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Lethal Heat, Humidity Could Hit Three-Quarters of Humanity by 2100

Without prompt action to reduce greenhouse gas emissions, lethal heat and humidity will pose a risk to 74% of the human population by 2100, a study published in the journal Nature Climate Change projects.

The research team, led by biologist Camilo Mora of the University of Hawaii at Manoa, began its work by determining a baseline combination of heat and humidity associated with high human mortality in hundreds of case studies.

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The use of both conditions was important, Mora explained, because in high humidity “your sweat doesn’t evaporate, so heat accumulates in your body instead. People can then suffer heat toxicity, which is like sunburn on the inside of your body. The blood rushes to the skin to cool you down, so there’s less blood going to the organs. A common killer is when the lining of your gut breaks down and leaks toxins into the rest of your body.” Under some conditions, that can happen at temperatures as low as 23ºC.

Next, the researchers compared that lethality index to projected climate change in different regions. They found that if greenhouse gas emissions are not reduced, the deadliest waves of heat and humidity, now an occasional threat to 30% of humanity, will pose a risk to 74% by the end of the 21st century.

Even if greenhouse emissions are reduced “aggressively”, the researchers told InsideClimate News, “at least 48% of the population will still face deadly heat waves by 2100 because of the amount of long-lived heat-trapping gases that already have accumulated in the atmosphere.”

Assuming that “all countries agreed to abide by the Paris agreement tomorrow,” Mora asserted in the Globe and Mail, “you are still going to have close to 60% of the world’s population facing deadly conditions for 20 or more days per year.”

“We’re running out of good options for the future,” he added. “For heat waves, our options are now between bad or terrible.”

For Americans, the report landed as much of the U.S. southwest faced near-record heat, with Phoenix looking at temperatures of 120ºF/48.9°C. “The United States,” Mora grimly predicted, “is going to be an oven.”

But the implications of his group’s findings are worldwide. Severe heat in Europe contributed to wildfires that killed at least 60 people in Portugal. And as Global News recalled, “in late May, temperatures in Turbat, Pakistan, climbed to about 53.5°C; if confirmed, that could be among the five hottest temperatures reliably measured on Earth.” Last year, a third of a billion people suffered through torrid heat in India.

Even Canada, with “a reputation for being cold”, is not immune to deadly heat, as the Globe pointed out in a week when most of the country was enjoying moderate temperatures. “For example, in Vancouver, during a record heat wave in the summer of 2009, there were some 130 deaths above the average for that period.”