Ensemble Simulation of Twenty-First Century Climate Changes: Business-as-Usual versus CO₂ Stabilization

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 twenly-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.