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A third of the world’s people already face deadly heat waves. It could be nearly three-quarters by 2100.

Nearly one-third of the global population suffers deadly levels of heat for at least 20 days during the year, new research suggests. And by the end of the century, thanks to climate change, this number could climb above 70 percent.

Certain parts of the world, the researchers note, will be harder hit than others. Tropical regions, where temperatures are already high for much of the year, will see many more days of deadly heat than other parts of the world. Under a business-as-usual climate scenario, they may face these conditions almost year-round by 2100.

The new study, published Monday in the journal Nature Climate Change, underscores the growing threat that rising temperatures pose to public health. The research focuses specifically on heat and humidity conditions known to increase the risk of human mortality — generally speaking, that’s when temperatures climb above 98.6 degrees Fahrenheit (the average human body temperature), but can also include cooler conditions with higher levels of humidity.

“We found this very unique threshold of temperature and humidity that allows us to identify why all these people die in all these cities around the world,” said lead study author Camilo Mora, a geography expert at the University of Hawaii at Manoa. “It makes a lot of sense from the human physiology side of things. The way in which the body cools down is by sweating — the evaporation of that sweat cools you down. But when it’s humid, that sweat doesn’t evaporate, so the heat that the body generates, instead of going away, it stays in your body.”

A number of deadly heat waves have made international headlines in the past few decades, the researchers point out. The Chicago heat wave of 1995, for instance, is believed to have caused more than 700 deaths in less than a week after temperatures soared above 100 degrees. A 2010 heat wave in Russia during July and August may have killed more than 10,000 people.

The researchers examined more than 900 published papers documenting cases of extreme heat and excess mortality between 1980 and 2014. Altogether, the papers identified 783 individual events in 164 cities around the world. The researchers used these studies to identify the heat and humidity thresholds that lead to increased mortality rates.

A majority of the reported cases occurred in mid-latitude cities, largely in Europe and North America. According to Elisaveta Petkova, a researcher at Columbia University’s Earth Institute who was not involved with the study, most research on climate-related mortalities has tended to focus on Western nations, and the new study highlights a relative shortage of data from other parts of the globe. However, the researchers were able to find at least some information on the climate conditions that led to heat-related deaths for locations throughout much of the world.

By looking at historical climate data, the researchers determined that about 13 percent of all the world’s land area — home to about 30 percent of the total human population — had faced these deadly conditions for 20 or more days during the year 2000. And this number is only expected to grow.

Harvey, Chelsea. A third of the world’s people already face deadly heat waves. It could be nearly three-quarters by 2100. The Washington Post, June 19, 2017.

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