Is Global Warming Taking the "Super" Out of Lake Superior?

10-01-2007 // Roger Di Silvestro

LAKE SUPERIOR is the biggest freshwater lake on the planet, as calculated by the lake's surface area of 31,700 square miles. How big is that? About the size of South Carolina. The lake's volume equals that of all the other Great Lakes combined plus three more Lake Eries, which means that by itself it holds more than 10 percent of the world's freshwater.

But it is holding less of it with the passing of time.

In the larger scheme of things, that's no big deal--the fate of all lakes is to fill up with silt, turn into a marsh, and finally to dry ground, given the passage of enough time. But for Lake Superior, the loss of size remains a mystery.

Last summer, Lake Superior was within 3 inches of setting a new record for its lowest measured depth. It fell more than a foot in just the past year, widening shorelines in some areas by dozens of yards and leaving boats unable to reach mooring sites that have been left high and dry. At one beach in the southeastern reaches of the lake, a rope that once marked the boundary of a swimming area now lies on the ground. Cargo ships are carrying lighter loads of coal and iron ore so they will float higher in shallower waters, to avoid running aground in once-open channels.

This drop in water depth has occurred in Lake Superior, and to a lesser degree in the other Great Lakes, during a time in which the lake is reaching record temperature highs: Superior's average temperature is up 4.5 degrees F since 1979.

Is there a link between receding shorelines and increased water temperatures? Is global warming the culprit? Hard to say. On the one hand, global warming models predict more rain for the Great Lakes region as warming arrives, but rainfall has declined, so some other factor must be at work. On the other hand, models also indicate that as the lakes warm, their winter ice cover will shrink, allowing more evaporation during cold months, and, indeed, winter ice cover is down, and evaporation rates are up, as predicted. But El Niño, a warming of the Pacific Ocean, may also be a factor. An El Niño event that produced warmer winters in the late 1990s arrived just as the lakes began receding.

Perhaps both global warming and other factors, such as drought cycles, are combining to bring Superior to new lows in water depth, some scientists say.

The precise cause is not the only unknown in the Great Lakes. Biologists worry about how shallower, warmer waters will affect fish species. In a world in which scientists are struggling to understand the finer points of how global warming will affect our lives and the ecosystems on which we depend, Lake Superior stands out at one of the great unknowns.

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