DEGREES OF CERTAINTY

Debate in the media over global warming often mixes what is well known with what is speculative, thereby leading to an artificially confusin g impression that scientists share no consensus of the probable magnit ude, timing, and potential seriousness of the environmental and societ al consequences of the documented and well-understood buildup of vario us greenhouse-enhancing gases in the atmosphere. Indeed, widespread co ncern exists over the plausibility of temperature increases of 1 to 5- degrees-C in the 21 st century, and that the mid to upper part of that range could imply dramatic restructuring of ecosystems or communities . I discuss the difficulty in interpreting the 0.5 +/- 0.2-degrees-C 2 0th century warming trend as ''proof'' of greenhouse-gas-induced globa l warming in light of possible climatic-change causal factors such as industrial aerosols, natural fluctuations, or changes in solar output. How to act is controversial, and economic model results showing poten tial abatement costs of carbon taxes are discussed