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A woman refreshes herself in the water of a fountain at Andre Citroen garden in Paris on June 19, 2017.LUDOVIC MARIN/AFP / Getty Images

The global risk of death due to extreme heat is set to rise dramatically over the next several decades because of climate change, according to a new analysis of past heat waves and their impact on human life.

The prediction implies that in mid-latitude regions, including North America and Europe, people who are vulnerable to heat will need to spend more time in air-conditioned settings during the summer or face life-threatening temperatures.

The analysis also suggests that heat is currently claiming more lives than is widely recognized in the tropical regions of the world, particularly in countries where health officials lack the resources to properly document the effect.

Even under the most ambitious scenarios for curtailing carbon emissions, a jump in the number of deadly heat waves is already baked into the global climate, the researchers found.

"If all countries agreed to abide by the Paris [climate] agreement tomorrow, you are still going to have close to 60 per cent of the world's population facing deadly conditions for 20 or more days per year," said Camilo Mora, an ecologist at the University of Hawaii at Manoa who led the analysis, published Monday in the journal Nature Climate Change.

"We don't have a lot of good choices here," he said.

Dr. Mora, who specializes in crunching numbers to make forecasts about biodiversity and climate, said the analysis began as a class exercise to see what patterns emerged from the mortality rates of historical heat waves worldwide. Eventually the project grew to include data from 1,900 cases in 36 countries where heat waves are thought to have killed people between 1980 and 2014.

At first, the group found tremendous disparities in the places where deaths seemed to spike during heat waves and at what critical temperatures. But by combining temperature and humidity, they arrived at an apparent threshold beyond which heat becomes a lethal risk. For example, at relatively low humidity, that threshold is crossed when average temperatures (including daily highs and nightly lows) top 30 C. Once the humidity climbs above 50 per cent, however, even an average daily temperature in the mid-20s can be lethal.

Next, Dr. Mora's group looked at projections under various scenarios for climate change to see how the numbers were likely to change.

Currently, about 30 per cent of the world's population is exposed to the risk of lethal heat more than 20 days a year. Even if "drastic reductions" of greenhouse gas emissions were applied, the analysis found that by 2100, almost half the world's population would face the equivalent risk. If emissions continue to rise, then almost three-quarters will face the same predicament.

Despite its reputation as a cold country, Canada is no stranger to deadly heat, even in locations that are not normally considered especially hot. 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, according to the Canadian Disaster Database, which is maintained by Public Safety Canada.

Sarah Henderson, a senior scientist with the BC Centre for Disease Control, has studied that event in detail. In a paper published last year in Environmental Health Perspectives, she and her colleagues charted how the heat wave disproportionately affected not the very elderly, as might be expected, but people 65 to 75 who were not working and whose socioeconomic circumstances meant they were likely to have poorer baseline health and were living in neighbourhoods with less urban greenery or other heat-mitigating features.

"The best thing we can do is understand that hot weather is going to come," she said. "So how do we best communicate with populations and start adapting our environments to be really resilient to these events?"

A report released last December by the Toronto-based Institute for Catastrophic Loss Reduction found that some Canadian municipalities are way ahead of others in developing communication plans for responding to deadly heat waves. Those lagging behind may be underestimating the risk to their most vulnerable citizens – with potentially disastrous consequences, said Paul Kovacs, an economist and the institute's executive director.

"It's not clear to me that there's acceptance that this is truly a threatening issue," he said.

Dr. Mora noted that while climate change is already increasing the risk of death due to heat, mortality rates are not rising in tandem worldwide, which means populations are adapting by minimizing exposure during heat waves. The downside, he said, is that the heat risk is "making people prisoners of their own homes."

He added that the effect could increasingly affect productivity and quality of life by forcing people to work at odd hours to avoid the hottest times of the day. Increasing reliance on air conditioning also leaves out those who are too poor or transient to have access to it and leaves large populations increasingly vulnerable to power outages during extreme heat events.

The analysis also found that most reports of death due to heat do not come from the world's hottest countries. For example, a heat wave that struck Europe in the summer of 2003 is thought to have produced 70,000 additional deaths.

And while people living in tropical countries may be more accustomed to taking protective measures in high temperatures, Dr. Mora said, a relative dearth of reports of deadly heat waves in those countries likely has more to do with the lack of public health institutions keeping track of such events.

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