Drought makes heatwaves hotter but less deadly

(12-01-2022) During heatwaves, the land dries out. That drought further enhances the rising of air temperatures. However, desiccated soils may still make the heatwaves less (rather than more) deadly to humans, due to the consequent reduction in air humidity.

Heatwaves and droughts are causing acute excess mortality and damage to society worldwide. For instance, the number of deaths related to the European heatwave of 2003 reached more than 50000. But also in the past two years heatwaves have raised mortality tolls, adding on top of the mortality associated to the COVID-19 pandemic.

Until now, it was believed that drying soils contribute to making heatwaves even more deadly, as they push heatwave temperatures even higher. After all, drier land results in lower evaporation, so that more energy is left at the Earth’s surface to heat up the air. But the temperature effect of drought may be deceiving: the lower evaporation also causes lower air humidity, which helps the cooling of the human body through transpiration. That beneficial effect often takes over, and makes heatwaves less deadly when they concur with drought.

Better measures against drought and heat

Dry and hot periods are becoming longer, more frequent, and more intense in a warming climate.

Adaptation measures such as afforestation and irrigation of croplands may not necessarily be effective. The current study suggests that these drought-resistant measures can be ineffective against deadly heat and even detrimental, despite the fact that they smooth out the extremely high temperatures.

The favorable effect due to a lower temperature can be balanced out by the higher humidity, which makes the heat sultrier. The study stresses the magnitude of the challenge to counteract the increasingly deadly heat and drought, and suggests that seemingly effective measures against heat can still be counterproductive.

As Hendrik Wouters, first author of the study, highlights: "It is necessary in the first instance to combat global warming in its foundations, through drastic reductions in greenhouse gas emissions. Furthermore, one should reconsider existing drought and heat measures and explore alternative drought and heat-resistant measures within the agriculture, food, and hydrological sectors. More should be done on plant species that are better adapted to a drier and hotter climate. Crop choices (e.g., wheat or maize) and agricultural measures (eg., no-tillage farming or genetic modification of crops) need to be considered to lower water use and higher reflection of solar energy. Nonetheless, follow-up research is needed to know how effective and desirable such measures are."

About the research

A team of scientists led by the Hydro-Climate Extremes Lab (H-CEL) at Ghent University, analyzed heatwaves from millions of weather balloons that were released from airports and weather institutes worldwide over a 35-year period (1981–2015).

These measurements were combined with satellite imagery and in turn used for weather simulations to assess the effect of soil desiccation during deadly heatwaves. The findings were published in the journal ‘Science Advances’. The study is part of the DRY–2–DRY project funded by the European Research Council (ERC).


Read the scientific publication

Hendrik Wouters
Department of Environment
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