Mark Leary was surveying a frozen moose that had drowned in the Kuskokwim River in Southwest Alaska when he got the call: Two all-terrain vehicles, with five people on board, had also crashed through the ice a few miles upstream.

Leary, a team leader with Bethel Search and Rescue, a volunteer-led organization based in the western Alaskan town of Bethel, immediately headed to the scene. By the time he arrived, there was no sign of either vehicle. Three passengers had managed to haul themselves out of the river and were rescued; the other two perished.

Falling through ice and drowning is a perennial risk in northern communities where winter ice is a defining part of the environment. But to Leary, the four-wheeler accident stuck out as an especially harrowing one, in part because of its timing: It occurred in late March 2019, a time of year when the Kuskokwim River, which runs through Bethel, should be frozen solid and safe for locals to use as a highway to drive from place to place.

“I thought to myself as I was [going] out there — this shouldn’t be happening,” Leary said in an interview. “We should have at least another month of safe travel on river ice.”

Far from an isolated incident, Leary’s experience reflects a reality facing northern communities around the world: As winters grow warmer because of climate change, seasonal lake, river and sea ice is becoming treacherous. Now, a new study is warning that this trend could have widespread and tragic consequences.

The research, published last week in the journal PLOS One, examined records of more than 4,000 fatal winter drownings worldwide alongside weather and climate records. It found that winter drowning events “increase exponentially” as air temperatures approach 32 degrees Fahrenheit, the freezing point of water. The findings point to a potential for more drownings as winters become milder, with small children and people in remote Indigenous communities facing an acute risk.

“This is the saddest research I’ve ever done,” Sapna Sharma, an associate professor of biology at York University in Canada and the lead author of the new study, said in an interview.

In many northern countries, warmer winters are one of the clearest signals of climate change. In Canada, wintertime air temperatures have risen nearly six degrees over the past 73 years, faster than in any other season. In parts of Alaska, winter has warmed by closer to 10 degrees. In Finland, winters have warmed by about five degrees.

This rapid winter warming is having a clear effect on winter ice. Rivers, lakes and even parts of the open ocean that have long served as seasonal ice roads or sturdy platforms for fishing and recreation are freezing up later in the fall, thawing earlier in the spring and becoming less stable throughout the winter. A huge number of people face potential harm: Research published last year in Nature Climate Change found that a 1.8-degree Fahrenheit uptick in global average temperatures could cause 100 million people to lose access to a reliably frozen lake in winter.

“We know that lakes are warming; lake ice is melting earlier in the spring, forming later in the fall; and that we have more freeze-thaw events and thinner ice,” Sharma said. “And we wondered if that directly affected people.”

To answer that question, Sharma and an international team of collaborators compiled nearly 30 years of records of fatal winter drownings from coroners’ offices, local police stations and lifesaving societies in 10 countries that are home to seasonally frozen rivers and lakes, including Canada, the United States, Germany, Italy, Russia and Japan. The researchers consolidated these death records by month and matched them with monthly temperature data from the University of East Anglia to explore the relationship between winter drownings and winter weather.

The findings were stark: Temperature has a huge effect on winter drownings.

At winter air temperatures below 14 degrees Fahrenheit, winter drownings were a rare occurrence, the study found. But as temperatures approached the freezing point of water, the number of drownings rose exponentially, with the largest number of fatal drownings occurring when temperatures were between 23 and 32 degrees. Most fatal winter drownings occurred in the last month of winter, when temperatures are typically warmer and longer days encourage people to spend more time outside.

Children and young adults appear to be especially vulnerable. A close examination of demographic data from Minnesota revealed that kids under age 9 accounted for nearly half of all drownings that didn’t involve a vehicle, while young adults between 20 and 24 years old were the most likely to drown while operating a snowmobile. “That was the toughest part of the study for me and also I think the most surprising,” Sharma said.

Remote Indigenous communities also appear to be disproportionately threatened. Despite bitter-cold winter air temperatures, communities in northern Canada had the highest per capita rates of winter drowning in the data set, something the researchers believe may reflect the fact that Arctic Indigenous people spend a significant amount of time on the ice hunting, fishing and traveling between communities in the winter. In these areas, snowmobile routes are the equivalent of highways connecting communities and hunting grounds.

Gordon Giesbrecht, a researcher on hypothermia at the University of Manitoba in Canada, called the new research “very important.”

“It is the largest study of its kind covering 10 countries over 26 years,” Giesbrecht, who wasn’t involved in the paper, wrote in an email. “This is a tremendous amount of data.”

There were some glimmers of hope in that data.

In Italy, the incidence of winter drownings was far lower than the researchers expected based on the country’s relatively mild winter climate. Sharma says that Italy has “strong local laws and regulations” around winter ice activities, including prohibitions on ice fishing and snowmobiling.

Other winter ice activities are allowed only when authorities deem them safe. “When you have information on whether the ice is thick enough to bear weight, the outcomes are much more positive,” Sharma said. “That seems like a key ingredient.”

Still, even in places like western Alaska, where river ice conditions are closely monitored and the information used for detailed reports about travel safety, people sometimes make dangerous choices because they have no other option, Leary says. Bethel, he noted, is “the only place with a real hospital” across a vast region.

“When you live in a small village with very limited goods and services and Bethel has everything you need, sometimes you just need to go,” he said. “People are getting on the river with snow machines when it’s barely safe.”

Maddie Stone (@themadstone) is a writer for the Capital Weather Gang and a freelance science reporter covering climate change, the environment, natural hazards and more. Previously, she was a science writer and editor at the technology website Gizmodo, where she went on to found Earther, a climate-change-focused vertical.