Ensemble Simulation of Twenty-First Century Climate Changes:
Business-as-Usual versus CO2 Stabilization
Aiguo Dai, Gerald A. Meehl, Warren M. Washington,
Tom M. L. Wigley, and Julie M. Arblaster
National Center for Atmospheric Research, Boulder, Colorado
Natural variability of the climate system imposes a large uncertainty on future
climate change signals simulated by a single integration of any coupled
ocean-atmosphere model. This is especially true for regional precipitation changes.
Here, these uncertainties are reduced by using results from two ensembles of five
integrations of a coupled ocean-atmosphere model forced by projected future
greenhouse gas and sulfate aerosol changes. Under a business-as-usual scenario, the
simulations show a global warming of ~1.9°C over the twenty-first century (continuing
the trend observed since the late 1970s), accompanied by a ~3% increase in global
precipitation. Stabilizing the CO2 level at 550 ppm reduces the warming
only moderately (by ~0.4°C in 2100). The patterns of seasonal-mean temperature and
precipitation change in the two cases are highly correlated (r=0.99 for temperature
and r=0.93 for precipitation). Over the midlatitude North Atlantic Ocean, the model
produces a moderate surface cooling (1°-2°C, mostly in winter) over the twenty-first
century. This cooling is accompanied by changes in atmospheric lapse rates over the
region (i.e., larger warming in the free troposphere than at the surface), which
stabilizes the surface ocean. The resultant reduction in local oceanic convection
contributes to a 20% slowdown in the thermohaline circulation.
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Hongjun Zhang:
zhangho@ucar.edu