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.

Using model simulations, the researchers investigated what might happen under several potential future climate scenarios. They found that even with substantial efforts to reduce greenhouse gas emissions, nearly 50 percent of the world’s population will experience 20 or more days of deadly heat by the year 2100. Under a business-as-usual scenario, that percentage climbs to nearly 74 percent.

“We’ve run out of good choices for the future,” Mora said. “Right now, when it comes to heat waves, our choices are between bad and terrible.”

It’s difficult to say how many more deaths will occur as a result of the extreme heat; that depends on how human societies deal with the problem. Communities could try to lessen the risks by increasing the use of air conditioning or putting better heat warning systems in place, the researchers note. According to Petkova, successful heat adaptation strategies rely strongly on the public’s awareness of the danger and its access to shelter, cooling devices or other tools they can use to protect themselves.

In fact, Mora said, the literature suggests that the number deaths caused by heat waves may already be decreasing, perhaps because of better adaptation efforts. But adapting to extreme heat doesn’t prevent the heat waves from occurring in the first place, meaning that more vulnerable members of society — the elderly, the frail or those who don’t have access to air conditioning — still face the risk of heat-related death. And changes in the population structure will likely play an important role in future heat-related mortality, Petkova said, as rapidly aging populations in some countries may be particularly susceptible to heat waves.

So while an increase in deadly heat waves may be inevitable, the research underscores the importance of working to reduce greenhouse gas emissions and minimize the future impact of climate change as much as possible, Mora said. And he added that even less vulnerable communities — those with higher incomes and greater access to air conditioning, for instance — shouldn’t get too comfortable. Even in those places, severe heat waves can pose serious risks for the world and its inhabitants.

For one thing, an increase in the use of air conditioning could place a heavy strain on the electrical grid, Mora said. And extreme heat may force people to spend more and more of their time indoors.

“We will become prisoners in our homes,” Mora said.

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