Galloping increases in human fossil fuel emissions now appear to be outrunning the ability of the world's oceans to absorb them. The first year-by-year accounting of the oceans' role as a carbon sink shows that, even as they soak up record amounts, the seas are absorbing a smaller proportion of the rising total.
Oceanographer Samar Khatiwala of Columbia University's Lamont-Doherty Earth Observatory, lead author of the report, published in the current issue of the journal Nature, says the oceans appear to be reaching a natural limit.
"The more carbon dioxide you put in, the more acidic the ocean becomes, reducing its ability to hold CO2," he said. "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."
The study, first of its kind, reconstructs the year-by-year accumulation of carbon in the oceans from 1765 to 2008.
Over this period, the oceans have been absorbing about a quarter of the carbon dioxide put in the air by human industrial activity, keeping pace with sharply rising global emissions until 2000. Since then, even as the tonnage continues to climb, the overall percentage of global emissions that the oceans have taken from the atmosphere has fallen by as much as 10 percent.
Khatiwala the new ocean study, along with recent land studies, suggests that "we cannot count on these sinks operating in the future as they have in the past, and keep on subsidizing our ever-growing appetite for fossil fuels.
IMAGE: Samar Khatiwala, Lamont-Doherty Earth Observatory
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