Jan. 29, 2009 -- The Great Lakes have long been a bastion of stability -- with water hovering at about the same level for as long as anyone can remember. But a new study shows that climate change once pushed lake levels far below where they are now. That opens up the possibility that future climate change might do the same thing.
More than 33 million people depend on the Great Lakes for water, hydropower and work in industries ranging from shipping to recreation. During the past event, about 8,500 years ago, water ceased to flow between the lakes. Today, going from interconnected bodies of water to isolated basins could be catastrophic.
"If you can't transport things freely from the Great Lakes out to the Atlantic, major economic dislocation is going to happen there," said John King, a geological oceanographer at the University of Rhode Island in Narragansett. "The way things are done in that neck of the woods would change dramatically."
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For decades, scientists have been working to reconstruct the aqueous history of the Great Lakes. Those reconstructions showed that, for thousands of years, lake levels have only varied by a couple of meters (about 7 feet) up or down, mostly due to the advance and retreat of glaciers, which alternately trap and release water. As they move, glaciers also cause land underneath them to rise and fall slightly.
Using a variety of techniques, King and colleagues took a closer look at the region's geologic past. They scanned aerial photographs for evidence of former beaches, cliffs and outlets for water flow from one lake to the next. They bounced sound off surfaces to get cross-sections of the underwater topography. They looked at microfossil evidence, took note of tree stumps at surprisingly low depths, and collected core samples from areas of interest for radiocarbon dating.
Together, the evidence showed that between about 8,800 and 8,300 years ago, the water dropped 20 meters (66 feet) below current levels. Outlets became too high for water to pass, and lakes were suddenly cut off from each other.
A major drop-off in rainfall had to be what caused the drop, the researchers concluded in December in the journal Eos. Glaciers had already left the region by then, leaving a period of significantly cooler and dryer weather behind.
"We looked at all the evidence we could find, and there's no other explanation," King said. "This means that the idea that the lakes are insensitive to climate change is incorrect."
The future is projected to grow warmer and drier -- not cooler and drier like the period King's team studied. But the new work might hold warning signs for what’s to come, said geologist Steve Colman, of the University of Minnesota, Duluth.
Ships carrying coal, iron, and other goods wouldn't be able to pass through their normal routes and neither would spawning fish. Evaporating water would increase the concentration of salts and change the chemistry of the lakes. Hydropower plants would run dry.
"The potential exists for lake levels to fall below their outlets like they did 8,000 years ago if climate continues to get warmer and drier," Colman told Discovery News. "There's an analogy there. Whether it fits exactly or not, nobody knows."
Scientists need to do more research, King added, to see how likely the scenario is and to prepare both Canada and the United States for a bracing array of economic and ecological consequences.
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