THE AVERAGE GLOBAL TEMPERATURE during the 1980s exceeded that of any other decade since reliable temperature recording began a century ago, and 1990 was the warmest year on record (Hansen, 1991). This unusual period of global warming has sparked a vigorous debate about its causes. A number of scientists claim that the effect-a rise in the global temperature induced by the build-up of manmade gases in the atmosphere-is the main cause. Others point to the substantial natural variability in the past temperature record and call for further research before attempting to draw firm conclusions or take drastic action. Despite these uncertainties, many governments consider the potentially severe consequences of continued global warming over the next century a sufficiently serious threat to warrant immediate remedial action (Benedick et al., 1991). International negotiations under the auspices of the United Nations have been conducted to reach an agreement to reduce the growth of emissions of carbon dioxide and other greenhouse gases. These efforts have resulted in a convention on climate change that was signed at the UN Conference on Environment and Development in Rio de Janeiro in June 1992. The ongoing policy debate about responses to the expected future global warming is guided by extensive scientific studies. Hundreds of scientists have taken part in recent assessments of the potential for manmade climate change and policy options (e.g., IPCC, 1990a,b; NAS, 1991; EPA, 1990a,b). These reports are in agreement on the following key issues: Carbon dioxide (CO2) and other greenhouse gases produced by natural processes were present in the atmosphere long before the role of humans became significant. By partially blocking the infrared radiation originating from the earth, these gases have kept the globe warmer (by about 32?C) than would be the case in their absence. The atmospheric concentration of greenhouse gases is rising as the result of a variety of human activities, of which the burning of fossil fuels is
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John Bongaarts (1992) studied this question.