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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.

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.

Scientists once thought that holding atmospheric CO2 level to 450 ppm was sufficient to ward off the worst effects of global warming. But a number of scientists and environment groups now say new research indicates that 350 ppm is the highest safe level. Even 450 ppm had been seen an ambitious target. The global CO2 level, now at 385 ppm, is growing by about 2 ppm each year.

The ability of CO2 in the air to increase the acidity of ocean water is well-established science, “not controversial,” says Victoria Fabry, a marine biologist and visiting researcher at Scripps Institution of Oceanography at the University of California, San Diego. But the study of the effects of acidification is a relatively young field, developing “in the last decade or so,” Dr. Fabry says.

The next task for researchers is to gain a big-picture view of what is happening across a variety of species and to study how acidification might act in concert with ocean warming and other stresses on marine life, she says.

Suggestions for protecting coral reefs include treating water near reefs to keep the pH level elevated, restricting human visitors, even putting shade cloths over reefs to lower the water temperature. Such solutions would likely be practical only in very limited areas.

“It’s difficult to see how they could be scaled up to have a global impact,” she says.

A controversial remedy

Russ George calls CO2 a “chemical shock treatment” to the world’s oceans. “The tragedy is that people think the oceans might be OK if we just leave them alone,” says the entrepreneur, a former environmental project manager for the government of Canada.

It’s too late to rely on reducing the level of CO2 in the atmosphere to prevent ocean acidification, Mr. George says. Because current CO2 levels will take a century or more to fall, he says, “the amount of CO2 already emitted is a lethal dose.”

His solution is to dump iron dust into the ocean. Iron encourages the growth of algae blooms, which in turn absorb CO2. For every ton of iron added to the ocean, 100,000 tons of carbon are captured and held by algae, George says. (Algal blooms also deplete oxygen in the water, creating other problems.)

George created Planktos Corp., bought a ship, and set about conducting experiments in dumping iron dust, first near the Galapagos Islands and later near the Canary Islands. If it proved effective, his plan was to scale up the process and sell carbon credits to companies looking to offset their CO2 emissions.

Protests from environmental groups scrapped his plans. They saw it as toxic ocean dumping, he says. “It scared the living daylights out of our investors … and we went broke.”

Iron dumping “is fairly controversial,” says marine biologist Victoria Fabry, a visiting researcher at Scripps Institution of Oceanography at the University of California, San Diego. “What you’re doing there is producing more organic matter,” she says. If the algae sink and decompose, the CO2 would be trapped. “You would hope it would sink as far as possible, so that it would be removed from the atmosphere for as long as possible,” she says.

Iron dumping also “seems to have a lot of major questions associated with the side effects and the actual risks that could be created in the ocean ecosystem as a result,” says Jacqueline Savitz, a senior scientist at Oceana, an oceans advocacy group. “They haven’t really been fully answered.”

George has reconstituted his company as Planktos Science and is looking for new investors. He says he’s hopeful he’ll get a chance to conduct his experiment. “We’re undeterred,” he says. “But it’s not going to happen unless people help.”

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