Heatwaves now last much longer than they did in the 1980s

An analysis of all the heatwaves that occurred around the world between 1979 and 2020 has found that they are now persisting for 12 days on average compared with 8 days at the start of the study.

As the planet continues to heat up, they will persist for even longer, says Wei Zhang at Utah State University. “Based on the trend, it could double to 16 days by around 2060,” he says.

Zhang’s team found that heatwaves are not only persisting for much longer, they are also becoming more frequent and moving more slowly, meaning that specific places have to endure heatwave conditions more often and for longer.

While heatwaves are usually thought of as events affecting one region, the area affected by extreme heat changes over time as the weather systems responsible for the hot conditions move.

The speed at which heatwaves move has slowed from around 340 kilometres per day in the 1980s to around 280 kilometres per day now, the team found. What’s more, the rate of slowdown is accelerating.

Because heatwaves are persisting for longer, they are travelling further despite their slower average speeds, with the total distance rising from around 2500 kilometres to around 3000 kilometres. This means a larger area is being affected.

The study didn’t look at the causes of these trends. But the shift towards more frequent, slower-moving and longer-lasting heatwaves as the planet warms will lead to ever more devastating impacts on societies and nature unless more is done to prevent further warming, the team warns.

Most previous studies of heatwaves have looked at specific places or regions. Zhang’s team is one of only a few to look at how heatwaves move over time.

To do this, the team divided the world into a grid. A heatwave was defined as one or more grid squares having temperatures well above the 1981 to 2010 average – specifically, higher than 95 per cent of temperatures in that period – for more than 3 days.

The team found heatwaves tend to move in certain directions due to prevailing conditions. For instance, in Australia there is strong tendency for heatwaves to move towards the south-east, whereas in South America they tend to move north-east.

“I find it very interesting to see all those heatwave properties that cannot be captured when squeezing heatwaves into static consideration,” says Andrea Böhnisch at the Ludwig Maximilian University of Munich, Germany, who was involved with one of the few other studies to look at heatwaves as moving systems.

However, when it comes to figuring out what needs to be done to adapt to a hotter world, regional assessments may be more helpful than global ones, says Böhnisch.

She also points out that the numbers for, say, heatwave persistence, depend heavily how the team defines a heatwave. With different definitions, the overall trends would remain the same but the numbers could vary substantially. “This should be considered when looking at the precise numerical values,” she says.

Other studies have shown that hurricanes are also moving more slowly, says David Keellings at the University of Florida.

“So this means that these incredibly dangerous events are going to spend longer over any one location, and that impacts will be felt more. Generally, the longer a population is exposed to heatwave conditions, the greater the rate of hospitalisation and death,” he says.