What we know about extreme heat’s health impacts after the hottest summer on record

Last summer’s heat waves demonstrated all the ways that extreme heat takes a toll on the human body. In cities across the U.S. from Phoenix to New York, people suffered from heat exhaustion, heat stroke, heat cramps, and more. In Texas, more than 300 people died from heat last year — the highest number since the state started tracking the deaths in 1989.

In a paper published Tuesday in Nature, researchers confirmed that 2023 was the hottest on record in the northern hemisphere in 2,000 years. And the stakes are as high as the temps.

“Many people do not realize how deadly extreme heat can be,” said Jennifer Wang, the executive director of the Yale Center on Climate Change and Health. Hundreds of people in the U.S. die each year from heat-related causes, federal records show. And it’s increasing, though some experts believe these deaths are undercounted.

Heat can multiply the effects of other conditions, which can make it tricky to nail down as a contributing factor to death or illness. Scientists know that heat can affect a person’s cardiovascular health and risk for chronic conditions. There’s also emerging research about how extreme heat could affect mental health, dementia, Alzheimer’s disease, addiction, sleep, and more, said Ashley Ward, director of the Heat Policy Innovation Hub at Duke University. And common medications like antipsychotics, beta blockers, diuretics, and more can disrupt how the body manages heat, making people even more vulnerable.

As researchers like Ward and Wang piece together the impacts of a warming planet on its human population, climatologists have worked to demonstrate how much change has already occurred, and how much we need to prepare for in the future.

“It’s very, very concerning,” Jan Esper, a professor of climatology at Johannes Gutenberg University in Germany and corresponding author of the paper, said in a press briefing. Last summer’s record-breaking heat is one step in the continuation of a clear trend, Esper said. “I would not be surprised by another huge step in the next 15 to 20 years.”

While 2023 brought the hottest temperatures yet, the researchers found that in the last 28 years, 25 have brought summers that were hotter than the year 246, the hottest summer of the pre-industrial era, according to Max Torbenson, a research associate at Johannes Gutenberg University and co-author of the study.

With two methods, Torbenson and his colleagues calculated the difference between last year’s heat waves and pre-industrial temperatures. Temps were 2.07 degrees Celsius higher in 2023 than the average pre-industrial summer recorded between 1850 and 1900. And looking at 2,000 years of tree ring widths — which can help estimate past temperature — they calculated that summer 2023 exceeded the average pre-industrial temperatures by 2.2 degrees Celsius.

Regardless of whether last summer was 2.07 degrees or 2.2 degrees hotter than pre-industrial times, “the reality didn’t change,” said Esper. Record summer heat brought record deaths in some places, including Texas.

Experts in climate change and public health agreed that continued research is critical when it comes to advocating for policies that can combat the health effects of increasing temperatures.

Clinicians need more incentives to help track deaths and illnesses related to extreme heat in how they code patient conditions at the hospital, Ward said. Currently, only a few diagnosis codes like the one for heat stroke are billable, meaning they can be used to seek payment from insurance companies. Clinicians need to know how to document the diffuse impacts of heat on other diseases, she said, and be given the time to do so.

Pharmacists can also play a role in educating people about the risks of medications that can make somebody more susceptible to high temperatures, Ward added. But she emphasized that change needs to happen outside of health care, too.

Cooling centers, where many people went to access air conditioning last summer, are one way that communities have begun to protect people from the dangers of extreme heat. Wang calls them “necessary but completely inadequate” as a response to rising temperatures.

Local municipalities need to make policy changes like requiring more energy-efficient building codes and setting legal maximum temperatures for rented apartments the same way that many areas already have legal minimum temperatures in the winter, experts said. Cities like Miami have created positions like chief heat officer to lead the cooperative changes needed.

The record-setting heat of summer 2023 didn’t surprise the study’s authors, but it does worry them.

“I am not concerned about myself because I am too old,” Esper said about the effects that climate change will have on the world if it continues unchecked. “But I have two children, and there are many other children out there.”