A GLOBAL CLIMATE MODEL (GENESIS) WITH A LAND-SURFACE TRANSFER SCHEME (LSX) .2. CO2 SENSITIVITY

The sensitivity of the equilibrium climate to doubled atmospheric CO2 is investigated using the GENESIS global climate model version 1.02. T he atmospheric general circulation model is a heavily modified version of the NCAR CCM1 and is coupled to a multicanopy land-surface model(L SX); multilayer models of soil, snow, and sea ice; and a slab ocean mi xed layer. Features that are relatively new in CO2 sensitivity studies include explicit subgrid convective plumes, PBL mixing a diurnal cycl e, a complex land-surface model, sea ice dynamics, and semi-Lagrangian transport of water vapor. The global annual surface-air warming in th e model is 2.1 degrees C, with global precipitation increasing by 3.3% . Over mast land areas, most of the changes in precipitation are insig nificant at the 5% level compared to interannual variability. Decrease s in soil moisture in summer are not as large as in most previous mode ls and only occur poleward of similar to 55 degrees N in Siberia, nort hern Canada,and Alaska. Sea ice area in September recedes by 62% in th e Arctic and by 43% in the Antarctic. The area of Northern Hemispheric permafrost decreases by 48%, while the the total area of Northern Hem ispheric snowcover in January decreases by only 13%. The effects of se veral 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 summ er drying in northern high latitudes are reproduced, although with som ewhat greater amplitude. Compared to convective adjustment, penetrativ e plume convection increases the tropical Hadley Cell response but dec reases the global warming slightly by 0.1 degrees to 0.3 degrees, cont rary to several previous GCM studies in which penetrative convection w as 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 dis cuss implications of the greater global warming (3.2 degrees 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.