Arctic Ocean Little Help In Soaking Up Carbon

By Jesse Emspak: Subscribe to Jesse's RSS feed

August 5, 2010 4:17 PM EDT

Scientists have long thought that the ocean mitigates global warming is by absorbing carbon dioxide from the atmosphere, and that the disappearance of the north polar ice cap would open more water and increase that absorption. But some new research shows that might not be the case.

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Wei Jun Cai, a marine chemist at the University of Georgia, made the trip to the Arctic in the summer of 2008 and studied the Canada basin, which stretches from the northern side of Alaska to the pole. His team found the water there is absorbing carbon dioxide, but it may not do so at the same rate for much longer.

"People speculated that it should be a (Carbon) sink, because with the ice the carbon dioxide concentration is low," he said. But that isn't really the case.

The problem is that as the polar oceans absorb more carbon dioxide, the rate at which they take in additional amounts decreases. That's because the rate of uptake depends on the difference between the amount of carbon dioxide (CO₂) in the air and that in the water. Eventually, the two reach equilibrium and no more is absorbed. The oceans, by some estimates, have absorbed 30 percent of the carbon dioxide that people have put into the air since the industrial revolution.

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Cai said the first studies were done in the zone near the edge of the ice caps, where water mixes relatively well. That means water that has already absorbed some CO₂will be moved by currents and water with less CO₂will come to the surface. This allows for more to be taken up.

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Some ocean scientists had thought that with less ice in the Arctic, there would be more open water that would pick up more carbon dioxide. In addition, the plant life - mostly algae and plankton - would grow more and absorb CO₂via photosynthesis.

The work Cai did, which was a collaborative effort between the University of Georgia and the government of China, was more comprehensive in that the team explored a larger part of the Arctic. And it showed the problem with that scenario is it did not account enough for the density of meltwater from the ice.

In the Arctic the ice is much less salty than the seawater underneath it, so when it melts it forms a layer of less-saline water on top of a denser, saltier one. The fresher water absorbs CO₂, but the rate at which it does so slows as it becomes more saturated with the gas.

That means that even if the polar ice cap were to disappear, it is far from clear that the water would absorb as much carbon dioxide as it has so far. Even if all the ice were to disappear for part of the year - something scientists say could happen in the next fifty to a hundred years - it wouldn't help.

Another issue with putting more carbon dioxide in the oceans is acidification. The CO₂that gets absorbed makes carbonic acid, which lowers the ocean's pH, a measure of acidity. The current pH of the oceans is about 8.1, lower than what it was a century ago, when it was about 8.2. (A difference of 0.1 in pH means a 29% difference in acidity because the scale is logarithmic).

Cai notes that changing the pH of the oceans affects plankton as well, and changes their capacity to absorb carbon dioxide from the air. In addition, the surface water will acidify faster, because in the Arctic it doesn't mix with the water below.

The new measurements from the Arctic underscore how severe the problem of Ocean acidification is, said Carl Safina, a professor of ecology and founder of the Blue Ocean Institute. The problem in the Arctic is that unlike tropical oceans, there aren't as many different species of plants and animals. "The effect is likely complicated," he said. "Generally, larger, more diverse systems are more resilient. Whether, in fact the Arctic is affected more quickly than coral reefs, is an open question."

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