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Southern ocean carbon sink weakened

May 21, 2007

Scientists have observed the first evidence that the Southern Ocean's ability to absorb the major greenhouse gas, carbon dioxide, has weakened by about 15 per cent per decade since 1981.
18 May 2007

In research published today in Science, an international research team - including CSIRO's Dr Ray Langenfelds - concludes that the Southern Ocean carbon dioxide sink has weakened over the past 25 years and will be less efficient in the future. Such weakening of one of the Earth's major carbon dioxide sinks will lead to higher levels of atmospheric carbon dioxide in the long-term.

Dr Paul Fraser, who leads research into atmospheric greenhouse gases at CSIRO Marine and Atmospheric Research, says the international 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," Dr 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."

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.

"Combined, the Earth's land and ocean sinks absorb about half of all carbon dioxide emissions from human activities," Dr Fraser says. "The Southern Ocean takes up 15 per cent of these emissions, hence a reduction in its efficiency will have serious implications for atmospheric carbon dioxide concentrations over coming decades."

Dr Fraser points to one piece of good news: that 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, but the impact of increasing levels of greenhouse gases will continue unabated."

The international team comprised researchers from 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 US, NIWA in New Zealand, the South African Weather Service, LSCE/IPSL and CNRS in France, and the Centre for Atmospheric and Oceanic Studies in Japan.

The team used observations from 40 stations around the world, including Cape Grim in north-west Tasmania. The Cape Grim station, operated by the Australian Bureau of Meteorology, monitors and studies changes in global atmospheric composition in a program led by CSIRO and the Bureau.

Meanwhile, research to increase understanding and improve management of the oceans will increase following the announcement today by WA Premier, the Hon Alan Carpenter. The Western Australian Marine Science Institution (WAMSI) is a new $A21 million, five-year research collaboration focussing on the marine environment to Australia's west.

CSIRO Australia