The number of deaths from heat continues to rise nationwide,

Heat-related deaths are on the rise in the United States. Since 1999, high temperatures have caused or contributed to the deaths of more than 21,500 people there, new research shows.

The last seven years in particular have seen a sharp increase in heat-related deaths. According to a study published on Monday in the medical journal JAMA, 2,325 people died in 2023, the hottest year in the world since weather records began.

While previous studies had shown no clear trend in heat-related mortality in the U.S. — and actually showed a slight downward trend from 1975 to 2018 — the latest study is the first to show a clear upward trend from 2016 to 2023. The researchers said the current trend is likely to continue due to climate change.

“The trend is what's really striking and I think warrants further investigation,” said Jeffrey Howard, the study's lead author and an associate professor of public health at the University of Texas at San Antonio. “It justifies further investment in surveillance, better tools and more effort to understand what's underlying these trends.”

The study analyzed data from death certificates from the Centers for Disease Control and Prevention 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 preliminary.)

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However, previous research and reports have shown that the number of deaths caused by heat is chronically underestimated. This is partly because heat is not always listed on death certificates when it causes other health problems, such as lung disease.

It is also unclear whether the recent increase in heat-related deaths is due solely to rising temperatures or whether health authorities are also getting better at recording and reporting heat, Howard said.

“It's probably going to be a bit of both,” he said, adding that “the numbers are probably underestimated.”

Still, the findings underscore that heat is the deadliest of all climate hazards. According to the National Weather Service, over the past 30 years, heat has killed more Americans on average than hurricanes, floods and tornadoes combined.

The study also highlights the need for local and national authorities to take urgent action to prepare for more heat and heat-related deaths each year. Last month saw the two hottest days on record, on July 22 and 23.

Howard said the vast majority of heat-related deaths occurred in California, Nevada, Texas and Arizona.

“These four states account for a very large percentage of that,” he said.

Last year, Phoenix experienced 31 consecutive days of temperatures of 109 degrees Fahrenheit or higher, which county authorities linked to a record 645 heat-related deaths in 2023.

“Local authorities in high-risk areas should consider investing in expanding access to drinking centers and public cooling centers or other buildings with air conditioning,” the study says.

But according to Jonathan Parfrey, executive director of the nonprofit Climate Resolve, cooling centers have limited capacity. In fact, many sit empty during heatwaves because they are difficult to access, poorly located or do not have convenient opening hours.

“The data clearly shows that people don't want to travel during a heatwave – they want to stay home,” said Parfrey. Instead, it is important that neighbors look out for each other and that city services provide people with community-based resilience centers that offer more than just air conditioning or water.

Like Howard, Parfrey said the study almost certainly does not reflect the full extent of heat-related deaths. He also said he wished the researchers could have quantified the extent to which the trend was due to improvements in tracking and reporting the cause of death.

The state of California, for example, is working on introducing a syndromic surveillance system that will enable health authorities to collect heat-related mortality data in real time, similar to what was done during the Covid-19 pandemic.

“This is a huge improvement on what has been happening so far, because when we do these reports after heatwaves, they are always retrospective, and it could take a year or two [before] someone is going through the data,” he said. “The idea is that the California Department of Public Health will realize that if the reports of deaths are not simultaneous with the actual event, this issue will never get attention.”

Parfrey recently participated in a panel discussion with state and federal leaders to discuss additional steps California can take to protect its citizens from the increasing heat.

While heat is nothing new for the state, climate change is making heat waves “more frequent, more intense, more severe, and longer lasting,” said Wade Crowfoot, secretary of the California Natural Resources Agency.

In fact, the Golden State has become significantly hotter over the last century.

In 1895, the average July temperature in California was 22.2 degrees. Over the next 100 years, it rose about 4 degrees, and over the next three decades, another 4 degrees, Crowfoot said. The average temperature in California this July – the hottest month on record for the state – was 27.6 degrees.

“Since 1895, the average July temperature in California has increased by 12 degrees – and most of that increase has occurred in the last 30 years,” Crowfoot said. “We are living in record-breaking extreme temperatures.”

The projections for the future are similarly grim. Even in a best-case scenario in which humanity reduces its use of planet-warming fossil fuels, California will still see a temperature increase of about 5.6 degrees by the end of the century – or nearly 9 degrees under 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 rising temperatures. The plan includes education campaigns, infrastructure improvements, and nature-based solutions such as trees.

The state is also working on implementing CalHeatScore, a tool for ranking and classifying heat waves, similar to the way weather agencies categorize hurricanes.

“Extreme heat events are dangerous,” Christina Curry, deputy director of the California Office of Emergency Services, said during the panel. “They are becoming more frequent, lasting longer and 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 respond to it.”

The California Department of Occupational Safety and Health (Cal/OSHA) adopted long-awaited heat standards for indoor workers in July, following a similar move for outdoor workers in 2006. But a recent investigation by the Los Angeles Times and Capital & Main found that compliance with the outdoor regulations is lacking.

In addition, heat-related programs have been impacted by this year's federal budget cuts, including significant cuts to the Extreme Heat Action Plan and reductions to urban greening programs.

At the national level, the Biden-Harris administration unveiled its own heat strategy earlier this month, aimed at improving the country's preparedness for and response to high temperatures.

The Federal Emergency Management Agency (FEMA) has also begun implementing heat programs, including Building resilient infrastructure and communities The program provides funding to states and localities in advance of disasters, according to Nick Shufro, deputy director of FEMA's risk management directorate.

“My department is responsible for risk assessment, which ranges from natural risks to man-made and technological hazards, and now we're starting to include extreme heat for the first time,” Shufro said during the panel discussion. “So this is very significant.”

But heat experts are urging FEMA to do more — such as expanding the definition of “major disasters” to include extreme heat. That, they say, would open up more funding opportunities 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 need to respond more strongly to rising heat mortality, but doctors, the media and community groups can also help raise awareness of the risk.

That was partly why the researchers published their findings in the medical journal JAMA, he said, noting that his own doctor in Texas never mentioned the risks of extreme heat to him.

“There are roles at different levels that can potentially be strengthened – both at the community level and at the patient and physician level – to maintain that awareness and try to keep it top of mind,” he said. “Especially during the summer months.”