CO2, climate, and vegetation feedbacks at the Last Glacial Maximum

To examine the potential for vegetation feedbacks on the climate system at the Last Glacial Maximum (LGM), we operate the new, fully coupled, Global E nvironmental and Ecological Simulation of Interactive Systems (GENESIS) - I ntegrated Blosphere Simulator (IBIS) climate-vegetation model with boundary conditions appropriate for similar to 21,000 years before present. Colder and drier conditions (LGM compared to present) lead grasslands and tundra t o largely replace present-day forests in temperate and boreal latitudes. Al so, the physiological effects of lowering atmospheric CO2 to LGM levels (si milar to 200 ppmv) cause a reduction in tropical and subtropical forest cov er (compared to present) in favor of C-4 grasslands. These climate- and CO2 -induced changes in LGM vegetation cover produce feedbacks on the climate t hat are, on regional scales, comparable in magnitude to the radiative effec ts of lowered CO2. For example, a positive albedo-driven feedback, due to c hanging vegetation cover, contributes to additional middle- and high-latitu de cooling. Furthermore, sparser forest cover in the tropics significantly reduces evapotranspiration and further reduces tropical precipitation (0.13 mm d(-1) on the annual average compared to the 0.59 mm d(-1) decrease with out vegetation feedbacks). Our simulations indicate that the physiological effects of lowered CO2 on the climate-vegetation system are more clearly ma nifested through changes in vegetation cover (i.e., changes in leaf area in dex), than through the dilation of leaf stomata and the enhancement of tran spiration.