Climate science tells us that in the future, combinations of heat and humidity could be so extreme that the evaporation of human sweat may not sufficiently cool our bodies. Damon Winter/The New York Times

After enduring another scorching summer — the fourth-hottest on record for the contiguous United States — it may be hard to imagine conditions getting much worse. But as a new report from the United Nations’ panel on climate change warns, we are locked in to additional warming and other changes like sea level rise. And we are running out of time to avert potentially catastrophic outcomes.

One critically important and underreported fact is that as temperatures rise, absolute humidity, the total amount of moisture in the air, will also increase. That may create combinations of heat and humidity so extreme that the evaporation of human sweat won’t sufficiently cool our bodies, leaving even healthy adults at risk of death from overheating.

Our research suggests that in about 50 years, these deadly conditions — almost unknown on the planet today — could occur once per decade in parts of the world. Millions of people could be exposed to these extreme conditions if greenhouse gas emissions continue to rise following historical trends.

Today’s Extreme Events, and a Harsh Future

The heat index, also known as the “feels-like” temperature, shows how hot it feels when you consider the combined effect of temperature and humidity.



The highest heat index measured at Central Park in New York this summer was 106 degrees on July 3, when the temperature reached 92 degrees and the relative humidity was a muggy 62 percent. That's in the National Weather Service's “danger” zone for heat exhaustion, which can be deadly, especially for the elderly and the sick. Symptoms of heat exhaustion include muscle cramps, dizziness and vomiting. Heat indexes above about 137 degrees make any strenuous outdoor activity critically dangerous and potentially fatal, even for healthy people. These conditions are rare but not unheard of.



During the 1995 Midwest heat wave, for example, parts of Wisconsin experienced heat indexes above 137 degrees. More than 700 people died in Chicago, where the heat index was lower, topping out at 125 degrees. There is a far deadlier threshold.



In theory, a healthy adult resting in the shade, wearing thin clothing and with an endless supply of water could survive temperatures in excess of anything experienced on earth — if the air is dry. Add sufficient humidity, and even at far lower temperatures the same person would overheat and could die in a matter of hours. Sweat takes away excess heat as it evaporates. When the heat index tops about 165 degrees, the evaporation of sweat will not cool us sufficiently. Core body temperature will rise, and if it exceeds its tolerable range, organs will strain and start to fail. On July 31, 2015 in Bandar Mahshahr, Iran, the heat index reached 165 degrees, among the highest ever recorded. Thanks to robust infrastructure and cheap electricity, the city’s residents were able to take refuge in air-conditioned buildings, middle-class luxuries that don't exist for many in the hottest regions.

According to our projections, about 11 million people — nearly the population of Ohio — could be at risk of suffering heat and humidity past this limit of human tolerance at least once by 2080. And a heat index of about 165 degrees is deadly for even healthy adults resting in the shade. For children, the elderly, the sick and the billions who work outside, heat indexes of 115 to 130 degrees have led to tens of thousands of deaths in recent heat waves.

A man suffering from heat exhaustion is showered at a medical camp in Karachi, Pakistan, where a heat wave killed dozens of people in May 2018. Rehan Khan/EPA, via Shutterstock

Suffering Ahead for Billions of People

Warnings about extreme heat sometimes refer only to air temperature, and climate assessments rarely consider humidity. But for most hot and humid places, the number of days with an extreme combination of heat and humidity is projected to increase much faster than the number of days with just extreme air temperatures.

By 2080, more than three billion people could experience a heat index above about 122 degrees — considered extremely dangerous by the National Weather Service — and almost one billion could suffer conditions that make prolonged exposure or strenuous outdoor activity critically dangerous.

Population exposed to extreme heat and humidity 3.1 billion people in 2080 30 percent of the world’s population 20 EXTREME Danger of heat exhaustion 1.0 10 billion Outdoor Activity CRITICALLY DANGEROUS 0 2030 2040 2050 2060 2070 2080 3.1 Population exposed to extreme heat and humidity billion people in 2080 percent of the world’s population 30 20 EXTREME Danger of heat exhaustion 1.0 10 billion Outdoor Activity CRITICALLY DANGEROUS 0 2030 2040 2050 2060 2070 2080 Populations refer to those predicted to be exposed to the specified level of heat and humidity at least once.

Two generations from now, heat indexes higher than about 137 degrees, above which strenuous outside activity is critically dangerous, could occur regularly in the eastern United States. There are several reasons for this region’s projected susceptibility, including low elevation and abundant moisture from the balmy Gulf of Mexico and Atlantic Ocean. Globally, in the tropics and subtropics — including India, eastern China, parts of Brazil, central Africa and many other densely populated regions — seven or more days per year could be spent above this threshold.

By 2080: Days with heat indexes above 137 degrees 0 1 every 2 years 1 a year 2 A YEAR 3 A YEAR 7 A YEAR Atlanta Shanghai New Delhi Dubai Hong Kong Khartoum, Sudan Manaus, Brazil Lagos, Nigeria By 2080: Days with heat indexes above 137 degrees 0 1 every 2 years 1 a year 2 A YEAR 3 A YEAR 7 A YEAR Atlanta Shanghai New Delhi Dubai Hong Kong Khartoum, Sudan Manaus, Brazil Lagos, Nigeria By 2080: Days with heat indexes above 137 degrees 0 1 every 2 years 1 a year 2 A YEAR 3 A YEAR 7 A YEAR Atlanta Shanghai Dubai Manaus, Brazil Lagos, Nigeria Gray regions had zero predicted occurrences of a heat index above 137 degrees or the occurrences were too rare to estimate.

Soaring Frequency of Humid Heat Around the World

What is currently considered severe humid heat will become frequent. Imagine the single highest heat index that your region experiences each year. We studied how often those conditions could occur by 2080.

Projections for the United States are sobering. By 2080, New York City could experience approximately six weeks per year of humid heat at least as extreme as what has been experienced only once a year, on average, between 1981 and 2005.

By 2080: How many weeks per year the city could experience that heat index The worst heat index each year, on average 90° 3 weeks Seattle 109° 4 weeks Chicago 105° 6 weeks New York City 103° 8 weeks Atlanta By 2080: How many weeks per year the city could see that heat index The worst heat index each year, on average 90° 3 weeks Seattle 109° 4 weeks Chicago New York City 105° 6 weeks 103° 8 weeks Atlanta

Parts of the southeastern United States could see more than three months per year with heat indexes higher than the current once-per-year extreme.

By 2080: Months per year with heat indexes higher than the current once-per-year extreme 0.5 1 1.5 2 2.5 3 Wash. Maine Mont. N.D. Ore. Vt. Minn. N.H. Idaho Mass. S.D. Wis. N.Y. R.I. Conn. Mich. Wyo. Iowa N.J. Penn. Nev. Neb. Ohio Md. Del. Utah Ill. Ind. Colo. W. Va. Va. Mo. Calif. Kan. Ky. N.C. Tenn. Ariz. Okla. N.M. Ark. S.C. Ga. Ala. Miss. Tex. La. Alaska Fla. Hawaii By 2080: Months per year with heat indexes higher than the current once-per-year extreme 0.5 1 1.5 2 2.5 3

The picture looks even more bleak in the tropics, a region that has contributed relatively little to cumulative greenhouse gas emissions. Most of the tropics could spend more than half the year under conditions worse than the current once-per-year extreme.

By 2080: Months per year with heat indexes higher than the current once-per-year extreme 1 2 3 6 Moscow London Rome Istanbul New York Beijing Tokyo Los Angeles Cairo Shanghai Riyadh, Saudia Arabia Kolkata, India Hong Kong Mexico City Mumbai Lagos, Nigeria Bogota, Colombia Nairobi, Kenya Singapore Jakarta, Indonesia Rio de Janeiro Santiago, Chile Sydney Cape Town By 2080: Months per year with heat indexes higher than the current once-per-year extreme 1 2 3 6 Istanbul New York Hong Kong Cairo Mumbai Lagos, Nigeria Bogota, Colombia Jakarta, Indonesia Rio de Janeiro By 2080: Months per year with heat indexes higher than the current once-per-year extreme 1 2 3 6 Moscow London Istanbul New York Beijing Tokyo Los Angeles Cairo Hong Kong Mumbai Mexico City Lagos, Nigeria Bogota, Colombia Nairobi, Kenya Singapore Jakarta, Indonesia Rio de Janeiro Sydney Cape Town Santiago, Chile

Avoiding the Worst, Adapting to the Inevitable

The most direct way to minimize the effects of worsening heat and humidity is to dramatically reduce greenhouse gas emissions — for example, by taxing carbon and investing far more boldly in renewable energy.

But because some impacts of climate change are unavoidable, we’ll also need to adapt. Shifting daily activities — everything from outdoor markets to children’s sports — to the nighttime could reduce the impact of hotter, more humid days. Automation could play a larger role in daytime work for industries like construction and agriculture. But these and other adaptations will entail significant societal and economic costs.

Air-conditioning, which is either unavailable or too expensive for many, will increasingly become a life-or-death requirement in parts of the world. But more air-conditioning would lead to more climate warming, especially if powered by fossil fuels.

Homeless boys and a stray dog sleep in the breeze of an exhaust fan outside a restaurant in New Delhi. Tsering Topgyal/Associated Press

More research is also needed on just how harmful these unprecedented levels of heat and humidity will be for human health, especially among the most vulnerable: the elderly, those with pre-existing health conditions and people without air-conditioning. What are the economic and social costs of lost productivity, everywhere from agricultural fields to classrooms? Questions about how wildlife and natural ecosystems will be affected by humid heat are also mostly unexplored.

It remains to be seen to what extent society can adapt to frequent, unprecedented heat stress. But the prospects could dim as more locations reach deadly levels of heat and humidity, potentially unleashing cascading impacts like food insecurity, forced migration and violent conflict. In the immediate future, however, we need to consider how climate variables like heat and humidity could dance together to create outcomes that might be more extreme than imagined — and more dangerous.

The authors are climate scientists Ethan Coffel at Dartmouth College, Radley Horton at Columbia University’s Lamont Doherty Earth Observatory and Colin Raymond at Columbia University.