It is difficult for scientists to accurately predict the impact climate change will have on lake levels because water level is a complex balance between riverine input and output, precipitation and evaporation, wind, humidity, and extraction rates. All of these variables will respond to climate change to some unknowable extent; lake level is the integrated consequence of these variables interacting with and counteracting against each other. As a result, lake levels will be in flux for centuries to millennia as the many facets of climate change and the human response play out.

There have been disparate lake level results from varying global climate model runs. Some have predicted large drops in lake levels (up to 1.4 m) due to increased evaporation rates as temperatures rise, while others have displayed slight increases in lake level (up to 0.35 m) because of increased precipitation patterns.

One study analyzed the results of 565 model simulations from 23 different global climate models in the hopes that the ensemble forecast would provide the most probabilistic future.⁶⁰ The results indicated that lake levels are most sensitive to changes in temperature but there is still a high degree of variability within emissions scenarios. Under a low emissions future, lake levels could range from -0.87 - + 0.31 m and under a high emissions future, lake levels could range from -1.81 - +0.88 m by the year 2100. Models project that Lake Superior’s water level will respond the least to changes in climate. The water levels of Lakes Michigan, Huron, Erie, and Ontario are projected to respond similarly to changes in climate. Despite having a large range of uncertainty regarding future lake levels, most of the 23 models predict a decrease in lake level with time due to climate change; the magnitude of the decline is dictated by the intensity of future emissions.⁶¹

Secondary Impacts

• Reduced shipping capacity: The St. Lawrence Seaway was developed in the late 1950s and constructed with an ideal of stable lake levels. The U.S. government maintains a 27-foot depth below low water datum (LWD) in the St. Marys, St. Clair, and Detroit rivers as well as in Lake St. Clair and some of the Great Lakes ports.⁶² Shipping interests have therefore become dependent on relatively stable lake levels. Increased evaporation rates due to warmer temperatures may lower lake water levels. By 2030, lake levels may be above the LWD for Lake Ontario and at the LWD for Lake Erie. If the lake level is lower, it is estimated that every one inch in lake level drop will cause a 1,000 foot ship to lose 270 tons of cargo capacity; an ocean-going vessel of about 740 feet will lose 100 tons of capacity.⁶³
   
• Reduced hydropower: Decreased water levels will reduce the amount of water available to generate power through hydroelectric dams for the region.
   
• Reduced wetland areas: Due to urban development, land use change, and channelizing of streams, areas historically occupied by marshes, wetlands, and swamps have diminished over time in the Great Lakes region. In the future, lower lake water levels will cause existing wetlands to be further reduced; at the same time, new habitat that was formerly submerged will be opened up as water levels drop.⁶⁴
   
• Water supply: It is anticipated that water demands will be 20-50% higher in the future as population and economic requirements grow.⁶⁵ The Great Lakes serve as important natural reservoirs of freshwater for the region. If water levels decline as a result of climate change, the total volume of freshwater available will decline as well, which could complicate existing infrastructure for municipalities. This will likely not become a major concern for the region because water intake pipes are located at relatively deep depths in the Great Lakes; for example, in Lake Michigan most water supply intake pipes are located between 900 to 1500 m offshore so there should be no direct impacts from lower lake levels.⁶⁶
   
• Loss of shoreline: If lake levels increase due to changes in climate, the region could witness an increase in the surface area of the Great Lakes as waters rise. Higher lake levels would cause a loss of land and shoreline, potentially compromising buildings and infrastructure built adjacent to the lakes.