The Climate System Model (CSM), a coupled global climate model without
``flux adjustments'' recently developed at NCAR, was used to simulate the
20th century climate using historical greenhouse gas and sulfate aerosol
forcing. This simulation was extended through the 21st century under two
newly developed scenarios, a business-as-usual case (ACACIA-BAU,
CO2 ~710 ppmv in 2100) and a CO2 stabilization case (STA550, CO2 ~540
ppmv in 2100). Here we compare the simulated and observed 20th century
climate, and then describe the simulated climates for the 21st century. The
model simulates the spatial and temporal variations of the 20th 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 middle- and high-latitude land areas, ENSO-induced precipitation
anomalies, and pole-midlatitude oscillations (such as the NAO) in sea-level
pressure fields. The model has a cold bias (2-6°C) 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.

The projected global surface warming from the 1990s to the 2090s is ~1.9°C
under the BAU scenario and ~1.5°C under the STA550 scenario. In both
cases, the middle stratosphere cools due to the increase in CO2, whereas the
lower stratosphere warms in response to recovery of the ozone layer. As in other
coupled models, the surface warming is largest at winter high latitudes (>5.0°C
from the 1990s to the 2090s) and smallest (~1.0°C) over the southern oceans,
and is larger over land areas than ocean areas. Globally averaged precipitation
increases by ~3.5% (3.0%) from the 1990s to the 2090s in the BAU (STA) case.
In the BAU case, large precipitation increases (up to 50%) occur over northern
mid- and high-latitudes and over India and the Aradian Peninsula. Marked
differences occur between the BAU and STA regional precipitation changes
resulting from inter-decadal variability. Surface evaporation increases at all
latitudes except for 60-90°S. Water vapor from increased tropical evaporation
is transported into mid- and high-latitudes and returned to the surface through
increased precipitation there. Changes in soil moisture content are small (within ±3%).
Total cloud cover changes little, although there is a upward shift of middle-level
clouds. Surface diurnal temperature range decreases by about 0.2-0.5°C over most
land areas. 2-8 day synoptic storm activity decreases (by up to 10%) at low
latitudes and over midlatitude oceans, but increases over Eurasia and Canada.
The cores of subtropical jets move slightly up- and equator-ward. Associated
with reduced latitudinal temperature gradients over mid- and high-latitudes, the
wintertime Ferrel cell weakens (by 10-15%). The Hadley circulation also weakens
(by ~10%), partly due to the upward shift of cloudiness which produces enhanced
warming in the upper troposphere.