Loss of sea ice is unlikely to enable Arctic waters to mop up more carbon dioxide from the air.
By Hannah Hoag 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 CO2 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 CO2 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 CO2 in the surface waters of the Canada Basin, in the western Arctic Ocean. “We found that ice-free basin areas had rather high CO2 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 CO2 absorbed by the planet’s oceans. It tends to take in proportionately more CO2 because gases dissolve more easily in cold water. Scientists had previously thought that open water would promote the exchange of CO2 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 CO2 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 CO2 concentrations in offshore waters. During a research cruise aboard the Chinese icebreaker Xuelong (Snow Dragon) in summer 2008, Cai and his colleagues took continuous measurements of CO2 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 CO2 (a measure of its concentration) ranged from 120 to 150 micropascals, well below its atmospheric concentration of 375 micropascals. But in the ice-free areas further offshore, CO2 concentration was 320-365 micropascals, nearly matching atmospheric concentrations. In 1994 and 1999, scientists had observed surface water CO2 concentrations of below 260 and 260-300 micropascals, respectively, in these areas. Cai and his colleagues also found that primary production — the removal of CO2 from the atmosphere by phytoplankton — was almost negligible, something they attributed to low nutrient levels. …
Arctic Ocean full up with carbon dioxide
