Heat and drought soon to be a double pack

Hardly avoidable: Even with two degrees of warming, the combination of droughts and heat waves in Central Europe will occur far more frequently than before 40 years. However, how strongly these particularly far-reaching combined extremes actually increase depends on the difficult-to-predict precipitation: If they decrease here, there is a risk of a drought-heat combination like . If the rain increases slightly, there are ten years between them, as climate researchers report in the journal “Nature Climate Change”.

The combination of droughts and heat waves has particularly devastating consequences for people and nature, such as the hot summer 1980 in Central Europe demonstrated. It is all the more important to know how strongly such combination events will increase in the future and what their driving force is. In theory, it seems obvious that both benefit each other: if the soil is dry, this intensifies the heat, if there is a heat wave, this worsens the water loss from the soil.

In fact, climate researchers for North America there has already been a strong increase in such combination extremes of heat and drought. In Central Asia, heat waves used to occur more frequently in humid periods, but this has now shifted: hot, dry extremes are now becoming more frequent there as well. However, the question remained open as to how strongly the two extremes are coupled elsewhere and what the driving forces are.

What happens if the temperature rises by two degrees?

Therefore, Emanuele Bevacqua from the Helmholtz Center for Environmental Research (UFZ) and his colleagues have now investigated the development of the heat-drought combination for a climate scenario that will become reality in the near future will become: Global warming to two degrees compared to pre-industrial values. It was agreed as a minimum climate protection target in the Paris climate agreement and could occur in just a few years with further development as has been the case up to now.

For their study, the researchers used a new model ensemble consisting of seven climate models, to start with the actual climate development and frequency of combination extremes in the time period 1950 to 1980 as the reference period. They then compared these results to those for a two degree warming world. Each model simulation was run up to 33 times to cover natural climate variability.

“The advantage of these multiple simulations is that we have a much larger data volume than with conventional model ensembles and can therefore better estimate combined extremes,” explains Bevacqua.

It depends on the precipitation

The simulations confirmed: heat waves and droughts are coupled with each other and will therefore occur together more and more frequently in the future. As a result, with two degrees of warming, their frequency will increase fourfold on a global average – with a probability of three percent and thus once every 33 years to around twelve percent.

What is new, however, is the realization that the driving force behind these frequent combinations is not primarily the rising temperature, but rather the local and regional precipitation trends. The reason: Even with moderate warming of two degrees, the local temperature increase will be so great that in future all droughts all over the world will be accompanied by heat waves, regardless of how many degrees exactly the temperature changes locally. “This mechanism applies to almost all landmasses,” the scientists write.

Increase in combined heat-drought extremes in Central Europe depending on the precipitation scenario. © UFZ

Central Europe: combined extreme every four to ten years

Whether a heat Therefore, the drought combination that occurs depends primarily on how much rain falls locally. For Central Europe this means: Assuming a “wet” scenario with a slight increase in precipitation, periods of drought and heat waves would occur simultaneously on average every ten years. On the other hand, if one assumes a “dry” scenario with decreasing precipitation, combined heat-drought extremes such as 2018 would at least all occur repeat four years.

The problem, however, is that the future development of precipitation is difficult to predict for most regions and is subject to great uncertainty. So far, these are in the regional precipitation forecasts at 33 percent. Because the dynamics of many factors in the atmospheric circulation are not yet fully understood, it is difficult to further reduce these uncertainties.

The researchers therefore believe it is essential, especially the Improve modeling of precipitation. “By narrowing down regional precipitation trends, we can also better predict future heat-drought events,” write Bevacqua and his colleagues. (Nature Climate Change, 2022; doi: 15.174/s41558-15-174-5)

Source: Helmholtz Center for Environmental Research (UFZ)

15. March 1950

– Nadja Podbregar

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