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Glacial climate swings: It’s in the ocean

A paper this week in Nature (subscription) sheds new light on the causes of pronounced greenhouse-gas and climate fluctuations during glacial times.

The last ice-age, which covered the period from around 110,000 to 10,000 years before now, is famed for a series of climate swings known as Dansgaard-Oeschger events.

Scientists have found evidence in Greenland ice cores for abrupt warming episodes of up to 15 degrees Celsius within few decades, followed by a more gradual cooling. These glacial warm and cold periods swung back and forth between the poles in a kind of thermal seesaw effect, whereby Antarctic temperatures rose when Greenland temperatures dropped, and vice versa.

It has long been assumed that Dansgaard-Oeschger events were triggered by changes in Atlantic ocean-circulation. The new modelling study by Andreas Schmittner and Eric Galbraith now adds new evidence to the idea. Weakening Atlantic meridional overturning circulation, the heat conveyor which carries warm water surface water northwards and cold deep-water back south, is indeed the primary physical mechanism driving glacial climate fluctuations, they conclude. Here’s an editor’s summary.

Schmittner and Galbraith carried out simulations with a coupled model of glacial climate and (simplified) biogeochemical cycles. When they manipulated the Atlantic circulation – artificially ‘switching’ it off and on in their model, that is – the model nicely reproduced the temperature changes typical for Dansgaard-Oeschger events. Remarkably, the model also reproduced reasonably well the ice-core-derived changes in atmospheric concentrations of the greenhouse-gases carbon dioxide and nitrous oxide which accompany such events.

The results confirm once more that the composition of the Earth’s atmosphere, and hence global climate, is intrinsically linked with oceanic processes. As changes in Atlantic circulation reduce nutrient concentration in biologically productive surface waters, the efficiency of the ‘biological pump’ which lowers atmospheric carbon dioxide by extracting and sequestering carbon will also decrease.

But remarkably well though the model performs, the “smoking gun” is still missing, notes Thomas Stocker in his news and views article (subscription)

“Modellers still do not have the all-encompassing climate model that would simulate a series of Dansgaard-Oeschger events in a self-contained way.”

Better data from polar ice cores, in conjunction with innovative climate simulations such as Schmittner and Galbraith’s, are the best way forward, he adds.

“This will be the least prejudiced approach to the problem – and so the best test-bed for our understanding of how oceanic, atmospheric and biogeochemical processes operate and interact in the climate system.”

Quirin Schiermeier


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