Great Lakes' past may offer clues on climate
By James Janega
Published June 26, 2007
CHICAGO — From one view of history, the Great Lakes are near record lows, approaching the bottom-scraping frustration of the mid-1960s.
From another, longer view, though, the lakes are nearly as high as they've ever been, just a few feet below the high-water mark reached at the end of the Little Ice Age in the 1850s.
Both pictures are scientifically accurate, and are getting increased attention from climatologists, lake scientists and environmentalists curious about history's large climate cycles and how they tip the lakes' eons-old balancing act of rainfall and runoff, heating and evaporation.
The fluctuations are posing new questions about whether climate change has begun to alter the depth of the lakes, though the picture still too complex to yield definitive answers.
"If you look at the record even from 1850 on, at lake levels and precipitation levels, this is not abnormal," said Thomas E. Croley II, research hydrologist at the Great Lakes Environmental Research Laboratory in Ann Arbor, Mich. "But it's always hard to say whether this is the start of something else, and that's where you get so much subjective opinion, so many questions of climate change."
Recently, owners retired the Lake Superior ferry between Minnesota and Isle Royale because its draft was too deep to dock. Deep lake cargo carriers have left tonnage at loading docks to cross shallow channels between lakes. Marinas have been dredging more than ever. And still other indicators of lake health seem out of whack.
In the last decade, researchers learned the lakes not only were dropping compared with modern records, they were also getting warmer — even faster than temperature increases on land. Lake Superior is 5 degrees Fahrenheit warmer than it was a century ago — "and most of that warming happened recently," within 25 years or so, said Jay Austin, a climatologist at the Large Lakes Observatory at the University of Minnesota in Duluth.
Ice on the lake forms later and melts earlier, he said, and the tipping point when the winter lake begins warming rapidly for the summer has come earlier and earlier each year. That in turn has spelled faster evaporation for Lake Superior — the feeder lake for lakes system — which has been steaming away 4.6 millimeters faster every year since 1977.
At that pace, more water is now evaporating than is falling into the lake as rainfall.
"Lake Superior is sort of this canary in the coal mine," Austin said. "It responds very quickly to climate change."
And that realization has prompted a wider look at the lakes' past.
For years, depth markings of falling water levels and satellite views of shrinking ice sheets have told a bleak tale of drought and dwindling waters in the Great Lakes, particularly Lakes Superior, Michigan and Huron.
But in the last decade, climate researchers, pollen experts and specialists in prehistoric forests have also added their expertise, uncovering more evidence below the lakes' diminishing waters, whispered by ancient peat bogs, drowned beachfronts and submerged tree trunks.
The researchers have teased chemical indicators from sediment records, carbon dated soggy stumps under Lake Superior and put microscopes to core samples from Lake Huron to find pollen from warmer and drier climates. Together, the evidence suggests water levels have risen and fallen by several meters over the centuries, often tied to cycles of warming and cooling in the region.
"The evidence is pretty strong it was once much drier around here than at the present time," said Thomas Johnson, a geologist at the Large Lakes Observatory in Duluth, citing evidence from long-ago beach ridges and drowned swamps not quite at the shores of the modern Great Lakes.
During periods of low water that lasted until 2,000 years ago, a forest stood at the bottom of Duluth harbor and in parts of Lake Huron. Peat bogs stretched between what are now the Apostles Islands. At a bay above Sault Ste. Marie, successive underwater ridges were a thriving forest when Leif Erikkson landed in North America, and an active beach later in dry spells as recent as the 1700s. In Lake Michigan, salvage divers in the 1980s found a grove of sunken tree trunks 15 miles off Navy Pier that date back thousands of years.
Piecing together those clues, climate detectives suspect Lakes Superior, Michigan and Huron may have been lower than 20th century historical averages in the 13th and 17th centuries — and much higher in the 16th century as well as over the last 50 years.
The renewed look at the lakes' far past is hoped to yield insight for the future, said Cynthia Sellinger, who tracks lake levels for the Great Lakes research lab, part of the National Oceanic and Atmospheric Administration.
From year to year, rainfall and snowpack replenish lake levels, while ice cover and high humidity preserve them from evaporation. But sometimes, bizarre factors get thrown in as well.
In 2005, Lakes Superior, Michigan and Huron, locked in drought, languished below long term annual averages. Meanwhile, the downstream lakes they fed were brimming with water — a surprise on its face until Sellinger compared lake levels to weather tracks and realized Lakes Erie and Ontario were filled by the last gasps of tropical storm Arlene and hurricanes Dennis and Katrina, a freak of the Atlantic hurricane season.
"Superior hasn't had a 'normal' ice cover since 1997," said George Leshkevich, manager of NOAA's Great Lakes Coastwatch. "You're never sure if it's some hiccup on the geological timeline. But something is happening, it seems."
The drop has been depressing to boaters, but an unexpected boon to others. Agate hunters on the rocky shores of Lake Superior seem grateful for wider gravel beds that might yield a few more finds, said Erin Zoellick, assistant director of education at the Great Lakes Aquarium in Duluth.