Levis, S., J.A. Foley, and D. Pollard (1999). CO2, climate, and vegetation feedbacks at the Last Glacial Maximum. Journal of Geophysical Research (Atmospheres) 104 (D24), 31,191-31,198.

Abstract:

Abstract. To examine the potential for vegetation feedbacks on the climate system at the Last Glacial Maximum (LGM), we operate the new, fully coupled, Global Environmental and Ecologi-cal Simulation of Interactive Systems (GENESIS) - Integrated BIosphere Simulator (IBIS) climate-vegetation model with boundary conditions appropriate for ~21,000 years before present. Colder and drier conditions (LGM compared to present) lead grasslands and tundra to largely re-place present-day forests in temperate and boreal latitudes. Also, the physiological effects of low-ering atmospheric CO2 to LGM levels (~200 ppmv) cause a reduction in tropical and subtropical forest cover (compared to present) in favor of C4 grasslands. These climate- and CO2-induced changes in LGM vegetation cover produce feedbacks on the climate that are, on regional scales, comparable in magnitude to the radiative effects of lowered CO2. For example, a positive albedo-driven feedback, due to changing vegetation cover, contributes to additional middle- and high-latitude cooling. Furthermore, sparser forest cover in the tropics significantly reduces evapotran-spiration and further reduces tropical precipitation (0.13 mm d-1 on the annual average compared to the 0.59 mm d-1 decrease without vegetation feedbacks). Our simulations indicate that the physiological effects of lowered CO2 on the climate-vegetation system are more clearly manifested through changes in vegetation cover (i.e., changes in leaf area index), than through the dilation of leaf stomata and the enhancement of transpiration.