The Southern Ocean’s carbon sink gets stronger

Since 1750 Earth’s oceans have absorbed nearly 30% of anthropogenic carbon dioxide emissions. Although the Southern Ocean—the circumpolar waters surrounding Antarctica—occupies just a quarter of the total ocean area, it’s thought to be responsible for up to half of that uptake (see the article by Adele Morrison, Thomas Frölicher, and Jorge Sarmiento, Physics Today, January 2015, page 27 ). Air–sea fluxes of CO₂ are proportional to the difference Δp in partial pressure of the gas in the atmosphere and in the ocean. In 2007 flux estimates indicated that the Southern Ocean’s carbon sink had weakened in recent decades—a trend attributable to an intensification and southward shift of the westerly winds: The stronger the winds, the greater the upwelling of deep, carbon-rich waters. According to two new studies, the slowdown ended in 2002, and by 2012 the Southern Ocean had regained its expected strength, absorbing about 1.2 petagrams (1.2 × 10¹² kg) of carbon per year. To reach that conclusion, an international collaboration led by ETH Zürich postdoc Peter Landschützer used new statistical methods to interpolate the relatively scarce Southern Ocean ΔpCO₂ measurements in space and time over a 30-year period. The other study, led by University of Colorado Boulder postdoc David Munro, found the same reinvigoration by analyzing a particularly dense time series of Δp measurements through just one region—the Drake Passage, which extends from the tip of South America to West Antarctica. What accounts for the trend reversal isn’t entirely clear. The westerly winds have not weakened, though circulation-driven changes in sea-surface temperatures, which affect CO₂ solubility, is a likely factor. (P. Landschützer et al., Science 349, 1221, 2015 doi:10.1126/science.aab2620 ; D. R. Munro et al., Geophys. Res. Lett., in press, doi:10.1002/2015GL065194 .)