Simulated changes in aridity from the last glacial maximum to 4xCO 2

Aridity is generally defined as the 'degree to which a climate lacks moisture to sustain life in terrestrial ecosystems'. Several recent studies using the 'aridity index' (the ratio of potential evaporation to precipitation), have concluded that aridity will increase with CO 2 because of increasing temperature. However, the 'aridity index' is—counterintuitively—not a direct measure of aridity per se (when defined as above) and there is widespread evidence that contradicts the 'warmer is more arid' interpretation. We provide here an assessment of multi-model changes in a broad set of aridity metrics over a large range of atmospheric CO 2 concentrations ranging from conditions at the last glacial maximum to 4xCO 2 , using an ensemble of simulations from state-of-the-art Earth system models. Most measures of aridity do not show increasing aridity on global scales under conditions of increasing atmospheric CO 2 concentrations and related global warming, although we note some varying responses depending on the considered variables. The response is, furthermore, more nuanced at regional scales, but in the majority of regions aridity does not increase with CO 2 in the majority of metrics. Our results emphasize that it is not the climate models that project overwhelming increases of aridity with increasing CO 2 , but rather a secondary, offline, impact model—the 'aridity index'—that uses climate model output as input.