June 19 (UPI) -- If greenhouse gas emissions continue at the same rate, 74 percent of the planet's populations will be exposed to deadly heatwaves by the end of the century. Even if world leaders manage to curb carbon emissions, nearly half the world's populations will still be exposed to deadly temperatures by 2100.
"For heatwaves, our options are now between bad or terrible," Camilo Mora, associate professor of geography at the University of Hawaii at Manoa, said in a news release. "Many people around the world are already paying the ultimate price of heatwaves, and while models suggest that this is likely to continue, it could be much worse if emissions are not considerably reduced."
Above certain thresholds, heat -- often exacerbated by humidity -- makes human life untenable. Since 1980, roughly 1,900 heatwave episodes have proved deadly.
In the United States, more than 9,000 people had died of heat-related causes since 1979. In 1995, a heat wave in Chicago killed 700 people.
Researchers at the University of Hawaii analyzed 783 lethal heat episodes -- comprising 164 cities across 36 countries -- to identify deadly temperature and humidity thresholds. Scientists then looked at places around the world where these thresholds are exceeded on at least 20 days per year. Currently, 30 percent of the world's populations are exposed to deadly heatwaves.
The analysis -- detailed in the journal Nature Climate Change -- confirm physiological findings. The findings showed the threshold at which heat becomes lethal is lowered as humidity rises.
"Finding a threshold beyond which climatic conditions turn deadly is scientifically important yet frightening," said study co-author Farrah Powell, a UH Manoa graduate student. "This threshold now allows us to identify conditions that are harmful to people. And because it is based on documented cases of real people across the globe, it makes it that more credible and relevant. The scary thing is how common those deadly conditions are already."
As part of their research, UH scientists built a web app that allows users to search how often different places experience a deadly combination of heat and humidity. The app also reveals how often places will experience deadly heat conditions in the future. By 2100, Orlando and Houston may experience deadly conditions for the entirety of the summer.
Though warming at the poles has been most dramatic, research suggests climate change will prove most life-threatening to populations in the tropics.
"With high temperatures and humidities, it takes very little warming for conditions to turn deadly in the tropics," said co-author Iain Caldwell, a UH Manoa post-doctoral researcher.