Thursday, November 19, 2009

Ocean uptake of man-made carbon may be slowing

Lamont-Doherty Earth Observatory: The oceans play a key role in regulating climate, absorbing more than a quarter of the carbon dioxide that humans put into the air. Now, the first year-by-year accounting of this mechanism during the industrial era suggests the oceans are struggling to keep up with rising emissions—a finding with potentially wide implications for future climate. The study appears in this week’s issue of the journal Nature, and is expanded upon in a separate website.

The researchers estimate that the oceans last year took up a record 2.3 billion tons of CO2 produced from burning of fossil fuels. But with overall emissions growing rapidly, the proportion of fossil-fuel emissions absorbed by the oceans since 2000 may have declined by as much as 10%.

Some climate models have already predicted such a slowdown in the oceans’ ability to soak up excess carbon from the atmosphere, but this is the first time scientists have actually measured it. Models attribute the change to depletion of ozone in the stratosphere and global warming-induced shifts in winds and ocean circulation. But the new study suggests the slowdown is due to natural chemical and physical limits on the oceans’ ability to absorb carbon—an idea that is now the subject of widespread research by other scientists.

“The more carbon dioxide you put in, the more acidic the ocean becomes, reducing its ability to hold CO2” said the study’s lead author, Samar Khatiwala, an oceanographer at Columbia University’s Lamont-Doherty Earth Observatory. “Because of this chemical effect, over time, the ocean is expected to become a less efficient sink of manmade carbon. The surprise is that we may already be seeing evidence for this, perhaps compounded by the ocean’s slow circulation in the face of accelerating emissions.”…

Carbon released by fossil fuel burning (black) continues to accumulate in the air (red), oceans (blue), and land (green). The oceans take up roughly a quarter of manmade CO2, but evidence suggests they are now taking up a smaller proportion. Credit: Samar Khatiwala, Lamont-Doherty Earth Observatory

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