Assessing surprises and nonlinearities in greenhouse warming

Society has a great interest in the risks posed by global climate change, both in taking steps to avoid unacceptable damage and in avoiding unwanted or ineffective mitigation measures. Unfortunately, this interest is not matched by available knowledge. Physical science aspects of climate change--how much warmer, wetter, or drier, and how variable climate might be in different regions with different atmospheric concentrations of greenhouse gases--are uncertain and are likely to remain so for many years to come. And even if one posts particular climatic shifts, the ecological, social, economic, and other human consequences are elusive. These uncertainties pose sharp dilemmas for determining research priorities and for individual and social decision making. On what basis should society make decisions about the amounts and types of mitigation or adaptation efforts that should be undertaken. What values are put at risk by climate change. What kinds of uncertainties most cloud the picture. In particular, how significant are potential nonlinearities or surprises in physical or human effects. What research directions and other steps are appropriate for shrinking these uncertainties.