Despite the uncertainties and complications, the slow improvement in the overall realism of climate change models, a lively improvement in the appreciation of important plant and ecosystem processes, the increasing government support for climate change studies, and the heightened public interest in climate change have all maintained an interest among scholars from various disciplines in undertaking studies of possible impacts of human-induced global changes, including climate change. Many kinds of possible impacts have ben studied; a summary of the kinds of climate change impacts that the United States may experience is included in the two-volume study conducted by the Office of Technology Assessment [ U.S. Congress, 1993].
As climate change scenarios are applied to impacts problems, a particular difficulty arises. There is an array of human impacts on the biosphere that do not flow through the channel of climate change. A changing climate could, for example, affect forest health; meanwhile people are cutting forests for timber, burning forests to gain crop or grazing land, and damaging forests with acid rain and regional air pollutants such as ozone. A study of the specific impact of a climate change, therefore, may require a complex consideration of possible synergistic or antagonistic effects of all the forces for forest change, and those conducting field studies have sometimes found it impossible to disentangle climate change impacts from changes resulting from other forces [ Savage, 1991; Baker, 1992]. And those projecting future impacts may have difficulty in applying a climate change scenario to agricultural systems already under stress [ Riebsame, 1991].
Finally, in the past attempts to foresee impacts on ecosystems routinely assumed that undisturbed ecosystems are in equilibrium, which is not true [ Botkin, 1992]. Research models are now, however, incorporating nonlinear and non-equilibrium features into their calculations [see, for example Smith and Shugart, 1993].