Currently, that number is about one in three people. Such action is under threat with President Trump's announcement this month that he will pull the USA out of the landmark Paris accord, though some cities and states are looking to fill the gap left by the federal government.
Lead author of the paper, Professor Camilo Mora, of the University of Hawaii at Manoa, said "For heatwaves, our options are now between bad or awful". "For heatwaves, our options are now between bad or awful". Mora said that many people are already suffering the consequences of the heat waves and although forecasts suggest that they are very likely to continue, the situation could be way worse if emissions are not significantly reduced.
"The human body can only function within a narrow range of core body temperatures around 37 degrees Celsius".
So while an increase in deadly heat waves may be inevitable, the research underscores the importance of working to reduce greenhouse gas emissions and minimize the future impact of climate change as much as possible, Mora said. By looking at heat and humidity during such lethal episodes, researchers worked out a threshold beyond which conditions become deadly.
Not surprisingly, the threshold is driven not only by the air temperature, but also the relative humidity.
The already substantial heat death rate "also points out the limitation of many of these governments to afford the cost of adaptation", he said in an interview with the Thomson Reuters Foundation.
"Even if we "successfully" mitigate climate change, and do follow a strong policy of reducing greenhouse gases here on, we should still expect to see a substantial increase in heat stress worldwide", Matthews said. For example, by 2100 NY is projected to have around 50 days with temperatures and humidities exceeding the threshold in which people have previously died.
"The heat means that we are becoming prisoners in our own homes - you go to Houston, Texas in the summer and there's no-one outside", he said.
Using historic data, the team found that 30 percent of the world's population sees at least 20 days each year that surpass the temperature and humidity thresholds for a deadly event at a given location.
Aggressive cutbacks now on emissions could help, though only slightly.
"We are running out of good choices for the future", he warned. "Dehydration is a compounding factor that makes the blood thicker, making it more difficult for the heart to work harder, among other things".
The study also found the greatest risk to human life from deadly heat was projected for tropical areas.
When analyzing the climatic conditions for those cities, the researchers discovered a common threshold beyond which temperatures and humidities became lethal.
In the deep tropics, such as in Jakarta, Indonesia, the entire year is pegged to be above the potentially deadly level.
A University of California Irvine study published this month discovered that the likelihood of a heat wave killing more than 100 people in India has doubled because of a 0.5 degrees Celsius climb in temperature in the last half century.
Dr Marshall Shepherd, director of the University of Georgia's Atmospheric Sciences Program, was not involved in the study but agreed with its findings.
Washington D.C., on average, will get hit with potentially deadly heat 15 days a year if the Paris target is met, and 85 day per year if no further action is taken to fight climate change. In India, where more than 2,400 people died from heat-related illness past year, local and state authorities are preparing for another brutally hot summer.
Most heat-related fatalities, however, do not take place during widely known disasters.
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