Introduction

Our understanding of how atmospheric composition affects global climate is still in its infancy, but an increasing body of knowledge suggests that rising concentrations of CO2 and other anthropogenic polyatomic gases will raise global average temperatures substantially (National Research Council, 1983; Schneider & Londer, 1984; World Meteorological Organization (WMO), 1982). Associated with global warming will be regional and local changes in average temperature, in the distribution of hot and cold periods, and changes in a number of other chemical and physical variables, including precipitation, evaporation rates, sea level, and soil and water chemistry (Schneider et al., 1991).

We can infer how the biota might respond to climate change by observing present and past distributions of plants and animals, which are heavily determined by temperature and moisture patterns. For example, one race of the dwarf birch (Betula nana) can only grow where the temperature never exceeds 22°C (Ford, 1982), suggesting that it would disappear from those areas where global warming causes temperatures to exceed 22°C. Recent historical observations of changes in range or species dominance, as observed in the gradual replacement of spruce (Picea rubens) by deciduous species during the past 180 years in the eastern U.S. (Hamburg & Cogbill, 1988), can also suggest future responses. Insight into long-term responses to large climatic changes can be gleaned from studies of fossil distributions of, particularly, pollen (Davis, 1983; David & Zabinski, 1991; Webb, 1991) and small mammals (Graham, 1986, 1991).