global warmingan increase in global average surface temperature resulting from an increase in the amount of carbon dioxide, methane, and certain other trace gases in the atmosphere. These gases are known collectively as greenhouse gases because they contribute to a warming of the Earth’s surface and lower atmosphere, a phenomenon called the greenhouse effect. In part because the emission of carbon dioxide is related to the essential use of carbon-based energy sources, the issue of global warming incorporates a broad scientific and political debate about its significance and consequences.
The scientific background

Through much of the 20th century, many scientists and policy makers alike shared a perception that climate—the aggregate statistics of weather over time and space—was fairly stable and dependable. This belief is clearly reflected in policies such as those set out in the U.S. National Flood Insurance Program, which was developed in the late 1960s and was based on the assumption that 30 years of flood history could be used as a reliable basis for calculating future flood risk. Scientific research subsequently contradicted this common perception of climate, and today scientists have ample evidence that climate is not stable and dependable but in fact changes—sometimes quite abruptly and dramatically over periods shorter than generations.

In the 1970s some climate scientists focused on the possibility that the Earth was moving inexorably toward a new ice age, then characterized as “global cooling.” At the same time, Manabe Syukuro, a Japanese meteorologist who was an early pioneer in creating complex computer models of the climate, explored the possibility that the emissions of great quantities of carbon dioxide and other greenhouse gases from the combustion of fossil fuels such as coal, oil, and natural gas could affect climate. Some 75 years earlier, Swedish chemist Svante Arrhenius and American geologist Thomas C. Chamberlin had independently hypothesized that increasing the atmospheric concentration of greenhouse gases could raise the atmosphere’s temperature by leading to a greater absorption of solar radiation that would otherwise be reflected from the Earth’s surface back into space. In the 1970s and 1980s, scientific opinion turned in support of this hypothesis as data gathered since the 1950s showed a steady increase in the amount of greenhouse gases in the atmosphere, and early modeling efforts supported the greenhouse hypothesis. The attention of the public became focused on the idea of rising temperatures worldwide, and the issue has since been referred to as “global warming.”

Today, as the result of the efforts of thousands of scientists over several decades who have studied climate and its interconnections with the global land-ocean-atmosphere system, the scientific understanding of global warming is both more clear and more complex. Scientists have obtained strong evidence that climate does indeed change, on timescales time scales ranging from a few seasons to millennia. Scientists have further shown that human activities can indeed influence the climate. However, research also indicates that the human influence on the climate system is deeply woven into the fabric of climate variability, which makes the detection and attribution of specific sources of change very difficult. If these difficulties were not challenging enough, researchers have postulated that, in addition to the production and emission of greenhouse gases, human-related activities such as changes in the use of the land surface—deforestation, afforestation, desertification, irrigation, urbanization—can also affect local, regional, and even global climate patterns.

To further the understanding of climate, scientists have gathered detailed observations of various weather phenomena (such as temperature, precipitation, and storms) and of related influences on climate such as ocean currents and the atmosphere’s chemical composition. Scientists have then used sophisticated computer models called general circulation models to incorporate these observations with the many factors that have an influence on climate in order to study past, present, and future climate patterns. These models have projected an increase in global average temperatures that will continue for decades as a result of greenhouse gas emissions. Concerned about such projections and their potentially severe consequences for human populations and the environment, nations around the world in the late 1980s and early 1990s focused their attention on developing policies to control greenhouse gases. One step was to organize the scientific community to provide information on a periodic basis to policy makers. Toward this end the World Meteorological Organization and the United Nations Environment Programme formed the Intergovernmental Panel on Climate Change (IPCC) in 1988.

The IPCC has produced several major assessments for policy makers. In February 2007 the panel released a summary from its first working group on the science of climate change in anticipation of the release of its fourth assessment report later in the year. The IPCC largely reaffirmed the conclusions of its earlier reports, indicating that it is now 90 percent certain that most of the warming observed over the previous half century could be attributed to greenhouse gas emissions produced by human activities (i.e., primarily industrial processes and transportation). In 2001 the panel had concluded that the previous 100 years had seen an increase in global average surface temperature of 0.6 °C (1.1 °F), with an error range of 0.2 °C (0.4 °F). The IPCC also estimated that by 2100 the global 2001 report predicted that the average surface temperature will have increased would increase by 1.4 to 5.8 °C (2.5 to 10.4 °F) , depending on by 2100, according to the best estimate values over a range of scenarios for greenhouse gas emissions. Revised estimates from the fourth assessment report in 2007 forecast a 1.8–4.0 °C (3.2–7.2 °F) rise in average surface temperature over the same period. The IPCC projected that “future changes in climate are expected to include additional warming, changes in precipitation patterns and amounts, sea-level rise, and changes in the frequency and intensity of some extreme events.” Such changes would likely have significant impacts on both human and ecological systems.

The policy debate

The IPCC’s assessments are intended to inform action. At the international level, policy discussions centre on the United Nations Framework Convention on Climate Change. The convention, signed by 154 nations at the 1992 Earth Summit in Rio de Janeiro, calls for voluntary reductions in greenhouse gas emissions. Under the convention international negotiations for stronger commitments to reductions in emissions led to a proposed agreement in 1997 called the Kyoto Protocol (so named for the Japanese city in which it was negotiated). The Kyoto Protocol has been the subject of intense debate and discussion, which tend to distinguish developed countries (which are included under the protocol) from developing countries (which are not included). Within developed countries the protocol has its proponents, who seek to reduce the risks of future human-caused climate change, and its opponents, who seek to avoid the risks that the protocol poses to economic development and growth. Debate has focused on the United States because it emits more greenhouse gases into the atmosphere than any other country and because, in 2001, the United States government rejected the protocol for economic reasons. Despite the lack of support by the United States, the protocol garnered sufficient participation to go into force in 2005.

The debate over the Kyoto Protocol involves questions of both science and economics, and it also involves considerations of environmental values and of international trade and diplomacy. Because the debate hinges on expectations about the future impact both of human activities on the climate and of government policy on national economies, there is considerable room for irreconcilable views and hence political stalemate. With hopes of resolving the stalemate, researchers have continued to develop more-precise observations of the Earth’s climates and more-sophisticated general circulation and economic models in order to gain a clearer understanding of future changes and thereby increase the chances for policy action.

However, after years of intensive research, three realizations have changed the international debate on global warming. The first is that even a full and successful implementation of the Kyoto Protocol may have only an imperceptible effect on the climate system, because the reductions in greenhouse gas emissions mandated by the protocol are dwarfed by those that scientists in the IPCC have said are needed to have a discernible influence on climate. Second, problems encountered in negotiating and ratifying the Kyoto Protocol suggest that political, economic, and technological considerations will make the protocol very difficult to implement. The protocol’s proponents can only characterize it as a small first step or symbolic gesture, whereas its opponents can label it a costly boondoggle. The third realization is that the growing environmental, economic, and human costs associated with changes in climate depend less on the changes themselves than on patterns of development (e.g., demographics, wealth and poverty), decisions about where and how development takes place, the characteristics of health care systems, ecosystem management or exploitation, and other factors.

As a sign of change in the international response to global warming, delegates at a meeting under the Framework Convention on Climate Change in 2002 in New Delhi issued a declaration concluding that “actions are required to diversify energy supply by developing advanced, cleaner, more efficient, affordable and cost-effective energy technologies” and also that “adaptation to the adverse effects of climate change…requires urgent attention and action on the part of all countries.” These conclusions suggest an alternative to the political stalemate because agreement on such actions does not depend on foreseeing changes in either the climate or national economies but can be justified on their own merits. No matter how the science and politics of global warming evolve over the coming years, one fact remains irrefutable: global warming presents a challenge that will be with us for decades, and perhaps centuries, to come.

Richard C.J. Somerville, The Forgiving Air: Understanding Environmental Change (1996, reissued 1998), is a readable introduction to the science of climate and global environmental change. Stephen H. Schneider, Armin Rosencranz, and John O. Niles (eds.), Climate Change Policy: A Survey (2002), a primer on various aspects of the policy debate, explains alternatives for dealing with climate change. John T. Houghton, Global Warming: The Complete Briefing, 3rd ed. (2004), offers a perspective on climate change from one of the leading participants in the IPCC process. Daniel Sarewitz and Roger Pielke, Jr., “Breaking the Global-Warming Gridlock,” The Atlantic Monthly, 286(1):55-64 (2000), is an alternative view on how to make progress on climate policy by focusing on reducing vulnerability to climate impacts.

The Intergovernmental Panel on Climate Change, Climate Change 2001, 3 vol. (2001), is the United Nations’ assessment of the science, impact, and economics of climate change and is its third assessment in 11 effort since 1990. The Intergovernmental Panel on Climate Change, Climate Change 2007: The Physical Science Basis: Summary for Policymakers (2007), is the United Nations’ assessment of the science of climate change and the initial part of its fourth assessment effort in 18 years.