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

Abstract

The aridity of a terrestrial climate is often quantified using the dimensionless ratio 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 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, 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 , which has a one-to-one but nonlinear correspondence with . It is argued that the magnitudes of changes are more uniformly relevant than the magnitudes of changes, which tend to be much higher in wetter regions. The ratio and its changes are also found to be excellent statistical predictors of the land surface evaporative fraction and its changes.

Seasonal Responses of Terrestrial Carbon Cycle to Climate Variations in CMIP5 Models: Evaluation and Projection

“…Plant photosynthesis in tropical forests is therefore likely inhibited more severely by future warming, partly leading to the decreases of g T NPP in ESMs. Furthermore, tropical lands are projected to become drier in CMIP5 ESMs (e.g., with decreasing soil moisture, and expansions of arid and semiarid areas) (Dai 2013;Feng and Fu 2013;Orlowsky and Seneviratne 2013;Scheff and Frierson 2015). Such water limitation likely contributes to the negative response of NPP to future warming in tropical forests.…”

Total Water Storage Change in Cameroon: Calculation, Variability and Link with Onset and Retreat Dates of the Rainy Season

“…Beforehand, the aridity index (I a ) adopted by the United Nations [32,33] is calculated to characterize each part of the area. It is defined as the ratio of the climatological annual mean values of precipitation (PR) and potential evapotranspiration (PET) [6,32,34,35]:…”

“…Depending on the values of this index, the areas are classified as follows [32,33]: hyper-arid for I a < 0.05, arid for 0.05 < I a < 0.2, semi-arid for 0.2 < I a < 0.5, dry sub-humid for 0.5 < I a < 0.65 and humid for I a > 0.65.…”

Plants and Drought in a Changing Climate

“…Given that PET increases as a function of temperature, drought metrics that use PET to estimate water demand show a large scale transition towards drier conditions under hotter temperatures [26][27][28]. Similarly, assessment of changes to aridity based on the aridity index, which is itself a function of PET, show strong trends towards drier conditions in the future [29][30][31][32]. However, because actual ET does not go up as fast as PET [33], this implied drying may not be representing actual plant stress.…”

“…However, because actual ET does not go up as fast as PET [33], this implied drying may not be representing actual plant stress. While metrics that gauge atmospheric demand ("atmospherecentric") show large increases in both drought [26][27][28] and aridity [27,[29][30][31][34][35][36] under future climate conditions, metrics that account for plant responses to CO 2 ("plant-centric" metrics) show more muted responses [12,37,38]. In fact, ET stays relatively constant in climate model projections as CO 2 rises, even as temperatures increase substantially [12].…”