CLIMATE AND BIOME SIMULATIONS FOR THE PAST 21,000 YEARS

This paper reports on a set of paleoclimate simulations for 21, 16, 14 , 11 and 6 ka (thousands of years ago) carried out with the Community Climate Model, Version 1 (CCM1) of the National Center for Atmospheric Research (NCAR). This climate model uses four interactive components that were not available in our previous simulations with the NCAR CCM0 (COHMAP, 1988 Science, 241, 1043-1052; Wright et al., 1993 Global Cli mate Since the Last Glocial Maximum, University of Minnesota Press, MN ): soil moisture, snow hydrology, sea-ice, and mixed-layer ocean tempe rature. The new simulations also use new estimates of ice sheet height and size from Peltier (1994, Science, 265, 195-201): and synchronize the astronomically dated orbital forcing with the ice sheer and atmosp heric CO2 levels corrected from radiocarbon years to calendar years. T he CCM1 simulations agree with the previous simulations in their most general characteristics. The 21 ka climate is cold and dry, in respons e to the presence of the ice sheets and lowered CO2 levels. The period 14-6 ka has strengthened northern summer monsoons and warm mid-latitu de continental interiors in response to orbital changes. Regional diff erences between the CCM1 and CCM0 simulations can be traced to the eff ects of either the new interactive model components or the new boundar y conditios. CCM1 simulates climate processes more realistically, but has additional degrees of freedom that can allow the model to 'drift' toward less realistic solutions in some instances. The CCM1 simulation s are expressed in terms of equilibrium vegetation using BIOME 1, and indicate large shifts in biomes. Northern tundra and forest biomes are displaced southward at glacial maximum and subtropical deserts contra ct in the mid-Holocene when monsoons strengthen. These vegetation chan ges could, if simulated interactively, introduce additional climate fe edbacks. The total area of vegetated land remains nearly constant thro ugh time because the exposure of continental shelves with lowered sea level largely compensates for the land covered by the expanded ice she ets. (C) Elsevier Science Ltd. All rights reserved.