Heat killed a record number of Americans last year

As the planet warms, heat-related deaths are increasing in the U.S., according to a new study that looked at federally reported data since 1999.

More Americans died from heat in 2023 than any year in over two decades of records, according to the findings published Monday. Last year was also the globe's hottest year on record, the latest grim milestone in a warming trend fueled by climate change.

The study , published in the American Medical Association journal JAMA, found that 2,325 people died from heat in 2023. Researchers admit that number is likely an undercount. The research adjusted for a growing and aging U.S. population , and found the death toll was still staggering.

“The current trajectory that we’re on, in terms of warming and the change in the climate, is starting to actually show up in increased deaths,” lead author Jeffrey Howard, an associate professor of public health at the University of Texas at San Antonio, told USA TODAY. “That’s something that we hadn’t had measured before.”

Howard – along with researchers from the Uniformed Services University of the Health Sciences, in Maryland, and Pennsylvania State University – examined death certificate data between 1999 and 2023. Deaths were counted if heat was listed as an underlying or contributing cause of death.

Reported deaths remained relatively flat until around 2016, when the number of people dying began increasing, in what Howard, who studies health effects from extreme weather, calls a “hockey stick.” The hockey stick analogy has been used to describe warming global temperatures caused by climate change, where temperatures have swooped upward at alarming rates in recent years.

Howard's study suggests the human toll follows the same outline. An important indicator is age-adjusted deaths per 100,000 people. That heat-related death rate has increased dramatically compared to the early 2000s, regardless of age or population size.

The upward trajectory appears to be sharpening recently. In 2022, 1,722 people died at an adjusted rate of 0.47. But 2023 saw 603 more deaths than the previous year, with an adjusted rate of 0.63, the highest on record.

Deaths weren’t evenly spread nationally. In an interview, Howard said deaths were overwhelmingly concentrated in traditionally hot regions: Arizona, California, Nevada and Texas.

The study is limited in how local governments classify heat-related deaths, which could mean the actual number of deaths is an undercount. It’s also potentially skewed as more people become aware of the fatal risks of heat. The study didn’t break down vulnerable groups. For example, people without air conditioning, those who live or work outdoors, and people with underlying health conditions, are all at greater risk of serious illness or death from heat.

Heat is often called the silent killer , said Sameed Khatana, an assistant professor of medicine at the University of Pennsylvania and a cardiologist at the Philadelphia VA Medical Center who has studied heat death reporting.

Officials are becoming more mindful of heat when filling out death paperwork, Khatana said. “I think there is growing understanding of some of these subtleties that have been pointed out in research over the past several decades."

Researchers, such as Khatana, have previously called for improvements in how heat deaths are reported.

Attributing heat in death certificates is handled differently in counties across the country, said Ashley Ward, director of the Heat Policy Innovation Hub at Duke University’s Nicholas Institute for Energy, Environment and Sustainability.“There needs to be guidance and standards developed for how we determine heat is a contributor.”

Researchers in the study published Monday called on local officials to consider investing in expanding access to hydration stations, public cooling centers and additional buildings with air conditioning.

In 2024, the sweltering heat in the summer months hasn’t ceased. It’s unclear how deaths compare, but there have been several heat waves across the country in recent months.

Phoenix, for example, set an all-time record for the number of days it was more than 100 degrees and the region continues to add to that record, said Mark O'Malley, a meteorologist in the National Weather Service office in Phoenix. Sunday was the 91st day in a row with temperatures over 100 degrees at the city’s Sky Harbor Airport.

In July, California and New Hampshire experienced the warmest July on record this summer and this July was one of the 10 warmest on record in 19 other states, according to the National Oceanic and Atmospheric Administration .

Globally, three consecutive days in late July became the hottest on record, surpassing records set the previous July. NOAA puts the chances of this year being the warmest on record at roughly 77%,

“On the whole,” Howard said, “it seems that things are getting worse and not better.”

This article originally appeared on USA TODAY: Heat killed a record number of Americans last year