The Science of Climate Change: Global and U.S. Perspectives
Tom M. L. Wigley, National Center For Atmospheric Research
Press Release 
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Basic Science on climate change:
The Wigley report provides more information on how climate is influenced by anthropogenic factors. You may download a pdf of the entire report by clicking on the report cover above, or read portions of the report in html by following the links in the "In This Section" box.
Eileen Claussen, Executive Director, Pew Center on Global Climate Change
This report on the science of climate change seeks to explain how climate is influenced by anthropogenic factors. Understanding the effect of greenhouse gas concentrations on the atmosphere is key to understanding the potential magnitude of the "greenhouse effect," evaluating possible environmental impacts, and considering policy responses.
A variety of factors determine the rate and magnitude of climate change, including the emissions of greenhouse and aerosol-producing gases, the carbon cycle, the oceans, biosphere, and clouds. As our understanding in each of these areas evolves, it is important that researchers, policy-makers, the press, and the public be kept informed since these developments affect our understanding of the seriousness and complexity of this issue.
As part of the Pew Center's series examining the potential impacts of higher atmospheric concentrations of greenhouse gases on the United States, this paper by the distinguished climate scientist Tom M.L. Wigley, senior scientist with the National Center for Atmospheric Research, addresses what is known and not known about the science of climate change. Its publication comes in an interim period between assessments of the science by the Intergovernmental Panel on Climate Change (which published its second assessment in 1996 and will publish its third assessment in 2001). The author uses preliminary estimates of greenhouse gas and sulfur dioxide emissions from the current IPCC review process as well as his own work to supplement previously published research.
The new research suggests the likelihood of slightly larger changes in temperature and sea level rise than projected in the most recent IPCC assessment. The temperature rise is expected to be greater in the U.S. than the average temperature increase across the globe. While changes in precipitation and extreme weather events such as hurricanes and other storms are more difficult to predict, it is possible that the intensity of rain and hurricane events could increase. Uncertainties in predicting the direction and magnitude of these changes make it difficult to predict the impacts of climate change. However, even small changes in climate can lead to effects that are far from trivial.
While the analysis presented is the work of one author, this report has been subject to extensive peer review. The Pew Center and the author are indebted to many scientists and organizations for their constructive comments on previous drafts of this paper or sections of this paper. Their comments have helped improve the text substantially, and so, while the opinions expressed in this report are solely those of the author, we gratefully acknowledge their input: E. Barron, B. Felzer, C. Hakkarinen, A. Henderson-Sellers, M. Hulme, M. MacCracken, M. McFarland, J. Mahlman, G. Meehl, N. Nakicenovic, B.D. Santer, M.E. Schlesinger, K.P. Shine, J.B. Smith, and S.J. Smith. The A1, A2, B1, and B2 scenarios developed in the current IPCC working group process have been used with the kind permission of their producers, represented by T. Morita, A. Sankovski, B. deVries, and N. Nakicenovic. D. Viner of the Climate Impacts LINK Project (UK Dept. of the Environment, Regions and Transport contract EPG1/1/68) supplied the HadCM2 data on behalf of the Hadley Centre and UK Meteorological Office. In addition, the Pew Center would like to acknowledge and thank Joel Smith and Brian Hurd of Stratus Consulting for their management of this Environmental Impacts series.
The average surface temperature of the globe has warmed appreciably since the late 1800s, by about 0.6°C. Since this warming cannot be adequately explained by natural phenomena such as increased solar activity, human-induced increases in greenhouse-gas concentrations appear to be at least partly responsible. In addition to the warming effect of greenhouse-gas increases, however, changes in temperature over the past century are likely to have been significantly influenced by the cooling effect associated with changes in the sulfate aerosol loading of the atmosphere, arising from fossil-fuel-derived sulfur dioxide (SO2) emissions. When greenhouse-gas, sulfate aerosol, and solar influences are considered together, observed climate changes are consistent with model predictions.
Projections of future global-mean temperature and sea level change made by the Intergovernmental Panel on Climate Change (IPCC) in its 1996 Second Assessment Report used emissions scenarios developed in 1992. Preliminary versions of new emissions scenarios produced by the writing team for the IPCC Special Report on Emissions Scenarios (SRES) are now available. The most important difference between the old (1992) and new (SRES) scenarios is that the new scenarios have much lower emissions of sulfur dioxide. The reduction in sulfur dioxide emissions (and their attendant cooling effects through the production of sulfate aerosols) results in a slight increase in temperature and sea level rise projections from those previously given by the IPCC. If central estimates of model parameters are used, global-mean warming from 1990 to 2100 ranges from 1.9°C to 2.9°C. Sea-level rise estimates over the same period range from 46 to 58 cm. For temperature and sea level changes over the next few decades, projections are virtually independent of the emissions scenario.
Based on results from a number of climate models, the rate of future warming over the United States is expected to be noticeably faster than the global-mean rate. Future regional-scale precipitation changes are highly uncertain. The only result that is common to all climate models is an increase in winter precipitation in northern latitudes, from the northern Great Plains to the northeastern states. Even in the absence of large precipitation changes, there could still be significant changes in the availability of water for agriculture, human consumption, and industry because of the increased evaporation that should accompany warming. This factor alone would lead to drier summer soil conditions and reduced runoff. The effects of increased evaporation, however, may be partly offset by the direct plant-physiological effect that carbon dioxide (CO2) has in improving plant water-use efficiency and, hence, lowering evapotranspiration rates.
Changes in weather and climate extremes over the United States are certain to occur as the global climate changes. The frequency of extremely hot days is almost certain to increase, and the frequency of frosts should decrease. Changes in the frequency of daily precipitation extremes are highly uncertain, although there is evidence for an increase in the frequency of wet extremes. For hurricanes and tropical storms, the evidence suggests that there could be small increases in their windspeeds. It is also likely that future such storms will be accompanied by larger rainfall amounts. While there is no credible model-based information on changes in the number of hurricanes and tropical storms per year worldwide, there is empirical evidence that suggests that a small increase in frequency is possible in the North Atlantic region. For all extreme events, however, it is unlikely that the projected changes will become evident in a statistically convincing way for many decades, with the exception of temperature extremes, which should become evident sooner.
Tom M.L. Wigley
Tom M.L. Wigley (B.Sc., Ph.D.), formerly Director of the Climatic Research Unit, University of East Anglia, Norwich, U.K., currently holds a Senior Scientist position with the National Center for Atmospheric Research, Boulder, CO. One of the world's foremost scientists in the area of climate change, he has published in diverse aspects of the broad field of climatology. His main interests are in carbon cycle modeling, projections of future climate and sea-level change, and interpretation of past climate change particularly with a view to detecting anthropogenic influences. Recently, he has concentrated on facets of the global warming problem, and has contributed on many occasions to Intergovernmental Panel on Climate Change (IPCC) reports and assessments.