The Influence of Mid-20th to Mid-21st Century Shifts in the Geographic Distribution of Aerosol Emissions on Their Climate Efficacy

Abstract:

Recent studies have identified that an identical amount and composition of aerosol emissions can have order-of-magnitude different impacts on global-scale temperature and precipitation, depending solely on the geographic location of emissions. In particular, it appears that aerosols sourced from the major past and present-day emitting regions produce stronger cooling and precipitation effects than do those from anticipated future emissions hotspots. Using novel simulations in the fully coupled NCAR CESM/CAM5 model, we test the resulting hypothesis that the shifting geographic distribution of global aerosol emissions over time should be expected to change the temperature and precipitation sensitivity of the climate system to aerosols, independent of any changes in total aerosol amount or composition. Our analysis finds that, contrary to what might be expected from recent studies, the global-mean temperature sensitivity to a mid-20^th century geographic distribution of aerosol emissions is not statistically significantly different from the sensitivity to the present-day or projected mid-21^st century distributions. However, differences emerge in the global-mean precipitation impact of and hydrologic sensitivity to the evolving geographic distributions, driven largely be differences in the effect of regional emissions hotspots (e.g. India) on strong local precipitation regimes (e.g. the South Asian monsoon). These results reinforce the importance of considering the geographic heterogeneity of aerosol emissions when evaluating their climate impacts, particularly on the hydrologic cycle, but also indicate that certain aspects of the climate system response (i.e. global-mean temperature sensitivity) may be relatively insensitive to the nature of this heterogeneity.