Foreword

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.