Heat waves worldwide have become longer and more frequent in last 70 years, may worsen due to global warming
In almost every part of the world, heat waves have been increasing in frequency and duration since the 1950s. This is according to a comprehensive worldwide assessment of heat waves down to regional levels. Worryingly, researchers found that this trend has accelerated and it is projected to worsen under enhanced global warming. Defined as prolonged periods of excessive heat, heat waves are a specific type of extreme temperature event.
The research team from the University of New South Wales (UNSW), Australia, produced a new metric, cumulative heat, revealing how much heat is packed into individual heat waves and heat wave seasons. This number is also on the rise, say authors. In Australia’s worst heat wave season, for example, an additional 80°C of cumulative heat was experienced across the country. In Russia and the Mediterranean, their most extreme seasons registered an additional 200°C or more.
“Not only have we seen more and longer heat waves worldwide over the past 70 years, but this trend has markedly accelerated. Cumulative heat shows a similar acceleration, increasing globally on average by 1°C-4.5°C each decade but in some places, like the Middle East, and parts of Africa and South America, the trend is up to 10°C a decade,” says lead author Dr Sarah Perkins-Kirkpatrick from the ARC Centre of Excellence for Climate Extremes, in the analysis published in Nature Communications.
The analysis reveals that over northern and southern Australia, the excess heat from heat waves has increased by 2-3℃ per decade. This is similar to other regions, such as western North America, the Amazon, and the global average. Alaska, Brazil, and West Asia, however, have cumulative heat trends of a massive 4-5℃ per decade. For the vast majority of the world, the worst seasons have occurred in the last 20 years, says the team. Dr Perkins-Kirkpatrick says the study is just the latest piece of evidence that should act as a “clarion call to policymakers” that urgent action is needed now if the world wants to prevent the worst outcomes of global warming.
The authors also identified that natural variability impacts on heat waves 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 team found that the changes were stark. For example, the Mediterranean saw a dramatic uptick in heat waves when measured over multi-decade spans. From 1950-2017, the Mediterranean saw an increase in heat waves 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 showed how the trends vary, and that some regions have experienced much more rapid increases. The Mediterranean has seen approximately 2.5 more heat wave days per decade, while the Amazon rainforest has seen an extra 5.5 more heat wave days per decade since 1950. The global average sits at approximately two extra heat wave days per decade. “Regions like the Amazon, north-east Brazil, west Asia and the Mediterranean are experiencing rapid changes in heat waves while areas like South Australia and North Asia are still seeing changes but at a slower rate,” the findings state.
According to the researchers, no matter how rapid or slow the changes are, it seems inevitable that vulnerable nations with less infrastructure will be hit hardest by extreme heat. “Climate scientists have long forecast that a clear sign of global warming would be seen with a change in heat waves. The dramatic region-by-region change in heat waves 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,” says Dr Perkins-Kirkpatrick.
The only heat wave metric that has not seen an acceleration is heat wave intensity, which measures the average temperature across heat waves. But the researchers say this is because globally one sees more heat wave days and heat waves are lasting longer. When the average temperature is measured across longer heat waves, any shifts in intensity are almost undetectable. Only southern Australia and small areas of Africa and South America show a detectable increase in average heat wave intensity, they explain.
There are many adverse impacts of heat waves, including on human health, agriculture, workplace productivity, wildfire frequency and intensity, and public infrastructure. The inequality of heatwave impacts has been assessed, adversely affecting developing nations due to a lack of adaptive capacity, as well as varying cultural constraints, say experts. “These impacts will increase under enhanced global warming, where more rapid heat wave trends will likely produce more severe and possibly irreversible impacts in some sectors,” the team cautions.
Another study, which is based on data from more than 36,000 weather stations around the world, also confirms that as the planet continues to warm, extreme weather events such as heat waves and heavy rainfall are now more frequent, more intense, and longer. The team evaluated 29 indices of weather extremes, including the number of days above 25℃ or below 0℃, and consecutive dry days with less than 1mm of rain. This latest update compares the three decades between 1981 and 2010 to 30 years prior, between 1951 and 1980. Globally, the clearest index shows an increase in the number of above-average warm days, says the research team.