The ability of the world's oceans to absorb carbon dioxide, thus tempering the greenhouse effect, is weakening and so more CO2 emissions will remain in the atmosphere to warm up Earth, according to a study published Thursday in the journal Science.
Focusing on the Southern Ocean around Antarctica, the study found that that body of water is so full of carbon dioxide that it can barely absorb any more.
Since 1981 the Southern Ocean has been taking up 5 to 30 percent less carbon dioxide per decade than researchers had earlier predicted, the study found. Global CO2 emissions increased by 40 percent during that time.
Human activity is the main culprit, said lead researcher Corinne Le Quere, who called the finding "very alarming."
The phenomenon wasn’t expected to be apparent until 2050 or so, said Le Quere, a researcher at Britain's East Anglia University.
The Southern Ocean is the single largest "sink" for atmospheric carbon, capturing about 15 percent of it. When carbon is in a sink — whether it’s an ocean or a forest, both of which can lock up carbon dioxide — it stays out of the atmosphere and does not contribute to global warming.
Carbon whipped up by winds
Increased winds over the last half-century are to blame for the change, Le Quere said. These winds blend the carbon dioxide throughout the Southern Ocean, mixing the naturally occurring carbon that usually stays deep down with the human-caused carbon.
When natural carbon is brought up to the surface by the winds, it is harder for the Southern Ocean to accommodate more human-generated carbon, which comes from factories, coal-fired power plants and petroleum-powered motor vehicle exhaust.
The winds themselves are caused by two separate human factors.
First, the human-spawned ozone depletion in the upper atmosphere over the Southern Ocean has created large changes in temperature throughout the atmosphere, Le Quere said.
Second, the uneven nature of global warming has produced higher temperatures in the northern parts of the world than in the south, which has also made the winds accelerate in the Southern Ocean.
"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," Le Quere said. "This is serious. All climate models predict that this kind of ‘feedback’ will continue and intensify during this century."
'More CO2 will stay in atmosphere'
"The Earth’s carbon sinks – of which the Southern Ocean accounts for 15 percent – absorb about half of all human carbon emissions," she added. "With the Southern Ocean reaching its saturation point, more CO2 will stay in our atmosphere."
Other researchers participating in the study were from the British Antarctic Survey and the Max-Planck Institute for Biogeochemistry in Jena, Germany.
Chris Rapley, director of the British Antarctic Survey, added that "the possibility that in a warmer world the Southern Ocean — the strongest ocean sink — is weakening is a cause for concern."
Another sign of warming in the Antarctic was reported on Tuesday by NASA, which found vast areas of snow melted on the southern continent in 2005 in a process that may accelerate invisible melting deep beneath the surface.