Heatwave trends accelerate worldwide

Nearly every part of the world has seen a significant increase in the frequency and duration of heatwaves over the past seven decades and this trend is likely to accelerate, according to a new study.

Of note, the findings published in the journal Nature Communications show that cumulative heat – a new measure of exactly how much heat is packed into individual heatwaves and heatwave seasons – is increasing significantly.

The global average is a one to 4.5 degree Celsius increase each decade, but in areas such as the Middle East and parts of Africa and South America, it is as much as 10 degrees.

The only metric assessed that isn’t uniformly increasing is heatwave intensity, which measures the average temperature across heatwaves. There have been increases in southern Australia and parts of Africa, but these are small.

However, the researchers say this is simply a result of mathematics. As heatwaves are lasting longer, the average temperature is measured across a longer period, which brings it down.

“Climate scientists have long forecast that a clear sign of global warming would be seen with a change in heatwaves,” says lead author Sarah Perkins Kirkpatrick, from Australia’s University of NSW.

“Not only have we seen more and longer heatwaves worldwide over the past 70 years, but this trend has markedly accelerated.”

Perkins Kirkpatrick and colleague Sophie Lewis say understanding regional heatwave trends “has critical implications for the biophysical and human systems they impact”, but previous assessments have been “hindered by the range of metrics employed, underpinning datasets, and time periods examined”.

Their study used Berkeley Earth data from 1950 to 2017 and defined a heatwave as at least three consecutive days above the 90th percentile of the maximum recorded temperature for that calendar day.

The results show, among other trends, that the most intense heatwave seasons as defined by cumulative intensity generally have occurred since 2000. But the picture varies around the world.

Regions such as the Amazon, west Asia and the Mediterranean have experienced rapid escalation, but it is slower in South Australia and North Asia.

In the Mediterranean, there was an average increase of two heatwave days a decade for the period 1950-2017, but this rose to 6.4 days for 1980-2017.

“The results of our study have important implications for all systems affected by chronic heat exposure. This is because the appropriate management and adaptation of these systems is influenced by the separate components that drive the overall change,” the authors write in their paper.

“For example, longer, slightly warmer heatwaves may require different management strategies across various sectors such as public health and energy supply than shorter and more intense events despite a similar change in cumulative intensity.”