When the Great Lakes should have been constricting under heavy ice sheets in late February, satellite imagery revealed an eerie scene of open, dark water spanning Erie and Ontario, with waves crashing against normally silent and frozen shorelines.
More quickly than ever before, the Great Lakes are warming. It sounds like a clinical phrase. However, it feels more immediate when you’re standing on a gravel beach near Lake Huron in the winter and hearing the sound of water slapping against stone where everything was once muffled by ice.
| Category | Details |
|---|---|
| Water System | Great Lakes (Superior, Michigan, Huron, Erie, Ontario) |
| Surface Freshwater Share | ~20% of the world’s fresh surface water |
| Region Population | 30+ million people in U.S. & Canada |
| Ice Record | Satellite monitoring since 1973 |
| Recent Record | Lowest average winter ice cover (2024 season) |
| Key Research Bodies | NOAA Great Lakes Environmental Research Laboratory |
| Regional Climate Research | Great Lakes Integrated Sciences and Assessments (GLISA) |
Lakes Michigan, Huron, Erie, and Ontario recorded record-high average surface temperatures in 2024, according to recent data. Long regarded as the icy, melancholy anomaly, Lake Superior has experienced a significant warming since the late 1970s. Winter temperatures have been rising at the fastest rate in the area, which has been warming more quickly than most of the contiguous United States. It turns out that the story gets sharper in the winter.
For decades, the amount of ice has been decreasing. The total length of the ice season has decreased by almost a month, and the number of days with substantial ice has significantly decreased since the early 1970s. While the long-term average would typically be close to 30%, the average ice cover during the peak season in 2024 was at historically low levels, falling into the single digits. It’s more than just a poor year. It is a recurring pattern.
Perhaps the warmth was enhanced by a powerful El Niño. Rarely do climate systems move by themselves. However, scientists indicate a more significant change that started in the late 1990s when the lakes absorbed an abnormally high amount of heat and a strong El Niño occurred from 1997 to 1998. Extreme temperature fluctuations, including heat waves and cold spells, have more than doubled in frequency throughout a large portion of the system since that time.
The lakes appear to have entered a new regime. More volatile, not just warmer.
Rather than acting like lakes, the Great Lakes act more like inland seas. Their coastlines are as long as the Atlantic coast. Lake Michigan is roiled by upwellings driven by the wind. Over Superior, storm systems develop at a startling rate. The lakes store heat differently due to their size, retaining warmth well into fall and occasionally into early winter, which delays the formation of ice and changes seasonal rhythms that used to feel consistent.
It’s difficult to ignore the minor hiccups that are building up as you watch this happen. Once heavy and predictable in places like Buffalo and Marquette, lake-effect snow now depends on a delicate balance between cold air and warm water. Strong snowstorms are fueled by open water during low-ice years. However, those snow events are shifting more and more in the direction of rain as air temperatures continue to rise. Sputtering is the familiar winter machinery.
The first to sense it are fishermen. The foundation of commercial fishing in Michigan and Ontario, lake whitefish depend on winter ice to shield their eggs from strong waves. Because shallow spawning grounds have less ice forming, storm surges agitate delicate eggs before they hatch. Additionally, fish larvae and plankton blooms are not matching due to warmer winters and earlier springs, which lowers survival rates. Compared to decades ago, it now takes a lot longer for whitefish to grow to harvestable size.
The speed at which species can adapt is still unknown. Slow warming allows fish to swim toward cooler depths. Cold snaps or sudden spikes in temperature are less forgiving. Juveniles and eggs have nowhere to go.
Then there are blooms of algae. More persistent blooms, especially in western Lake Erie, have been fueled by warmer surface waters and more precipitation that washes nutrients off farm fields. Blue-green algae called cyanobacteria, which can produce toxins, appear to flourish during long warm seasons. Blooms are becoming longer and beginning earlier. Beaches close. Water treatment facilities adapt. Green streaks spreading across bays make boaters look uneasy.
Additionally, the lakes now mix differently. Overturn events in the spring and fall of an average year move nutrients and oxygen from the top to the bottom. Long-term stratification, however, where warm water persistently sits on top of cooler depths, can lower oxygen levels below and increase the risk of hypoxia. Species that rely on those cold havens may suffer if oxygen in the deep layers is lost. Scholars are keeping a close eye on things, simulating situations that seemed theoretical at first.
The Great Lakes basin is home to thirty million people. These lakes supply drinking water to cities like Toronto, Cleveland, and Chicago. Grain and iron ore are transported across international borders via shipping lanes. Predictable seasons—beach days in the summer, ice fishing in the winter—are crucial to tourism economies.
The term “warming lakes” has a subtle irony to it. Calm is suggested by water. However, instability rather than warmth is the main factor at play here. Since the late 1990s, temperature extremes have increased, and surface heat waves are becoming more frequent. While cold snaps continue to occur, they feel unpredictable and serve to highlight rather than to define a broader upward trend.
It’s difficult to ignore the sense that something fundamental is changing. Although the Great Lakes have always undergone change—storms reshaping dunes, shifting water levels—the rate seems to be accelerating these days. These enormous bodies of water hold heat that is difficult to release. It lingers after being absorbed.
Using atmospheric data and sophisticated simulations, scientists are continuing to improve models and extend temperature records back decades. Fisheries managers and coastal planners may be better prepared for the future if extreme event forecasting is done. However, foreseeing does not avert.
More quickly than ever before, the Great Lakes are warming. That much is obvious. It is unclear if this is the beginning of a longer-term change or a transient stage heightened by natural cycles. One thing is for sure, the lakes don’t act the same way they used to.
