Abstract
Aridity, defined as the ratio of precipitation (P) to potential evapotranspiration (PET) over land, is critical to natural ecosystems and agricultural production. Global climate models project global decreases of P/PET (drying) in the 21st century. We examine the uncertainty of aridity projections due to scenarios of greenhouse gases (GHGs) and aerosols with three sets of ensemble simulations from a single climate model, the Community Earth System Model (CESM1). Ensembles consist of two Radiative Concentration Pathways (RCPs) and a scenario with RCP-like GHGs but with aerosol precursor emissions and atmospheric oxidants fixed at the year 2005 level. Under a high GHGs emission scenario (RCP8.5), global land P/PET decreases (drying) by 6.4 ± 0.8 % in 2060–2080 relative to 1985–2005. A GHG mitigation scenario (RCP4.5) would reduce the drying (P/PET decrease) to 3.7 ± 0.6 %. Although future aerosol emissions reduction would increase P, we find that it has little impact on global aridity due to offsetting effects on PET. Regionally, deceasing aerosols can have significant effects and aerosol-induced P/PET changes are due to different factors across different regions. When normalized by global mean temperature response, GHGs decrease global land P/PET by 2.7 ± 0.6 %/°C and surface temperature changes dominate GHG-induced P/PET change.
Highlights
Recent global warming has led to concern about future water cycle changes critical to agricultural production and stability of ecosystems
We explore the roles of greenhouse gases (GHGs) and aerosols in future projection of aridity and other key climate variables, and focus on understanding the extent to which future aridity differs between RCP8.5 and 4.5
We find that under RCP8.5, the global annual mean P/potential evapotranspiration (PET) over global land decreases by 6.4 ± 0.8 % in 2060–2080 relative to 1985–2005, consistent with earlier studies
Summary
Recent global warming has led to concern about future water cycle changes critical to agricultural production and stability of ecosystems. Four RCP scenarios with different GHG levels all assume a sharp reduction in aerosol emissions during the 21st century due to mitigation of aerosol emissions. We focus on climate simulations from a single CMIP5 model, the Community Earth System Model (CESM1), under RCP8.5 and RCP4.5 to understand the avoided aridity impacts of lower GHG emissions. We explore the roles of GHGs and aerosols in future projection of aridity and other key climate variables, and focus on understanding the extent to which future aridity differs between RCP8.5 and 4.5. One way to test the generality of the results to other emission reductions is to test whether the forcing changes due to aerosols vs GHGs make a difference to avoiding impacts from changes in aridity.
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