When temperatures are extreme, the accuracy of a weather forecast can be the difference between life and death. New research shows that even small errors in temperature predictions – as little as one degree Celsius – lead to more deaths and that improving forecasts would save thousands of lives and billions of dollars annually.

The research, published by the National Bureau of Economic Research, found that forecasts are particularly effective at helping people avoid deaths on hot days like those occurring so often this summer, and which are increasing because of human-caused climate change.

“We find that surprising extreme heat is deadly but forecasted extreme heat is not as deadly,” wrote the study authors, economists from Columbia University and the University of Arizona.

Heat is the nation’s most prolific weather killer. The Centers for Disease Control and Prevention estimates an average of 702 people died each year from heat-related causes from 2004 to 2018. Since then, heat-related deaths have steadily climbed, to more than 1,700 last year based on provisional CDC data. Experts fear this year’s toll could surpass that as the number of confirmed heat-associated deaths and deaths under investigation mount.

The study examined 12 years of U.S. weather forecasts and observed weather and mortality data at the county level. They found that more deaths occurred when day-ahead forecasts underestimated summer heat and winter cold by just 1.8 degrees Fahrenheit.

Forecasts that were one degree too cool on hot days corresponded to a 3% increase in deaths. Forecasts that were one degree too warm on cold days corresponded to a 0.5% increase in deaths.

“Accurately forecasted weather shocks can have very different implications from inaccurately forecasted weather shocks if people act on their information about coming weather,” the economists wrote.

Forecasts that overestimated summer heat and winter cold – those that were too warm on hot days, and too cool on cold days – were not associated with an increase in deaths. But “that doesn’t mean forecasters should exaggerate their forecasts,” the authors wrote in a separate article about their research. “If people find that their forecasts are consistently off by a degree or two, they might change how they use forecasts or come to trust them less, leaving people at even higher risk.”

Forecast errors had essentially the same effect on deaths regardless of humidity level, even though humid heat is considered more dangerous because it limits the body’s ability to cool off through sweating. One possible explanation could be that “at high temperatures, there may be limited influence of humidity on the health conditions that cause most heat-related deaths,” according to a review of previous research.

“Even if humid heat is more dangerous, forecasts may not be any more or less effective in humid heat versus dry heat,” Jeffrey Shrader, the study’s lead author, said in an email.

Day-ahead temperature forecasts tended to be “close to correct on average,” the authors wrote, and their accuracy improved by 27% during the study’s sample period from 2005 to 2017. Still, an additional 50% improvement in accuracy between now and the end of this century would save 2,200 lives per year, the study found, and the public would be willing to pay $2.1 billion annually for that reduced risk of death. Those numbers increase to 2,400 lives saved per year and a willingness to pay $2.9 billion annually when accounting for the expected increase in hot days due to climate change.

“Although these findings should be confirmed in other studies, they point to a potential intervention that can be enacted in the near term,” Sameed Khatana, an assistant professor of medicine at the University of Pennsylvania and cardiologist at the Philadelphia VA Medical Center, said in an email. “With extreme temperatures becoming more frequent every year, having such potential solutions is increasingly becoming crucial.”

Heat waves are occurring more often, becoming more intense and lasting longer as the world gets warmer. A new study published in July by the World Weather Attribution network found that prolonged and punishing heat waves in the United States and other parts of the world earlier in the month would have been “virtually impossible” without climate change. Looking ahead, the number of Americans experiencing an average of at least three straight days of 100-plus degree Fahrenheit heat each year is expected to increase from 46 percent to 63 percent in 30 years, according to a Washington Post analysis of data published by the nonprofit First Street Foundation.

The authors also looked at the impact of forecasts on how people use their time. They found that forecast differences as small as one degree can lead people to make meaningful changes to their daily activities, including where they spend their time and how much electricity they use. For example, on days when the forecast was one degree too warm, people reduced their leisure time by 40 minutes, instead spending more time at work or home, and increased their electricity use by 0.14%.

“This is the first study that estimates the benefits from more accurate weather forecasts using the real, observable actions people take in response to forecasts,” Shrader said. “Previous studies . . . assumed that people would take certain actions in response to forecasts, then derived the benefits of forecast improvements from that assumption.”

Jeffrey Lazo, an economist who has conducted previous research on the value of weather information, would like to see more studies use a similar approach.

“This is potentially groundbreaking work based on rigorous theory and using solid national data on fatalities and weather forecast quality to generate reliable and valid estimates of the economic value of improved weather forecasts,” Lazo said in an email. The study’s findings “should provide support for further research into improving weather forecasting.”

Climate change, however, could make it harder to improve forecast accuracy because of rapidly shifting weather patterns, said Kristie Ebi, professor of global health at the University of Washington Center for Health and the Global Environment. Another concern cited by Ebi is that forecasts don’t reach everyone.

“A challenge is the people who are unaware of forecasts, including the unhoused, tourists, older adults with limited internet access, and others,” Ebi said in an email.

Better forecasts are just one component of reducing heat-related deaths, according to Ebi, who has argued that early-warning and response systems for heat waves haven’t kept up with the historic extremes being driven by climate change. Ebi says that heat action plans, such as those now coordinated in some cities by designated “heat officers,” are becoming increasingly important.

“Heat action plans save lives through increasing preparedness at all scales and through providing services, such as heat wave early-warning systems, and through urban planning to prepare for a hotter future,” Ebi said. “Increasing implementation of heat wave early-warning systems could save lives today and in the future.”

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