Southern Ocean changing but still major CO2 sink: study

SINGAPORE (Reuters) - The Southern Ocean has proved more resilient to global warming than previously thought and remains a major store of mankind's planet-warming carbon dioxide, a study has found.

Oceans absorb a large portion of the extra CO2 released by mankind through burning fossil fuels or deforestation, acting as a brake on climate change, and the Southern Ocean is the largest of these "carbon sinks."

Previous research has suggested the vast ocean between Australia and Antarctica was losing its potency because climate change had affected its currents and increased powerful westerly winds.

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The latest study compares ship-based measurements of the ocean since the 1960s and more recent data from hundreds of robotic floats. The analysis shows the Southern Ocean has maintained its ability to soak up excess carbon despite changes to currents and wind speeds.

"It's a positive thing. It's one thing it looks like we don't have to worry about as much as we thought," said Steve Rintoul of the Center for Australian Weather and Climate Research, part of a team researchers that also included scientists from the Institute for Marine Research at the University of Kiel in Germany.

Rintoul said the data showed, as had earlier studies, the Southern Ocean was becoming warmer, and also fresher. The study was published this week in Nature Geoscience.

He said with data on salinity and temperature, the team could measure density of sea water and how that density changed from one place to another in relation to how fast water was moving between two places.

"By looking at the density we could say something about the way the major currents were or were not changing.

"And this was the surprise. We found that the currents had not changed. They had shifted their position, they'd shifted closer to Antarctica but not become stronger or weaker."

GIANT FLYWHEEL

Scientists are closely studying the Southern Ocean for any changes in the Antarctic Circumpolar Current, a vast body of water that runs west to east around the continent from about 40 degrees south and driven in part by powerful westerly winds blowing on the ocean's surface.

The current acts like a giant flywheel for world's weather.

"The current is closer to Antarctica now than it was in the previous decade but it's carrying about the same amount of water," Rintoul told Reuters from Hobart in southern Australia.

Carbon dioxide is absorbed by the Southern Ocean's turbulent surface layer and then carried to the depths by circulation patterns.

It is also absorbed by billions of tiny phytoplankton and other organisms, which fall to the ocean bottom when they die.

Some of the carbon-rich water from the depths rises near Antarctica, releasing CO2, while further away from the continent, it sinks again because it is less dense. Overall, though, the ocean absorbs much more than it releases.

"Our results suggest that that part of the circulation, the upwelling near Antarctica and the sinking further north, has also not changed."

Previous research suggested the faster winds blowing on the surface increased the upwelling of the deep carbon-rich water.

Rintoul said it was hard to tell what would happen to the ocean in the future largely because computer climate models weren't powerful enough to take into account the impacts of small-scale turbulence or eddies.

These help shift the circumpolar current to the south but not change its strength.

He said the same computer models suggested the circumpolar current should have sped up because of the stronger winds and caused more CO2-rich water to upwell from the depths.

"The point of this study is that we don't see that."

(Editing by David Fox)