The combination of droughts and heat waves has particularly devastating consequences for people and nature. It is all the more important to know how strongly such combination events will increase in the future and what their driving force is. A research team has now investigated this for a global warming of two degrees compared to pre-industrial conditions. Accordingly, the number of extreme heat and drought periods in Central Europe, for example, increases once every five to ten years, depending on the precipitation scenario.
Global temperatures are rising, and at the same time the fluctuations in the climate pendulum are increasing – extreme weather events are becoming more frequent. This applies in particular to periods of drought and heat waves, which are now more common in many regions of the world than they were 50 or 100 years ago. The consequences of such extremes are particularly devastating when heat and drought occur together, as was the case in Central Europe in summer 2018, for example. Intuitively, this combination of drought and heat may seem logical: when the soil is dry, it increases the heat, when there is a heat wave, it increases water loss from the soil.
Focus on heat-drought combination
Despite this, it has been clear for many regions that both heat waves and dry periods will increase, but not to what extent they are mutually dependent and how much the combination of both weather extremes will increase in the future. “In the past, periods of drought and heat waves were often considered separately, but in fact both events are strongly correlated, which can be seen, for example, in the two extreme years 2003 and 2018,” explains senior author Jakob Zscheischler from the Helmholtz Center for Environmental Research (UFZ). Together with first author Emanuele Bevacqua and his team, he has investigated how the combined heat-drought events will develop compared to pre-industrial levels given global warming by two degrees. This warming was agreed in the Paris climate agreement as the upper limit of what is still acceptable and as the minimum climate protection target.
For the study, the research team used a new model ensemble consisting of seven climate models to first depict the actual climate development and frequency of combination extremes in the period 1950 to 1980 as a reference period. They then compared the results to those for a two degree warming world. Each model simulation was run up to 100 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.
Rainfall is crucial
The simulations confirmed that heat waves and droughts are linked and mutually supportive. As a result, they will occur more frequently at the same time in the future than would be the case without this mutual feedback. Specifically, the researchers determined that the extreme combination between 1950 and 1980 only had a probability of three percent – statistically speaking, it therefore occurred every 33 years on average. If the climate is two degrees warmer, such heat-drought events occur with a probability of around twelve percent – this corresponds to a fourfold increase in frequency compared to the historical reference period.
What is new, however, is that the driving force behind these clustered combinations is not primarily rising temperature, but rather 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. Whether a heat-drought combination occurs therefore depends primarily on how much rain falls at a location. The problem, however, is that the future development of precipitation is the most difficult to predict for most regions and is subject to great uncertainty. “The precipitation regime depends on the atmospheric circulation, which determines the regional weather dynamics through interactions over large parts of the globe,” says Bevacqua. Because the dynamics of many of these processes are not yet fully understood, it is difficult to further reduce these uncertainties.
For Central Europe, this means that the risk of combined heat-drought events depends heavily on which precipitation scenario is used: assuming a “wet” scenario with a slight increase in precipitation, there would be simultaneous periods of drought and heat waves every ten years on average appear. If, on the other hand, one assumes a “dry” scenario with decreasing precipitation, combined heat-drought extremes such as 2018 would repeat themselves at least every four years. So far, these are in the precipitation forecasts at 48 percent. According to the researchers, it is therefore essential to improve the modeling of precipitation in particular. “By narrowing down regional precipitation trends, we can also better predict future heat-drought events,” write Bevacqua and his colleagues.
Source: Emanuele Bevacqua (Helmholtz Center for Environmental Research (UFZ), Leipzig) et al., Nature Climate Change, doi: 10.1038/s41558-022-01309-5