Separating the Influences of Land Warming, the Direct CO ₂ Effect, the Plant Physiological Effect, and SST Warming on Regional Precipitation Changes

Regional precipitation responses to land warming are separated from other aspects of CO ₂ forcing using idealized atmosphere-only climate model experiments. Land warming is crucial in determining the regional responses to direct CO ₂ forcing and the plant physiological effect and partially mitigates the response to sea surface temperature (SST) warming. The plant physiological effect causes large reductions in transpiration over forest regions but also produces significant land warming which increases moisture convergence in tropical forest regions, opposing transpiration-related reductions in precipitation. The balance between these two large terms differs by region and explains why some regions actually experience increased precipitation in response to reduced transpiration. Regional circulation changes driven by increased CO ₂ atmospheric radiative heating appear to be mainly related to land-sea differences in water cycle adjustments rather than spatial variations in radiative heating. This drives increased rainfall in several monsoon regions, particularly over West Africa. SST warming generally leads to reduced precipitation over land and increases over the oceans, while the associated land warming enhances moisture convergence and precipitation over land. However, the combination of SST and land warming effects is highly nonlinear in the tropics, which is likely to be because of strong coupling between precipitation change and land warming.