CO2 sponge losing ability to soak up extra emissions

James Randerson

Climate change "feedback effect" in the Southern Ocean.

ONE OF the most important of the natural sponges that soak up carbon dioxide in the atmosphere is working 30 per cent less efficiently than a quarter of a century ago, researchers say.

The Southern Ocean is responsible for soaking up the annual CO2 contribution of the United Kingdom and the Netherlands combined, but the study shows that the ocean is absorbing the same quantity of the gas as it was 24 years ago. Scientists had expected that the amount of CO2 absorbed would increase in line with rising levels in the atmosphere. The change is due to increased winds over the ocean linked to climate change and the depleted ozone layer.

"This is serious. All climate models predict that this kind of `feedback' will continue and intensify during this century," said Corinne Le Quere, part of the team at the University of East Anglia, Norwich, which carried out the study. "This is the first time that we've been able to say that climate change itself is responsible for the saturation of the Southern Ocean sink."

So-called carbon sinks such as the oceans, vegetation, and soils soak up around half of the extra CO2 we are pumping into the atmosphere each year — some 9.3 billion tonnes. The Southern Ocean alone is responsible for parcelling up 0.7 billion tonnes a year and storing it in the deep.

"Since the 1980s the Southern Ocean sink for carbon has not changed at all, although CO2 emissions over the same period have increased by 40 per cent," said Professor Le Quere. "We would expect that as the emissions and the CO2 in the atmosphere have increased, the Southern Ocean sink should also increase."

The team looked at measurements of CO2 in the atmosphere from 40 stations around the globe — 11 around the Southern Ocean itself. They calculate that the efficiency of the Southern Ocean as a carbon sink has dropped by 30 per cent compared with the amount it would be putting away if it had kept pace with levels of CO2.

The team used CO2 measurements in water collected during research cruises in the Southern Ocean in 1998, 2000, 2001, and 2002 to check their calculations.

The scientists believe the reason for the change is an increase in average wind speed across the ocean.

"The winds act to mix the oceans. So when you have strong winds you have more water circulation and more mixing," said Professor Le Quere. More mixing brings colder water up from the depths that is saturated with CO2, so that it cannot accept any more from the atmosphere. The increase in wind speed is partly a consequence of climate change itself — one of a handful of "positive feedback" effects that look likely to accelerate global warming. Because the world is warming unevenly, pressure differences between different regions are increasing. One consequence of this is increasing wind speed in the Southern Ocean. The depletion of the ozone layer has also contributed to higher winds.

"Since the beginning of the industrial revolution the world's oceans have absorbed about a quarter of the amount of carbon emitted into the atmosphere by humans. The possibility that in a warmer world the Southern Ocean — the strongest ocean sink — is weakening is a cause for concern," said Chris Rapley, director of the British Antarctic Survey. —

© Guardian Newspapers Limited 2006