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For Australia, the worst heatwave season - such as the 2009 belter across southern Australia - caused an additional 80 degrees of cumulative heat across the country, the paper found. For Russia in 2010 and the Mediterranean in 2003, their most extreme seasons involved 200 degrees of extra heat.

“Not only have we seen more and longer heatwaves worldwide over the past 70 years, but
this trend has markedly accelerated,” Sarah Perkins Kirkpatrick, the paper's lead author, said.

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

Anthropogenic climate change caused by rising levels of greenhouse gas emissions was driving the shift in heatwaves, the paper said.

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As heatwaves are getting longer, average intensity changes during hot spells are statistically insignificant for most parts of the world. The exceptions, though, include southern Australia and small parts of Africa and South America where heatwave intensity is showing a detectable rise over time, the researchers said.

Teasing out the regional variations was important because areas that were enduring longer, slightly warmer heatwaves would require different management systems for public health and energy supply than shorter, more intense events – even if cumulative intensities were similar, the paper said.

Trend changes were not globally uniform in magnitude, with the biggest shifts occurring in regions already "known to experience disproportionately more adverse effects of climate change", it stated.

“This research is just the latest piece of evidence that should act as a clarion call to
policymakers that urgent action is needed now if we are to prevent the worst outcomes of
global warming," Dr Perkins-Kirkpatrick said. "The time for inaction is over."