Great Lakes Ice Coverage

Snow and Ice Cover
  • From 1973 to 2010, annual average ice coverage on the Great Lakes declined by 71%.
  • From 1975 through 2004, the number of days with land snow cover decreased by 15 days, and the average snow depth decreased by 2 inches (5.1 cm).
  • Snow and ice levels on the Great Lakes and on land will likely continue to decrease.
  • Reduced lake freezing will result in more exposed water that could increase lake-effect precipitation.
Observed Decline in Great Lakes Ice CoverObserved Decline in Great Lakes Ice Cover, 1973-2010. From Wang et al. (2012).

Ice coverage declined by 71% overall on all five Great Lakes and Lake St. Clair since from 1973-2010. Total losses of annual lake ice coverage varied from lake to lake, ranging from 37% in Lake St. Clair and 50% in Lake Erie to 88% in Lake Ontario.1 Though the long-term trend has been downward, high ice winters, such as 2013-2014 and 2014-2015, can still occur and illustrate the complexity of this system.

Great Lakes
Ice Coverage Decline
1973-2010
All Great Lakes 71%
Lake Ontario 88%
Lake Superior 79%
Lake Michigan 77%
Lake Huron 62%
Lake Erie 50%
Lake St. Clair 37%

Lake Temperature

The Great Lakes have warmed faster than the nearby air temperature in recent years. Lake Superior summer (July–September) surface water temperatures increased approximately 4.5°F from 1979-2006, a significantly faster rate than regional atmospheric warming. Declining winter ice cover is the largest driving factor. The onset of first ice cover on inland lakes in the region is 6-11 days later than during the middle 19th century and the breakup of ice in the spring is 2-13 days earlier. With shorter winters and open lake water earlier in the spring, the lakes are becoming stratified earlier, allowing the lakes a longer period to warm and amplifying the effects of warmer summer air temperatures.2

Effects on Climate

Much of the weather and climate experienced by communities in the Great Lakes is driven by the seasonal behavior of the lakes. Significant changes to physical properties of the lakes, including ice cover, water temperature, and evaporation from the lake surface, could have profound implications for the climate of the Great Lakes region. Most prominently, lake-effect snow is greatly diminished once ice cover develops as it reduces evaporation from open water surfaces. With less ice cover, longer periods of the year with open lake water, and warmer temperatures, the key ingredients for lake-effect precipitation are more abundant.

Projected Future Ice Coverage

At the current rate of decline of ice cover, Lake Superior will have little to no open lake ice during a typical winter in about three decades. Under those conditions, Lake Superior will have little significant ice cover to melt each spring, and the seasonality of the lakes will be driven primarily by winter temperatures. The date of the onset of summer stratification will change more slowly, and the rate of increase of summer water temperatures will also slow as a result.2

Ecological Implications

As summer water temperatures increase and the summer stratified season grows longer, there is potential for significant impact to the ecology of the Upper Great Lakes.3 4 5 6 The Upper Great Lakes all form ice to varying extents each winter, and many ecosystems depend on the ice cover in different ways. Plankton are more resilient when protected by a layer of ice. Coldwater fish species such as whitefish and lake trout will be forced to compete with warmwater species migrating north with rising temperatures. Declining ice cover could also stress whitefish reproduction in Lake Superior where ice protects eggs from winter storm disturbance. With greater lake stratification, oxygen can become depleted in the lakes' productive lower levels, leading to "dead zones", and, with increased nutrient loading and stronger storms, toxic algal blooms.

Lake Ice Cover, Evaporation, and Lake Levels

Cold winter temperatures increase ice cover on the Great Lakes. The ice acts as a cap, reducing evaporation by preventing water vapor from escaping into the air. But the reciprocal process is also true. Higher autumn evaporation increases winter ice cover. In years with high Great Lakes ice cover, the lakes often lose a great deal of heat energy to evaporation in the preceding autumn, cooling the water enough to form ice. This means that extensive winter ice cover is actually an effective indicator of high evaporation rates during the previous seasons.7

Evaporation preceding and following high and low ice cover years. High winter ice cover means evaporation and heat loss was actually higher during the preceding fall. The image in the upper left of this figure was taken on March 29th, 2013. The bottom left image was taken on March 29th, 2014.

A Tale of Two Winters, December 2013 and December 2011

The bitterly cold winter of 2013-2014 provides a striking example of how Great Lakes evaporation can defy expectations. Evaporation rates during December 2013, which was colder than normal, were roughly 60 percent higher than they were in December 2011, a much warmer month.

The Difference Between Water and Air Temperature Drives Evaporation

Intuitively, one would expect warmer temperatures to yield higher evaporation rates, but the difference between lake surface and air temperatures is actually the more important factor. Late autumn and early winter can see surprisingly large differences between lake and air temperatures, especially in the deeper lakes. In early January 2014, Lake Superior was 30-40°F (17-22°C) warmer than the overlying air. That sharp temperature contrast led to high evaporation rates.

Evaporation is a Dominant Factor for Lake Levels

Evaporation is one of the dominant physical processes affecting Great Lakes lake levels. A single day’s loss of approximately 0.5 inches of water from the surface area of the Great Lakes is roughly 20 times the amount of water that flows over Niagara Falls. Seasonal and long-term changes in ice cover and evaporation rates therefore carry large implication for future lake levels.

Lake levels have been declining since the early 1980s and have been at a sustained low for several years. Furthermore, Lake Superior underwent a regime shift during the late-1990s El Niño event, resulting in warmer summer water temperatures and winters with less ice cover. Given the long-term trend of warming lake temperatures, it’s unclear if the lakes will ever return to previous conditions. While projections of future lake levels remain uncertain, less ice cover on Lake Superior and the other Great Lakes could amplify evaporation rates and lead to greater long-term declines in lake levels.7

External Resources

NOAA-GLERL Ice Cover Resources: NOAAs Great Lakes Environmental Research Laboratory has been exploring the relationships between ice cover, lake thermal structure, and regional climate for over 30 years. Information on current ice conditions, historical observations, and future lake ice projections can be found here.

The Space Science and Engineering Center (SSEC) at University of Wisconsin-Madison provides downloadable satellite images of the region, many of which are widely-used in discussions of ice coverage.