POTENTIAL ROLE OF VEGETATION FEEDBACK IN THE CLIMATE SENSITIVITY OF HIGH-LATITUDE REGIONS - A CASE-STUDY AT 6000 YEARS BP

Previous climate model simulations have shown that the configuration o f the Earth's orbit during the early to mid-Holocene (approximately 10 -5 kyr) can account for the generally warmer-than-present conditions e xperienced by the high latitudes of the northern hemisphere. New simul ations for 6 kyr with two atmospheric/mixed-layer ocean models (Commun ity Climate Model, version 1, CCM1, and Global ENvironmental and Ecolo gical Simulation of Interactive Systems, version 2, GENESIS 2) are pre sented here and compared with results from two previous simulations wi th GENESIS 1 that were obtained with and without the albedo feedback d ue to climate-induced poleward expansion of the boreal forest. The cli mate model results are summarized in the form of potential vegetation maps obtained with the global BIOME model, which facilitates visual co mparisons both among models and with pollen and plant macrofossil data recording shifts of the forest-tundra boundary. A preliminary synthes is shows that the forest limit was shifted 100-200 km north in most se ctors. Both CCM1 and GENESIS 2 produced a shift of this magnitude. GEN ESIS 1 however produced too small a shift, except when the boreal fore st albedo feedback was included. The feedback in this case was estimat ed to have amplified forest expansion by approximately 50%. The forest limit changes also show meridional patterns (greatest expansion in ce ntral Siberia and little or none in Alaska and Labrador) which have ye t to be reproduced by models. Further progress in understanding of the processes involved in the response of climate and vegetation to orbit al forcing will require both the deployment of coupled atmosphere-bios phere-ocean models and the development df more comprehensive observati onal data sets.