AUDIE CORNISH, HOST:
Now, a story about heat, the sweaty, miserable kind. Heat plus humidity. Working outdoors or playing sports on a hot, muggy day can be dangerous, even deadly. And as the climate continues to warm, being outside will become even more challenging. Those are the findings of a new study in the journal Nature Climate Change.
NPR's Richard Harris tells us more.
RICHARD HARRIS, BYLINE: The one-two punch of heat and humidity is so serious, the military keeps a close eye on it and calls off physical activity when it gets too bad. Similar rules govern some outdoor workers as well. Howard Frumkin at the University of Washington School of Public Health says there's good reason for that.
HOWARD FRUMKIN: First, you get relatively mild reactions, such as a heat rash. Then, you can get heat cramps, then heat exhaustion and then more severely, if people keep on pushing through, heat stroke and possibly death.
HARRIS: Scientists measure this heat and humidity combination with a thermometer they swath in a wet cloth and then swing around their heads. This is called a wet bulb globe temperature. Scientists at the National Oceanic and Atmospheric Administration's lab at Princeton decided to forecast what will happen to these measurements worldwide as the planet warms.
John Dunne says the answer is a lot.
JOHN DUNNE: Heat stress itself affects more people as climate warms and affects the people that it already affected more severely.
HARRIS: To put a number on this, Dunne and his colleagues measured something called labor capacity. In essence, it asks how much time people have to rest when working outside during the hottest months of the year. Right now, globally, people can work 90 percent of the time and have to take a break 10 percent of the time. But that will change a lot by 2050.
DUNNE: The labor capacity will reduce from 90 percent down to 80 percent during peak months.
HARRIS: And as temperatures rise, that will get worse and worse. By the end of the century, it's quite possible the planet will have heated up 3 degrees Celsius, about 5 degrees Fahrenheit. That will make hot and sticky places truly miserable.
DUNNE: The lower Mississippi Valley would see decreases in the ability for people to work outside safely down to about 30 percent during peak months.
HARRIS: And some time in the subsequent century, unless warming is brought under control, New York City would become as hot and humid as the Middle Eastern country of Bahrain is today. Dunne says the impact on human beings is much more serious when you look at it this way as opposed to just thinking about average temperature increases.
DUNNE: The potential seems frightening. But, you know, I have great hope in the ability of humanity to adapt.
HARRIS: The change will happen gradually, he says, and technologies like air conditioning can help, at least among people who now work in open air factories. But Howard Frumkin worries that this effect will hit hardest among the world's poor people who barely make it by today working in farm fields or sweat shops.
FRUMKIN: Especially in times of economic desperation or in places where people really need to work to provide food to their families. I think there's a real danger that people will try to power through and endanger themselves.
HARRIS: And Frumkin says the new research probably underestimates the effect because it doesn't take into account that cities are hot and home now to half the planet's population.
FRUMKIN: Cities will be far hotter places than the countryside and we can expect the diminished work capacity to hit especially hard in cities.
HARRIS: And while air conditioning can help, it also requires electricity, which is often generated by burning fossil fuels, which adds to global warming. Richard Harris, NPR News. Transcript provided by NPR, Copyright NPR.