Nov. 14, 1997 Earth's climate cools significantly and abruptly every 1,500 years or so in a persistent, regular rhythm, a team led by scientists at Columbia University's Lamont-Doherty Earth Observatory reports in the Nov. 14 issue of the journal Science.
The newfound naturally occurring climate cycle has continued uninterrupted over at least the past 32,000 years, said the scientists, who believe the last such cycle may have taken place 300 years ago. The periodic sudden cold spells have occurred both when the Earth was covered with massive glaciers during the last ice age and have persisted even after human civilization began to flourish in a relatively warm, ice-free era that scientists had previously thought was resistant to dramatic climate shifts.
The newly discovered cycle appears to be "a pervasive component of the Earth's climate system," the scientists wrote. "The cycle may well be the pacemaker of rapid climate change."
The discovery will prompt a search to explain this recurring, large-scale climate pattern, and finding the cause will provide a fundamentally new understanding of how Earth's climate system can shift abruptly and dramatically, said Gerard Bond, a paleoclimatologist at Lamont-Doherty, Columbia's earth sciences research institute in Palisades, N.Y.
The finding of abrupt climate shifts in the modern era adds an important new factor in predicting future global climate change, he said. And it throws new light on historical events, such as the Little Ice Age, a cold spell that gripped the world in the 17th and 18th centuries and might prove to be the most recent manifestation of the phenomenon.
The scientists re-analyzed sediments cored decades ago from the bottom of the Atlantic Ocean by Lamont's legendary research vessel, the Vema, and preserved in Lamont's deep-sea core repository, which holds the world's largest collection of ocean sediment cores from every ocean in the world. The scientists found evidence that on average, every 1,470 years, plus or minus 500 years, cold, ice-bearing waters, which today circulate around southern Greenland, pushed as far south as Great Britain. The polar waters penetrated a warm North Atlantic current that prevails today, and may have disrupted the global ocean circulation pattern that keeps the North Atlantic region warm. The ocean circulation disruption may well have had far-flung, worldwide effects, they said.
The ocean sediment evidence agrees with chemical clues from Greenland ice cored by other researchers, which show that the air above Greenland cooled in precisely the same pattern. The 1,500-year cold snaps dropped average temperatures throughout the North Atlantic region within a century or two, and probably faster, the scientists said. Temperatures stayed cold for several hundred years, then warmed again as quickly as they cooled.
Reporting the finding with Dr. Bond, the lead author, were: Maziet Cheseby, Rusty Lotti, Peter Almasi, Peter deMenocal, Paul Priorie and Heidi Cullen, all of Lamont-Doherty; William Showers of North Carolina State University; and Irka Hadjas and Georges Bonani of ITP ETH in Zurich, Switzerland.
The scientists analyzed two ocean sediment cores from opposite sides of the North Atlantic, one from off the southwest coast of Greenland and the other more than 600 miles to the south, off the coast of England. The found regularly spaced layers of microscopic rock particles that originated from Greenland and Svalbard, an island in the Arctic Ocean, as well as glass from Icelandic volcanoes. The tiny particles 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 number of particles doubled or tripled in both ocean sediments, indicating that the amount of floating ice increased and extended further south.
At the same time, the scientists also analyzed the sediments for the skeletal remnants of microscopic marine plankton. They found that the abundance of cold-water-loving plants increased and the amount of warmer-water plankton decreased in the same 1,500-year cycle. That indicated that the North Atlantic surface water temperatures dropped as far south as Great Britain.
The increase in floating ice may have resulted from cooler air temperatures that caused glaciers to advance and sea ice to spread. Or cooler ocean temperatures may have allowed more ice to survive long transits before they melted. The melting ice, in turn, may have added fresh water to the North Atlantic and disrupted the delicately balanced global ocean circulation system, known as the Great Ocean Conveyor, which is set in motion by the sinking of denser, salty water in the North Atlantic.
The team was only able to confirm the pattern of abrupt climate shifts to 32,000 years ago, the limit of radiocarbon dating techniques. However, the scientists are currently seeking to learn whether the cycles persisted even before the last ice age began, as far back as the Eemian Period, more than 115,000 years ago, when the Earth's climate was relatively warm like today's.
Dr. Bond, Dr. deMenocal and William B.F. Ryan of Lamont-Doherty will lead an expedition this spring to collect new ocean sediment cores to see if the climate trend has continued to the present. They will use a new coring device that does not ruin the top sections of the sediments, which represent the most recent times, as it is thrust into the ocean floor.
The research was supported by the National Science Foundation and the National Oceanic and Atmospheric Administration. Lamont's Deepsea Sediment Repository is supported by the NSF and the Office of Naval Research. Lamont-Doherty Earth Observatory is part of the Columbia Earth Institute, launched this year to develop innovations for wise stewardship of our planet.
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