Wayne County -- For thousands of years, the evidence lay at the bottom of Brown's Lake, buried in thick, gooey mud.
You could stand on the soggy bank in the summer and stare at the water, or tiptoe out onto the frozen crust that shrouded it in winter, and never realize what was hidden below. The ancient lake hid its secret well.
A drilling rig finally pried it loose. First, College of Wooster glacial geologist Greg Wiles and his colleagues used the rig to plunge a hollow tube into the mud of the lakebed, down 10, 20, 30 feet and more. Then they gingerly extracted the tube, packed full of an oozing, smelly mess.
The scientists took it to a lab and studied it. Looking at the deeper layers of the core sample was like peering back in time. And there, in the middle of the dark muck, was the proof: two distinct horizontal bands of white -- like twin stripes of vanilla frosting in a chocolate layer cake.
The white layers are called loess, a fine, windblown silt left behind when a moving glacier grinds rocks to dust. The loess stripes were the fingerprints of the most dramatic climate change in the last 10,000 years, and perhaps a foretaste of the future.
About 8,200 years ago, as the Earth warmed from the last ice age, something strange happened. The warming actually triggered an abrupt cooling. A catastrophic flood unleashed by the collapse of ice dams dumped trillions of gallons of fresh water into the North Atlantic Ocean. The outburst disrupted ocean currents that normally warm the Northern Hemisphere.
Within a decade, average temperatures around the North Atlantic basin dipped substantially and stayed down for a century or more.
Portions of the ocean's surface may have frozen in winter. Greenland felt the brunt of the sudden climate shift, tumbling as much as 14 degrees Fahrenheit, with places like Iceland, Norway and the United Kingdom chilled to a lesser degree. North America probably had decades of colder, drier, windier winters.
Details about the scope, impact and cause of the so-called "8.2K event" have begun to emerge in the last decade as researchers analyze more "proxy" climate records from the distant past -- ice sheet cores, ancient pollen grains and deeply buried sediment layers like those from the bottom of Brown's Lake.
Scientists have used such samples to uncover the 8.2K event's footprint in a variety of locations, from Greenland and Ireland to Canada and the North Atlantic seabed.
The new research by Wiles, University of Cincinnati geologist Tom Lowell and their collaborators, published in March, is the first documentation in Ohio and one of only a few confirmations so far of the event's effects within the United States.
Findings could help predict present warming
The long-ago big chill has implications for Earth's future. Abrupt climate swings are the rule, not the exception, over the long course of our planet's history.
"These abrupt changes are fascinating and unsettling and imperfectly known," Wiles said. "It would be interesting to be able to explain them, and maybe even vital."
If scientists can learn how the climate responded to a natural "forcing" mechanism like the 8.2K event's meltwater gusher, they should be able to better predict the consequences of present climate forcing due to human-accelerated global warming.
"There's no chance that nature did exactly the same experiment in the past" as today's warming conditions, said climate researcher Richard Alley of the University of Pennsylvania. "But we're going to learn something by seeing how things responded when they got kicked" during the 8.2K event.
Ancient lakes' demise plays key role
The kick, in this case, came from the catastrophic demise of a pair of vast ancient lakes in southern Canada. When the last ice age ended around 11,600 years ago, the Laurentide ice sheet that had covered much of the continent began its slow northward retreat. Meltwater pooled in what is today Manitoba and Ontario, prevented from draining by ice dams that still choked the Hudson Bay to the north.
These glacial lakes, called Ojibway and Agassiz, were as big as the present-day Great Lakes combined. When warming finally buckled the Hudson Bay ice, a glut of water equivalent to 64 billion Olympic swimming pools roared into the bay and down the Hudson Strait into the sea. It may have taken as little as half a year to completely drain Ojibway and Agassiz.
The huge freshwater pulse sharply reduced the salinity of the North Atlantic's surface water. That, in turn, spelled trouble for the global temperature-regulating cycle known as the great ocean conveyor.
Normally, the lands around the North Atlantic basin are warmed by temperate water that ocean currents deliver up from the tropics. (Some scientists believe northwest Europe's climate is warmed more by the North Atlantic's own seasonal absorption and release of heat than by heat transported from the Southern Hemisphere.)
Once relieved of its heat, the dense, salty North Atlantic surface water sinks to the sea bottom, displacing the water that was there and driving currents southward to be warmed again, repeating the loop.
Fresh water gums up the cycle. It's less dense than salt water, so it doesn't sink. That can slow or even stall the engine powering the great ocean conveyor. There's evidence that even before the 8.2K event, Earth's climate was cooling off, possibly because of previous glacial runoff into the ocean or a periodic dimming of the sun's heat output. But the outburst of meltwater 8,200 years ago dramatically dialed down the thermostat.
Deprived of its imported warmth, the northeast North Atlantic basin became a frigid place. Northeastern North America's climate may have been influenced more by atmospheric disturbances than the changes in ocean current.
Dying vegetation loosened its grip on the soil, allowing blustery winds to scoop up dust and ferry it for hundreds of miles. Without evaporative moisture from the vanished glacial lakes, the drier air sweeping down from the north may have been colder than before. Or it's possible the disappearance of the ice dams and the cold air that used to hover above them created a void, letting gulf storms slip northward to bring some warmth.
"It's actually a very complicated event, not as simple as people used to think," said Anders Carlson, a glacial geologist at the Woods Hole Oceanographic Institution.
The sediment cores from the bottom of Brown's Lake testify to that complexity. The two layers of dry, windblown glacial soil are separated by a dark stripe of dirt containing plant matter. It's an indication the climate may have briefly recovered, warming enough that vegetation resumed growing for a while before the cold and dryness returned. "Kind of a flicker," Wiles said.
The cause is unknown. Maybe, some scientists suggest, the ice dams didn't collapse all at once, instead sending a one-two punch of fresh water to the ocean at different times.
Could 8.2K event happen again?
With present-day global warming rapidly eroding the world's polar ice sheets and mountain-top glaciers, and boosting runoff from increased precipitation, the obvious question is whether a repeat of the 8.2K event is possible, and what the outcome would be.
Providing an answer is difficult because scientists don't have detailed records of conditions 8,200 years ago for comparison to today. Also, the global climate system is so complex, with so many variables, that even the best computer models contain uncertainties and tend to underestimate the size, speed and extent of abrupt climate swings.
Here's what is known:
* Before the last 10,000 years of relative stability, sudden climate shifts were common, with temperature changes of nearly 30 degrees Fahrenheit in some locales occurring in just a few years.
* Abrupt climate changes were more likely in the past when the climate system was undergoing rapid forcing, like the meltwater pulse of 8,200 years ago. "Thus, greenhouse warming and other human alterations of the Earth's system may increase the possibility of large, abrupt and unwelcome regional or global climate events," according to a 2002 National Academy of Sciences panel on abrupt climate change, chaired by Alley, the University of Pennsylvania researcher.
* Melting and rainfall runoff have caused a "sustained and widespread" addition of fresh water to the North Atlantic ocean during the last 40 years, according to research published in 2002 and 2003. However, the rate of melting in Greenland is much less than the 8.2K event, and there is no evidence yet of a significant slowdown in the great ocean conveyor, although the report's authors said the North Atlantic could reach a critical threshold in a century.
* Even if melting does eventually disrupt the ocean currents that warm the North Atlantic, continued atmospheric heat buildup could blunt the effects. "If we wait 100, 200 years until we melt enough of Greenland to freshen the North Atlantic so that it could freeze," said Alley, "it may not freeze anyway because it's too hot" by then.
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