Published online 22 July 2010 | Nature | doi:10.1038/news.2010.372

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Loss of sea ice is unlikely to enable Arctic waters to mop up more carbon dioxide from the air.

Parts of the Arctic Ocean have carbon dioxide concentrations approaching atmospheric levels. Zhongyong Gao / State Ocean Administration of China -Third Institute of Oceanography, Xiamen, China

As climate scientists watched the Arctic's sea-ice cover shrink year after year, they thought there might be a silver lining: an ice-free Arctic Ocean could soak up large amounts of CO 2 from the atmosphere, slowing down the accumulation of greenhouse gases and climate change.

But research published in Science today suggests that part of the Arctic Ocean has already mopped up so much CO 2 that it could have almost reached its limit1. Wei-Jun Cai, a biogeochemist at the University of Georgia in Athens and an international team sampled the amount of CO 2 in the surface waters of the Canada Basin, in the western Arctic Ocean. "We found that ice-free basin areas had rather high CO 2 values that approached atmospheric levels," says Cai. "It was not expected."

Although the Arctic Ocean accounts for only 3% of the world's ocean surface area and is mostly covered in ice, it takes up 5-14% of all the CO 2 absorbed by the planet's oceans. It tends to take in proportionately more CO 2 because gases dissolve more easily in cold water.

Scientists had previously thought that open water would promote the exchange of CO 2 between the air and the ocean and that the increase in light reaching the water would also trigger the microscopic ocean plants called phytoplankton to transfer more CO 2 from the atmosphere to the ocean through photosynthesis2.

But that "prediction was based on observations of either highly productive ocean margins or ice-covered basins prior to a major ice retreat," says Cai. Very few scientists had surveyed CO 2 concentrations in offshore waters.

Low productivity

During a research cruise aboard the Chinese icebreaker Xuelong (Snow Dragon) in summer 2008, Cai and his colleagues took continuous measurements of CO 2 concentrations in the upper layers of the Canada Basin (the Arctic Ocean sector bordering the northern Alaskan coast and northern Canada), where sea ice had melted dramatically and retreated to a near-record low. At the ocean margins, where deep water meets the continental shelf, the partial pressure of CO 2 (a measure of its concentration) ranged from 120 to 250 microatmospheres, well below its atmospheric concentration of 375 microatmospheres. But in the ice-free areas further offshore, CO 2 concentration was 320-365 microatmospheres, nearly matching atmospheric concentrations. In 1994 and 1999, scientists had observed surface water CO 2 concentrations of below 260 and 260-300 microatmospheres, respectively, in these areas.

Cai and his colleagues also found that primary production — the removal of CO 2 from the atmosphere by phytoplankton — was almost negligible, something they attributed to low nutrient levels.

"The results come as no surprise to me," says Jean-Éric Tremblay, a biological oceanographer at Laval University in Quebec, Canada, who has studied primary production in the Beaufort Sea since 2002. "We need to stop thinking of these systems as light-deprived. When the seasonal ice melts in the spring, what really limits phytoplankton growth is nutrients. This paper demonstrates that very well."

Chinese interest

But there is some hesitancy over the conclusion that productivity will disappear when all the summer ice is gone.

"There is so much seasonal and spatial variability in the concentration of CO 2 in the Arctic that it is really difficult, if not absolutely impossible, to assess long-term trends from a few snapshots," says Bob Anderson, a geochemist at the Lamont-Doherty Earth Observatory at Columbia University in New York. "I'd like to know whether biological productivity was high in the central basin the month before they were there."

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Tremblay warns that what Cai and his colleagues have observed in the Canada Basin may not be true for inshore waters, where winds bring nutrients up from the deep ocean to the shelf. "These shelves occupy 70% of the whole surface area of the Arctic Ocean. What happens offshore is not necessarily the main driver of the overall CO 2 budget," he says. "I wouldn't say that the intake capacity of the Arctic Ocean in terms of CO 2 is at its capacity. There is one missing component in that story and that is what happens on those productive shelves."

The paper may also mark the maturing of Chinese climate science; it is one of the first high-impact climate studies from China and the research was partly supported by the Chinese government. "The Chinese are putting a lot of resources into catching up and eventually passing the West in terms of ocean research," says Anderson.

"This is the first time in the ocean sciences the Chinese have come out that strong," says Tremblay. "It can definitely be taken as a scientific awakening."