The linked article discusses both the reality and consequences of the changing nature of CO₂ absorption/release and heat uptake by the Southern Ocean. The extracts below only focus on the risks that the Southern Ocean may soon start venting more CO₂ than it is absorbing.
Jeff Tollefson (17 November 2016), "How much longer can Antarctica’s hostile ocean delay global warming?", Nature, Volume: 539, Pages: 346–348, doi:10.1038/539346ahttp://www.nature.com/news/how-much-longer-can-antarctica-s-hostile-ocean-delay-global-warming-1.20978
The waters of the Southern Ocean have absorbed much of the excess heat and carbon generated by humanity.
Although controversial when she encountered it back in 1994, this powerful upwelling is now recognized as a hallmark of the Southern Ocean, a mysterious beast that swirls around Antarctica, driven by the world’s strongest sustained winds. The Southern Ocean absorbs copious amounts of carbon dioxide and heat from the atmosphere, which has slowed the rate of global warming. And its powerful currents drive much of the global ocean circulation.
Already, initial data from an array of ocean floats suggest that upwelling waters could be limiting how much CO2 the Southern Ocean absorbs each year. This raises new questions about how effective these waters will be as a brake on global warming in decades to come.
“The Southern Ocean is doing us a big climate favour at the moment, but it’s not necessarily the case that it will continue doing so in the future,” says Michael Meredith …
Indirect evidence suggests that the Southern Ocean is a net carbon sink and has absorbed as much as 15% of the carbon emissions emitted by humanity since the industrial revolution. But at some times of year and in specific places in this region, carbon-rich surface waters release CO2 into the atmosphere.
Now, researchers are getting some of their first glimpses in near-real time of what happens in the Southern Ocean, particularly in winter. “Right off the bat, we are seeing CO2 fluxes into the atmosphere that are much greater than we had estimated before,” Sarmiento says. “It’s just revolutionary.”
The unpublished analysis is based on just 13 floats that have been in the water for at least a year, so the question now is whether the higher CO2 emissions during winter represent larger trends across the entire Southern Ocean.
“It’s pretty tantalizing,” says Alison Gray, a postdoctoral researcher at Princeton who is leading the study. “It would imply that potentially there is a much weaker carbon sink in the Southern Ocean than has been estimated.”
Hints of something similar have been seen before. In 2007, a team led by Corinne Le Quéré, now director of the Tyndall Centre for Climate Change Research in Norwich, UK, published a study in Science indicating that the rate of carbon uptake by the Southern Ocean decreased between 1981 and 2004. The authors blamed the changes on the winds that encircle the Antarctic continent. The speed of those winds had increased during that time, probably as a result of the hole in the stratospheric ozone layer over Antarctica and possibly because of global warming. Stronger winds are better able to pull up deep, ancient water, which releases CO2 when it reaches the surface. That would have caused a net weakening of the carbon sink.
If that trend were to continue, atmospheric CO2 levels would rise even faster in the future. However, a study in Science last year found that the carbon sink started to strengthen in the early 2000s (see ‘The unreliable sink’).
Le Quéré says it’s unclear whether that rise in CO2 absorption is a return to normal or a deviation from the long-term weakening of the sink. Regardless, she says, it’s now clear that the Southern Ocean might be much more fickle than scientists thought."