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Small hero. This centimeter-sized mollusk, Clio pyramidata, is one of many species of drifting organisms that help store carbon in the deep ocean.

Plugging a Carbon Sink?

First the good news: Researchers report this week in Science that the ocean has sopped up nearly half the carbon dioxide gas that humans have pumped into the air since the 1800s. The less gas in the atmosphere, the less greenhouse warming, according to most scientists. Now, the not-so-good news: The extra gas could eventually sabotage the ocean's ability to continue to soak up CO2.

To understand how the ocean squirrels away carbon, consider the life of plankton. These small organisms spend their days drifting on the ocean currents. Some photosynthesize, drawing carbon out of the atmosphere. Many build elaborate shells of calcium carbonate, which also ties up carbon compounds. Eventually, these organisms die and fall into the deep ocean. Some settle into the siliceous ooze that blankets the abyssal plains, effectively dropping out of the carbon cycle. The rest dissolve in the low-calcium carbonate conditions that characterize deep waters. In either case, the carbon is not likely to see the atmosphere again for a long time.

But shell-making is a sensitive business. In the last few years, researchers have reported that even small decreases in the amount of calcium carbonate available in seawater can hinder the ability of small organisms--and larger corals as well--to construct skeletons.

Now, scientists report that the amount of human-produced carbon dioxide being absorbed into the ocean is large enough to make the surface ocean more acidic, which reduces the amount of calcium carbonate available to the organisms in the upper ocean. And as the human-produced carbon dioxide works its way down into the ocean, it appears to accelerate the dissolution of calcium carbonate.

To uncover these trends, oceanographer Richard Feely of the Pacific Marine Environmental Laboratory in Seattle, and colleagues worldwide, constructed maps of the ocean's carbon chemistry. To compare these results with what the ocean would have looked like before the industrial revolution, they subtracted out the carbon from fossil fuels, which has a characteristic isotopic signature. The analysis, reported in the 16 July issue of Science, shows that where the human-derived carbon dioxide has moved deep enough, the layer of carbonate-dissolving water is now as much as 200 meters closer to the surface. If sinking shells dissolve in this shallower water, their carbon could possibly return to the atmosphere. If these trends continue over the next few centuries, the researchers suggest, the ability of organisms to produce shells could be compromised.

The paper drives home the point that the ocean may become less effective in the coming decades as a sink for human-produced carbon dioxide, says oceanographer Paul Falkowski. Previous work, including his own, has pointed in that direction, he says. "I fear that the authors of this study have confirmed that in spades."

Related sites
The source of the data: The Joint Global Ocean Flux Study--Studying the Carbon Cycle in the Ocean
The Carbon Dioxide program at the Pacific Marine Environmental Laboratory