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Around 19,000 years ago, oceanic conditions underwent dramatic changes that coincided with a shift in global climate, marking the onset of the Holocene warming. In the North Atlantic, major changes in the Meridional Overturning Circulation (MOC), which carries warm and highly saline surface water north to cooler regions, played a substantial role in regulating climate and levels of atmospheric carbon dioxide. Scientists are now convinced that the ocean absorbed, stored, and released vast quantities of carbon in the past, playing a major role in the end of the last Pleistocene Ice Age glacial period. Understanding the ocean's role in the past is important to understanding how it may influence climate in the future. A new report in Science shows that the MOC experienced a series of abrupt changes that lasted from decades to centuries, and may have stored and released more CO2 than previously thought.

It is beginning to look like the Atlantic ocean's overturning current flow has had a very active and varied history during the last deglaciation, 18,000-10,000 years ago. Two articles in the January 14, 2011, issue of the journal Science explain the latest discoveries about the part ocean circulation in the Atlantic has played in climate change. Reporting in a perspective article, “Northern Meltwater Pulses, CO2, and Changes in Atlantic Convection,” Professor Michael Sarnthein, a researcher in Paleoclimatology and Paleoceanography at the University of Kiel, reviews “impressive and detailed evidence of how the North Atlantic MOC behaved after the Last Glacial Maximum (LGM).” According to Sarnthein, some old oceanographic dogma needs to be changed:

Today, intensive convection in the North Atlantic quickly transfers surface waters to depth, and newly formed deep water typically has a ventilation age of ∼500 years. In contrast, Thornalley et al. report that during the Heinrich stadial 1 (HS1), a cold interval that occurred between about 17,500 and 14,700 years ago, a mass of extremely old water, up to 5200 years old, reached the Atlantic Ocean's northern “dead end.” They suggest this was intermediate water that originated in the Southern Ocean near Antarctica, and its age would once have been considered unreasonable. In addition, they found that the ventilation ages of past surface waters could vary by as much as 2100 years, which agrees with results of other methods and challenges a common, but little-substantiated dogma: that past surface waters had ventilation ages of ∼400 years, similar to a current age average.

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