Climates of the Twentieth and Twenty-First Centuries Simulated by the NCAR Climate System Model

The Climate System Model, a coupled global climate model without ''flux adjustments'' recently developed at the National Center for Atmospheric Research, was used to simulate the twentieth-century climate using historical greenhouse gas and sulfate aerosol forcing. This simulation was extended through the twenty-first century under two newly developed scenarios, a business-as-usual case (ACACIA-BAU, CO 2 710 ppmv in 2100) and a CO 2 stabilization case (STA550, CO 2 540 ppmv in 2100). Here we compare the simulated and observed twentieth-century climate, and then describe the simulated climates for the twenty-first century. The model simulates the spatial and temporal variations of the twentieth-century climate reasonably well. These include the rapid rise in global and zonal mean surface temperatures since the late 1970s, the precipitation increases over northern mid-and high-latitude land areas, ENSO-induced precipitation anomalies, and Polemidlatitude oscillations (such as the North Atlantic oscillation) in sea level pressure fields. The model has a cold bias (2-6C) in surface air temperature over land, overestimates of cloudiness (by 10%-30%) over land, and underestimates of marine stratus clouds to the west of North and South America and Africa.