Low soil moisture conditions can induce drought but also elevate temperatures. Detailed modelling of the drought–temperature link now shows that rising global temperature will bring drier soils and higher heatwave temperatures in Europe.
Since the deadly heatwaves of 2003 and 20101,2, Europe has seen a growing number of hot summers, including the ‘Lucifer’ event of 2017. While detrimental to society, the dry conditions accompanying heatwaves often have severe additional impacts; high temperatures not only accelerate soil drying, but dry soils in turn warm the atmosphere by retaining less water for evapotranspiration. Although the link between droughts and heatwaves is widely acknowledged3, understanding of how climate change affects this link has been limited by methodological and computational constraints. Writing in Nature Climate Change, Luis Samaniego and co-authors use model projections to show that future equivalents of the 2003 drought will be twice as frequent under 3 K of warming compared with 1.5 K of warming4.
In both 2003 and 2010, summertime temperatures soared over a large swathe of Europe and Eurasia; seasonal mean temperatures anomalies exceeded 3–4 K1,2, while maximum temperature anomalies over 10 K were observed over shorter timescales. Such events have considerable socio-economic impacts, including increased mortality5. However, through strong accompanying reductions in soil moisture, heatwaves further decrease gross primary productivity6, transforming ecosystems from net carbon sinks to net carbon sources.
Changes in soil moisture are also known to impact atmospheric temperature variability, driven by perturbations to the land-surface energy balance7. In dry soils, for example, plants cannot extract sufficient moisture from the soil to satisfy the atmospheric demand for evapotranspiration; excess energy thereby enhances the sensible heat flux (red arrows in Fig. 1) and warms the atmosphere. This, in turn, increases the atmospheric demand for evaporation (green arrows in Fig. 1). This process operates on hourly to daily timescales, but can also be interpreted in the context of climate change, where increasing temperatures enhance evapotranspiration and change precipitation patterns (blue arrows in Fig. 1). Increasing precipitation deficits and increasing evaporative demand will both contribute to the development of soil moisture deficits and drought.
Whereas the importance of land–atmosphere interactions has long since been recognized, our understanding of how these impact droughts and heatwaves has been hampered by methodological limitations that prevent the implementation of all feedback processes in models (Fig. 1). Offline land-surface models, for example, lack direct feedback with the atmosphere6,8 (red arrows), while the role of soil moisture in temperature is often assessed statistically3, or with the feedback between soil moisture and evapotranspiration removed (green arrows)9. Furthermore, drought projections currently lack the resolution to account for soil and vegetation properties that control local drought development. Given the devastating socio-economic impacts of summer extremes, however, there is much interest in fully incorporating all feedback processes to understand how soil moisture deficits and heatwaves may change in the future.
Samaniego and colleagues4 overcome the limitations of previous coarse-resolution assessments to quantify how European soil moisture drought may change under various levels of anthropogenic warming, motivated by the Paris Agreement. They use a large ensemble of high-resolution hydrological and land-surface models forced with climate projections from the Coupled Model Intercomparison Project Phase 5 to demonstrate a 40% increase in drought area (±24%) for a 3 K warming compared with 1.5 K. This leads to a 42% increase in the number of people living under drought conditions in Europe. Furthermore, events like the 2003 drought — which are extreme by today’s standard — will become twice as frequent, and thereby considered the norm. Projected decreases in soil water availability are strongest in the Mediterranean, where aridity levels under a 3 K warming scenario will become desert-like.
In a related study also published in Nature Climate Change, Rasmijn and co-authors examine how future changes in soil moisture may feedback to influence heatwaves10. Using a technique to assimilate upper-troposphere large-scale circulation into a global climate model, the authors recreate the large-scale weather conditions of the 2010 Russian heatwave event in the climate of 2075 under the RCP8.5 scenario. The study demonstrates that enhanced evaporative demand due to higher temperatures before and during the heatwave leads to even drier soils (yellow/red and green feedback loops in Fig. 1) than in the current climate, further reducing cooling via evapotranspiration from the land surface. The near absence of evapotranspiration leads to modelled temperatures up to 8 K warmer than recorded maxima. Current levels of soil moisture during mid-latitude heatwaves prevent the sustained high sensible heat fluxes required for such extremes. The research shows that adaptation to extreme temperatures shouldn’t focus too much on past extremes, but rather should consider how much warmer future extremes might be.
It has long since been recognized that droughts and heatwaves will become more frequent with climate change. Detailed projections now start to unravel the feedbacks between droughts and heatwaves, revealing that these events will not only become more frequent, but also more intense. How individual drought events evolve in a warmer climate is demonstrated in an impressive ensemble modelling study by Samaniego et al. Focusing on the other part of the feedback cycle, Rasmijn et al. show how high heatwave temperatures can rise when droughts become even more severe. By detailing how past events should be seen in a future climate, both studies show that adaptation should consider possible future events.
While projections of future droughts and heatwaves are becoming increasingly detailed, important uncertainties remain. Current climate model projections for Europe might severely underestimate future reductions in precipitation11. Adaptation to future drought conditions might be more complex than simply acknowledging a new norm. The increasing anthropogenic influence on the water cycle, in particular during droughts12, might affect future patterns in continental-scale water availability in ways that are not yet represented in hydrological models. And finally, the robustness of drought and heatwave projections critically depends on the representation of evapotranspiration processes in models13. The evapotranspiration response to heat and drought can vary considerably between vegetation types14, and little is known about how key vegetation properties, such as rooting depth, will change with increasing aridity.
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The author declares no competing interests.
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Teuling, A.J. A hot future for European droughts. Nature Clim Change 8, 364–365 (2018). https://doi.org/10.1038/s41558-018-0154-5
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