Drought makes heatwaves hotter, but less deadly
During heatwaves, there is no rain and the soil dries out. This further enhances the rising of heatwave temperatures. But remarkably, desiccated soils also have an advantage: they reduce air humidity and make a heatwave less deadly to humans.
Heatwaves and droughts are causing acute excess mortality and damage to society worldwide. The number of deaths related to the European heatwave of 2003 amounts to more than 70.000. Until now, it was believed that dried out soils make heatwaves even more deadly as it pushes heatwave temperatures even higher, because drier land results in lower evaporation. Consequently, more energy is left at the Earth’s surface to heat up the outside air further.
But the temperature effect of drought is deceiving. Less evaporation due to drought also means less humidity. High humidity increases the risk of overheating because it hampers cooling of the human body through transpiration. As humidity decreases due to dry conditions, a beneficial effect occurs: heat waves actually become less deadly.
Alternative measures needed
The results make more clear which measures against periods of drought and deadly heat are most effective. This is important, because such periods are becoming longer, more frequent and more intense in a warming climate. Many measures that are already taking place, such as (re)afforestation and irrigation of croplands, are necessary for nature conservation, biodiversity, agriculture and food production. However, the current study shows that these drought-resistant measures are ineffective against deadly heat and can even be detrimental, despite the fact that they smooth out the extremely high temperatures.
The study stresses once again how great the challenge is to counteract the increasingly deadly heat and drought. The authors argue that in the first instance, it is necessary to combat global warming in its foundations. 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 (eg., 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. Folow-up research is needed to know how effective and desirable such measures are.
About the research
For this study, a team of scientists (UGent, Wageningen University & Research, VITO, Loyola Marymount University, and Euro-Mediterranean Center on Climate Change) analyzed heatwaves from millions of weather balloons that were released from airports and weather institutes worldwide over a 35-year period (1981-2015).
The study used a conceptual model developed by WUR and used in research and education to organize the large amount of data and recognize periods of drought and heat (15 million atmospheric balloon sounds). The measurement data were combined with satellite images and in turn used for weather simulations. This allowed the effect of soil dehydration during deadly heat to be ascertained. 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.