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Great Salt Lake Superior?

Duluth's lift bridge.

Lower water levels predicted for Lake Superior this summer may affect the shipping industry. Photo by Tim Slattery

The minus-20-degree temperatures outdoors the night of his presentation did not stop Sea Grant researcher Tom Johnson from discussing the effects global warming may be having on Lake Superior. Johnson, director of the Large Lakes Observatory, at the University of Minnesota Duluth, interpreted findings from the National Oceanic and Atmospheric Administration (NOAA) and several research papers this past winter.

“Clearly we had a very hot 1998, but was it just normal variability?” Johnson queried. “There’s been a general warming trend since the 1890s, in part due to natural causes. But in recent decades, researchers attribute the warming to an increase in greenhouse gases due to human activities - mainly energy use,” Johnson said. Greenhouse gases include methane, chlorofluorocarbons, carbon dioxide, ozone, and nitrous oxide.

NOAA data shows that 1998 was the warmest year on record at 58.1 degrees F - 1.2 degrees hotter than normal, and that the 1990s have been the warmest decade on record. Scientists can account for this by looking at natural temperature records preserved in tree-rings, locked in the skeletons of tropical coral reefs, frozen in glaciers and ice caps, and buried in the sediments of lakes and oceans. This type of climate data can extend scientists’ understanding far beyond the 140-year instrumental record provided by thermometers and rain gauges.

Recent Impacts on Lake Superior

The recent warming increased Lake Superior temperatures this past summer, which delighted swimmers but changed the dynamics of the lake. The annual fall turnover was late. According to Elise Ralph, also of the Large Lakes Observatory, Lake Superior hadn’t turned over by the end of December, when it usually starts by October. Turnover is a mixing of surface and bottom water that begins each year when surface water reaches a temperature of 4 degrees Centigrade. Water is most dense at this temperature, and it begins to sink. This drives a circulation pattern that brings oxygen to the lower depths of the lake and affects many ecosystem processes.

Other Great Lakes were affected by the unusual climate in 1998. Precipitation over the basin was below normal, leading the U.S. Army Corps of Engineers to predict significantly lower water levels for Lake Erie in 1999. Most of Lake Erie’s water enters through the Detroit River from the upper Great Lakes.

Low levels are also predicted for Lake Superior. Current projections are that the water level this July will be about eight inches below average, which is about three inches below what it was last July. This is bad news for the shipping industry, since lower water levels prevent cargo ships from carrying as much. According to Davis Helberg, director of the Seaway Port Authority of Duluth, a one-inch decrease in draft means a 250-ton decrease in the cargo a ship can carry.

Future Impacts Predicted

Imagine a saltwater Lake Superior ringed by oaks and maples. That could well be the case in the far future if climate model predictions are correct. Johnson discussed four scenarios developed by NOAA researchers that predict the amount of carbon dioxide in the atmosphere will double within the next century, leading to a 3-4 degree Centigrade increase in the high temperature for western Lake Superior and a 4-5 degree Centigrade increase in the high temperature for eastern Lake Superior. Precipitation could increase 10-20 percent in the northern part of the lake, and up to 10 percent around the rest of the lake. However, this would be offset by increased evaporation, which could reduce basin runoff by up to 20 percent.

Winter ice cover is likely to be limited to the shore and shallow areas of the lake. And there may be more winters with no ice cover in these areas. This lack of ice could lead to year-round navigation, a reduction in the range of some fish species that depend on cold water (such as whitefish), a reduction in winter recreational activities, and an increase in winter lake evaporation.

Vegetation change scenarios predict global warming could alter forest composition so that the shores of Lake Superior will feature more southern species of trees (oaks and maples) or even prairie - if these plants are able to adapt to the different soils and day lengths found in the northern climes.

Tom Johnson

Tom Johnson, director of the Large Lakes Observatory, on the shores of Lake Superior. Photo courtesy of Bob King, Duluth News-Tribune

A global warming fact sheet produced by Ohio Sea Grant says that such changes would occur slowly in human terms. The effects may not become noticeable until 2040, or possibly as late as 2090. “In the meantime,” according to the fact sheet, “the established trees in the forest canopy could appear the same, giving the illusion that all is well. However, changes in precipitation patterns may affect the frequency with which adult plants flower and fruit, lessening the chance of offspring survival. As a result, when the presently established trees become overmature and begin to die off naturally around 2090, there would be fewer young trees to take their place.”

“The NOAA study predicts that lake levels will drop anywhere between 2-33 feet,” said Johnson. “If the drop is indeed at the upper end of this range, the St. Marys River will no longer be able to flow, leading to Lake Superior becoming a closed basin and a salt lake,” he said. Salt and dissolved solids entering the lake from rivers would slowly accumulate due to evaporation. “The turnover rate will decrease with the bottom becoming static” (or anoxic, meaning devoid of oxygen), said Johnson.

Can the Impacts Be Changed?

These climate change scenarios represent a range of possible futures. “The models are crude and developing, but this is what they say now, and it worries me a lot,” said Johnson. “This magnitude of environmental change bothers me deeply.”

The audience at Johnson’s presentation, sponsored by the Sigma Xi Society, Sierra Club and Northeastern Minnesotans for Wilderness, were concerned by these predictions and wanted to know what they could do to help prevent such an outcome. Johnson suggested people encourage their congressional representative to support international treaties that reduce carbon dioxide emissions into the atmosphere. “Also, anything people can do on a personal level to reduce the consumption of fossil fuels is good,” said Johnson. “Whether it’s only running the dishwasher when it’s full, to limiting the amount you drive.”


Mortsch, L. and Quinn, F. (1996) Climate Change Scenarios for Great Lakes Basin Ecosystem Studies. Limnology and Oceanography, 41(5), 903-911.

Assel, R. (1991) Implications of CO2 Global Warming on Great Lakes Ice Cover. Climatic Change, 18: 377-395.

Global Change in the Great Lakes Scenarios. A series of fact sheets from Ohio Sea Grant, 614.292.8949.

NOAA’s global warming Web site: www.ngdc.noaa.gov/paleo/globalwarming/home.html.

By Marie Zhuikov
April 1999

Return to April 1999 Seiche

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