Heatwaves are happening more frequently and lasting longer around the world, according to the first comprehensive global study on prolonged periods of excessive heat.
The research 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.
This study found that changes in heatwaves are not only increasing but accelerating due to the human-driven climate crisis.
“Not only have we seen more and longer heatwaves worldwide over the past 70 years, but this trend has markedly accelerated,” said lead author Dr Sarah Perkins Kirkpatrick from the ARC Centre of Excellence for Climate Extremes.
Heatwaves have a myriad of negative impacts, including on human health and agriculture, and can lead to more frequent and intense wildfires.
Extreme temperature events were the worst in regions which experience disproportionately more negative impacts of the climate crisis.
The research, published in the journal Nature Communications, also came up with a new metric — cumulative heat — to reveal how much heat is packed into individual heatwaves and seasons. That number is also rising.
Crucially the study found that the most intense heatwave seasons, as defined by cumulative intensity, have happened since 2000.
Dr Perkins Kirkpatrick said: “Cumulative heat shows a similar acceleration, increasing globally on average by 1C-4.5C each decade but in some places, like the Middle East, and parts of Africa and South America, the trend is up to 10C a decade.”
During Australia’s most intense heatwave season, an additional 80C (176F) of cumulative heat was experienced. In Siberia and the Mediterranean, their most extreme seasons had an additional 200C (392F).
Alaska experienced its worst season where 150 C (302F) extra heat was contributed.
According to the study, the only metric of heatwaves that has not accelerated is heatwave intensity, which measures the average temperature across heatwaves.
Scientists say that this is because, around the world, there are more heatwave days and they are lasting longer. When the average temperature is measured across longer heatwaves, any shifts in intensity are barely detectable.
A detectable increase in average heatwave intensity was found only in southern Australia and some regions of Africa and South America.
At a regional level, researchers discovered that natural variability can have a large impacts on heatwaves. This variability can overwhelm heatwave trends, so regional trends shorter than a few decades are generally not reliable.
Instead, researchers looked at how the trends had changed over multi-decade intervals between 1950-2017.
The changes were dramatic. In the Mediterranean, there was a significant increase in heatwaves when measured over multi-decade spans.
From 1950-2017, the Mediterranean saw an increase in heatwaves by two days a decade. When researchers narrowed it down to 1980 to 2017, the trend accelerated to 6.4 days a decade.
Heatwave trends were varied in different regions. There are rapid changes in heatwaves in the Amazon, north-east Brazil, west Asia and the Mediterranean.
South Australia and North Asia are still seeing changes to heatwave trends but at a slower pace.
Overall, the research found that vulnerable nations, where infrastructure is less robust, will be the hardest hit by extreme heat.
The results indicate that strategies must be devised to deal with chronic heat exposure, which will have impacts on human health and energy supply, and that further research is needed on what impacts the decomposition of cumulative heat will have.
“Climate scientists have long forecast that a clear sign of global warming would be seen with a change in heatwaves,” said Dr Perkins Kirkpatrick, adding that “these events, are unequivocal indicators that global warming is now with us and accelerating”.