Since October, the water temperatures on all five of the Great Lakes have hovered at record-high autumn levels, about five to six degrees above average. This comes after abnormally large spikes in temperature over the summer, as well.
“I’m not surprised at all that the water temperatures are so warm,” said Sapna Sharma, an associate professor at York University who has studied ice for more than a decade. “Lakes are experiencing more extreme warm years.”
Sharma and her colleagues analyzed 60 lakes in the Northern Hemisphere in a September study and found a pronounced warming trend over the past 100 to 200 years. The lakes have warmed six times as fast in the past 25 years compared with any other quarter in the past century. Previous work shows lake warming trends extend to the Southern Hemisphere.
The warmer water temperatures are pushing the onset of ice to later in the year, as well as ending the ice season earlier. The study found that on average, lakes are losing about 17 days of ice cover per century. The lakes were freezing about 11 days later and thawing about seven days earlier.
Sharma says the cause is a sharp increase in abnormally warm weather. Since 1995, several lakes have begun to experience winters with minimal or no ice cover.
“If we continue at this same rate, over the next 75 years we’ll lose 106 more days of ice cover on average across our lakes,” said co-author Dave Richardson, a professor at the State University of New York at New Paltz. “That is losing the entire ice period for many of these lakes over the next hundred years.”
The study examined lakes with consistent ice-cover observations from 107 to 204 years old. The longest observations, which dated to 1443, were recorded by Shinto priests for Lake Suwa in Japan. The priests logged the date the lake froze to anticipate the timing of a religious parable. The Shinto priests believed their male and female deities lived on opposite sides of the lake. When the water froze, the male could cross the lake to visit the female deity. The priests also tried to use the timing to forecast rice crops.
On other lakes, people recorded the timing of lake freeze to determine when ice could be harvested for refrigeration during the summer or when trappers would return from catching beavers all winter.
“The earliest observers that wrote these down were not scientists. Ice was important for the way of life and living and killing whales and fishing in the wintertime,” said John J. Magnuson, a limnologist the University of Wisconsin at Madison. “The longer records all began before there was a science, and the science is capitalizing on what’s occurred.”
Today, researchers study ice cover to help track the health of the ecosystem. Less ice cover means more lake water can evaporate, reducing the amount of freshwater available to aquatic organisms and people. Less ice also allows the surface of the lake to warm earlier and more intensely, resulting in more algal blooms that can sometimes contaminate the water for humans. Changes in ice cover also have an economic effect on those who use the lake for fishing, sports and transportation for goods.
“You lose a lot of those goods and services that you might otherwise have, even before you lose the ice in its entirety,” Richardson said. He said these simple long-term measurements have “been a really valuable piece of understanding how climate change works.”
Greatest warming on the Great Lakes
The Great Lakes, which supply water for more than 30 million people, were among the fastest-warming lakes in the study. Lake Superior was the fastest, losing nearly two months of ice cover per century. Lake Michigan also experienced rapid warming and more ice-free years.
Lake Superior is warming up three times as fast as the global average, which is around 0.61 degrees (0.34 Celsius) per decade.
Sharma said Lake Superior’s summer water temperatures were warming at the world’s second-fastest rate, referring to her previous research. “It probably is linked to this less ice cover," she said.
Sharma said lower ice cover in the winter leads to warmer water in the summer as the surface of the lake intercepts the sun’s energy sooner. Last winter’s maximum ice cover was 7.5 percent lower than the long-term average over the Great Lakes.
Abnormally warm air temperatures have elevated Great Lakes water temperatures since the summer. October was especially warm, with air temperatures 5 to 10 degrees above long-term averages at land stations surrounding the lakes. Several locations had their warmest Octobers on record.
Because of Lake Superior’s depth, it has a high heat capacity and takes longer than shallower lakes to cool down when its water temperatures are elevated. As of Thursday, Lake Superior’s average water temperature was around 53 degrees — the warmest in early November in more than a quarter-century of records.
Colder air has finally moved into the region, potentially good news for this year’s winter ice cover. While water temperatures are still at record highs, they have cooled about 5 to 10 degrees over the past few weeks. The season’s first lake-effect snows occurred as frosty air swept across the relatively warm lake waters. On Monday and Tuesday, up to 11 inches fell in parts of northern Michigan.
Sharma thinks the Great Lakes will experience near-average ice cover this winter, as the current La Niña pattern favors cold temperatures in the northern United States. But longer-term projections are less optimistic with global warming.
“If we continue emitting greenhouse gases at this rate, Lake Superior will not freeze after the 2060s. Lake Michigan will not freeze after the 2060s,” said Sharma, citing her previous research.
The planet is projected to warm around 4.9 degrees (2.7 Celsius) by the end of the century. At this rate, Sharma and her colleagues calculate that about 5,700 lakes could permanently lose ice cover by 2100.
At the ongoing United Nations COP26 conference in Glasgow, Scotland, nations are negotiating how to limit warming to 2.7 degrees (1.5 Celsius) above preindustrial levels. Sharma said if global temperature changes were below 3.6 degrees (2 Celsius), only 179 lakes would permanently lose ice cover in the next century.
“We find that [for] preserving ice cover, there isn’t a magic solution. The solution is mitigating greenhouse gas emissions,” she said. “It’s not just about the ice. It’s about what will happen to water quality and water quantity. So that’s why COP is so important.”