Heat-related mortality is on the rise in the United States, where high temperatures have caused or contributed to the deaths of more than 21,500 people since 1999, according to a new study.
The past seven years in particular have been marked by a surge in heat-related deaths, including 2,325 deaths in 2023, the hottest year on record on the planet, according to a study published Monday in the medical journal JAMA.
While previous research had not portrayed a clear trend in heat-related mortality in the U.S. — and in fact showed a slight downward trend between 1975 and 2018 — the latest paper is the first to demonstrate a clear uptick between 2016 and 2023. The researchers said the current trajectory is likely to continue because of climate change.
“The trend is what really stands out and, in my opinion, warrants further investigation,” said Jeffrey Howard, senior author of the study and an associate professor of public health at the University of Texas at San Antonio. “It warrants more investment in surveillance, better tools and more efforts to try to understand what underlies these trends.”
The study analyzed data from Centers for Disease Control and Prevention death certificates that listed “exposure to excessive natural heat,” “effects of heat and light” or “environmental hyperthermia of a newborn” as an underlying or contributing cause of death, Howard said. (The 2023 data is still provisional.)
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However, previous research and reports have found that heat deaths are chronically underreported, in part because heat is not always listed on death certificates when it triggers other health problems, such as lung disease.
What's more, it's unclear whether the recent increase in heat-related deaths was due solely to rising temperatures or whether health departments are also getting better at tracking and reporting heat as a factor, Howard said.
“It's probably a bit of both,” he said, adding that “the numbers are probably underestimated.”
Still, the results underscore heat's role as the deadliest of all weather hazards. Heat has killed more Americans on average over the past 30 years than hurricanes, floods and tornadoes combined, according to the National Weather Service.
The study also underscores the urgent need for local and national authorities to take action to prepare for more heat-related and heat-related deaths each year. Last month saw the two hottest days on Earth on record, on July 22 and 23.
Howard said the vast majority of heat-related deaths came from California, Nevada, Texas and Arizona.
“Those four states represent a very large percentage of these,” he said.
Last year, Phoenix experienced 31 consecutive days of temperatures of 110 degrees or higher, which county officials linked to its record 645 heat deaths in 2023.
“Local authorities in high-risk areas should consider investing in expanding access to public hydration centers, cooling centers or other air-conditioned buildings,” the study said.
But cooling centers have their limits, according to Jonathan Parfrey, executive director of the nonprofit Climate Resolve. In fact, many remain empty during heat waves due to lack of accessibility, location or suitable hours.
“The data is overwhelming: People don’t want to travel when there’s a heat wave; they’d rather stay in the same place,” Parfrey said. Instead, it’s important for neighbors to check in on each other and for city services to provide people with community-oriented resilience centers that offer more than just air conditioning or water.
Like Howard, Parfrey said the study almost certainly doesn't reflect the full extent of heat-related mortality. She also said she wished the researchers had been able to quantify how much of the trend was due to improvements in tracking and reporting the cause of death.
The state of California, for example, has been working to implement a syndromic surveillance system that will allow health officials to capture heat-related mortality data in real time, similar to what was done during the COVID-19 pandemic.
“It’s a big improvement on what has happened previously, because when we do these reports after heatwaves, it’s always done in retrospect, and it could be a year or two. [before] “Someone is looking at the data,” he said. “The idea is that the California Department of Public Health will realize that this issue will never get attention unless the reports of deaths are contemporaneous with the actual event.”
Parfrey recently participated in a panel with state and federal leaders on other ways California is working to protect residents from intensifying heat.
While heat is nothing new in the state, heat waves “will become more common, more intense and severe, and longer lasting” because of climate change, said Wade Crowfoot, secretary of the California Natural Resources Agency.
In fact, the Golden State has become noticeably warmer over the past century.
In 1895, the average July temperature in California was 72.4 degrees. It rose about 4 degrees over the next 100 years, and then another 4 degrees over the next three decades, Crowfoot said. California's average temperature this July, the warmest month on record in the state, was 81.7 degrees.
“Since 1895, the average July temperature has increased 12 degrees in California, and most of that increase has occurred in the last 30 years,” Crowfoot said. “We are living in record-breaking temperature extremes.”
Future projections are similarly grim. Even under the best-case emissions scenario, in which humanity reduces its use of planet-warming fossil fuels, California will still experience a temperature increase of about 5.6 degrees by the end of the century, or nearly 9 degrees in a high-emissions scenario, Crowfoot said.
In 2022, Governor Gavin Newsom introduced California’s Extreme Heat Action Plan to increase the state’s resilience to high temperatures, which includes public awareness campaigns, infrastructure improvements, and nature-based solutions like trees.
The state is also working to implement CalHeatScore, a tool that will classify heat waves similarly to how weather officials categorize hurricanes.
“Extreme heat events are dangerous,” said Christina Curry, chief deputy director of the California Office of Emergency Services, during the panel. “They are becoming more frequent and longer lasting, and they are deadly. And what makes them particularly dangerous is that they are invisible — they are not like a fire or a flood where the public can see the danger and react to it.”
In July, the California Division of Occupational Safety and Health (Cal/OSHA) approved long-awaited heat standards for indoor workers, following a similar move for outdoor workers in 2006. However, a recent investigation by the Los Angeles Times and Capital & Main found that enforcement of the outdoor standards has been slow.
Furthermore, heat-related programs took a hit amid state budget cuts this year, including notable cuts to the extreme heat action plan and reductions in urban greening programs.
At the national level, the Biden-Harris administration unveiled its own heat strategy earlier this month, which aims to improve the country's preparedness and response to high temperatures.
The Federal Emergency Management Agency has also begun implementing heat-related programs, including its Building resilient infrastructure and communities program, which provides funding to state and local areas ahead of disasters, according to Nick Shufro, acting assistant administrator for FEMA's risk management directorate.
“My department is responsible for conducting risk assessments ranging from natural to technological to man-made, and we are now starting, for the first time, to include extreme heat,” Shufro said during the panel. “That’s important.”
But heat experts have been urging FEMA to do more, including expanding its definition of “major disasters” to include extreme heat, which they say would open up more funding avenues for cities and states to prepare for and respond to rising temperatures.
Howard, the study's lead author, said it's not just government agencies that should step up their efforts to respond to rising heat-related deaths. Doctors, the media and community groups can also help spread awareness about the risk.
That's partly why the researchers chose JAMA, a medical journal, to publish their findings, he said, noting that his own doctor in Texas never mentioned the risk of extreme heat to him.
“There are functions at different levels that can be improved, both at the community level and at the doctor-patient level, to maintain that awareness and try to keep it at the forefront,” she said. “Especially in the summer months.”
