If Climate Goals Aren’t Met, Extreme Heat Will Kill Thousands in U.S. Cities

This week, the city of Churu in Northern India saw mid-day temperatures rise above 120 degrees Fahrenheit, triggering government warnings to avoid drinking coffee, tea, or alcohol; in New Delhi, more than 21 million people are experiencingdangerously high temperatures that are expected to persist into next week.

The first seriously scary heat wave of the Northern Hemisphere’s summer is a good time to remember that extreme heat in the U.S. already causes more deaths than any other severe weather event, killing an estimated 1,500 people each year. And the future looks dangerously hotter: The United Nations warned last November that global temperatures are on track to rise by at least 3 degrees Celsius above pre-industrial levels, soaring past the two-degree goal that nearly 200 cities signed onto in 2015 as part of the Paris Agreement.

Now University of Bristol researchers behind a new report published Wednesday in the journal Science Advances hope to give city leaders a motivational boost to double-down on their commitment to dramatically cut greenhouse gas emissions by calculating just how many heat-related deaths could be avoided. With funding from the Union of Concerned Scientists, researchers modeled the relationship between mortality and temperature rise in 15 U.S. cities across various regions. They estimate that as many as 1,980 deaths per city could be avoided in a 1-in-30-year heat wave event if global heating is limited to 2 degrees Celsius, rather than 3 degrees. If temperature rise is limited to 1.5 degrees Celsius, the potential number of lives saved jumps up to as much as 2,716.

The estimations are based on 1987-2000 data on daily death count and average temperatures, which the researchers ran through statistical models to find the relationship between temperature and mortality. They also controlled for factors like the future rates of population growth, urbanization, and aging. And to isolate the effect of local temperature rise on the number of deaths, the study also excludes individual efforts from each city to make its urban areas cooler—everything from increasing the tree canopy and employing more heat-reflective surfaces on roofs and road surfaces to opening public cooling centers and beefing up social services.

Thus, how many deaths a city can prevent depends in part on temperature change and population size, which means New York City and Los Angeles—the most populous of the 15 cities in the study—would experience the largest benefit. NYC could see 1,980 and 2,716 fewer deaths in a 2-degree and 1.5-degree warming scenario, respectively. Los Angeles, meanwhile, could see between 759 and 1,085 fewer heat-related deaths. “If you have more people in the city, you have more people exposed—and who are vulnerable—to [the effects of] climate change,” says lead researcher Eunice Lo, who studies the public health effects of global warming at University of Bristol.

When she and her team calculated the number of lives saved per 100,000 people, Miami and Detroit come out on top. In Miami, between 197 and 272 heat-related deaths could be prevented for every 100,000 people in a heat wave event expected to occur once every 30 years, and in Detroit, that range is between 69 and 95. Lo thinks the two cities stand out because of their population’s demographics. “We can’t be 100 percent sure, but Miami has the [one of the] largest proportions of elderly people among the studied cities, and those above 65 are most vulnerable to heat than any other age group,” she says. Meanwhile, cities in Michigan, such as Detroit, now have the fastest-aging population in the U.S.

The researchers also calculated that the number of “hot days”—when the mean temperature falls above what their model say is associated with the lowest mortality risk—would also significantly fall if global warming is limited to 1.5 degrees Celsius. San Francisco, on the higher range, could see 510 fewer hot days; St. Louis, on the lower end, could see a reduction of 202 days.

If the world hits that 3-degree threshold, once-rare heat cataclysms would become routine. An event as deadly as the three-day 1995 heat wave in Chicago, which killed 739 people—many of them elderly, isolated, and living on fixed incomes—could happen once every 1.4 years. That frequency could be halved to a 1-in-2.8 year event if heating is limited to 2 degrees Celsius, or once every 4.7 years under a 1.5 degree scenario—still plenty alarming in itself, Lo says, but that the “multiple times difference” in frequency across the three warming scenarios should not be ignored.

Different factors could affect hypothetical future mortality rates, for better and worse. The researchers controlled for population increase and demographic changes that make a higher proportion of the city residents more vulnerable, as well as increases in urbanization and urban heat island effect—all of which would increase heat-related deaths. Cities like Paris that are thinking more systematically about adapting to intensifying heat via an aggressive campaign of car-removal and tree-planting, on the other hand, could put fewer lives at risk.

“Future choices on urbanization go both ways,” Lo says. “Increasing urbanization without putting green space could exacerbate temperature increases within urban areas and make our estimates more conservative.”

While the study focuses on U.S. cities, in part because of funding and available data, the authors hope it can motivate city leaders worldwide to meet their climate goals. She adds that her team consciously chose to phrase the results as how many deaths can be prevented, rather than how many lives would be lost, should present trends continue. “It’s the same number, but I think if you tell people things are going to be really bad, and that there’s little hope, people won’t act,” she says. “But if you tell them that if lives can be saved, then hopefully they will feel more optimistic and more motivated to increase climate action.”