The aridity Index under global warming

Abstract

Aridity is a complex concept that ideally requires a comprehensive assessment of hydroclimatological and hydroecological variables to fully understand anticipated changes. A widely used (offline) impact model to assess projected changes in aridity is the aridity index (AI) (defined as the ratio of potential evaporation to precipitation), summarizing the aridity concept into a single number. Based on the AI, it was shown that aridity will generally increase under conditions of increased CO 2 and associated global warming. However, assessing the same climate model output directly suggests a more nuanced response of aridity to global warming, raising the question if the AI provides a good representation of the complex nature of anticipated aridity changes. By systematically comparing projections of the AI against projections for various hydroclimatological and ecohydrological variables, we show that the AI generally provides a rather poor proxy for projected aridity conditions. Direct climate model output is shown to contradict signals of increasing aridity obtained from the AI in at least half of the global land area with robust change. We further show that part of this discrepancy can be related to the parameterization of potential evaporation. Especially the most commonly used potential evaporation model likely leads to an overestimation of future aridity due to incorrect assumptions under increasing atmospheric CO 2 . Our results show that AI-based approaches do not correctly communicate changes projected by the fully coupled climate models. The solution is to directly analyse the model outputs rather than use a separate offline impact model. We thus urge for a direct and joint assessment of climate model output when assessing future aridity changes rather than using simple index-based impact models that use climate model output as input and are potentially subject to significant biases.

Projected increases in magnitude and socioeconomic exposure of global droughts in 1.5 and 2 °C warmer climates

“…Future studies may explore their potential physical mechanisms (i.e., connecting drought evolution with landatmosphere interactions). For other uncertainty sources, several previous studies Gu et al, 2019;Chen and Brissette, 2019) have been devoted to detecting and attributing uncertainty to GCM structure, RCPs, internal climate variability, and even drought indices. Here, it is challenging to consider all these uncertainties systematically; future work could focus on including the integrated uncertainty and quantifying relative contributions to drought evolution and impact assessments.…”

Robust Future Changes in Meteorological Drought in CMIP6 Projections Despite Uncertainty in Precipitation

“…The previous suggestions of more severe droughts largely arise from uncoupled modeling studies (Sheffield et al, 2012) that do not capture the various climate interactions and generally quantify droughts using potential evapotranspiration in addition to precipitation (Dai, 2013). Recent studies (Greve et al, 2019; Justin Sheffield et al, 2012; Milly & Dunne, 2016; Swann et al, 2016; Yang et al, 2019) have shown that these uncoupled approaches strongly overestimate regional drought and aridity increases due to inappropriate assumptions under increasing CO 2 and are inconsistent with coupled climate model projections. As such, those studies have encouraged the use of direct climate model outputs in drought assessments.…”

Global Characterization of the Varying Responses of the Standardized Precipitation Evapotranspiration Index to Atmospheric Evaporative Demand

“…Overall, the debate on the role of AED on drought severity (B. Cook et al, 2018;Dai et al, 2018;Greve et al, 2019) originates mainly from the notion that the role of AED is complex, depending largely on the drought type (e.g., meteorological, hydrological, and environmental) and prevailing climatic conditions (S. M. Vicente-Serrano, McVicar, et al, 2020).…”

“…As such, it is expected that the atmospheric CO 2 fertilization would increase plant water use efficiency (Milly & Dunne, 2016;Roderick et al, 2015;A. L. S. Swann et al, 2016), alleviate plant water demand (Abigail L. S. Swann, 2018), and reduce the stress corresponding to hydrological deficits (Greve et al, 2019). However, the impact of atmospheric CO 2 fertilization is still uncertain (Brodribb et al, 2020;S.…”