Separating the Influences of Land Warming, the Direct CO2 Effect, the Plant Physiological Effect, and SST Warming on Regional Precipitation Changes

Regional precipitation responses to land warming are separated from other aspects of CO2 forcing using idealized atmosphere-only climate model experiments. Land warming is crucial in determining the regional responses to direct CO2 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 CO2 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. Plain Language Summary Land is warming faster than the oceans in response to climate change. A new set of climate model experiments have been used to determine how this enhanced land warming could affect wind and rainfall patterns. In general, land warming drives winds that blow from ocean on to land, similar to a very large-scale sea breeze. On its own, this would tend to increase rainfall over land. The expected overall rainfall change varies across different regions and depends on the balance between the effect of land warming and other aspects of climate change such as warming of the atmosphere and oceans and changes to water loss from plants. In several monsoon regions, the impact of direct heating of the atmosphere from increased carbon dioxide plays a very important role in rainfall change. In tropical forest regions, reduction in transpiration from plants in response to increased carbon dioxide has a large effect on the whole regional water cycle.