Terrestrial Aridity and Its Response to Greenhouse Warming across CMIP5 Climate Models

The aridity of a terrestrial climate is often quantified using the dimensionless ratio P/PET of annual precipitation (P) to annual potential evapotranspiration (PET). In this study, the climatological patterns and greenhouse warming responses of terrestrial P, Penman–Monteith PET, and P/PET are compared among 16 modern global climate models. The large-scale climatological values and implied biome types often disagree widely among models, with large systematic differences from observational estimates. In addition, the PET climatologies often differ by several tens of percent when computed using monthly versus 3-hourly inputs. With greenhouse warming, land P does not systematically increase or decrease, except at high latitudes. Therefore, because of moderate, ubiquitous PET increases, P/PET decreases (drying) are much more widespread than increases (wetting) in the tropics, subtropics, and midlatitudes in most models, confirming and expanding on earlier findings. The PET increases are also somewhat sensitive to the time resolution of the inputs, although not as systematically as for the PET climatologies. The changes in the balance between P and PET are also quantified using an alternative aridity index, the ratio P/(P1PET), which has a one-to-one but nonlinear correspondence with P/PET. It is argued that the magnitudes of P/(P1PET) changes are more uniformly relevant than the magnitudes of P/PET changes, which tend to be much higher in wetter regions. The ratio P/(P1PET) and its changes are also found to be excellent statistical predictors of the land surface evaporative fraction and its changes.