During the past century, human activities have caused increases in the atmospheric concentrations of both naturally occurring and artificially introduced "greenhouse" gases. The carbon dioxide (CO2) concentration, for example, has increased about 25 percent. Currently, these long-lived gases, some of which are more effective than others at trapping heat, combine to provide an increase in greenhouse warming over the past 100 years equivalent to that which would have been provided by a 40 percent increase in CO2 alone.
The highly publicized concern that these greenhouse gas accumulations might lead to a significant modification of the earth's climate underlies an expanding body of literature. Much of this work deals with quantitative discussions of the greenhouse gas accumulation itself, quantitative analyses of the heat balance of the planet, and inferences from these analyses for climate change. Part Two of this report has been written by the Effects Panel with a full awareness of the deliberations that have led to much of the greenhouse warming literature, and its assessments rest on the same foundations as do several other assessments of the same questions.1 It was not the panel's purpose to retrace these steps, but rather to assess this body of knowledge in terms of policy needs.
The panel first examined the climate record and the difficulties associated with its interpretation, as well as the available analytical tools and their limitations. It then developed a concise, objective account of the current knowledge, understanding, and predictive capability that attend the greenhouse warming question. The panel made no attempt to perform a comprehensive critical assessment from the perspective of the various relevant scientific disciplines. The report of the Intergovernmental Panel on Climate Change (1990) is comprehensive and covers in greater detail the topics that
are addressed in the report of the Effects Panel. The Effects Panel's report is more constrained in scope and presents a succinct representation of data and knowledge to serve as a backdrop for current policy decisions. In doing so, it examines in depth only a few topics that illustrate the extent to which the current science base is sufficient to inform policy decisions. The panel believes the principal difference between its analysis and that of the IPCC is that in the Effects Panel's analysis a greater credence is accorded to the uncertainties in the current scientific knowledge and tools.
The task assigned to the Effects Panel did not require it to assess policy issues as such. Nevertheless, its members record here their support of the conclusions, including the need for prudent response, that are expressed in the report of the Synthesis Panel (Part One).
1. See, for example, Intergovernmental Panel on Climate Change, 1990; U.S. Congress, Office of Technology Assessment, 1991; and National Research Council, 1983.
Intergovernmental Panel on Climate Change. 1990. Climate Change: The IPCC Scientific Assessment, J. T. Houghton, G. J. Jenkins, and J. J. Ephraums, eds. New York: Cambridge University Press.
National Research Council. 1983. Changing Climate: Report of the Carbon Dioxide Assessment Committee. Washington, D.C.: National Academy Press.
U.S. Congress, Office of Technology Assessment. 1991. Changing by Degrees: Steps to Reduce Greenhouse Gases. OTA-O-482. Washington, D.C.: U.S. Government Printing Office.