Sea change: carbon dioxide imperils marine ecosystems.

Almost half the carbon dioxide produced by human activity in the past 2 centuries is now dissolved in the oceans. It's wreaking chemical changes there that, if unchecked, could threaten the capacity of corals and other marine organisms to make their hard shells and skeletons, scientists say.

From 1800 to 1994, fossil fuel use and industrial processes such as cement production generated some 900 billion metric tons of carbon dioxide. Atmospheric concentrations of the planet-warming greenhouse gas rose during that period from about 280 parts per million to around 360 ppm, says Christopher L. Sabine, an oceanographer at the National Oceanic and Atmospheric Administration (NOAA) in Seattle. He reports that data garnered during more than 95 recent transoceanic research cruises suggest that much carbon dioxide ended up in the oceans as well.

Between 1989 and 1998, seagoing researchers measured the oceans' temperature, pH, salinity, and other aspects of marine chemistry from the water's surface to the seafloor. In a report in the July 16 Science, Sabine and his colleagues estimate that between 1800 and 1994, the world's oceans absorbed about 433 billion metric tons of industrial carbon dioxide.

The threat to shell-making marine life follows from the carbonic acid ([H.sub.2]C[O.sub.3]) that forms when carbon dioxide dissolves in water. In the ocean, much of that acid reacts with carbonate ions in the water to form nonacidic compounds (SN: 8/17/02, p. 104).

Carbonate ions are abundant in most surface waters, and corals and some free-swimming organisms use the material to form their calcium carbonate skeletons or shells, says Richard A. Feely, a marine chemist also at NOAA in Seattle. As ocean chemistry has slowly changed over the past 2 centuries, however, there's been a decrease in the range of depths at which the two most common forms of biomineralized calcium carbonate--aragonite and calcite--can dissolve.

During that period, the lowest depth at which aragonite saturates the water has migrated upward as much as 150 meters in the tropical Atlantic, for example. Below that boundary, the shells and skeletons of marine organisms can dissolve. In parts of the northern Pacific, the lower boundary for calcite saturation is as much as 100 m shallower than it was in 1800. Feely, Sabine, and others report their findings in a second study in the July 16 Science.

Shallower depths of carbonate-ion saturation in the future may be bad news for organisms that use the material to make their hard parts, says Victoria J. Fabry, a biological oceanographer at California State University in San Marcos and a coauthor of one of the studies. In shipboard observations of small, free-swimming snails collected from the North Pacific and placed in closed jars of surface water, she found that as the animals' breathing elevated the concentration of carbon dioxide in the jar, carbonate ions in the water rapidly decreased, thereby inhibiting new growth of the creatures' shells.

Scientists had previously noted similar effects in coral. In large-scale studies on an enclosed artificial reef in Monaco, coral growth dropped as much as 21 percent when researchers boosted the concentration of carbon dioxide in the enclosure to 560 ppm. In a similar experiment in Arizona's Biosphere II near Tucson, doubling the air's carbon dioxide concentration decreased coral growth by 40 percent.

Assessing the full ecological impacts of the carbon dioxide-induced acidification of seawater is an important task for the future, comments Taro Takahashi of Lamont-Doherty Earth Observatory in Palisades, N.Y.