Great Lakes Ice Coverage

NOTE: This page is no longer updated as of 2021, and more recently updated information is now available through GLISA’s Sustained Assessment of the Great Lakes and Great Lakes Retrospectives and Prospectives pages.

Summary

• The number of days per winter with lake ice coverage has declined since the start of record in 1973. ¹
• In most areas, ice cover declines were a sudden shift as opposed to a gradual decline.  For Lakes Michigan, Erie, and Ontario the shift occurred in the mid-1980s, but for Lakes Superior and Huron the shift occurred during the 1997/98 winter. ^{2 3}
• Ice cover has decreased the most in the north (i.e., Lake Superior, Northern Lake Michigan and Huron) and in coastal areas
• Ice cover on the Great Lakes will likely continue to decrease in the future, however, these decreases are expected to be interrupted by high-ice winters associated with cold air outbreaks.
• Reduced ice cover results in more winter lake-effect precipitation and increased winter wave activity. ⁴

A more in-depth analysis of Great Lakes ice cover is available here from GLISA in our Sustained Assessment of the Lakes resource.

Historical Trends

Role of Lake Surface Temperatures

The Great Lakes have warmed faster than the nearby air temperature in recent years. Summer (July–September) surface water temperatures on Lake Superior increased approximately 4.5°F from 1979-2006, a 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. ⁹

Impacts of Reduced Ice Cover

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, have profound implications for the climate of the Great Lakes region. Most prominently, the formation of lake-effect precipitation requires open water on the lakes. Declining ice cover,  or longer periods of the year with open lake water, combined with warmer surface temperatures, will lead to increased lake-effect precipitation in the future.  In the near term this may mean increased lake-effect snow, but as air temperatures rise lake-effect snow will transition to lake-effect rain.

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. ^{10 11 12 13} 

The Upper Great Lakes all form ice to varying extents each winter, and many ecosystems depend on 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 warm-water 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”. Increases in extreme precipitation, runoff, and nutrient loading can create conditions favorable to toxic algal bloom formation.¹⁴

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 ice cover, the Great 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. ¹⁵

Evaporation is one of the important 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 each day. Seasonal and long-term changes in ice cover and evaporation rates therefore carry large implications for future lake levels.

Lake levels began declining after reaching record highs in the early 1980s and were at sustained lows for several years in the early 2000s to mid 2010s. Furthermore, Lake Superior underwent a regime shift during the late-1990s, 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. Though projections of future lake levels remain uncertain, less ice cover on Lake Superior and the other Great Lakes would increase evaporation rates and, in the absence of increases in precipitation, lead to long-term declines in lake levels.¹⁶

External Resources

NOAA-GLERL Ice Cover Resources: NOAA’s 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.

References