Anthropogenic Aerosols and past Precipitation Change (Invited Presentation)

The climate response of precipitation to the effects of anthropogenic aerosols is a critical while yet understood aspect in climate science. Previous studies were limited by not only the lack of long historical precipitation record but also overly simplified parameterizations of aerosol-cloud interactions in the climate models. Recent Coupled Model Intercomparison Project Phase 5 has included, for the first time, a sizable group of models equipped with physically based parameterizations of cloud and aerosol processes to perform long historical transient climate simulations. Using results of these models along with the data from the 20th Century Reanalysis Project, my study reveals that the changes in precipitation, in reference to its preindustrial level, has largely been dominated by aerosols throughout the tropics and also in the mid- and high-latitudes of the Northern Hemisphere since at least the middle of the 20th Century, whereas greenhouse gas forcing alone is only responsible for the precipitation changes in the high-latitudes of the Southern Hemisphere where aerosol forcing is largely insubstantial. Many of these aerosol-caused changes in precipitation occur in regions away from aerosol-laden areas, thus reflect an effect via firstly the response of large-scale atmospheric dynamics to aerosol forcing. Aerosol-induced cooling has offset some of the warming caused by greenhouse gases from the tropics to Arctic, and thus formed the gradients of surface temperature anomaly that enable the revealed precipitation changes to occur. Improved representation of aerosol-cloud interaction has been demonstrated to be the key factor for the models to reproduce consistent precipitation change patterns with reanalysis data. Remaining discrepancy in amplitude between modeled and observed tropical precipitation change suggests the further development of parameterization in the models to represent the aerosol-cloud interactions in deep convection.