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A process-based ecosystem model, DEMETER, is used to simulate the sensitivity of the terrestrial biosphere to changes in climate. In this study, DEMETER is applied to the two following climatic regimes: (1) the modern observed climate and (2) a simulated mid-Holocene climate (6000 years before present). The mid-Holocene climate is simulated using the GENESIS global climate model, where shifts in the Earth’s orbital parameters result in warmer northern continents and enhanced monsoons in Asia, North Africa, and North America. DEMETER simulates large differences between modern and mid-Holocene vegetation cover: (1) mid-Holocene boreal forests extend farther poleward than present in much of Europe, Asia, and North America, and (2) mid-Holocene North African grasslands extend substantially farther north than present. The simulated patterns of mid-Holocene vegetation are consistent with many features of the paleobotanical record. Simulated mid-Holocene global net primary productivity is approximately 3% larger than present, largely due to the increase of boreal forest and tropical grasslands relative to tundra and desert. Global vegetation carbon is higher at 6 kyr B.P. compared to present by roughly the same amount(4%). Mid-Holocene global litter carbon is larger than present by 10%, while global soil carbon is approximately 1% less. Despite the regional changes in productivity and carbon storage the simulated total carbon storage potential of the terrestrial biosphere (not including changes in peat) does not change significantly between the two simulations.