Without Significant Greenhouse Gas Reductions, Countries in the Tropics and Subtropics Could Face ‘Extreme’ Heat Danger by 2100, a New Study Concludes

Under the study’s most extreme climate and heat scenarios, as many as 90 days each year could be unsafe for people to be outside.

A laborer quenches his thirst with water from a bottle on a street amid rising temperatures in New Delhi on May 27, 2020. Credit: Jewel Samad/AFP via Getty Images
A laborer quenches his thirst with water from a bottle on a street amid rising temperatures in New Delhi on May 27, 2020. Credit: Jewel Samad/AFP via Getty Images

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If the effects of climate change go unchecked through the end of the century, some parts of the world will likely experience roughly two weeks each year when temperatures are so high that it would be too dangerous for anyone to venture outdoors, according to a new study released Thursday.

The study, published in the open-access journal Communications Earth & Environment, found that by 2100 there are likely to be 15 days a year in which some countries near the equator experience heat indexes exceeding 124 degrees Fahrenheit, or 51 degrees Celsius.

The heat index is derived from measurements of air temperature and relative humidity. The National Weather Service identifies a heat index above 124 degrees as a condition of “extreme danger” in which heat stroke is “highly likely” and it is unsafe for people to be outside.


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Researchers from the University of Washington and Harvard University identified that extreme heat scenario as one of a range of possibilities which are contingent both on the level of global greenhouse gas emissions in the decades to come and the steps that policymakers take to mitigate them. In one of the researchers’ most extreme scenarios, as many as 90 days out of each year would be too hot for people to go outside.

Lucas Vargas Zeppetello, an atmospheric scientist at Harvard who served as the lead author of the study, said his research centered on developing statistical projections of global mean temperature change based on demographic and economic growth trends for the remainder of the 21st century. He noted that there are few examples in the historical record of locales that have exceeded a heat index at the extremely dangerous level. 

“If you look 50 to 100 years into the future, there are some regions in the world where, depending on how good or bad we do curbing our CO2 emissions, that could be a regular occurrence,” Zeppetello said, referring to the extremely dangerous heat index designations.

“There’s two sides of the coin here: The good side is that we still have time to prevent the worst possible scenario and make it so that we mitigate the worst possible impacts,” Zeppetello said. “But the bad side of the coin is if we don’t do anything, the consequences—particularly for people in the global subtropics, the Indian subcontinent—are going to be fairly dramatic.”

Heat Stress Projected to Impact Billions by 2100

Zeppetello and his co-researchers also found that countries in the tropics and subtropics would likely experience as many as 180 days of dangerous temperatures (a heat index above 103 degrees Fahrenheit on the National Weather Service scale) by 2050. By 2100, those regions would likely experience a heat index at that level for most of the year.

The tropical and subtropical countries experienced heat indexes at the dangerous level on about 55 days each year from 1979 to 1998, 

The findings of the study build on a growing body of research that attempts to identify the potential effects of rising temperatures on future generations. Earlier this month, the First Street Foundation released a model indicating that by 2053 as much as a quarter of U.S. land area—a ribbon of states stretching from Wisconsin to Texas—would become part of an “extreme heat belt” marked by extremely dangerous heat index events.

The effects of extreme heat can take a severe toll on the human body. Researchers said that a dangerous heat index designation can lead to heat cramps, heat stroke, exhaustion, fatigue, nausea, headache, excessive muscle aches, confusion, weakness, slowed heartbeat, dizziness and fainting.

An extremely dangerous heat index includes those effects and vertigo, shortness of breath, vomiting, delirium, loss of consciousness and convulsions. If people in extremely dangerous heat index conditions go untreated, death can occur within hours.

David Battisti, an atmospheric scientist at the University of Washington who was one of Zeppetello’s co-researchers, said that while models projecting temperature change may vary in estimating such factors as how much carbon dioxide might be released into the atmosphere in the future, the pattern of warming has remained consistent.

Although researchers found that the sharpest increases in temperature would be in sub-Saharan Africa, the Amazon rainforest, northern Australia, southeast Asia and the Indian subcontinent, the effects of what researchers call “heat stress” would also increase in the United States.

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In the southeastern United States, for example, locales that experienced two or three days per year of dangerous heat index levels at the end of the 20th century, might experience 20 to 30 such days by the middle of the century.

Battisti said that the high humidity levels in some parts of the country can have a dramatic effect on the heat index.

“If you live in Los Angeles or if you live in Denver, you can have these really hot days, they won’t kill you because it’s pretty dry,” Battisti said. “Whereas if you live in the southeast U.S., a day that you have even less temperature still could be more dangerous because of the humidity in the air.”

Zeppetello noted the recent record-breaking temperatures in cities in Europe and the United States and said that they may provide a glimpse of what is likely to come in the latter half of the century and beyond.

“There’s a great quote that I can’t take credit for, but there was someone in the scientific community who said, ‘Don’t think of it as the hottest year in history; think of it as the coldest year for the next hundred years,’” Zeppetello said, “And I think there’s value in that. Just seeing that we are tipping the scales pretty quickly toward really unprecedented forms of weather in the global north.”