More than a third of the world’s heat deaths each year are directly due to global warming, according to the latest study to calculate the human cost of climate change.
But scientists say it’s just a bit of the global toll of climate change (there are even more people dying from another extreme climate amplified by global warming like storms, floods and drought) and that the number of heat deaths will grow exponentially with rising temperatures.
Dozens of researchers who examined heat deaths in 732 cities around the world between 1991 and 2018 estimated that 37 percent were caused by higher temperatures due to human-caused warming, according to a study published Monday in the Nature Climate Change magazine.
That’s about 9,700 people a year from cities alone, but it’s a lot more worldwide, said the study’s lead author.
“They are heat-related deaths that can really be prevented. It’s something we cause directly, ”said Ana Vicedo-Cabrera, an epidemiologist at the Institute for Preventive and Social Medicine at the University of Bern in Switzerland.
The highest percentages of heat deaths caused by climate change were in South American cities.
Vicedo-Cabrera pointed to southern Europe and southern Asia as other hotspots for climate-related heat deaths.
The researchers found that Sao Paulo (Brazil) is the one that records the most climate-related heat deaths, with an average of 239 per year.
“Negative” health effects
According to the study, about 35 percent of heat deaths in the United States can be blamed on climate change. That means a total of more than 1,100 deaths a year in about 200 U.S. cities, surpassed by 141 in New York. Honolulu had the highest share of heat deaths attributable to climate change, 82%.
Scientists used data from decades of mortality in the 732 cities to draw curves detailing how each city’s mortality rate varies with temperature and how heat-death curves vary from city to city. Some cities adapt better to heat than others because of air conditioning, cultural factors and environmental conditions, Vicedo-Cabrera said.
The researchers then took the observed temperatures and compared them to 10 computer models that simulate a world without climate change. The difference is the warming caused by humans.
By applying this scientifically accepted technique to the individualized heat-to-death curves for the 732 cities, the scientists calculated the deaths from additional heat from climate change.
“People are still asking for evidence that climate change is already affecting our health. This attribution study directly answers this question using state-of-the-art epidemiological methods and the amount of data the authors have accumulated to analyze is impressive, ”said Jonathan Patz, director of the Global Health Institute at the University of Wisconsin.
Patz, who was not part of the study, said he was one of the first to now detail heat deaths related to climate change, rather than the future.
“Climate change is not something in the distant future,” Antonio Gasparrini, a professor of biostatistics and epidemiology at the London School of Hygiene and Tropical Medicine, told AFP news agency.
“We can now measure the negative impacts on health, in addition to the known environmental and ecological effects,” Gasparrini said.
Scientists warn that in the middle of the century, deadly heat waves that could have occurred once a century before climate change began could occur much more frequently.
The growing field of attribution of climate science measures the amount, for example, of the intensity of a typhoon, the duration of a drought, or the destruction of a storm surge that has been amplified by global warming.
But little research has attempted 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 change in thinking is essential … so that world leaders can understand the risks,” he said in a comment to Nature Climate Change.