The Southern Ocean plays a major role in absorbing atmospheric carbon dioxide (CO2) and locking it away in the cold depths, acting as one of the world’s most important carbon sinks. But during the 1990s, a decrease in uptake was observed, suggesting its carbon-sucking abilities were stagnating. Two new studies have now, however, shown that the Southern Ocean seems to have bounced back, reviving its ability to absorb more greenhouse gasses.
“Although it comprises only 26 percent of the total ocean area, the Southern Ocean has absorbed nearly 40 percent of all anthropogenic carbon dioxide taken up by the global oceans up to the present,” explains David Munro, a scientist at the Institute of Arctic and Alpine Research at the University of Colorado Boulder, and who authored the paper published in Geophysical Research Letters.
By sampling the atmosphere and surface ocean in a region between the tip of South America and the tip of the Antarctic Peninsula known as the Drake Passage, they were able to build up a picture of how carbon absorption has changed over more than a decade. They did this by measuring the amount of CO2 in the surface waters. This long-term data set was only made possible by piggybacking instruments on the supply vessel that transports people and goods to the Antarctic research stations, making 20 crossings a year. The data collected shows an increase of CO2 in these upper layers of the ocean, suggesting a strengthening of the ocean's ability to act as a carbon sink.
The second study, published in Science, took a wider look at how the changes have occurred across the whole of the Southern Ocean. They found that after the stagnation during the '90s, the ocean began to revive around 2002. The researchers suspect that the Southern Ocean carbon sink fluctuates, possibly in a periodic cycle, rather than changing in response to atmospheric CO2.
The scientists think that this cycle might be driven by changes in weather patterns. An alteration in the areas of high and low pressure, over the Atlantic and Pacific oceans respectively, has caused wind patterns to shift. In addition to this, changes in surface temperature are also probably playing a role. In the Atlantic sector of the Southern Ocean, these changes in wind pattern mean that there is less upwelling of deep CO2-rich waters, allowing that carbon to stay locked at depth. In the Pacific sector, on the other hand, an increase in surface temperature has meant that more carbon can be absorbed by the water.
The data so far doesn’t allow for predictions on how the Southern Ocean as a carbon sink might change in the future. As Nicolas Gruber, one of the authors of the Science paper, points out, “current models are not able to reproduce the observed variations,” so they certainly won’t be able to predict future ones. That means our best bet is to continue with these long-term data sets and to continue monitoring how the oceans change over time.