QUANTIFYING THE ROLE OF BIOSPHERE-ATMOSPHERE FEEDBACKS IN CLIMATE-CHANGE - COUPLED MODEL SIMULATIONS FOR 6000 YEARS BP AND COMPARISON WITH PALAEODATA FOR NORTHERN EURASIA AND NORTHERN AFRICA

The LMD AGCM was iteratively coupled to the global BIOME1 model in ord er to explore the role of vegetation-climate interactions in response to mid-Holocene (6000 y BP) orbital forcing. The sea-surface temperatu re and sea-ice distribution used were present-day and CO2 concentratio n was pre-industrial. The land surface was initially prescribed with p resent-day vegetation. Initial climate ''anomalies'' (differences betw een AGCM results for 6000 y BP and control) were used to drive BIOME1; the simulated vegetation was provided to a further AGCM run, and so o n. Results after five iterations were compared to the initial results in order to identify vegetation feedbacks. These were centred on regio ns showing strong initial responses. The orbitally induced high-latitu de summer warming, and the intensification and extension of Northern H emisphere tropical monsoons, were both amplified by vegetation feedbac ks. Vegetation feedbacks were smaller than the initial orbital effects for most regions and seasons, but in West Africa the summer precipita tion increase more than doubled in response to changes in vegetation. In the last iteration, global tundra area was reduced by 25% and the s outhern limit of the Sahara desert was shifted 2.5 degrees N north (to 18 degrees N) relative to today. These results were compared with 600 0 y BP observational data recording forest-tundra boundary changes in northern Eurasia and savana-desert boundary changes in northern Africa . Although the inclusion of vegetation feedbacks improved the qualitat ive agreement between the model results and the data, the simulated ch anges were still insufficient, perhaps due to the lack of ocean-surfac e feedbacks.