Oceans to acid

When the team added the carbon stored in the oceans to the carbon stored in the atmosphere, the total exceeded emissions from human activities alone. After carefully reviewing their data and calculations, they concluded that the "extra" CO2 came from changes in land use, such as deforestation. This suggests that during the same period, the planet's terrestrial bio-sphere became a net source of, rather than a sink for, carbon dioxide.

Marine biologists, meanwhile, worry about what happens to that carbon once the oceans take it up.

When carbon dioxide mixes with seawater, it forms a weak carbonic acid. Over millions of years, erosion has supplied the oceans with vast amounts of dissolved calcium from weathered rock on the continents. This provides a natural buffer against the acid, creating chemical conditions to which some key forms of marine life are finely tuned.

Over the past five years, however, evidence has been mounting that rising CO2 levels could pose major challenges to these life forms by altering this balance.

By some measures, rising CO2 levels during the industrial age already have increased the oceans' acidity by roughly 0.1 pH units. By the end of this century, the reduction could reach 0.4 units. That may not sound like much, but researchers point out that each whole-number shift in pH represents a 10-fold change.

Thus, to some marine organisms, a pH shift of 0.4 toward the acid end of the scale could lead to dramatic changes.

By the middle of the next century, for example, coral reefs in shallow waters could lose up to 30 percent of the calcium carbonate they need to build their structures, calculates an international research group led by Joan Kleypas, a marine biologist at the National Center for Atmospheric Research in Boulder, Colo. That could lead to stunted growth or other effects, which could make them more vulnerable to erosion or storm damage.

Also, increased CO2 levels caused key plankton species to create badly formed or incomplete calcium carbonate shells, according to a team led by Ulf Riebesell with the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany. In lab experiments the ratio of shell to the rest of the organism dropped by as much as 52 percent.

"We've only just scratched the surface on this issue," says Victoria Fabry, a biologist at the California State University at San Marcos who specializes in plankton ecology. Preliminary results from her own studies show that when water is "undersaturated" with a certain form of calcium carbonate, the shells of tiny plankton-like creatures start dissolving within 48 hours.

Ironically, chemistry that threatens the organisms can help the atmosphere. As shells and other calcium-carbonate clothing dissolve, it returns minerals to the seawater that can help the oceans soak up more CO2. Research by a team led by Richard Feely, also with NOAA's lab in Seattle, notes up to 60 percent of the calcium carbonate formed each year dissolves in the upper 2,000 meters of the ocean, making it readily available as a buffer. But it also could be reducing the amount of carbon that falls to the deep ocean, where it is cached for centuries, or gets buried in sediments.

The net effect of these processes remains unclear both for the atmosphere and marine ecosystems - leading researchers to undertake more detailed studies. Next year researchers will head to sea to gather more information on the amount of carbon the ocean is taking in to see what changes may be taking place.

"The world ocean is one of the great commons of the human race," says Dr. Raven. "We're all stakeholders, and what we're doing influences it."

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