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How air imperils the sea

Rising levels of carbon dioxide make oceans more acidic, putting shellfish, corals, and more at risk.

By Staff writer of The Christian Science Monitor / November 18, 2008

Corals must extract calcium carbonate from seawater. Rising acidity interferes with this process.

Reuters/File

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If the rising level of carbon dioxide in the earth’s atmosphere is a slowly ticking time bomb, some scientists say, the CO2 building in seawater is a depth charge about to explode.

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The world’s oceans are growing more acidic at an increasing – and some say alarming – rate. More and more environmentalists and scientists are saying it may take a severe lowering of CO2 levels to keep ocean life from facing major disruptions, including possible mass extinctions of species.

Seawater absorbs carbon dioxide from the air. But the huge amounts oceans have taken in since the Industrial Revolution began 250 years ago are beginning to make it more acidic.

That, in turn, is beginning to stress aquatic life. The species most at risk are those that use calcium carbonate to form protective shells or other coverings – corals, lobsters, oysters, crabs, mussels, and snails. These species find it more difficult to construct their calcium crusts in more acidic waters.

Other less visible, but equally important, species could be affected, too. Tiny creatures called pteropods, whose shells also are made from calcium carbonate, serve as food for larger species that are caught and consumed by humans. The consequences if pteropods diminish or die out could be dramatic.

Seawater already has dropped in pH, the measure of acidity, by a notable amount in the last couple of centuries, researchers say. And the pace of change is quickening: pH could drop significantly more in coming decades, they warn.

If humans continue to release carbon in the way that we have, “we will be looking at a massive extinction of corals in this century,” says Jacqueline Savitz, a marine biologist and coauthor of a study on ocean acidification released last week by Oceana, an ocean advocacy group.

Coral reefs do much more than dazzle divers who explore their beauty, though their monetary value as tourist attractions is significant. Reefs have “a lot of hidden economic value,” Dr. Savitz says. They provide vital habitat for a number of commercially valuable species. They provide barriers that act as storm protection. And they’ve been shown to be the source of medically useful substances.

Acidification is expected to add to a number of other stresses on coral reefs, including: warming ocean temperatures (which cause coral bleaching); pollution; and overfishing.

Lower ocean pH is also allowing sound waves to travel farther underwater. That’s bad news for marine mammals such as whales and dolphins, which rely on emitting and hearing sounds to hunt and communicate, says a recent study from the Monterey Bay Aquarium Research Institute in northern California.

By 2050, the report estimates, underwater sounds will travel about 70 percent farther than today.

To prevent dangerous acidification, countries must lower CO2 levels in the air from about 385 parts per million (ppm) today to 350 ppm, according to the Oceana report, entitled “Acid test: Can we save our oceans from CO2?” To do that, industrialized countries will have to cut carbon emissions by at least 25 percent by 2020 and 80 percent by 2050, in line with the recommendations from the Intergovernmental Panel on Climate Change, which won a 2007 Nobel Peace Prize for its climate research, the Oceana report says.

“If we don’t make major strides [in reducing atmospheric CO2] in the next few years, we’ll never do what we need to do by 2050,” Savitz says. Serious efforts to conserve energy would provide an effective starting point, she says.

In the United States, the National Oceanic and Atmospheric Administration (NOAA) and the National Science Foundation will conduct a study over the next 18 months to see how ocean acidification affects fisheries, marine mammals, coral reefs, and other natural resources. “These emissions are being absorbed into the oceans with potentially catastrophic effects,” says Dr. Steven Murawski, chief science adviser to NOAA. Vulnerable species represent about $2 billion in annual catch, about half the value of the total annual catch in US waters, NOAA says.

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