Shift towards ecosystem water limitation exacerbates hot temperature extremes

Hot temperature extremes have severe implications for human health, crop yields and tree mortality. Whereas they are mostly introduced by atmospheric circulation patterns, the intensity of hot temperature extremes is modulated by ecosystem functioning; when soil moisture is abundant, evaporation of water through transpiration and evaporation from surfaces is high, which causes relevant evaporative cooling. This cooling is greatly reduced under drought stress, because ecosystems adapt to water-limited conditions by saving water e.g. through stomatal regulation which leads to decreased terrestrial evaporation. This in turn leaves more energy to potentially exacerbate hot temperature extremes. 

While it has been shown that ecosystem water limitation is projected to increase in the future, the respective implications on hot temperature extremes are unclear. In this study, we capture the ecosystem's water limitation through the so-called Ecosystem Limitation Index (ELI, Denissen et al. 2020). To mitigate the confounding influence of changes in mean temperatures, which possibly originate from heat advection and circulation, we focus on the differences between mean and hot temperature extremes. Based on global climate projections from the sixth Coupled Model Intercomparison Project (CMIP6) from 1980 - 2100, we find regions with significant correlations between future evolution of temperature differences and ELI, with hot spots in North and South America. We furthermore test the role of the initial ELI for these correlations and find weak effects in Earth System Models included in the CMIP6 ensemble, but higher relevance in reanalysis data from the ECMWF Reanalysis 5^th generation (ERA5) from 1980 - 2020, where the highest correlations are found in initially water-limited regions. These findings show that in large areas across the globe, temperature extremes increase much faster than mean temperatures alongside ecosystem drying. Therefore, considering ecosystem drying is relevant for assessing the intensity of projected temperature extremes and their corresponding impacts. This way, improving the representation of vegetation dynamics in state-of-the-art models is necessary to more accurately estimate evaporative cooling and consequently hot temperature extremes.

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Denissen, J. M., Teuling, A. J., Reichstein, M., & Orth, R. (2020). Critical soil moisture derived from satellite observations over Europe. Journal of Geophysical Research: Atmospheres, 125(6), e2019JD031672.