Simulated responses of terrestrial aridity to black carbon and sulfate aerosols

Aridity index (AI), defined as the ratio of precipitation to potential evapotranspiration (PET), is a measure of the dryness of terrestrial climate. Global climate models generally project future decreases of AI (drying) associated with global warming scenarios driven by increasing greenhouse gas and declining aerosols. Given their different effects in the climate system, scattering and absorbing aerosols may affect AI differently. Here we explore the terrestrial aridity responses to anthropogenic black carbon (BC) and sulfate (SO4) aerosols with Community Earth System Model simulations. Positive BC radiative forcing decreases precipitation averaged over global land at a rate of 0.9%/degrees C of global mean surface temperature increase (moderate drying), while BC radiative forcing increases PET by 1.0%/degrees C (also drying). BC leads to a global decrease of 1.9%/degrees C in AI (drying). SO4 forcing is negative and causes precipitation a decrease at a rate of 6.7%/degrees C cooling (strong drying). PET also decreases in response to SO4 aerosol cooling by 6.3%/degrees C cooling (contributing to moistening). Thus, SO4 cooling leads to a small decrease in AI (drying) by 0.4%/degrees C cooling. Despite the opposite effects on global mean temperature, BC and SO4 both contribute to the twentieth century drying (AI decrease). Sensitivity test indicates that surface temperature and surface available energy changes dominate BC- and SO4-induced PET changes.