The large volume of the Great Lakes translates into a tremendous capacity to store and retain heat. Future warming air temperatures will lead to warmer lake temperatures, altering heating capacities, water circulation patterns, wintertime ice area, and summertime stratification.⁴¹ Lake temperatures are expected to warm earlier in the spring, reach higher maximum summer temperatures, and cool more slowly in the fall. From 1979 to 2006, summertime surface water temperatures increased by approximately 2.5°C.⁴² Warmer water temperatures coupled with warmer air temperatures will reduce the duration, thickness, and extent of the annual ice cover on the lakes.

Lake ice cover is a sensitive indicator of regional climate and can vary dramatically between years. In 1979 an estimated 95% of the Great Lakes was covered with ice while in 2002, only 11% was covered.⁴³ Typically, ice will begin to form on the lakes in late November to early December and maximum ice extent is reached in early February to March; lakes will often be ice free by late May to early June. Since the 1970s, annual ice cover on the Great Lakes has been declining and in 2010, there was an average of 71% less ice coverage relative to 1973.⁴⁴ Shallow lakes are, in general, more sensitive to climate fluctuations because of their reduced water heat capacity relative to deep-water lakes; the shallower lakes (e.g., Huron and Erie) will respond more quickly to changes in climate than the deeper lakes (e.g., Superior, Michigan, and Ontario). If trends of reduced ice coverage continue, large lakes such as Lake Michigan could experience ice-free winters as early as 2020.⁴⁵

Secondary Impacts

• Algal blooms: Increased water temperatures, reduced ice cover, and increased nutrients from runoff could stimulate cellular growth of filamentous algae along the shorelines of the lakes. The algae are visually unappealing, appearing like slime, and can harbor and produce pathogens. Some bloom-forming algae species can release volatile toxins, causing respiratory irritation in animals and people. Further, once an algal bloom terminates, bacteria consume the algae causing localized zones with diminished concentrations of dissolved oxygen that can cause fish mortality.
   
• Increased lake stratification: Historically, lakes in the Great Lakes region “turned over” during shifts of season, bringing nutrient-rich but oxygen-depleted waters to the surface and shuttling oxygen-rich, nutrient-poor waters to depth. Increasing the thermal capacity and heat storage in lakes will reduce seasonal mixing and increase stratification. Therefore, mixing and oxygen levels may decrease in response to regional warming trends.^46,47
   
• Reduced primary productivity: A warmer lake surface will serve to enhance the water stratification that occurs seasonally in the spring and summer.⁴⁸ A more stratified water column limits the amount of water exchange and nutrient mixing with the deep. Therefore, the reduced access to nutrients could limit primary production, having cascading consequences for higher trophic levels due to a lack of food.
   
• Increased hypoxia: Primary producers create oxygen as a natural byproduct of photosynthesis. Reduced primary production will also translate to less available oxygen in the water column, which could negatively affect zooplankton and fish. Also, warmer waters cannot hold as much gas as colder water; as lakes warm, they will physically hold less oxygen relative to historical norms.⁴⁹
   
• Altered fisheries: Fish have preferred thermal regimes; as lake temperatures change, fish distribution will be altered as fish seek out thermal refugia. Further, increased storm disturbance without ice protection could increase fish egg mortality. However, a reduction in annual ice cover will reduce the severity and duration of wintertime oxygen depletion in the waters of the Great Lakes, reducing seasonal winterkill of fish populations.⁵⁰
   
• Longer shipping season: From the 1950s to 1995, ice made the waters of the Great Lakes non-navigable for 11-16 weeks each winter by blocking navigation lanes, ports, and locks in the system. Warmer water and air temperatures will likely reduce the ice cover by one to three months, extending the shipping season.⁵¹
   
• Increased shoreline erosion: Wintertime ice shields the shoreline from the effects of storms and winds. As ice cover is reduced, shoreline erosion rates could increase, causing a subsequent decrease in water quality as more sediments are suspended in the water column.
   
• Shifts in phenology: Warmer water temperatures can increase maturation and growth rates of some plants and animals. Increased growth rates will cause some animals, like insects, to emerge earlier in the year relative to historical norms. If some animals, like insects, respond disproportionately to changes in water temperature as compared to their predators, climate change could cause a decoupling within food chains. This would have cascading consequences throughout the entire food web of the Great Lakes, altering productivity patterns and trophic structures.