Concurrent Hot and Dry Summers More Common in Future

According to ... statistics ... extreme climate events, similar to the heatwave that affected large areas of western and central Europe in the summer of 2003, are only supposed to occur around every 100 years.  But as global warming pushes average temperatures higher, the frequency of several extreme weather events is set to increase, experts claim.

Concurrent extremes more frequent
Perhaps the statisticians need to check their figures.  Researchers have traditionally studied extreme climate events such as heatwaves and drought in isolation, producing separate forecasts of how frequently each one is likely to occur.  But when these extremes coincide -- a combination of hot and dry summers, for example -- their impact is far greater.

ETH researcher Jakob Zscheischler and Professor Sonia Seneviratne from the ETH Institute for Atmospheric and Climate Science have now calculated the probability of compound climate extremes, as the co-occurrence of severe heat and drought generally depends on the correlation between temperature and precipitation in the summer.  The results of their study have just been published in the academic journal Science Advances.

Co-occurrence as much as five times greater than expected
In their study, Zscheischler and Seneviratne have calculated that the combination of heat and drought is as much as two to four times more frequent than if these two extreme climate events are studied in isolation.  In America's mid-west, for instance, the probability of this combination occurring is even up to five times higher.

Calculating the probability of these two extremes separately and then combining them is not the same as establishing the likelihood of their co-occurrence.  "Our calculations clearly show that compound climate extremes occur much more frequently than previously expected," says the ETH professor.

Zscheischler and Seneviratne have analyzed the combination of heatwaves and drought because observations show that "there are definite correlations between such compound climate events," she goes on to explain.  This was the reason for the extremely hot and dry summer that central Europe experienced in 2015.  "These scenarios are becoming more common."

Correlation amplified by unconstrained climate change
In their study, the authors show that the correlation between temperature and precipitation will intensify if climate change remains unconstrained.  As a result, very hot and dry summers will become increasingly common.

The greater frequency of compound climate extremes also poses a bigger threat to agriculture, society and the economy.  If two climate extremes are examined together, the probability of their co-occurrence often rises dramatically due to previously unaccounted dependencies -- along with the associated risks.  The latest calculations show that the risk is far greater than previously assumed, the ETH professor warns:  "We're not properly prepared for this."

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