A new study has found that 30 percent of the world's population is now exposed to potentially deadly heat for 20 days per year or more-and like a growing forest fire, climate change is spreading this extreme heat.
A tourist cools down at a fountain on a hot summer day in downtown Rome.
The study also found that the greatest risk to human life from deadly heat was projected for tropical areas.
"For heatwaves, our options are now between bad or awful", said Camilo Mora, an academic at the University of Hawaii and lead author of the study. "The young and elderly are at particular risk, but we found that this heat can kill soldiers, athletes, everyone".
The grim assessment would also expose 73.9 per cent of the world's population to climates beyond what the human body can withstand by the end of the century.
A shift to more, and more extreme, heat is one of the clearest outcomes of global warming: as overall temperatures rise, extreme heat becomes more likely, and extreme cold less likely. These heat waves bring with them droughts, wildfires, and an increased risk of communicable disease contraction - but the heat stress itself can also cause organ failure.More news: Trump calls Warmbier's death a 'total disgrace'
Armed with profiles of what constitutes deadly heat in different areas, the researchers looked to the future using models of a climate with low, moderate and high greenhouse gas emissions.
The study included maps of the number of days per year when heat waves burn above a deadly threshold, and paints risky colors for Vietnam and many countries in Asia-Pacific, Africa and South America.
Mora's research shows that the overall risk of heat-related illness or death has climbed steadily since 1980, with around 30% of the world's population now living in climatic conditions that deliver deadly temperatures at least 20 days a year.
"That's why the government needs to draw up measures to protect marginalized people in case of lethal heatwaves, in addition to reducing green house gas emissions". Warmer than that is termed as fever and as the temperature of the surroundings rise the body starts to sweat to cool off.
Now, about 30 percent of people worldwide are exposed to lethal heatwaves each year, the investigators said. Researchers believe that even with a reduction in emissions, almost half of the world will still be affected. If no action is taken - say, if the Paris agreement fails and isn't replaced - 74 percent of the global population will suffer these excessive conditions by 2100.
By 2100, for instance, NY is likely to experience around 50 days a year with combined temperature and humidity exceeding the threshold at which people have previously died, researchers said.More news: White House torn over Mueller's role as pressure grows
A number of deadly heat waves have made global headlines in the past few decades, the researchers point out.
Warnings like that could increase the number of people who survive a deadly heat event, the researchers say. "You either jump from the 4th floor or you jump from the 8th floor", he said.
Caldwell: High temperatures (and high humidity) can become deadly when the body produces or absorbs more heat than it can release. When the core body temperature gets hot enough it can create problems for numerous body's organs and, in some cases, the organs can malfunction and eventually lead to death.
Mora, the author of the University of Hawaii study, noted that the impacts of rising heat will extend far beyond simply a higher death toll, to things like "people having to take longer breaks to withstand the heat and compensate for it by working longer hours".
Developing countries, though, often lack the resources to fund adaptation efforts.More news: Fleury among players exposed for Vegas NHL expansion draft