Shifting sink

By Alicia Newton, Nature Reports Climate Change, 15 January 2009

DEMIS

Global Biogeochemical Cycles 22, BG4027 (2008)

Over the past decade, the North Atlantic Ocean has been absorbing less and less atmospheric carbon dioxide. Now scientists report that blame may lie with changes in the region's dominant climate pattern, known as the North Atlantic Oscillation, rather than anthropogenic warming.

The time series of carbon dioxide observations in the North Atlantic extends back only as far as 1995. To reconstruct the patterns of carbon dioxide uptake back through 1979, Helmuth Thomas at Dalhousie University in Halifax, Nova Scotia, and colleagues created a 'hindcast', essentially the opposite of a model forecast, for the region using a complex model of North Atlantic Ocean chemistry and physics. Close correspondence of their model simulations with the few available ten-year observational records gave the researchers increased confidence in the results.

They found that the decrease in CO₂ absorption reported since the mid 1990s coincided with a shift in the North Atlantic Oscillation index, a statistic that describes the location of air pressure anomalies, to more negative values, which altered patterns of circulation within the North Atlantic basin. They suggest that a return to a more positive index in the future could spur increased carbon dioxide uptake in the eastern North Atlantic.

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