For hundreds of years, Shinto priests living on the banks of Lake Suwa, in the Nagano Prefecture of Japan, watched the surface of the lake freeze solid each winter. Partway through the winter, the ice would crack and refreeze, creating a long, jagged ridge stretching from shore to shore: the footprints of a god walking across the lake to see his beloved, the story goes.
The priests carefully recorded the date that the lake froze and the ice ridge appeared, each year from 1442 onward. The ridge would come early some years, a few days later others, but the lake nearly always froze solid enough for the god to make his annual pilgrimage.
But since the Industrial Revolution, the inexorable impacts of warming have thinned out Lake Suwa’s ice, leaving the lake dark and unfrozen more years than not.
And Lake Suwa is far from alone, according to new research published Monday in Nature Climate Change: Around the world, lakes are already freezing less, and for shorter stretches of winter, than they did before climate change gripped the planet. And as Earth warms, the authors of the new analysis say, tens or even hundreds of thousands of lakes around the world will freeze less—or maybe even not at all.
World ‘o lakes
There are around 117 million lakes dotted across the planet’s surface, from tiny kettle lakes a stone’s throw across to Lake Superior, which covers over 31,000 square miles. More than half of these lakes freeze over every winter. The frozen period is crucial for both lake ecology and for the culture of people who live around or near their edges.
“Ecologically, ice is almost like a reset button on a lake in the winter,” says Sapna Sharma, a lake expert and biologist at York University in Canada and the lead author of the study. Ice sits like a lid on top of the lakes, keeping the water inside cool and still. And the length of that cold, quiet period matters. If the ice breaks up early—or doesn’t form at all—the plankton newly-hatched fish rely on for food may emerge too early; harmful algal blooms may be more likely; and lake water evaporates away, leaving the lakes smaller and even more sensitive to future heating.
And the frozen lakes matter to people. In the northern reaches of Canada, many communities are connected only after lakes and rivers freeze over, allowing a web of temporary roads to wind across the landscape. Other communities rely on fish plucked from frozen lakes as protein for sustenance over the rest of the year. And skating, ice fishing, and other recreation on frozen lakes is crucial to many communities’ sense of identity.
“Over the last few years, here in Colodaro, a few tournaments at some of the lower elevation lakes had to get pushed because we've had too nice weather,” says Wayne Brewster, the webmaster for the U.S. Ice Fishing Association, which hosts announcements for ice fishing tournaments across North America.
But many in the ice fishing community, he thinks, haven’t felt the effects fully yet. “We're eternal optimists,” he says. “But if we see these tournaments get canceled year after year because of ice conditions, then I think the conversations will start.”
The unfrozen future
The team surveyed hundreds of lakes across the Northern Hemisphere to see how their ice cover had changed since 1970—whether they froze and stayed frozen for the whole season, whether they slipped back and forth between ice and free water, or didn’t freeze at all. The thing that mattered most, they found, was the average air temperature the lake felt over the entire year: if that number slid higher than 47° Fahrenheit, the lakes that usually froze solid would freeze only part of the time. And if the temperature at the lake hit 50°F, it was likely that the lake wouldn’t freeze at all.
Even small changes, says Olaf Jensen, a biologist and lake expert at Rutgers University who was not involved in the study, can have big effects. “We often think of 2°C (3.6°F) as perhaps just a minor shift,” he says. “If it got 2°C warmer over the course of a day, for example, we’d barely notice it. But that same increase in average air temperatures means we're going to lose lake ice over large areas of the world.”
The impacts of climate change can be difficult to feel in some cases, says Sharma. But ice? It’s either frozen or it’s not. So shifts in air temperature that are too subtle for people to feel day by day show up very clearly in the winter, when a lake freezes over—or it doesn’t.
All in all, the team estimated, about 15,000 lakes across North America, Europe, and Asia might be only partly frozen during the winters now.
Crossing the climate change boundaries
That number could rise exponentially into the future, they say, both because air temperatures are rising precipitously—faster in the northernmost reaches of the planet than anywhere else—and because there are more lakes dotting the far northern landscape in the first place.
By 2080, they found, if greenhouse gas emissions continue unabated, the number of lakes that don’t stay frozen over winter would likely nearly quadruple. Under worse scenarios, that number could increase by as much as a factor of 15—meaning that nearly 20 percent of all the Northern Hemisphere lakes that are currently iced-over in winters would stay dark and liquid over the season.
But how many lakes lose their ice—and how quickly that transition occurs—strongly depends on how much the planet warms up, which in turn depends on how aggressively humans attempt to curtail our greenhouse gas emissions. So their future is in our collective hands, says Sharma.
“What I find is this really reflects the urgency of how fast climate is changing,” she says. “Because within this generation, and definitely our kids’ generation, something that we take for granted—access to frozen lakes—may be of the past.”