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

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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 Ecological 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 replace present‐day forests in temperate and boreal latitudes. Also, the physiological effects of lowering 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 evapotranspiration 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.