Ice age to warming - and back?
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"The Little Ice Age is the only abrupt climate change that people have experienced in industrial times," says Dr. Keigwin.Skip to next paragraph
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Other abrupt changes, like the rapid cooling event that peaked 8,200 years ago, could be far more disruptive.
As scientists have studied the climate record trapped in glacial ice from Antarctica and Greenland, and in mud samples extracted from beneath the ocean floor, their respect for the speed of change has grown. In the mid-1950s, a change of roughly 3 degrees C over more than 1,000 years was deemed abrupt. In 1999, a team led by Jeffrey Severinghaus at the Scripps Institution of Oceanography in La Jolla, Calif., determined that the last ice age ended with a temperature burst that raised the thermostat at Greenland by some 9 degrees C over a mere decade.
"We still don't understand the causes" behind the increase, he says. A range of abrupt regional and global changes poses the same challenge.
One potential source of change may be the North Atlantic, researchers say. There, warm water moves north along the surface, cooling as it travels. By the time the surface water reaches the far northern portions of the North Atlantic basin, it has cooled and grown denser than the underlying layers of ocean, and it begins to sink. The water then travels south along the bottom, driving an aquatic "conveyor belt" that spans the globe.
But researchers suspect that if enough fresh water, perhaps from melting ice, is injected into key spots in the North Atlantic, it can virtually shut down the conveyor. Fresh water is more buoyant than salt water and can form a layer that blocks the circulation. The Northern Atlantic region would then cool. Reduce the fresh water, researchers say, and the reverse can happen, warming the North Atlantic region.
Yet much of the evidence for this comes from records when the planet's climate was already cool, some scientists argue. So the evidence may not hold lessons for today, when the planet is in a warm hiatus between glacial periods. A more timely test of the North Atlantic's role in abrupt climate change, they say, may come through unraveling the mystery of a rapid cooling event that peaked 8,200 years ago.
By the standards of the Little Ice Age, the 8,200-year event was frosty and global. Although the event lasted only about 100 years, Greenland's temperatures dropped by about 3 degrees C. Indeed, the consultants who argue for upgrading the national security status of abrupt climate change used this event as their model.
Several researchers say the cooling 8,200 years ago may have been triggered by the collapse of ice dams holding back the waters of Lake Agassiz - a vast glacial reservoir covering the Great Lakes region of the US and Canada at the end of the last ice age. It would have flushed enough fresh water into the right places in the North Atlantic to shut down the conveyor.
But here, too, the picture grows murky. At a meeting last fall at the WHOI, researchers from the US and Canada looked at the problem and raised more questions than answers. For instance, no one has yet seen conclusive topographical evidence of such a huge outflow. If the waters of Lake Agassiz did surge into the Hudson Straits and the Labrador Sea, they surely would have carved a path in the land. This lack of evidence has sent some searching for an outflow path to the north, into the Arctic Ocean.
Core samples from the ocean floor also fail to confirm the Hudson Straits-Labrador Sea theory, says Mr. Keigwin of the WHOI. Instead, he has found evidence of freshening in the water much farther south and hugging the coast. This suggests that the water may have entered the Atlantic too far south to affect the conveyor - at least directly.
"This is an important problem," he says. If scientists can prove that freshening of the ocean did occur during this current period between ice ages, then the possibility of global warming triggering an abrupt climate change would have to be taken more seriously, he adds. "We need a coordinated effort" to answer the riddle.