Simulation Of Terrestrial Aridity Index By Using The Community Earth System Model

Aridity Index(AI), defined as the ratio of annual precipitation to potential evapotranspiration(PET), is a measure of the dryness of terrestrial climate. The aridity index may be near zero in a desert but can exceed unity in wet climates. The United Nations Convention to Combat Desertification classifies dry lands by AI < 0.65 and further divides dry lands into hyperarid(AI < 0.05), arid(0.05 < AI < 0.2), semiarid(0.2 < AI < 0.5), and dry subhumid(0.5 < AI < 0.65) regions. It is critical to understand how the aridity may change and what the mechanism are for better management of water and land use in a warmer climate in the future. This study examines changes in terrestrial aridity due to both natural and anthropogenic forcing through comparing terrestrial aridity in the Medieval Warm Period(MWP)(950-1250) with that in the Little Ice Age(LIA)(1550-1850), present day(PD)(1950-2005) with the last millennium(LM)(850-1850) using the Community Earth System Model(CESM) Ensemble simulations. Here the anthropogenic forcing includes forcings due to carbon dioxide(CO2), light-scattering sulfate(SO4) and light-absorbing black carbon(BC). Furthermore, we examine the uncertainties of aridity projection due to the internal variability, model structural uncertainty and emission scenarios. The major conclusions are as follows:(1) The aridity index becomes smaller(i.e., a drier terrestrial climate) by 0.34% for MWP versus LIA(because of warmer climate), and 1.4% for PD versus LM(because of decrease in precipitation).(2) Positive BC radiative forcing decreases precipitation averaged over global land at a rate of 0.9%/℃ of global mean surface temperature increase(moderate drying), while it increases PET at a rate of 1.0%/℃(also drying). Finally, BC forcing leads to a global decrease of 1.9%/℃in AI(drying). Negative SO4 forcing causes a decrease in precipitation at a rate of 6.7%/℃ cooling(strong drying). PET also decreases by 6.3%/℃ cooling(contributing to moistening) due to SO4 forcing. Thus, SO4 cooling effect leads to a small decrease in AI(drying) by 0.4%/℃ cooling.(3) We find that the impact of the internal variability on aridity projection is smaller than the model structural uncertainty by analyzing the simulations from the CESM Large Ensemble and the phase 5 of the Coupled Model Intercomparison Project(CMIP5). However, the natural variability can strongly impact the regional-scale precipitation and AI change in the future.(4) The global land aridity index decreases(drying) by 6.4% in 2060-2080 relative to 1985-2005 under a high green house gases(GHG) emission scenario(Represent Concentration Pathway 8.5, RCP8.5), whereas the corresponding value is 3.7% under a GHG mitigation scenario(RCP4.5). Although future reduction in aerosol emissions will increase P, we find that it has little impact on global aridity due to offsetting effects on PET. However, aerosols can strongly impact the regional-scale aridity change.