Published in Science on Aug. 23, 2024
By Vivian La
More than 47,000 Europeans died from heat-related causes last year, the warmest on record globally, a study published this month found. The number was surpassed only by the 60,000 Europeans who died of heat-related causes in 2022. Another study this month found that the toll in Europe could triple by the end of the century if Earth continues to warm to 3°C or 4°C degrees above preindustrial levels. The numbers, though shocking, almost certainly understate the toll of hot weather, worsened by global warming. But scientists aren’t sure how to do better.
Some argue the best way to understand the impact of heat is to track how death rates vary with fluctuations in temperature, as the European studies did. But others say a truer measure is to rely on officially declared heat waves and count excess deaths—those above the expected number—each day.
The two kinds of studies “provide answers to different questions, looking at different exposure metrics,” says environmental epidemiologist Jaime Madrigano of Johns Hopkins University. Seeing how deaths vary with temperature captures the health effects of gradual warming, whereas focusing on heat waves highlights the consequences of extreme cases.
The dispute isn’t just academic. Heat waves grab headlines and the attention of policymakers, spurring them to adopt heat warning systems that urge people to stay indoors or take other precautions. But some scientists note that just examining heat waves omits the deaths that occur beyond what’s considered extreme.
“We shouldn’t look at them as two efforts in parallel,” says occupational epidemiologist Barrak Alahmad of Harvard University. Deaths from heat are increasing “anywhere you put your eye.”
Heat stroke is the most extreme and direct effect of too much time exposed to high temperatures. But it only accounts for a minority of the toll. Deaths from many other causes also rise, for example when heat tips someone with an underlying heart condition into a fatal heart attack.
Proponents of using heat waves to measure how temperature increases those risks say these events are the deadliest, worst-case scenarios, so understanding them is paramount for preparedness. One study found that 48 people—most certainly an undercount—died during a 2018 heat wave in South Korea and its severity prompted changes in Seoul’s heat plan, such as opening more cooling centers. Counting deaths during heat waves also captures the cumulative health impact of multiple hot days in a row, a nuance that studies focused on daily temperatures can miss.
But limiting the scope of studies to heat waves likely undercounts deaths because there is no universal definition of a heat wave, says epidemiologist Vijendra Ingole of the U.K. Office for National Statistics. Heat waves are declared when temperatures exceed the historical average in an area. However, because of climate change, a hot spell that would once have been considered a heat wave might not be today, he says—yet remains deadly.
A study that Ingole co-authored in February with Amruta Nori-Sarma, an environmental health researcher at Boston University, underscored the limitations of the heat wave approach. They estimated a different number of deaths at various definitions of a heat wave in India, by duration and relative daily temperatures. They found that even shorter, less intense heat waves not captured by some definitions have risks similar to longer, more intense ones, including death.
Epidemiologists like Nori-Sarma argue instead for looking at excess deaths on hot days, regardless of whether they fall within an official heat wave. That approach also allows researchers to zoom in on specific populations—children, one geographic region, or those with cardiovascular disease—to capture differences in the impact of heat. Although epidemiologists can still look at deaths in these groups during a heat wave, Alahmad says those numbers don’t fully reveal the risk.
“Looking at heat waves only … you’re only looking at one slice of the cake,” says Alahmad, whose research focuses on outdoor workers. The temperatures they experience don’t necessarily meet the standard definition of a heat wave but can still be a danger given the longer exposure to heat. But looking at the entire temperature range, “That’s the whole cake,” he says.
A limitation of both approaches is the lack of robust data, especially from hot places in the Global South. For example, a 2021 study in The Lancet Planetary Health that looked at both heat waves and daily temperatures suggested that from 2000 to 2019, almost half a million people died annually from heat. The modeling study was cited in a United Nations call to action last month on extreme heat and relied on data in the Multi-Country Multi-City Collaborative Research Network, which pools climate and health data from 53 countries. But the network leaves out countries such as India, which is known for extreme summer temperatures, and it includes just one African country: South Africa.
In much of Africa, issues such as a decentralized health care system and limited weather stations mean mortality and weather data might not exist, says Kiswendsida Guigma, a climate scientist with the Red Cross Red Crescent Climate Center. Without the data, it’s hard to know just how dangerous heat is. “It’s like a chicken and egg relationship,” he says. Still, heat waves like the one that affected the Sahel region of Africa in late March—where temperatures hit up to 45°C for five straight days—could be opportunities for researchers to collect missing data, he says, even leveraging citizen science in remote places.
Ultimately, it’s likely a combination of the two metrics that will most accurately capture the fatal consequences of rising global temperatures. “Going toward one side or the other is perfectly fine, it’s about the interpretation of the findings,” says environmental epidemiologist Ana Maria Vicedo-Cabrera at the University of Bern, who co-authored the Lancet study.
And both sides emphasize that one fact is not in dispute. “By any metric we look at, by any definition we choose, by any method that we pick, [mortality] is getting worse,” Alahmad says.
Vivian La 