A Global Climate Model (GENESIS) with a Land-Surface Transfer Scheme (LSX). Part II: CO2 Sensitivity

Abstract

The sensitivity of the equilibrium climate to doubled atmospheric CO2 is investigated using the GENESIS global climate model version 1.02. The atmospheric general circulation model is a heavily modified version of the NCAR CCM1 and is coupled to a multicanopy land-surface model (LSX); multilayer models of soil, snow, and sea ice; and a slab ocean mixed layer. Features that are relatively new in CO2 sensitivity studies include explicit subgrid convective plumes, PBL mixing, a diurnal cycle, a complex land-surface model, sea ice dynamics and semi-Lagrangian transport of water vapor. The global annual surface-air warming in the model is 2.1°C, with global precipitation increasing by 3.3%. Over most land areas, most of the changes in precipitation are insignificant at the 5% level compared to interannual variability. Decreases in soil moisture in summer are not as large as in most previous models and only occur poleward of ?55°N in Siberia, northern Canada, and Alaska. Sea ice area in September recedes by 62% in the Arctic and by 43% in the Antarctic. The area of Northern Hemispheric permafrost decreases by 48%, while the total area of Northern Hemispheric snowcover in January decreases by only 13%. The effects of several modifications to the model physics are described. Replacing LSX and the multilayer soil with a single-layer bucket model causes little change to CO2 sensitivities on global scales, and the regions of summer drying in northern high latitudes are reproduced, although with somewhat greater amplitude. Compared to convective adjustment, penetrative plume convection increases the tropical Hadley Cell response but decreases the global warming slightly by 0.1° to 0.30°, contrary to several previous GCM studies in which penetrative convection was associated with greater CO2 warming. Similarly, the use of a cruder parameterization for cloud amount changes the local patterns of cloud response but has slight effect on the global warming. The authors discuss implications of the greater global warming (3.2°C) found in an earlier version of the model and suggest that it was due to more detailed interactions that no longer occur in the current version.