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Droughts and heat waves happening together – It depends on the rain

Our warming planet will lead to more frequent extremes in hot or dry weather. But what happens to make both droughts and heat waves occur together, and how does precipitation affect this? A recent study explains.

Climate Change and Environment

Global warming is expected to increase the frequency of droughts and heat waves. However, to date scientists have not been sure under what conditions these two extreme events occur together. A new EU-backed study has now revealed that, given a global temperature increase of 2 ℃, the frequency of simultaneously occurring droughts and heat waves – called compound hot–dry events – is mainly determined by mean precipitation trends. Supported by the EU-funded XAIDA and ERA4CS projects, the study has been published in the journal ‘Nature Climate Change’. When hot and dry extreme conditions occur at the same time, there is often a disproportionate impact on ecosystems and people. Compound hot–dry events can cause wildfires, tree deaths and crop losses. When long-lasting, they can lead to water shortages that threaten agriculture and food security. “In the past, periods of drought and heat waves were often considered separately; there is, however, a strong correlation between the two events, which can be seen in the extremes experienced in 2003 and 2018 in Europe. The negative consequences of these compound extremes are often greater than with one single extreme,” observes Earth system scientist and study co-author Dr Jakob Zscheischler of Helmholtz Centre for Environmental Research (UFZ), Germany, in a recent ‘ScienceDaily’ news item. The researchers used output from an ensemble of seven climate models to investigate what the occurrence of compound hot–dry events depends on and to show how important a role precipitation trends play in the frequency of future compound events. The team focused on the historical period between 1950 and 1980 and compared the results with a future climate about 2 °C warmer than pre-industrial conditions. “The advantage of these multiple simulations is that we have a much larger volume of data than with conventional model ensembles, enabling us to better estimate compound extremes,” explains UFZ researcher and study first author Dr Emanuele Bevacqua in the same news item.

A fourfold increase

The study confirms that global warming will increase the frequency of compound hot–dry events. Between 1950 and 1980, they occurred at a 3 % frequency, that is, once every 33 years. In a climate that is 2 ℃ warmer, their frequency is expected to increase to roughly 12 % – four times higher compared to the historical period. The research has also established that the frequency with which droughts and heat waves will occur simultaneously in the future will depend on precipitation rather than temperature trends. The authors explain in their study: “This occurs because local warming will be large enough that future droughts will always coincide with at least moderately hot extremes, even in a 2 °C warmer world. By contrast, precipitation trends are often weak and equivocal in sign, depending on the model, region and internal climate variability. Therefore, constraining regional precipitation trends will also constrain future compound hot–dry events.” The results of the study supported by XAIDA (EXTREME EVENTS: ARTIFICIAL INTELLIGENCE FOR DETECTION AND ATTRIBUTION) and ERA4CS (European Research Area for Climate Services) can also be applied to other compound weather extremes, such as the interaction of tropical storms and heat waves on land, or heat waves and acidity extremes in the oceans. For more information, please see: XAIDA project website ERA4CS project website

Keywords

XAIDA, ERA4CS, global warming, drought, heat wave, precipitation, climate, compound hot–dry event