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ENVIRONMENT: Southern Ocean Nears CO2 Saturation Point

Stephen Leahy

BROOKLIN, Canada, May 17 2007 (IPS) - Climate change has arrested the Southern Ocean’s ability to absorb greenhouse gases from the atmosphere, researchers announced Thursday.

That will make it more difficult to stabilise carbon dioxide (CO2) levels in the atmosphere and to reduce the risks of extreme forms of global warming.

The Southern Ocean has been absorbing less CO2 from the atmosphere since 1981, even though levels have increased 40 percent due to burning of fossil fuels. Oceans absorb half of all human carbon emissions, but the Southern Ocean is taking up less and less and is reaching its saturation point, reported an international research team in the journal Science.

This is the first evidence of the long-feared positive feedbacks that could rapidly accelerate the rate of climate change, pushing impacts to the extreme end of the scale.

“This is serious. All climate models predict that this kind of feedback will continue and intensify during this century,” said Corinne Le Quere of Britain’s University of East Anglia and the paper’s lead author.

“With the Southern Ocean reaching its saturation point, more CO2 will stay in our atmosphere,” Le Quere said in a statement.

This new research is not reflected in the recent reports from the Intergovernmental Panel on Climate Change (IPCC).

“This finding could make a significant difference in some of the IPCC projections,” said co-author Thomas Conway of the Global Monitoring Division of the U.S. National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado.

“If this study holds up, it means the rate of increase CO2 in the atmosphere will be faster,” Conway told IPS.

The four-year study looked at data from 51 carbon dioxide monitoring stations around the world to tease apart how the Southern Ocean was responding to the increased levels of atmospheric CO2. The Southern Ocean, the fourth largest of the world’s five oceans, was thought to absorb 15 percent of all human emissions, but in fact has absorbed less and less each decade since 1981.

Oceans ought to be able to absorb CO2 for hundreds of years into the future before becoming saturated. “This was something that should not be happening,” Conway said.

So why does it appear to be happening now in the Southern Ocean? The answer is wind.

Since 1958, there has been a documented increase in windiness in the southern hemisphere – the unexpected product of the depletion of the ozone layer and global warming.

“The ozone hole has led to a strong cooling of the Antarctic stratosphere,” Le Quere told IPS.

Emissions of chemicals such as chlorofluorocarbons (CFCs), along with other chlorine- and bromine-containing compounds, have depleted stratospheric ozone levels, resulting in a massive thinning or “ozone hole” over the Antarctic. That in turn produces a cooling 20 to 40 kilometres above the icy surface of the continent, resulting in a strengthening of stratospheric winds.

And that has strengthened westerly winds down to the surface, Le Quere said.

“Greenhouse gases have also changed the temperature structure of the atmosphere, and climate models show that these changes also strengthen the westerly winds over the Southern Ocean,” she noted.

Oceans absorb CO2 at the surface and transport it downwards, eventually storing the gas in the deep ocean. However, the increased winds are churning the Southern Ocean and bringing up the stored CO2 from the deeps up to the surface. As the surface becomes more saturated with CO2, it absorbs less and less from the atmosphere.

Because the Southern Ocean had been absorbing 15 percent of all human emissions, if the reduction continues to accelerate, it could lead to a further increase in atmospheric CO2 by a few tens of parts per million, making stabilisation targets more difficult to reach, Le Quere said.

Current carbon dioxide levels are 383 parts per million (ppm) and climbing at roughly two ppm per year. The IPCC, the European Union and other organisations have said CO2 levels need to be stabilised at 450 ppm to minimise extreme impacts and risks of positive feedbacks. Such feedbacks are the planet’s response to higher CO2 levels and the IPCC has concluded that they will likely be positive – meaning they will enhance the warming.

“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, in statement.

A secondary, but not unimportant aspect of this change in the Southern Ocean is that the surface is becoming more acidic more quickly, because when CO2 combines with seawater, it forms an acid. While this acidification has been documented in all oceans, scientists believed it would not affect marine life until after 2050. But with this latest finding, the impacts could come much sooner in the Southern Ocean.

Other oceans may also be reacting in a similar way and absorbing less CO2, says Le Quere, noting that there is evidence to that effect in the North Atlantic. This means that the climate models the IPCC uses are overestimating how much carbon the oceans are absorbing and underestimating the rate at which CO2 will rise in future.

“We are depending on carbon sinks like the oceans to absorb a huge amount of our emissions,” she said. “This means there is more urgency than ever to reduce our emissions.”

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