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Barnard Professor Julian Sachs

Wide swings in ocean temperature during the last ice age extended well south of the polar and subpolar Atlantic region and all the way into the warm, subtropical ocean, a new study demonstrates, suggesting that the effect of future global warming may extend farther south than some previous predictions.

The study, published in the Oct. 22 issue of Science, was undertaken by Julian Sachs, assistant professor of environmental science at Barnard College, and Scott J. Lehman, associate research professor of geological sciences at the Institute of Arctic and Alpine Research at the University of Colorado, Boulder. The two examined sediments recovered northeast of Bermuda from under more than two miles of water.

Measurements in Greenland ice cores by Pieter Grootes and coworkers at the University of Washington over the last decade documented large, rapid swings in air temperature during the last glacial period. Similar swings in polar and subpolar sea temperatures have been inferred from the distribution of micro-fossil shells in those sediments. But Sachs and Lehman are the first to demonstrate that dramatic temperature changes of up to 5 degrees Celsius (or 9 degrees Fahrenheit) occurred not only in the north, but well into the warm ocean, during the period 60,000 to 30,000 years ago.

“It has been known for the better part of a decade that Greenland and the polar Atlantic region experienced ocean-driven flip flops in temperature every few thousand years during the last glacial period approximately 80,000 to 100,000 years ago,” said Sachs and Lehman. “What is new here is the clear evidence that, like the polar Atlantic, the warm Atlantic was also undergoing related, very large, and very rapid – in terms of degree per decade-temperature changes.”

Where climate models predict subtropical sea temperature change of up to 5 degrees Celsius between maximum glacial and warm interglacial (i.e., modern) conditions, a period of 10,000 years, the large, rapid temperature swings the two scientists observed over a scant 250 years can probably only be caused by disruption or even halting of the North Atlantic conveyor-like circulation, a concept pioneered by Columbia’s Wallace Broecker, Newberry Professor of Earth and Environmental Sciences.

The circulation of the North Atlantic conveyor transports warm, tropical water north to the polar areas, via the Gulfstream and North Atlantic Drift currents. Once north, the salty warm water cools and then sinks to the bottom of the ocean, a process that draws more warm surface water from the south.

“This north-south conveyor is what keeps northern Europe far warmer than the Canadian provinces at the same latitude-in short, what keeps London from having a climate like Newfoundland,” said Sachs.

But if the salty water does not become cold enough to sink, due to global warming, or is diluted with too much freshwater, the North Atlantic conveyor halts. This appears to have happened repeatedly throughout the time period studied by Sachs and Lehman, since no other mechanism appears capable of producing the large, sudden temperature swings they document.

Most numerical models used to predict the climate response to increased greenhouse gas concentration do not predict the large temperature changes of the warm ocean documented by the two researchers, suggesting that those models may have to be altered.

“Most climate models developed over the past 10-15 years suggest the effects of a shutdown of the North Atlantic’s conveyor-like circulation, such as that due to global warming, will be localized in the far northeast Atlantic-Iceland and Scandinavia,” said Sachs. “Our data suggests the footprint may be much larger.”

“The fact that we observed such large temperature fluctuations in connection with changes in ocean circulation documented by Lloyd Keigwin, senior scientist at the Woods Hole Oceanographic Institute, and Ed Boyle, professor of chemical oceanography, at MIT, suggests future climate changes may not only be severe for Northern Europe but could affect more southerly latitudes,” Sachs added.

Observed Keigwin, “Because the climate system can respond this quickly means it could respond this quickly to man’s influence and it may respond unpredictably.”

While no single study will send climate modelers back to the drawing board, said Keigwin, “the kinds of changes they see may be greater than what the models predicted and that may lead to some recalculation.”

One caution, according to Sachs, is that the climate system today may have a different sensitivity than it did 30-60 thousand years ago, when ice sheets, solar radiation receipts and greenhouse gas concentrations were different.