What happens when two extreme weather events happen at once?

As the world warms, droughts and heat waves are going to become more common - but what happens when they happen at the same time?

Researchers from the UFZ Helmholtz Centre for Environmental Research investigated so-called ‘compound hot-dry events’ where heat waves and droughts strike at the same time.

The researchers warn that such events will become much more common due to global warming.

The researchers define ‘compound hot-dry events’ as summers in which the average temperature was higher than in 90 percent of the summers between 1950 and 1980, and precipitation was simultaneously lower than in 90 percent of those years.

One example is the 2018 drought and heat wave in Europe which led to forest fires and wildfires across the continent.

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The researchers warn that compound hot-dry events will increase with global warming: while the frequency lay at 3 percent between 1950 and 1980, which statistically is an occurrence every 33 years, in a climate that is two degrees warmer, this figure will be around 12 percent.

This would be a fourfold increase compared to the historical period studied.

Climate researcher Dr Jakob Zscheischler of UFZ said, "In the past, periods of drought and heat waves were often sidered 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.”

The researchers used simulations to compare historical results with a future climate two degrees warmer than pre-industrial times.

Dr Emanuele Bevacqua, first author and climate researcher at the UFZ said, "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,"

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Last year’s UN climate change report warned that extreme weather events like heatwaves and droughts which previously would have happened every 50 years could soon happen every four.

The report was the first to quantify the likelihood of extreme events across a wide variety of scenarios.

The researchers also warned that other ‘tipping point’ events are a possibility.

The researchers wrote, “Abrupt responses and tipping points of the climate system, such as strongly increased Antarctic ice sheet melt and forest dieback, cannot be ruled out”.

Dr. Robert Rohde, Lead Scientist of Berkeley Earth said, “What were once-in-50-year heat extremes are now occurring every 10 years.

“By a rise of two degrees celsius, those same extremes will occur every 3.5 years.”

The report found that (for example) once-in-a-decade heavy rain events are already 1.3 times more likely and 6.7% wetter, compared with the 50 years leading up to 1900 when human-driven warning began to occur.

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Droughts that previously happened once a decade now happen every five or six years.

Xuebin Zhang, a climatologist with Environment Canada in Toronto warned that as the world warms, such extreme weather events will not just become more frequent, they will become more severe.

Zhang said that the world should also expect more compound events, such as heat waves and long-term droughts occurring simultaneously.

Zhang said, “We are not going to be hit just by one thing, we are going to be hit by multiple things at the same time.”Last week’s UN climate change report warned that extreme weather events like heatwaves and droughts which previously would have happened every 50 years could soon happen every four.

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