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New evidence from deep-sea cores shows that the earth's climate cools significantly and abruptly in a naturally occurring 1,000- to 3,000-year cycle, a scientist at Columbia University's Lamont-Doherty Earth Observatory reported today.
The evidence shows that abrupt coolings occurred not only during the ice ages, but also during the current warmer period - long after most ice sheets disappeared and conditions on earth more closely resembled today's. Regularly spaced layers of rocky fragments in ocean sediments revealed the rapid cooling cycle, the most recent yet discovered. The findings were reported at the American Geophysical Union's fall meeting in San Francisco by Gerard Bond, a paleoclimatologist at Lamont-Doherty, Columbia's earth sciences research institute in Palisades, N.Y.
The discovery of warming and cooling cycles in the modern era adds a new factor in predicting future global climate change. And it throws new light on historical events, such as the Little Ice Age, a cold spell that gripped the world several hundred years ago. It may even have some bearing on the Neolithic hunter called the Ice Man, whose 5,200-year-old frozen remains were discovered recently in the Alps.
"If this is indeed a regular climate rhythm, it is still going on today," Dr. Bond said. "By understanding what causes these sudden climate change cycles, we could more reliably predict how the earth's climate system could shift in the near future. Because we now think that climate flips can occur on an earth relatively free of ice, the odds of a future climate jolt could be higher than we thought.
"The evidence is growing that climate in the post-ice age world is not as stable and is more variable than once thought," Dr. Bond said at an AGU session highlighting abrupt climate change during the Holocene era - the past 10,500 years after the last ice age ended and human civilization began to flourish. "The abrupt coolings in the Holocene are not as great as those that occurred during the ice ages, but still might be significant enough to cause severe winters, agricultural disruptions, and other adverse impacts on people."
The abrupt coolings occurred within 200 years, based on the layers of rock fragments that had been transported by glacial icebergs and sea ice to the North Atlantic, deposited on the seafloor and buried by subsequent sediments. At times of coolings, the amounts of rock fragments doubled or tripled in the ocean sediments. Also, different types of fragments suddenly appeared, indicating an increase in ice from several sources, including Iceland and perhaps Greenland, northern Canada and Svalbard, an island in the Arctic Ocean. The regularly spaced layers of ice-delivered debris showed that the amount of floating ice increased suddenly every 1,000 to 3,000 years. Dr. Bond dated the peaks of ice-delivered debris at about 12,300; 10,800; 8,000; 5,700; 3,900; 2,750 and 800 years ago.
The coolings dropped average temperatures in the North Atlantic region within 200 years or less. They stayed cold for several hundred years, then warmed again as quickly as they cooled, he said. The most recent of these cooling cycles might prove to be the Little Ice Age, which began sometime around 1100 A.D. and peaked a few hundred years later. During the Little Ice Age, glaciers in the Alps, Alaska, New Zealand and Sweden advanced well beyond their present limits, according to Ice Ages, a book by John and Katherine Imbrie. Snow blanketed Ethiopia's high mountains, where it is now unknown. Global climate was generally 2(F cooler than now. Europe suffered severe winters, as did North American colonists. The legendary winter Washington camped at Valley Forge was mild compared to others around the same time. To the north, New York harbor was frozen solid and people could walk from Staten Island to Manhattan.
Superimposing these cycles on human history, Dr. Bond noted that the Ice Man died "in the middle of one of the longest intervals in which ice debris was low and temperatures were presumably warm. Perhaps the now inhospitable and dangerous alpine region in which the Ice Man was found in 1991 was less so 5,200 years ago when he died. It's interesting to speculate, though he's only one sample."
In 1988, a German scientist named Hartmut Heinrich found large amounts of iceberg debris appearing in deep-sea cores every 7,000 to 10,000 years during the last ice age. These Heinrich events involved the huge Laurentide Ice Sheet that covered North America, which discharged armadas of icebergs into the North Atlantic. In January, Dr. Bond reported evidence of smaller iceberg advances, involving other, smaller glaciers, every 2,000 to 3,000 years during the ice age.
The amount of ice-delivered debris in deep-sea cores was far lower during the warmer, relatively ice-free Holocene than during the ice age. But by looking more closely, Dr. Bond found telltale, periodic layers that continued after most glaciers waned. The new evidence points to an underlying, persistent climate cycle that operates on earth whether or not large ice sheets exist.
The increase in floating ice may stem from cooler air temperatures that cause glaciers to advance and sea ice to spread. Or cooler ocean temperatures may allow more ice to survive long transits before they melt. The melting ice, in turn, may add freshwater to the North Atlantic and disrupt a delicately balanced global ocean circulation system that now keeps Europe warmer than North America, Dr. Bond said.
His research was supported by the National Science Foundation and the National Oceanic and Atmospheric Administration.