More than a third of the world's annual heat deaths are due directly to global warming, according to the latest study to calculate the human cost of climate change.
But scientists say that's only a sliver of climate's overall toll - even more people die from other extreme weather amplified by global warming such as storms, flooding and drought.
And heat death numbers will grow exponentially with rising temperatures.
Dozens of researchers looking at heat deaths in 732 global cities from 1991-2018 calculated 37 per cent were caused by higher temperatures from human-caused warming, according to a study in the journal Nature Climate Change.
That amounts to about 9700 people a year from just those cities but it is much more worldwide, the study's lead author says.
"These are deaths related to heat that actually can be prevented. It is something we directly cause," said Ana Vicedo-Cabrera, at the Institute of Social and Preventative Medicine at the University of Bern in Switzerland.
The highest percentages of heat deaths caused by climate change were in cities in South America.
Vicedo-Cabrera pointed to southern Europe and southern Asia as other hot spots.
Sao Paulo, Brazil, has the most climate-related heat deaths, averaging 239 a year, researchers found.
About 35 per cent of heat deaths in the United States can be blamed on climate change.
That's a total of more than 1100 a year in about 200 US cities, topped by 141 in New York. Honolulu had the highest portion of heat deaths attributable to climate change, 82 per cent.
Scientists used decades of mortality data in the 732 cities to plot curves detailing how each city's death rate changes with temperature and how the heat-death curves vary from city to city.
Some cities adapt to heat better than others because of air conditioning, cultural factors and environmental conditions, Vicedo-Cabrera said.
Then researchers took observed temperatures and compared them with 10 computer models simulating a world without climate change.
The difference is warming humans caused.
By applying that scientifically accepted technique to the individualised curves for the 732 cities, scientists calculated extra heat deaths from climate change.
Australian Associated Press