Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Parry, M. L., et al, eds. Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Weart, Spencer. The Discovery of Global Warming. Cambridge, MA: Harvard University Press, 2004.

Web Sites

“Climate Change.” U.S. Environmental Protection Agency. < http://epa.gov/climatechange/index.html> (accessed November 20, 2007).

Intergovernmental Panel on Climate Change. < http://www.ipcc.ch> (accessed November 19, 2007).

United Nations Framework Convention on Climate Change. < http://unfccc.int/2860.php> (accessed November 1, 2007).

Larry Gilman

Global Warming

Earth’s Greenhouse Effect

Atmospheric Concentrations of Greenhouse Gases

Predictions and Evidence of Global Warming

Resources

Global warming refers to a long-term increase in the Earth’s average surface temperature that results in large-scale changes in global climate, including redistribution of climatic zones defined by temperature, precipitation, and associated ecosystems. Global climate changes and episodes of global warming, have occurred throughout geologic history as a result of natural variations in incoming solar radiation, atmospheric chemistry, and oceanic and atmospheric circulation. Anthropogenic, or human-caused, global warming and climate change are an ongoing outcome of human activities during the last 150 years, that is, since the Industrial Revolution, when human burning of fossil fuels multiplied many times. Scientific data show that atmospheric concentrations of carbon dioxide, methane, nitrous oxide, and human-made chemicals called halocarbons are increasing as a result of emissions associated with human activities, and models predict that this environmental change may lead to global warming. Studies of Antarctic ice cores published in 2005 showed that atmospheric carbon dioxide is now 27% higher than any other point in the last 650,000 years.

Earth’s Greenhouse Effect

Solar radiation is the major source of energy to Earth’s surface. Much of that incoming short-wavelength energy is absorbed by the surface where it drives atmospheric and oceanic circulation, and fuels biological processes like photosynthesis. The land and sea surfaces then reradiate extra longer-wavelength heat, or infrared, energy. If Earth’s atmosphere were transparent to the emitted infrared radiation, the planet would cool relatively efficiently and would have an average surface temperature of about 0°F (–18°C). However, the Earth’s naturally occurring “greenhouse effect” maintains the planet’s average temperature at a more livable 59°F (15°C) by trapping some of the escaping heat within the atmosphere. Small concentrations of so-called “greenhouse gases,” also known as radiatively active gases, absorb some of the infrared energy and thereby delay its passage to space. Water vapor (H₂O), carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and ozone (O₃) are the most concentrated and effective greenhouse gases. The greenhouse effect has been extremely important to the evolution and survival of life on Earth. A surface temperature of 59° F is sufficient to maintain the Earth’s reservoirs of life-sustaining liquid water, and to impel climatic processes, whereas 0°F is too cold for most organisms to live or for ecological processes to function well.

Atmospheric Concentrations of Greenhouse Gases

Prior to the modern influence of human activities on atmospheric chemistry, the naturally occurring greenhouse gases had fairly stable atmospheric concentrations: carbon dioxide about 280 ppm (or parts per million by volume), methane 0.7 ppm, and nitrous oxide 0.285 ppm. (Human activities do not appear to affect the concentration of water vapor, which varies naturally over time.) Today, the atmospheric concentration of CO₂ has increased to about 364 ppm, while that of CH₄ is 1.7 ppm, and N₂O is 0.304 ppm. The concentrations of chlorofluorocarbons (CFCs), and other completely man-made greenhouse gases, have increased from essentially zero to about 0.7 ppb (parts per billion by volume).

Atmospheric concentrations of the greenhouse gases have increased particularly quickly since the middle of the twentieth century, coinciding with rapid human population growth and intensive global industrialization. The combined effects of fossil fuel use and deforestation have increased the atmospheric concentration of CO₂. Fossil fuels, like oil, natural gas, and coal contain carbon in their chemical structure that, when liberated by combustion, combines with oxygen to create CO₂. Trees, like all plants, take in CO₂, incorporate carbon in their structure, and emit O₂ back into the atmosphere; deforestation destroys carbon “sinks” that lower the atmospheric concentration of CO₂. Fossil-fuel mining, decomposition of organic materials in human and livestock waste treatment facilities, and flooding in rice agriculture have led to increased emissions of CH₄. Agricultural fertilizers, and combustion of fossil fuels and solid wastes account for increased N₂O emissions. Industrial processes emit a variety of powerful synthetic greenhouse gases like CFCs, hydrofluorcarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF₆).

The greenhouse gases vary greatly in their ability to absorb infrared radiation. On a per-molecule basis, methane is about 25-40 times more absorptive than carbon dioxide, nitrous oxide is 200-270 times, and CFCs are 3-15 thousand times. CO₂, however, has by far the largest atmospheric concentration, and has experienced the greatest increases due to human activity. CO₂ is responsible for about 60% of the human contribution to increased atmospheric heat retention.

Predictions and Evidence of Global Warming

Most atmospheric scientists today, with few exceptions, are convinced that the well-documented increase in greenhouse gases is resulting in an intensification of Earth’s naturally occurring greenhouse effect, with resulting climate change. The exact climatic response to increased concentrations of radia-tively active gases, and its potential effects on humans are, however, difficult to predict. If global climate change proceeds as most recent scientific studies forecast, it will have substantial and disastrous climatic, ecological, and sociopolitical consequences.

The Earth’s surface is surface temperature is variable from place to place and over time. Furthermore, the systems that interact to maintain the planet’s temperature and climate are extremely complex; cause-and-effect relationships between changes in one system, the atmosphere in this case, and results in another, global climate, are very difficult to predict, observe, and “prove.” In spite of these scientific challenges, there is significant evidence that the Earth has warmed significantly during the past 150 years or so, and that global climate has responded to the temperature increase. Climate records show a 1°F increase in the average temperature of the Earth’s oceans, atmosphere, and solid surface since the late 1900s. Geologic and historical studies document dramatic thinning and shrinkage of the polar ice caps, and retreat of Earth’s alpine glaciers. Less conclusive, but still suggestive, data supporting anthropogenic global warming include a several centimeter increase in global sea-level since 1900, and alterations in large-scale weather phenomena like the southeast Indian monsoon, Atlantic hurricane season, El Niño Southern Oscillation, and North African drought cycle.

The empirical, or observed, data listed above generally agree with predictions computed by mathematical models of global climate processes. These “virtual experiments,” called three-dimensional general circulation models (GCMs), simulate the complex movements of energy and mass involved in the global circulation of the atmosphere and oceans. Scientists use GCMs to predict the effects of a change in a specific variable, like the concentration of atmospheric CO₂, on the rest of the global climate system. Because of the complexity of the computational problem, GCMs that attempt to predict global climate change have had somewhat variable results. However, most experiments do suggest that the increased concentration of atmospheric greenhouse gases has resulted, and will continue to result, in global warming. For example, one GCM that doubles the present CO₂ concentration to about 700 ppm predicts a 2-6° F rise in global temperature, and suggests that the warming would be 2-3 times more intense at high latitudes than in the tropics.

Other predicted consequences of warming include large-scale shifts in atmospheric and oceanographic circulation patterns, melting of the polar ice caps, global sea-level rise, reorganization of the Earth’s climatic zones, and establishment of new large-scale weather patterns. Such changes in the distribution of heat, precipitation, and weather phenomena like storms and floods would affect the productivity and distribution of natural and managed vegetation. Animals and microorganisms would experience dramatic changes in their habitats, and perhaps face much higher rates of species extinction. Most ecologists consider that global warming, if were it to occur as predicted, would represent a serious threat to biodiversity and to the health of ecosystems worldwide.

The predicted climatic and biological changes associated with anthropogenic global warming could have potentially disastrous outcomes for the Earth’s human population. In 2006, about two thirds of the world’s population, some 4 billion people, lived within 120 miles (400 km) of the coast. Even small increases in global sea level, and in the intensity of coastal storms and floods, would threaten the lives and property of large numbers of people. Changes in regional temperature, precipitation, and weather, as well as biological health, would affect the managed agriculture, fishing, and forestry that provide food and shelter for the Earth’s burgeoning human population.

Most scientists, and many international policymakers, now consider global warming to be a credible threat to the Earth’s natural environment and human

KEY TERMS

Global warming —A projected increase in Earth’s surface temperature caused by an increase in the concentration of greenhouse gases, which absorb infrared energy emitted by Earth’s surface, thereby slowing its rate of cooling.

Greenhouse effect —The warmingof Earth’s atmosphere as a result of the capture of heat re-radiated from the Earth by certain gases present in the atmosphere.

population. However, because the specific consequences of global warming are difficult to predict, and in some cases unknown, the scientific community remains divided about the potential effects of the phenomenon. Attempts to prevent anthropogenic global warming, especially measures that require socioeco-nomic sacrifice, have therefore been extremely controversial. The 1992 United Nations Framework Convention on Climate Change (UNCCC),also called the Kyoto Protocol, acknowledges that human activities can alter global climate, and requires signatory nations to reduce greenhouse gas emissions. As of October 2006, 166 nations had ratified the Kyoto protocol. The United States, by far the world’s largest net producer of greenhouse gases and one of the biggest per capita, had not signed the treaty. Neither had Australia, the world’s largest per capita producer of greenhouse gasses as of 2005. These governments argued that the science of global warming remains inconclusive, and that the economic consequences of action would be too great.

Resources

BOOKS

Gore, Al. An Inconvenient Truth. Emmaus, PA: Rodale Books, 2006.

Intergovernmental Panel on Climate Change. Safeguarding the Ozone Layer and the Global Climate System: Special Report of the Intergovernmental Panel on Climate Change. Cambridge, UK: Cambridge University Press, 2005.

Lal, Rattan, et al. Climate Change and Global Food Security. New York: CRC, 2005.

McCaffrey, Paul. Global Climate Change. Minneapolis: H. W. Wilson, 2006.

OTHER

Intergovernmental Panel on Climate Change. <http://www.ipcc.ch> (accessed November 1, 2006).

United Nations. “United Nations Framework Convention on Climate Change.” <http://unfccc.int/2860.php> (accessed November 1, 2006).

United States Environmental Protection Agency. “Climate Change.” <http://epa.gov/climatechange/index.html> (accessed November 1, 2006).

Bill Freedman
Laurie Duncan

Global Warming

Introduction

Global warming is a long-term increase in Earth’s average surface temperature. Because global warming does not cause uniform warming in all locations and because many other changes in climate are occurring, scientists often prefer to speak of “global climate change” rather than of global warming when referring to the whole complex of climate changes being caused by human activities. Thus, scientists often reserve the phrase “global warming” for higher temperatures as such, apart from other climate changes. However, in general usage it still refers to the sum of all changes occurring in Earth’s climate as a result of human activities. A major effect of global warming is redistribution of climatic zones as defined by temperature, precipitation, and associated ecosystems.

Global climate changes, including episodes of global cooling and warming, have occurred many times throughout Earth’s history as a result of natural variations in solar radiation, atmospheric chemistry, oceanic and atmospheric circulations, volcanic eruptions, and other factors. Global warming in the last few decades, however, is primarily caused by human activities that started during the Industrial Revolution, when human burning of fossil fuels began to increase drastically, releasing large amounts of carbon dioxide (CO₂). Atmospheric concentrations of CO₂, methane (CH₄), nitrous oxide (N2_O), and artificial chemicals called halocarbons have long been increasing as a result of emissions from fossil-fuel burning and other activities. Increased atmospheric concentrations of these gases are causing Earth to warm.

As of 2008, atmospheric levels of CO₂ and CH₄ were about a third higher than at any time in at least 650,000 years, as shown by air bubbles trapped in ancient Antarctic ice. Global average surface temperature had increased by about 0.23°F (0.13°C) per decade for the last 50 years, and had increased more rapidly in recent years: The two warmest years since instrumental records began in the 1800s were 1998 and 2005.

Although a few scientists continue to doubt that recent climate warming is caused by human activities, this is no longer disputed by the great majority of climatologists, meteorologists, and geophysicists. The 2007 Assessment Report of the United Nations’ Intergovernmental Panel on Climate Change (IPCC) pronounced that global warming is now “unequivocal” and that it is more than 90% likely that most of the global warming observed since the mid-twentieth century is caused by anthropogenic releases of greenhouse gases.

Historical Background and Scientific Foundations

Solar radiation is the major source of energy arriving at Earth’s surface (a small fraction, about 1/7500, comes from Earth’s interior). Much of this incoming energy is at short wavelengths (ultraviolet and visible light). These forms of light are absorbed by Earth’s surface, where their energy drives atmospheric and oceanic circulations and biological processes like photosynthesis. The surface then re-radiates most of this energy as longer-wavelength light, variously termed heat radiation, infrared light, or infrared radiation. If Earth’s atmosphere were completely transparent to infrared radiation, this re-radiated energy would simply shine out into space and the planet would have an average surface temperature of about 0°F (–18°C).

However, certain gases in the atmosphere, mostly notably CO₂, CH₄, and water vapor (H₂O), absorb some of the infrared light radiated by Earth’s surface and are warmed. This warms the atmosphere directly and warms Earth’s surface both by contact with warmer air and by re-radiation of infrared radiation from the atmosphere. The overall effect is to slow the escape of energy from Earth, warming the planet like an invisible blanket. Gases that have this effect are termed greenhouse gases because, like the glass windows of a greenhouse, they allow solar energy to enter Earth’s atmosphere but impede the loss of heat. Earth’s atmosphere naturally contains certain levels of greenhouse gases. The resulting natural greenhouse effect maintains the planet’s average temperature at a livable 59°F (15°C). This natural greenhouse effect has been crucial to the evolution and survival of life on Earth. Today’s average surface temperature of 59°F is sufficient to maintain most of Earth’s water in liquid form, but 0°F (-18°C) would be too cold for most organisms to live or for ecological processes to function.

Although atmospheric levels of CO₂ and CH₄ have varied greatly over geologic time (hundreds of millions of years), by the time of the beginning of the Industrial Revolution around 1750, their concentrations had been fairly stable for hundreds of thousands of years. The pre-industrial concentration of CO₂ was about 280 parts per million by volume (ppm), of CH₄ about 0.7 ppm, and of NO₂ about 0.285 ppm. (The special case of water vapor is discussed later.) As of 2005, the atmospheric concentration of CO₂ had increased to about 379 ppm, a 36% increase. The concentration of CH₄ has gone up to 1.8 ppm and that of N₂O to 0.319 ppm. Concentrations of chlorofluorocarbons (CFCs) and other completely human-made greenhouse gases have increased from zero in pre-industrial times to about 0.7 ppb (parts per billion by volume).

Atmospheric concentrations of greenhouse gases have increased more quickly since the middle of the twentieth century, coinciding with rapid human population growth and global industrialization. The largest single cause of greenhouse gas releases has been the burning of fossil fuels, including oil, natural gas, and coal. Fossil fuels contain carbon which, in burning, combines with oxygen to create CO₂.

Deforestation is the second-largest emitter of greenhouse gases. As they grow, trees, like all plants, take in CO₂, incorporate the carbon in their structure, and release O₂ into the atmosphere. Deforestation therefore destroys a carbon sink that tends to lower the atmospheric concentration of CO₂. Deforestation also releases CO₂ and CH₄ through the burning and decay of trees and soil carbon in deforested areas.

Agriculture is a primary cause of deforestation. It also releases large quantities of CH₄ through decomposition of organic materials in livestock digestive systems, livestock waste-treatment facilities, and flooded rice fields. Agricultural fertilizers and combustion of fossil fuels and solid wastes are the primary sources of increased N₂O emissions.

Industrial processes emit a variety of powerful synthetic greenhouse gases, including chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), perfluorocarbons

WORDS TO KNOW

ANTHROPOGENIC: Made by humans or resulting from human activities.

CARBON SINK: A location like a forest where there is net storage of carbon as sequestration exceeds release.

INFRARED RADIATION: Electromagnetic radiation of a wavelength shorter than radio waves but longer than visible light that takes the form of heat.

INDUSTRIAL REVOLUTION: The period, beginning about the middle of the eighteenth century, during which humans began to use steam engines as a major source of power.

KYOTO PROTOCOL: Extension in 1997 of the 1992 United Nations Framework Convention on Climate Change (UNFCCC), an international treaty signed by almost all member countries with the goal of mitigating climate change.

PHOTOSYNTHESIS: The process by which plants fix carbon dioxide from the atmosphere using the energy of sunlight.

RADIATIVE FORCING: A change in the balance between incoming solar radiation and outgoing infrared radiation, resulting in warming or cooling of Earth’s surface.

RESIDENCE TIME: For a greenhouse gas, the average amounof time a given amount of the gas stays in the atmosphere before being absorbed or chemically altered.

(PFCs), and sulfur hexafluoride (SF₆). Many of these chemicals also attack the ozone layer, and their manufacture and use has been limited by the Montreal Protocol treaty of 1987.

Greenhouse gases vary greatly in their ability to absorb infrared radiation. On a per-molecule basis, methane is about 21-40 times more absorptive than carbon dioxide, nitrous oxide is 200-270 times more absorptive, and CFCs are 3,000-15,000 times more absorptive. CO₂, however, is present in the atmosphere at much higher concentrations than the more-powerful greenhouse gases and has experienced the greatest increases due to human activity. CO₂ is responsible for about 60% of the human contribution to increased atmospheric heat retention, termed radiative forcing.

Water vapor is responsible for more of the greenhouse effect than any other single gas, but is not a primary driver of global warming. This apparent contradiction is explained by the fact that the residence time of water vapor in the atmosphere is short. Increased water vapor concentrations, left to themselves, would disappear in a matter of weeks as rain or snow fall from clouds. The other greenhouse gases, however, linger for decades: These gases force the atmosphere to warm, increasing its ability

to hold water vapor. This increased level of water vapor acts to warm the world further. Therefore, water vapor acts as a feedback factor in global warming.

Impacts and Issues

Most scientists today are convinced that an increase in greenhouse gases is resulting in an intensification of Earth’s greenhouse effect, with resulting global warming and climate change. The exact climatic response to increased concentrations of greenhouse gases and its potential effects on humans are difficult to predict, but can be foretold in outline. Moreover, computer models of climate are constantly improving, allowing more realistic predictions to be made about future climate change. If global climate change proceeds as recent scientific studies forecast, it will likely have substantial negative ecological consequences.

Earth’s surface temperature is variable from place to place and over time. Furthermore, the systems that interact to maintain the planet’s temperature and climate are complex; cause-and-effect relationships between the oceans, atmosphere, land, living things, and nonliving chemistry are numerous. In spite of these scientific challenges, there is overwhelming evidence that Earth has warmed significantly during the past 150 years or so. Climate records show that from 1906 to 2005 there was a 1.3°F (0.74°C) increase in Earth’s average surface temperature, with twice as much warming as this global average occurring in the Arctic and over the West Antarctic Peninsula. The oceans have also warmed, though not as much.

Recent observations show dramatic melting of the Arctic sea ice, accelerated melting of the Greenland ice cap, and accelerated seaward flow of glaciers on the western peninsula of Antarctica. Global sea-level has risen since the mid-1800s; for 1961–2003, at about .07 in (1.8 mm) per year. Alterations have been seen in large-scale weather phenomena like the southeast Indian monsoon season, Atlantic hurricane season, El Niño/ Southern Oscillation, and North African drought cycle. Wildlife and plants are shifting their ranges to higher latitudes (closer to the poles, to track cool weather as climate warms) and to higher altitudes in mountainous regions. Many other types of observations, some of which are reviewed elsewhere in this book, independently demonstrate the reality of global warming and the complex changes it has produced in regional climates.

The aforementioned observations agree with predictions by computerized mathematical models of global climate processes. These three-dimensional general circulation models (GCMs) simulate the complex movements of energy and mass involved in the global circulation of the atmosphere, oceans, and living things. Scientists use GCMs to predict the effects of changes in specific variables, like the atmospheric concentration of CO₂, on the rest of the global climate system. Because of the complexity of the computational problem, GCMs that attempt to predict global climate change have some what variable results. However, modelers now agree that increased concentrations of atmospheric greenhouse gases have resulted, and will continue to result, in global warming. For example, one GCM that doubles the present CO₂ concentration to about 700 ppm predicts a 2-6°F (1.1-3.3°C) rise in global temperature and suggests that the warming would be 2 to 3 times more intense at high latitudes—nearer the poles—than in the tropics. These predictions are consistent with changes seen so far, or, if anything, tend to underestimate the pace of warming.

Other predicted consequences of warming include large-scale shifts in atmospheric and oceanographic circulation patterns, partial melting of the polar ice caps, global sea-level rise, reorganization of Earth’s climatic zones, and establishment of new large-scale weather patterns. Changes in the distribution of heat, precipitation, and weather phenomena like storms and floods will affect the productivity and distribution of natural and managed vegetation. Animals and microorganisms will experience dramatic changes in their habitats and face higher rates of species extinction. According to the Intergovernmental Panel on Climate Change (IPCC), a rise of about 4°F (2.2°C) above the pre-industrial average will likely commit 15-37% of all plant and animal species to extinction. Most biologists agree that global warming is a serious threat to biodiversity and to the health of ecosystems worldwide.

The predicted climatic and biological changes associated with anthropogenic global warming are likely to have negative outcomes for Earth’s human population, although it is likely that some regions will be more severely affected than others. As of 2000, about 400 mil-

IN CONTEXT: AUTHORITATIVE ARGUMENTS AND EVIDENCE ABOUT GLOBAL WARMING

“Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.”

…“Eleven of the last twelve years (1995–2006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850).”

SOURCE: Solomon, S., et al, eds. “IPCC, 2007: Summary for Policymakers.” In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

lion people lived within 65.6 ft (20 m) of sea level and within 12.4 mi (20 km) of a coast. Even small increases in global sea level, accompanied by increasingly intense coastal storms and floods, can therefore threaten the lives and property of a large number of people. Changes in regional temperature, precipitation, and extreme weather events would affect the managed agriculture, fishing, and forestry that provide food and shelter for Earth’s human population.

Most scientists and many international policymakers now consider global warming to be a credible threat to

Earth’s natural environment and human population. However, attempts to prevent anthropogenic global warming, especially measures that require socioeconomic sacrifice or unfamiliar forms of behavior, have been controversial. The 1997 Kyoto Protocol—an extension of the 1992 United Nations Framework Convention on Climate Change (UNFCCC), an international treaty signed by most nations—acknowledges that human activities can alter global climate and commits signatory nations to reducing their greenhouse-gas emissions. As of January 2008, 177 nations had ratified the Kyoto Protocol. The United States, the world’s second largest producer of greenhouse gases (after China) and one of the biggest emitters on a person-by-person basis, was the only industrialized country in the world to have not ratified the treaty. (Australia, another holdout, signed in late 2007.) The U.S. government argued that the science of global warming remains inconclusive and that the economic consequences of action would be too great. Negotiations for a follow-up treaty to the Kyoto Protocol began in November 2007.

Although there are ongoing research efforts to study methods of carbon capture and storage to reduce greenhouse-gas emissions, most proposals to inject large quantities of CO₂ into underground reservoirs or oceans have, so far, met with economic, technological, and environmental barriers.

Increased extreme weather is a predicted consequence of global climate change. In August 2007, scientists at the World Meteorological Organization, an agency of the United Nations, announced that during the first half of 2007, Earth showed significant increases above long-term global averages in both high temperatures and frequency of extreme weather events, including heavy rainfalls, cyclones, and wind storms. Global average land temperatures for January and April of 2007 were the warmest recorded for those two months since records began in the 1800s.

See Also Climate Change; Greenhouse Effect; Greenhouse Gases; Ice Cores; Intergovernmental Panel on Climate Change; IPCC 2007 Report; Kyoto Protocol; United Nations Framework Convention on Climate Change

BIBLIOGRAPHY

Books

Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. Emmaus, PA: Rodale Press, 2006.

Lal, Rattan, et al. Climate Change and Global Food Security. New York: CRC, 2005.

McCaffrey, Paul. Global Climate Change. Minneapolis, MN: H. W. Wilson, 2006.

Metz, B., et al, eds. Climate Change 2007: Mitigation of Climate Change: Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Parry, M. L., et al, eds. Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Weart, Spencer. The Discovery of Global Warming. Cambridge, MA: Harvard University Press, 2004.

Web Sites

Intergovernmental Panel on Climate Change (United Nations). “Intergovernmental Panel on Climate Change.” 2008. http://www.ipcc.ch (accessed March 24, 2008).

United Nations. “United Nations Framework Convention on Climate Change.” 2008. http://unfccc.int/2860.php (accessed March 24, 2008).

U.S. Environmental Protection Agency. “Climate Change.” March 14, 2008. http://epa.gov/climatechange/index.html (accessed March 24, 2008).

Larry Gilman

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 evidence—based on continuous satellite monitoring and data from about 2,000 meteorological stations around the world—indicates that globally averaged surface temperatures have warmed by about 0.3 to 0.6°C since the late nineteenth century. Different regions have warmed—some have even cooled—by 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.

Causes

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 process—dubbed the greenhouse

effect—keeps 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 works—understandable 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 Bangladesh—one of the world's poorest countries—will 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 emissions—during 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); NO_x (Nitrogen Oxides); Ozone; Treaties and Conferences.

Bibliography

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 http://www.ipcc.ch.

Intergovernmental Panel on Climate Change. "Climate Change 2001: IPCC Third Assessment Report." Available at http://www.grida.no/climate/ipcc_tar.

David Frame