The Southern Ocean—all of the water immediately surrounding the Antarctic continent, bounded to the north at roughly 30° latitude—is one of the least studied oceanic regions in the world. It also happens to be our oceans’ most powerful carbon sink, accounting for about 40% of the total amount of the greenhouse gas sequestered by the world’s oceans.
The amount of carbon that the Southern Ocean absorbs fluctuates. Its carbon sink seemed to weaken during the 1990s and then appeared to begin strengthening again at the turn of the new millennium until at least 2010. Scientists think this reversal in strength was driven by changes in sea surface temperature and upper-ocean ventilation.
To confirm these trends and find out what was behind them, Ritter et al. used the products of the Surface Ocean pCO2 Mapping (SOCOM) intercomparison project to track carbon dioxide (CO2) trends in the Southern Ocean. SOCOM combines the results from several groups that used two different databases containing millions of ocean surface measurements to map out the distribution of surface ocean CO2.
The researchers mapped and cross compared nine different SOCOM data sets from the Southern Ocean and found strong agreement among them. For example, eight of the nine data sets showed a weakening in carbon sink strength during the 1990s and strengthening during the early 2000s. Also, on average, the results were fairly uniform in terms of location.
Although the team’s results point to a probable cause for the weakening observed during the 1990s (most likely, a southward shift of the dominant westerly winds), the driving force behind the 2000s era behavior is not yet clear. More observations would further improve the results—especially for the 1990s, which has the sparsest data sets—and continue to illuminate this climatically critical region. (Geophysical Research Letters, https://doi.org/10.1002/2017GL074837, 2017)
—Sarah Witman, Freelance Writer