Antarctic ocean losing ability to absorb carbon dioxide

Recent climate change brought on by human activities has weakened one of the Earth's natural defenses against global warming. Scientists have observed the first evidence that the Southern Ocean's ability to absorb the major greenhouse gas carbon dioxide, CO2, has weakened by about 15 percent per decade since 1981. The study was published today in the journal Science. "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. This is serious," said lead author Dr. Corinne Le Quere of the University of East Anglia and the British Antarctic Survey. "The Earth's carbon sinks–of which the Southern Ocean accounts for 15 percent–absorb about half of all human carbon emissions. With the Southern Ocean reaching its saturation point, more CO2 will stay in our atmosphere," she said. The international team included researchers from the Commonwealth Scientific & Industrial Research Organization (CSIRO) in Australia, the Max-Planck Institute in Germany, the University of East Anglia and British Antarctic Survey in England, the Climate Monitoring and Diagnostics Laboratory in the United States, New Zealand's National Institute of Water and Atmospheric Research, the South African Weather Service, LSCE/IPSL and CNRS in France, and the Center for Atmospheric and Oceanic Studies in Japan. The saturation of the Southern Ocean was revealed by scrutinizing observations of atmospheric carbon dioxide from 40 stations around the world. The data show that since 1981 the Southern Ocean sink ceased to increase, whereas CO2 emissions increased by 40 percent. The weakening of the Southern Ocean's absorption rates–which could be in the range of five to 30 percent–is likely to result in an increase in the rate at which temperatures rise, scientists say. Dr. Paul Fraser, who leads research into atmospheric greenhouse gases at CSIRO Marine and Atmospheric Research, says the team's four year study concludes that the weakening is due to human activities. "The researchers found that the Southern Ocean is becoming less efficient at absorbing carbon dioxide due to an increase in wind strength over the ocean, resulting from human-induced climate change," Fraser says. "The increase in wind strength is due to a combination of higher levels of greenhouse gases in the atmosphere and long-term ozone depletion in the stratosphere, which previous CSIRO research has shown intensifies storms over the Southern Ocean," he said. The increased winds influence the processes of mixing and upwelling in the ocean, which in turn cause an increased release of carbon dioxide into the atmosphere, reducing the net absorption of carbon dioxide into the ocean, he explained. Fraser points to one piece of good news–ozone levels in the stratosphere have stopped declining and should recover slowly in coming decades. "Thus the impact of ozone depletion on the Southern Ocean carbon dioxide sink will lessen in the future," he said, "but the impact of increasing levels of greenhouse gases will continue unabated." Professor Chris Rapley, director of British Antarctic Survey, said: "Since the beginning of the industrial revolution the world's oceans have absorbed about a quarter of the 500 gigatons 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." He added that the reduced efficiency of the ocean to act as a carbon sink would make it harder to reduce emissions to levels that were low enough to limit temperature rises to 3.6 degrees, the amount where more extreme climate change effects are predicted to begin to take place.