Being outdoors will be an “extreme danger” in 2100 due to heat

A very hot future, that's what new research from the University of Washington and Harvard University predicts. The findings project a range of heat impacts around the world by the end of this century, depending on future greenhouse gas emissions. The study was published on August 25 in the open-access journal Communications Earth & Environment.

"The unprecedented heat events of recent summers are going to be much more common in places like North America and Europe," said lead author Lucas Vargas Zeppetello, who conducted the research as a doctoral student at UW and is now a research fellow. postdoc at Harvard. "For many places near the equator, by 2100 more than half the year will be challenging to work outdoors, even if we start to reduce emissions."

"Our study shows a wide range of possible scenarios for 2100," he added. "This shows that the emissions choices we make now are still important to creating a livable future."

The study looks at a combination of air temperature and humidity known as the "heat index" that measures the impact on the human body. The US National Weather Service defines a "dangerous" heat index at 39.4°C. An "extremely dangerous" heat index is 51C, which is considered unsafe for humans for any length of time.

“These standards were first created for people who work indoors in places like boiler rooms; they were not conceived as conditions that would occur in outdoor environmental settings. But we are seeing them now,” Vargas Zeppetello said.

The study finds that even if countries manage to meet the Paris Agreement goal of keeping warming to 2C, crossing the "dangerous" threshold will be three to 10 times more common by 2100 in the US, Western Europe, Chinese and japan Under the same scenario, hazardous days could double by 2100 in the tropics, covering half the year.

In a worst-case scenario, in which emissions remain unchecked until 2100, "extremely dangerous" conditions, in which humans should not be outdoors for long, could become common in neighbouring countries. to the equator, especially in India and sub-Saharan Africa.

“It is very scary to think what would happen if 30 or 40 days a year exceeded the threshold of extreme danger,” said Vargas Zeppetello. “These are scary scenarios that we still have the ability to prevent. This study shows the abyss, but it also shows that we have a say in preventing these scenarios from happening."

Heat depends on emissions

The study uses a probability-based method to calculate the range of future conditions. Instead of using the four future emissions pathways included in the Intergovernmental Panel on Climate Change reports, the authors use a statistical approach that combines historical data with projections for population, economic growth, and carbon intensity (the amount of carbon emitted by every dollar of economic investment exercise) to predict the likely range of future CO2 concentrations.

The statistical approach "provides plausible ranges for future temperature and carbon emissions and has been estimated and statistically validated with historical data," said co-author Adrian Raftery, a UW professor of statistics and sociology.

The authors translated higher carbon dioxide levels into a range of global temperature increases and then looked at how that would affect global monthly weather patterns.

"The number of days with dangerous levels of heat in the mid-latitudes, including the southeastern and central US, will more than double by 2050," said co-author David Battisti, professor of atmospheric sciences at UW. "Even for very low estimates of carbon emissions and climate response, by 2100 much of the tropics will experience 'dangerous' levels of heat stress for nearly half the year."

“It is extremely scary to think what would happen if 30 or 40 days a year exceeded the extremely dangerous threshold,” Vargas Zeppetello said. “These are scary scenarios that we still have the ability to prevent. This study shows you the abyss, but it also shows you that we have some agency to prevent these scenarios from happening.”

The results underscore the need both to reduce future greenhouse gas emissions and to protect populations, especially outdoor workers, against dangerous heat.

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