Global Climate Change Briefing Book
Congressional Research Service
Redistributed as a service of the National Library for the Environment

Greenhouse Effect and Global Climate Change

John R. Justus

There are a large number of scientists who believe that human activities, which have increased atmospheric concentrations of carbon dioxide (CO2) by more than one-third over the past 100 years, may be leading to an increase in globally average temperatures. However, this so-called "global warming" theory is not without challengers who argue that scientific proof supporting such theories is incomplete, and that many uncertainties remain surrounding the nature and direction of Earth's climate. Nevertheless, concern is growing that human activities, such as the burning of fossil fuels, industrial production, deforestation, and certain land-use practices, are increasing atmospheric concentrations of carbon dioxide (CO2) that, along with increased concentrations of other trace gases [chlorofluorocarbons (CFCs), methane, nitrous oxide, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluorane (SF6), and trifluoromethyl sulfur pentafluoride (SF5CF3)] are leading to changes in the fundamental chemical and physical composition of Earth's atmosphere. Those changes have the potential to alter the way heat and energy are distributed among the land, ocean, atmosphere, and ice components of Earth's climate system. Ultimately, such changes might affect Earth's climate.

Latest computer models of the Earth's climate (GCMs) have projected a globally averaged warming of 3 to 11 degrees F over the next 100 years, if greenhouse gas emissions were to continue to accumulate at the current rate. Those models show an enhancement of Earth's naturally occurring "greenhouse effect" -- the process by which the atmosphere traps and re-radiates infrared radiation and warms the Earth's surface -- with warming in some regions, little change in other regions, and cooling in still others. Climate scientists have predicted that such a warming could shift temperature zones, rainfall patterns, and agricultural belts and, under certain scenarios, cause sea level to either rise and inundate low-lying coastal areas or, in the event polar ice sheets grow, to fall and expose more land area near the coasts. They further predict that global warming, to the extent it occurs, could have far-reaching effects -- some positive, some negative depending on how it may be experienced in a given region -- on natural resources; biodiversity; food and fiber production; energy supply, use, and distribution; transportation; land use; water supply and control; and human health and welfare. Scientists are also debating whether a recent emergence/re-emergence of certain tropical diseases in the mid-latitudes may be an indication that Earth's climate is warming on average.

There have been controversial claims that a CO2-induced global warming signal has been detected. Indeed, globally averaged air temperatures at the Earth's surface have become about 0.9 degrees F warmer over the last 100 years. The twelve warmest years since historical records have been kept have occurred in the past two decades, with 1990, 1995, 1997, and 1998 among the warmest. In some cases, relationships have been inferred between natural seasonal to interannual climate changes and present-day severe weather events, such as those occurrences that have been noted in conjunction with the El Nino phenomenon. Changes have also been reported in the measured frequency and intensity of extreme precipitation events. Such changes, report the scientists who have analyzed those data, are broadly consistent with computer model projections of the Earth's hydrological cycle under an enhanced greenhouse effect. Heretofore, natural variability of climate has always been large enough that even the record-setting warmth and severe weather events in the 1980s and 1990s have not allowed most scientists to state with any certainty that such weather extremes are attributable to human-induced global warming or that a global warming signal attributable to human activities has been identified. In 1996, however, the Intergovernmental Panel on Climate Change, jointly established in 1988 by the United Nations World Meteorological Organization (WMO) and the United Nations Environment Program (UNEP), reported in its Second Assessment (1996) that ". . . [such] a change is unlikely to be entirely natural in origin . . . [and that] the balance of evidence, from changes in global mean surface air temperature and from changes in geographical, seasonal, and vertical patterns of atmospheric temperature, suggests a discernible human influence on global climate." And now, the preface of the latest draft report (November 2000) from the Intergovernmental Panel's new Third Assessment exercise concedes that a firmer association between human activities and climate seems to have emerged. That is news, because reservations about the source of the past century's warming and whether it bore a human fingerprint are often cited in policy debates, usually in support of deferring actions aimed at mitigating possible global warming. Those statements by the IPCC have focused attention in the executive, congressional, and public sectors on possible outcomes of potential climate change, to the extent that such projections are borne out, and on the need for better understanding of global and regional climate and its impacts.

Some scientists have criticized general circulation models, or GCMs, which are supposed to simulate Earth's climate system, including complicated interactions among the land, ocean, and atmosphere. Early GCMs projected temperature increases of 9 to 16 degrees F over the next century. Current scientific debate on global warming addresses why observed temperatures so far have lagged behind theoretical climate model projections, at least as far a daytime maximum temperatures are concerned. That research has yielded findings about sulfate aerosols in the atmosphere which are capable of producing different climatic effects. For example, large aerosol droplets act like a greenhouse gas and contribute to atmospheric warming. However, a modest amount of sulfate aerosol cooling results from cloud cover associated with those sulfate aerosols during daylight hours around highly industrialized areas. This effect is especially pronounced in the mid-latitudes of the Northern Hemisphere, where it is believed sulfate aerosols may be suppressing the global warming effect. On the other hand, an 0.5 degree F increase of daily night-time minimum temperatures has been observed, when heat gained during daylight hours is released from the Earth's surface and trapped by those very same clouds, as well as by increasing amounts of atmospheric water vapor and the other infrared absorbing greenhouse gases. Other scientists, representing the more skeptical point of view about global warming, have argued that sulfate aerosol particles represented in GCMs cannot, by themselves, account for the magnitude and distribution of offset cooling that has kept the observed temperature rise over the last 100 years below model-predicted temperature rise for the same period. They further criticize the GCMs for not even being able to demonstrate how heat is distributed throughout the atmosphere over a 24-hour period.

Skeptics have called to question the idea of a warming that is truly global, noting that measurements taken by satellites orbiting the Earth since 1979 have found little or no such positive temperature trend in large parts of the middle atmosphere over 21 years of record (1979-1999). Instruments aboard those satellites measure the radiative properties of certain atmospheric gases in a vertical column of air well above the Earth's surface, from which average temperatures of the atmosphere are then deduced. The all-encompassing satellite coverage, it is argued, yields results that are more reliable than temperature measurements made at the Earth's surface that do show a warming trend over the past 100 years. In January 2000, a special study by the National Research Council's Climate Research Committee reported that the apparent disparity between the two data sets over the 20-year history of the satellite measurements ". . . in no way invalidates the conclusion that surface temperature has been rising . . ." and that ". . . the surface warming is undoubtedly real." The scientific dissenters within the community, however, are far from conceding the debate, and, while some prominent skeptics might even acknowledge that the average surface temperature of the Earth has indeed risen, the crucial question for them is: What is causing it? On that issue, and on the larger question of how might the climate change in the future, the skeptics differ markedly with the dominant view among climate scientists, adhering to the view that any future warming will be inconsequential or modest at most, and that its effects will be beneficial overall. In a turn worthy of note, new research has even gone so far to imply that fossil fuel burning may not be as important in the mechanics of climate change as previously thought. NASA-funded research by Dr. James Hansen of the Goddard Institute for Space Studies, New York, NY, and his colleagues, suggests that climate change in recent decades might have been mainly caused by air pollution containing black carbon (soot) particles and the non-carbon dioxide greenhouse gases, particularly tropospheric ozone, methane, chlorofluorocarbons (CFCs). Appearing in the Aug. 29, 2000, issue of Proceedings of the National Academy of Sciences, Hansen's estimates of global climate forcings, or factors that promote warming, indicate that it is the processes producing non-CO2 greenhouse gases that have been more significant in climate change. With the growth rate of non-carbon dioxide greenhouse gases having declined in the past decade, if sources of methane and tropospheric ozone were reduced in the future, further changes in climate due to those gases in the next 50 years could be near zero, according to Hansen. The implication being that if such reductions were coupled with a reduction in both particles of black carbon and even modest carbon dioxide gas emissions, this could lead to a decline in the rate of any future climate change that has been attributed to human influence.

When preparing their Second Assessment of Climate Change, IPCC scientists found evidence to suggest that if the nations of the globe intend to hold collective greenhouse gas concentrations to no more than a third of that currently predicted for the year 2100 (under a business-as-usual scenario), or at around 700 parts per million in equivalent concentrations of CO2 (which translates into a 0.5 to 2.7 degree F temperature rise), then actions to reduce greenhouse gas emissions would have to be taken in the very near future. Others have asserted that stronger controls are not necessary until 2020, as long as technological solutions were immediately available to be implemented at that future date, which could significantly reduce or eliminate atmospheric concentrations of greenhouse gases, after that time.

Hoping to address many of these unresolved issues, a third IPCC assessment of global climate change is expected to be ready for release in early 2001. Also, results of a National Assessment of the Consequences of Climate Change for the United States were released in November of 2000. The preparation of that assessment, Climate Change Impacts on the United States: The Potential Consequences of Climate Variability and Change, was coordinated under the auspices of the U.S. Global Change Research Program's National Assessment Office.

Selected Web Sites

Energy Information Administration (EIA)

Intergovernmental Panel on Climate Change (IPCC)

United Nations Framework Convention on Climate Change

U.S. Global Change Research Information Office

Page last updated January 12, 2001.

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