One method to evaluate our understanding of the climate system is to compare model simulations of past climate with reconstructions of the climate from the data available for those time periods. This is the approach of the COHMAP project (Cooperative Holocene Mapping Project), a multi-institutional (Brown, Columbia, Minnesota, Oregon and Wisconsin) effort focused on examination of the climate of the last 18,000 years. This time interval encompasses the last glacial maximum, the termination of the glaciation, and the present Holocene interglacial. The investigators are reconstructing climate from pollen, lake levels, and marine plankton. The global data sets are organized in 3,000 year intervals and include ice-sheet height area, distributions of fossil plankton, pollen and plant microfossils, and status of lake levels. The project includes a parallel effort to simulate the global patterns of climate with the Community Climate model of the National Center for Atmospheric Research. The simulation experiments are "snapshots" of the climate at 3,000 year intervals spanning the same 18,000 year period and incorporating the effects of changes in earth's orbital parameters and slowly changing boundary conditions such as ice- sheet height and area. Proposed work over the next several years includes (1) completion of global analysis of the observed paleoclimate and comparison with recent numerical simulation experiments, (2) initiation of new work with ocean models and comparison of simulations with fossil plankton distributions, (3) test the sensitivity of the model simulations to variations in atmospheric carbon dioxide and aerosol that have accompanied climate changes over the past 18,000 years, (4) intrepretation of data from North America, North Atlantic, and Europe for quantitative reconstructions of past climates. This work should lead to an improved understanding and theory of the dynamics of glacial/interglacial (i.e., large scale) climatic change, to an appreciation of the strengths and weaknesses of climate models, and to new insights about the nature of the relationships between the biosphere and climate.
