More than a third of summer heat-related deaths are caused by climate change, say researchers who warn that rising global temperatures risk pushing the toll higher.
Previous research on how climate change affects human health has mostly estimated the future risks posed by heatwaves, droughts, wild fires and other extreme events.
But a new study by an international team of 70 experts is one of the first, and and the largest, to look at health consequences that have already happened, the authors said.
Published in Nature Climate Change, the findings were stark: data from 732 locations in 43 countries spread across every inhabited continent revealed that, on average, 37 percent of all heat-related deaths could be attributed directly to global warming.
"Climate change is not something in the distant future," author Antonio Gasparrini, a professor of biostatistics and epidemiology at the London School of Hygiene and Tropical Medicine, told AFP.
"We can already measure negative impacts on health, in addition to the known environmental and ecological effects."
The authors said their methods – extended worldwide – add up to more than 100,000 heat-related deaths per year laid squarely at the feet of anthropogenic climate change.
Extreme heat deaths
That number could be an underestimate because two of the regions for which data was largely missing – south Asia and central Africa – are known to be especially vulnerable to extreme heat deaths.
The 100,000 figure is consistent with recent analysis from the Institute for Health Metrics and Evaluations (IHME), published in the journal The Lancet.
Th IHME calculated more than 300,000 global heat-related deaths in 2019. If just over a third of those deaths are due to climate change, as Gasparrini's team reported, the global total would tally more than 100,000.
India accounted for more than a third of the IHME tally, while four of the five worst-hit countries were in south Asia and central Africa.
The share of heat-related deaths attributable to global warming in the new study varied widely from country to country.
In the United States, Australia, France, Britain and Spain, for example, that figure – between 35 and 39 percent – was roughly in line with the average across all countries.
For Mexico, South Africa, Thailand, Vietnam and Chile, the figure rose above 40 percent.
And for half-a-dozen countries – Brazil, Peru, Colombia, the Philippines, Kuwait and Guatemala – the percentage of heat-related mortality caused by climate change was 60 percent or more.
The researchers used complex methodology combining health data and temperature records from 1991 to 2018, coupled with climate modelling, to calculate the actual number of heat-related deaths against how many fewer deaths there would have been without global warming.
Adapt or die
The researchers found that it is not the increase in average summer temperature – up 1.5C since 1991 in the locations examined – that boosted death rates, but heatwaves: how long they last, nighttime temperatures and humidity levels.
Also crucial was the ability of the population to adapt.
If 95 percent of the population had air conditioning, mortality would be lower. But if they did not, or if farmers needed to work outside in temperatures of 45C to feed their families, the impacts could be catastrophic.
Even wealthy nations remained vulnerable: in 2003, a relentless heatwave in western Europe claimed 70,000 lives.
Deadly heatwaves that might have occurred once a century before climate change kicked in could, by the middle of the century, happen far more frequently, scientists warned.
The burgeoning field of attribution climate science measures by how much, for example, a typhoon's intensity, a drought's duration, or a storm surge's destruction had been amplified by global warming.
But little research has tried to do the same for human health, notes Dan Mitchell, a researcher at the Cabot Institute for the Environment at the University of Bristol.
"This shift in thinking is essential ... so that global leaders can understand the risks," he told Nature Climate Change.
