Extreme heat is a huge worry for doctors and public health experts around the world, and it’s steadily become a bigger problem over time, according to a sweeping new climate report published today in the leading medical journal, The Lancet.

The analysis, from 43 academic institutions and United Nations agencies, focuses on the threats the climate crisis poses to human health. While the group has published similar reports over the past five years, this is the first time it includes warnings on the impact hotter days have on mental health and physical activity. It follows an increase in devastating heatwaves around the world.

“I saw paramedics who had burns on their knees from kneeling down to care for patients with heatstroke. I saw far too many patients die in the ED as a result of their heat exposure this past year,” Jeremy Hess, a professor of environmental and occupational health sciences, global health and emergency medicine at the University of Washington, said during a press briefing this week. “These are preventable problems, and we need all hands on deck to respond and correct our course.”

Extreme heat is already a leading weather-related killer, and climate change is making heatwaves more frequent and intense. Children and older adults especially are more vulnerable to heat-related illness and death than people of other ages. Babies under the age of one and adults over 65 experienced dramatically more days of heatwaves in 2020 compared to the baseline average from 1986 to 2005, according to the new report.

The number of hours in a day in which it was too hot to safely work or exercise outside has also risen steadily over the past four decades, the report found. People in lower-income countries lost the most time, an average of 3.7 fewer hours in a day with safe temperatures. In 2020 alone, 295 billion hours of potential work were lost because of extreme heat.

Governments are now trying to find ways to help their populations cope with the heat. The United States last month announced plans to craft new protections for workers faced with sweltering conditions. To get the public better prepared for dangerously high temperatures, officials in Seville and Athens recently announced that they are looking into giving heatwaves names. It’s similar to the tactic used to keep people updated on hazards posed by major storms.

But physical health isn’t the only heat-related concern for doctors involved with the Lancet report. “There are pretty clear signals in the literature overall that extreme heat exposure is a risk for a number of different mental health concerns. This extends up to and includes suicide,” Hess said. “So it’s been a real interest for a while in the climate and health community to get a better sense of exactly who is at risk, what those risks look like, and how to track them over time.”

Twitter gave the Lancet paper’s researchers one window into how people’s sentiments changed during hot days. They studied 6 billion geolocated tweets sent from 40,000 different locations between 2015 to 2020. During heatwaves, they found a rise in “negative sentiment” and a decrease in “positive sentiment” in people’s tweets. To do that, they used a standard research method that essentially counts the number of words deemed positive or negative in tweets.

Over time, people only grew more “negative” during heatwaves. “If people were adapting globally to the effects of heat, we would expect, over time, the impact of a heatwave on people’s emotional states would be attenuated,” says Kelton Minor, another co-author of the report and a PhD candidate at the University of Copenhagen’s Center for Social Data Science. Instead, they found a 155 percent jump in “negative expressions” on Twitter during heatwaves in 2020 compared to the average from 2015 to 2019 (even when controlling for other stressors like COVID-19). “The scale of the impact of heatwaves on negative sentiment is not marginal; this is a sort of large increase,” Minor says.

“We need to shine a light on the hidden corners of the true mental health impacts of climate change. We know that it is impacting mental health, and this is a first step on that road in order to better understand it,” Renee Salas, an attending physician in the Department of Emergency Medicine and an assistant professor of emergency medicine at Harvard Medical School, said in the press briefing on the new report.