Fears for Southern Ocean, the 'world's most important carbon sink', as the planet heats

Intense winds in the Southern Ocean drive the turnover of water to the surface, absorbing C02 from the air and carrying it to the depths of the sea. Photo: AFP

The Southern Ocean absorbs 10 percent of humanity's greenhouse gases, but there are fears that could change as the planet heats.

Depending which way it goes, a shift in the ocean's balance could buy slightly more time in the fight to cut greenhouse gases - or shorten the runway.

New Zealand scientists say they are getting closer to helping solve the mystery by improving how they model Southern Ocean wind.

Intense winds drive the turnover of water to the surface, creating a carbon sink by absorbing CO2 from the air and carrying it to the depths of the ocean.

But these wind is also one of the reasons why the ocean is inhospitable to those trying to measure its air and climate.

That lack of measurements has contributed to gaps in the models scientists rely on to tell them whether the carbon sink will survive, or falter.

If the winds move in the wrong direction, they could drive deep water (already rich in carbon) to the surface, which releases CO2 - worsening, rather than helping, climate change.

GNS principal scientist Jocelyn Turnbull said the Southern Ocean had done humanity an enormous favour.

"The Southern Ocean, most people would argue, is the world's most important carbon sink."

However, its carbon uptake had fluctuated over the past 30 years and no one was quite sure why, Turnbull said.

Her team had records going back 30 years from tree rings, taken from the ocean's rare land masses, as well as air samples going back nine years.

Niwa's Erik Behrens said the "pump" action churning up deep ocean water was already happening in parts of the ocean. As for which way the balance would shift, "we simply don't know".

Turnbull said until recently, all air samples had to be collected by hand, using flasks around the size of a two-litre Coke bottle.

As well as a Niwa research vessel, her research uses volunteers on navy vessels, fishing boats and adventurous cruise ships to see how much carbon is in the air, indicating how much is being absorbed by the ocean.

But Behrens said people seldom got measurements of any kind in winter, when there was no reason for ships to go and the sea was covered in ice. Winter was also when the winds were strongest.

However, Turnbull said scientists recently made an "exciting" finding, when they increased the wind speed in their modelling of the ocean.

As they suspected, this delivered results much closer to reality, a discovery Dr Turnbull had been sharing with other researchers.

Together with other advances - for example, work modelling ocean eddies - the researchers said they were finally getting closer to understanding the Southern Ocean.