Brauer et al., www.nature.com/naturegeoscience (1 August 2008)

Polar express. Microscopic sediments from Lake Meerfelder Maar reveal an abrupt drop in temperatures about 12,700 years ago.

The Year the World Froze Over

It sounds like the stuff of science fiction, but nearly 13 millennia ago Europe was plunged suddenly into a deep freeze that lasted 1300 years--and the change happened in little more than a year, according to new data. The evidence also suggests that strong winds, not ocean currents, drove the rapid climate change.

Soon after the end of the last ice age, some 12,700 years ago, Europe suddenly fell into another perpetual winter. Average temperatures on the continent plunged as much as 5°C, and global temperatures overall dropped about 3°C. This extended cold period is known as the Younger Dryas, and paleoclimatologists studying Greenland ice cores and other evidence have known for some time that it began quickly, perhaps within just decades. But until now no one was sure exactly how rapidly the climate had changed or why. One hypothesis was a sudden shift in--or even a shutoff of--the Atlantic Ocean current that brings warm water from the tropics to Western Europe and helps the present-day continent maintain its relatively moderate temperatures.

Paleoceanographer Gerald Haug of the Geological Institute in Zürich, Switzerland, and colleagues hunted for clues in sediments from Lake Meerfelder Maar in western Germany. Nestled inside an ancient volcanic crater, the lake is the only place known to hold well-preserved sediments that span the Younger Dryas. The fine-grained layers even reveal seasonal changes.

In particular, the team studied deposits of siderite, a mineral formed by microscopic lake plankton during relatively warm months. Using x-ray scanning techniques to examine the deposits, the researchers found that, after several hundred years of seasonal changes, the siderite production stopped abruptly. "It gave us quite a surprise," Haug says. That suggests that the Younger Dryas began within a single year, the researchers report in the August issue of Nature Geoscience.

Moreover, the cessation of siderite formation in the lake was caused by a significant strengthening of the North American westerly winds, the researchers argue. Siderite forms at the lake bottom when the surface is calm and relatively warm and the water lacks oxygen. But strong, cold winds churn up and chill the surface and slightly oxygenate the water. The sedimentary record shows that the churning--and hence the cooling--must have lasted at Lake Meerfelder Maar for a long time. "It's a strong example of how dynamic the [planet's] climate system can be," he says, referring to the speed and duration of the change.

The findings will require the climate community "to question their dogma" that changes in ocean circulation caused the Younger Dryas, says oceanographer Carl Wunsch of the Massachusetts Institute of Technology in Cambridge, who has been a skeptic of ocean-driven climate change. This research suggests instead that the advent of strong winds may have shifted ocean circulation.

Posted in Earth, Europe