Heat waves kill people—and climate change is making it much, much worse

Heat waves, like the one that has held the Northwest United States in its grip for the past week, are deadly.

The human toll of the record-breaking temperatures that slammed both coasts of the U.S. and Canada in recent weeks is already enormous. At least 80 people have died in the U.S during the past few days of extreme heat; in British Columbia, the number is in the hundreds. And, as more data trickles in, those numbers are likely to rise even further.

A mountain of scientific research has shown that climate change is making heat waves longer, hotter, more likely, and more dangerous. A recent study published in Nature Climate Change adds additional detail by assessing the human cost of that extra heat: In June, a team of some 70 researchers reported that for the 732 sites on 6 continents they studied, on average, 37 percent of all heat-related deaths can be pinned directly on climate change.

The study underscores the urgency with which we need to address human-caused climate change, says Ana Vicedo Cabrera, lead author of the study and a climate change epidemiologist at the University of Bern, in Switzerland.

“Climate change is not something in the future: It’s something in the present, and it is already affecting our health in very dramatic ways,” she says. Extreme, deadly heat events like the one battering North America are a foreshadowing of what will come. “We can expect that what we’ve seen in the past—that 37 percent—is going to increase exponentially in the future.”

Extreme heat is deadly

 Extreme heat kills more people each year in the U.S. than any other kind of natural disaster. Globally, its impacts are enormous. During historic heat waves—like 1995 in Chicago, 2003 in Europe, or 2019 in France—thousands of people can die, and many more suffer severe health impacts that can last long after the heat dissipates, says Camilo Mora, a climate scientist at the University of Hawaii who authored a study titled “27 ways a heat wave can kill you: Deadly heat in the era of climate change.”

“These events can have long-term consequences, from kidney failure to your brain being damaged, to heart damage,” he says.

Previous studies have linked particular climate change-fueled heat waves that sweep through a city with higher deaths. In the sweltering 2003 heat wave in Europe, for example, human-caused climate change increased the risk of dying by 70 percent in Paris. This new study expands this type of analysis globally, looking at more than 700 locations across all inhabited continents.

The researchers looked at all the recorded deaths that occurred during summertime, as well as temperature data for those same places and times, in order to sift out all the deaths that were likely to be caused by extreme heat. There are temperature thresholds beyond which people are much more likely to die, but those thresholds are different in different parts of the world.

The team developed a mathematical formula that linked extreme temperatures—how hot it was beyond the comfortable average temperature for that city or town—to the number of people who might die if it got that hot. This approach allowed the researchers to figure out how many people have died because of extreme heat in each location they looked at.

Then, they used a climate model to simulate an imaginary world in which human-driven climate change hadn’t occurred. They used their formula to figure out how many people would have died of extreme heat in that alternate, theoretical universe.

The differences were stark. The planet has warmed roughly 1.8 degrees Fahrenheit (1 degree Celsius) since the late 1800s and stands to warm at least that much again by the end of the century without serious efforts to eliminate greenhouse gas emissions.

Without just that 1.8 degrees of warming that have already occurred, heat-related deaths would have made up just under one percent of all summertime mortality worldwide on average. But instead, heat-related deaths made up an average of over 1.5 percent of all summertime deaths—roughly 60 percent more.

If extended world-wide, that would mean more than 100,000 deaths per year could be attributed to human-caused climate change, though Vicedo Cabrera cautions that much more data and analysis are needed to come up with an accurate global estimate.

Climate injustice 

The study found that, on average, more than one in three heat-related deaths can be pinned on climate change. But in some South American countries, Kuwait, Iran, and parts of southeast Asia, the human toll is much higher: as much as 77 percent in Ecuador, or 61 percent in the Philippines. This disparity emerges not just because these places are particularly hot, but because there is often less access to air conditioning, well-constructed housing that manages heat distribution better, and other factors that can lower people’s vulnerability to heat.

The patterns of vulnerability the study uncovered reveal a profound inequity, says Tarik Benmarhnia, an environmental health expert at the University of California, San Diego.

“Think about who contributed to climate change over the last century, and who is seeing the most consequences today, and you see it is not fair. There is a huge environmental injustice in term of who is suffering the heat-related mortality caused by anthropogenic climate change.”

The U.S. is responsible for about 25 percent of all planet-warming emissions currently in the atmosphere, while Guatemala, for example, has contributed roughly 0.0002 percent. But more 75 percent of the heat deaths in that country can be linked to climate change.

The impacts in the U.S. are also devastating: About 35 percent of the U.S.’s heat-related deaths could be attributed to the climate change that has already occurred. Other research has clearly shown that those costs are not borne equally: in many cities, older people of color are twice as likely to die during extreme heat events than older white people.

“Worldwide, the effects are unequal. Within the U.S., the effects are unequal. At the county, at the city, in the neighborhood—the effects are unequal,” says Benmarhnia.

Deadly signs of climate change

 Scientists are working to determine how much worse and more likely climate change has made this Northwest heat wave, but there is little question that it played a major role, says Mora.

“How many times do we need to prove that when it rains, we get wet?” he asks. “For decades now, we climate scientists have been hammering the drum that this is going to get bad. Now, it is that bad.”

Even if all greenhouse gas emissions stopped tomorrow, the planet will continue to warm well past the 1.8°F it already has. That would make the kind of severe heat events we experience today more in line with a norm, rather than an extreme. But how bad the heat gets in the future depends on the climate actions we take now, says Mora.

“Our choices for the future are more of this, or a lot more of this. We can still choose between bad and worse,” he says.

Either way, it is well past time to start helping people across the country prepare for extreme heat, says University of Washington’s Kristie Ebi, a global environmental health expert. Some actions can be simple, like making sure people have access to fans, air conditioning, and shade. Other actions, such as figuring out how to make the electrical grid robust enough to stand up to the extra stresses imposed by too much heat, will be much more complex.

But the basic message is simple, according to Ebi: We can choose to save lives.

“Heat kills, but it doesn’t have to,” she says.