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The Rising Tide of Acid

17 February 2007 (All day)
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NOAA/Ross Hopcroft

Reflux.
Acid waters would dissolve the shell of creatures such as this swimming mollusk, Limacina helicina.

SAN FRANCISCO, CALIFORNIA--Drop a tooth in a can of carbonated soda, and it will eventually dissolve. That's because the carbon dioxide that makes the beverage bubbly also makes it acidic. The same thing is happening with the world's oceans as they take up CO2 released by the burning of fossil fuels. A research cruise through the Pacific Ocean has revealed just how much more acidic the ocean has become over the last 15 years, confirming the predictions of computer models.

It's not news that carbon dioxide poses a threat to some sea life. In recent years, researchers have shown through computer models and lab experiments that rising acidity coupled with an increase of dissolved inorganic carbon makes it harder for animals to build calcium carbonate shells and can even dissolve them (ScienceNOW, 28 September 2005). Deep, cold waters are naturally like this, so shelled organisms live in shallower waters, above what is called the aragonite saturation zone. (Aragonite is a kind of calcium carbonate.)

The changes in acidity, or pH, are quite small, so researchers use so-called spectrophotometric procedures that can detect differences down to 0.001 pH units. These methods were first used at sea on a 1991 cruise in the Pacific from Tahiti to Alaska. Last year, researchers retraced their route to see how things had changed.

Acidity was almost exactly the same as before in the deep ocean, 3 to 5 kilometers below the surface. But in the upper 700 meters, the waters had on average become more acidic by 0.025 pH units--matching the prediction of computer models, says Richard Feely, an oceanographer with the U.S. National Oceanic and Atmospheric Administration in Seattle, Washington. He described the findings yesterday here at the annual meeting of the American Association for the Advancement of Science (which publishes ScienceNOW).

In addition, dissolved inorganic carbon had become more prevalent in surface waters over the past 15 years. Factoring this in with the change in acidity, Feely and his colleagues calculate that the aragonite saturation horizon is rising rapidly. In the North Pacific, this danger zone is migrating upward through the ocean about 1 meter per year. This change is largely due to human-caused increases in C02, Feely says. Off the California coast, the rate is 5 meters per year, higher perhaps because of changes in ocean currents.

"This is a huge change in the ocean," said James Orr of the International Atomic Energy Agency Marine Environmental Laboratory in Monaco, in a related presentation at the meeting. John Guinotte of the Marine Conservation Biology Institute in Bellevue, Washington, who was not involved, agrees. The rate of change is "amazing," he says.

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