Characterizing GCM land surface schemes to understand their responses to climate change

The impact of land surface representation on GCM simulations of climate cha nge is analyzed using eight climate change experiments, carried out with fo ur GCMs each utilizing two different land surface schemes (LSSs). In the re gions studied (Amazonia, the Sahel, and southern Europe) the simulations di ffer markedly in terms of their predicted changes in evapotranspiration and soil moisture. These differences are only partly as a result of difference s in the predicted changes in precipitation and available energy. A simple "bucket model" characterization of each LSS demonstrates that the different hydrological sensitivities are also strongly dependent on properties of th e LSS, most notably the runoff, which occurs when evaporation is marginally soil moisture limited. This parameter, "Y-c," varies significantly among t he LSSs, and influences both the soil moisture in the 1 x CO2 control clima te, and the sensitivity of both evaporation and soil moisture to climate ch ange. It is concluded that uncertainty in the predicted changes in surface hydrology is more dependent on such gross features of the runoff versus soi l moisture curve than on the derailed treatment of evapotranspiration.