Heatwaves worldwide have become longer and more common over last 70 years

Scientists have found that heatwaves have been increasing in frequency and duration since the 1950s in nearly every part of the world.

The study, published in Nature Communications analysed the extreme temperature events at a regional level since the 1950s and found that nowhere on the planet experienced a significant decrease in heatwaves.

"Not only have we seen more and longer heatwaves worldwide over the past 70 years, but this trend has markedly accelerated," said study lead author Dr Sarah Perkins Kirkpatrick from the ARC Centre of Excellence for Climate Extremes in Australia.

"Cumulative heat shows a similar acceleration, increasing globally on average by 1 degree C-4.5 degree C each decade but in some places, like the Middle East, and parts of Africa and South America, the trend is up to 10 degree C a decade," Kirkpatrick added.

For the findings, the research team had produced a new metric, cumulative heat, which reveals exactly how much heat is packed into individual heatwaves and heatwave seasons. As expected, that number is also on the rise.

According to the researchers, in Australia's worst heatwave season, an additional 80 degree C of cumulative heat was experienced across the country. In Russia and the Mediterranean, their most extreme seasons baked in an additional 200 degree C or more.

The study also identified that natural variability impacts on heatwaves can be large at regional levels. This variability can overwhelm heatwave trends, so regional trends shorter than a few decades are generally not reliable.

To detect robust trend changes, the researchers looked at how the trends had changed over multi-decade intervals between 1950-2017. The changes were stark. For example, the Mediterranean, saw a dramatic uptick in heatwaves when measured over multi-decade spans. From 1950-2017, the Mediterranean saw an increase in heatwaves by two days a decade. But the trend from 1980 to 2017 had seen that accelerate to 6.4 days a decade.

The regional approach also showed how the trends vary. Regions like the Amazon, north-east Brazil, west Asia and the Mediterranean are experiencing rapid changes in heatwaves while areas like South Australia and North Asia are still seeing changes but at a slower rate.

"The dramatic region-by-region change in heatwaves we have witnessed over the past 70 years and the rapid increase in the number of these events, are unequivocal indicators that global warming is now with us and accelerating," the study authors wrote.

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