VEGETATION AND SOIL FEEDBACKS ON THE RESPONSE OF THE AFRICAN MONSOON TO ORBITAL FORCING IN THE EARLY TO MIDDLE HOLOCENE

FOSSIL pollen, ancient lake sediments and archaeological evidence from Africa indicate that the Sahel and Sahara regions were considerably w etter than today during the early to middle Holocene period, about 12, 000 to 5,000 years ago(1-4). Vegetation associated with the modern Sah ara/Sahel boundary was about 5 degrees farther north, and there were m ore and larger lakes between 15 and 30 degrees N. Simulations with cli mate models have shown that these wetter conditions were probably caus ed by changes in Earth's orbital parameters that increased the amplitu de of the seasonal cycle of solar radiation in the Northern Hemisphere , enhanced the land-ocean temperature contrast, and thereby strengthen ed the African summer monsoon(5-7). However, these simulations underes timated the consequent monsoon enhancement as inferred from palaeoreco rds(4). Here,ve use a climate model to show that changes in vegetation and soil may have increased the climate response to orbital forcing, We find that replacing today's orbital forcing with that of the mid-Ho locene increases summer precipitation by 12% between 15 and 22 degrees N. Replacing desert with grassland, and desert soil with more loamy s oil, further enhances the summer precipitation (by 6 and 10% respectiv ely), giving a total precipitation increase of 28%. When the simulated climate changes are applied to a biome model, vegetation becomes esta blished north of the current Sahara/Sahel boundary, thereby shrinking the area of the Sahara by 11% owing to orbital forcing alone, and by 2 0% owing to the combined influence of orbital forcing and the prescrib ed vegetation and soil changes, The inclusion of the vegetation and so il feedbacks thus brings the model simulations and palaeovegetation ob servations into closer agreement.