Paris - Scientists said on Sunday they had unravelled the mechanism by which Earth-warming carbon is sucked deep into the Southern Ocean to be safely locked away - a process that may itself be threatened by climate change.
Wind, eddies and currents work together to create carbon-sucking funnels, said the research team from Britain and Australia in a discovery that adds to the toolkit of scientists attempting climate warming predictions.
About a quarter of the carbon dioxide (CO2) on Earth is stored away in its oceans - some 40% of that in the Southern Ocean encircling Antarctica.
At a depth of about 1 000m, carbon can be locked away for hundreds to thousands of years, yet scientists had never been sure exactly how it gets there after dissolving into surface waters.
They had suspected the wind was the main force at play, pooling up surface water in some areas and forcing it down into the ocean depths.
Using 10 years of data obtained from small, deep-sea robotic probes, the researchers found that in addition to the wind, eddies - big whirlpool-like phenomena about 100km in diameter on average, also played a part.
"You add the effect of these eddies and the effect of the wind and the effect of prominent currents in the Southern Ocean, you add these three effects, it makes... 1 000km-wide funnels that bring the carbon from the sea surface to the interior," said study author Jean-Baptiste Sallee.
The team had also used temperature, salinity and pressure data collected from ship-based observations since the 1990s.
"This is a very efficient process to bring carbon from the surface to the interior. We found in the Southern Ocean there are five such funnels," said Sallee.
The team also found that the eddies counterbalanced a different effect of strong winds - that of releasing stored carbon by violent mixing of the sea.
"This does seem to be good news, but the thing is what will be the impact of climate change on the eddies? Will they stop, will they intensify? We have no idea," said Sallee.
A changing climate could theoretically affect the nature and effect of the Southern Ocean eddies by changing ocean currents, intensifying winds or creating stark temperature spikes.
The findings mean that eddies must be taken into account in future climate models, said Sallee. They are not, currently.
The study focused on the part of the Southern Ocean south of 35° south latitude.
The team could not say whether the same funnelling process would be at play in other seas, but Sallee said the Southern Ocean was "one of the most energetic places on Earth", and the effect of eddies would likely be larger there than anywhere else.
There is also another carbon capturing process, not covered by this study, of CO2-producing micro organisms that live near the ocean surface sinking to the sea floor and settling there when they die.