Even as the climate warms, the cold has become a more deadly threat to Americans. New research this month shows that cold-related deaths have increased dramatically in the US over the past two decades.

Researchers at Harvard Medical School conducted the study, which aims to better quantify deaths caused by cold temperatures in the U.S. They found that the country’s cold-related mortality rate has more than doubled since last year. part of the 1990s, with deaths especially increasing in recent years. Important risk factors for increased mortality likely include extreme weather events tied to climate change, homelessness, and social isolation, the researchers said.

Extreme temperatures at either end of the thermometer can be life-threatening. While recent research shows that heat-related deaths have increased in recent decades, cold weather tends to be more deadly overall. A 2021 study found that cold was responsible for about three-quarters of extreme temperature deaths worldwide in 2019, for example (there were an estimated 1.7 million deaths in total that year).

The Harvard researchers noted that cold-related deaths in the US have been studied sparingly, and little is known about how this burden has changed over time. To address this knowledge gap, they analyzed death certificate data collected by the Centers for Disease Control and Prevention, specifically focusing on deaths where the cold was recorded as a cause or contributing factor.

Between 1999 and 2022, there will be more than 40,000 cold-related deaths in the US, the researchers found. After adjusting for age, they found that mortality increased from 0.44 cold-related deaths per 100,000 people in 1999 to 0.92 such deaths per 100,000 people in 2022. Most of this jump is the result of a large annual increase in deaths between 2017 and 2022. The team’s findings are PUBLISHED this month’s journal JAMA.

“There is a rightful focus on heat-related deaths because of the undeniable nature of global warming,” lead study author Rishi Wadhera, a cardiologist and public health researcher at Harvard, told Gizmodo in a your email. “However, our study findings serve as a sobering reminder that cold-related deaths also remain an important public health issue.”

While their study was not designed to identify any specific reasons for this increase, Wadhera and his colleagues highlighted some possible reasons.

Climate change is known to increase the risk of heat and cold extreme weather eventsfor example. But it’s also likely that more Americans today are more at risk of being left out in the cold than ever before, especially people with unstable living conditions, Wadhera said.

“For example, we know that the number of people who are homeless in the US has increased in the last few years—it’s a population that is more exposed to outdoor weather, including cold snaps,” he said.

US homelessness levels have generally increased since 2016 (the early years of the covid-19 pandemic were a notable exceptionthanks in part to several assistance programs). By 2023, chronic homelessness exceeded record numbers set in 2007, when modern data collection efforts began, according to the Department of Housing and Urban Development (their report this year is not yet available). Substance use and social isolation can also increase people’s risk of exposure to unsafe conditions, including temperature.

A few trends emerged in the team’s data that showed who was most vulnerable to cold-related deaths. Regionally, the death rate is highest in the Midwest, where winters can be particularly brutal, for example. The highest death rates across racial and ethnic groups are seen among Native and Black Americans, respectively, and the highest age death rates are seen among people over 75.

The authors say more research needs to be done to unpack exactly why cold-related deaths are on the rise. At the same time, there are measures that policymakers can and should take to prevent this terrible loss of life, such as ensuring that vulnerable populations live in homes with reliable indoor heating or expanding access to heating centers.

“Our findings should raise awareness that injury and death from exposure to cold weather is possible — especially for older populations with multiple health conditions, homeless people who are more exposed to the outdoors, and low-income populations who may not have access to adequate indoor heating,” Wadhera said.