Published: November 2007

Acid Threat

Ftr Hdr Acid

If carbon dioxide continues to rise unchecked, computer models show that acidification will deplete carbonate ions in much of the ocean by 2100, turning the waters corrosive for many shell-building animals.

By Jennifer S. Holland
National Geographic Staff
Photograph by David Liittschwager, Maps by National Geographic maps
Tiny creatures near the base of the marine food chain lead perilous lives at best. Now they face a man-made threat. No, not global warming this time, though the root cause is the same. As the level of atmospheric carbon dioxide (CO2) rises, it is not only heating the globe but also dissolving in ocean waters, turning them more acidic. For shell-building animals that can mean a corrosive, even deadly environment.

Oceans are a natural sink for CO2, already soaking up more than a quarter of what's released into the atmosphere. Today we're pumping out massive quantities—a surge that began more than a century ago as factories, power plants, and cars began devouring fossil fuels. By now the oceans are taking in 25 million tons a day of excess CO2, and it is starting to show. Already scientists have measured a rise in acidity of some 30 percent in surface waters, and they predict a 100 to 150 percent increase by the end of the century.

No ill effects have been documented so far in the open ocean, but the threat is clear. Absorbed by seawater, CO2 reacts to form carbonic acid, which turns the normally alkaline water more acidic. In the process, fewer carbonate ions are left floating around—and many marine organisms, including mollusks and corals, rely on carbonate from seawater to build their shells and other hard parts. Eventually, vital species will no longer be able to build or maintain their shells and skeletons.

Users of the mineral aragonite—a very soluble type of calcium carbonate—are especially vulnerable. They include tiny pteropod snails, which help feed commercially vital fish like salmon. Computer models predict that polar waters will turn hostile for pteropods within 50 years (cold water holds the most CO2, so it is already less shell-friendly). By 2100, habitat for many shelled species could shrink drastically, with impacts up the food chain. And as the acidification reaches the tropics, "it's a doomsday scenario for coral reefs," says Carnegie Institution oceanographer Ken Caldeira. If current trends continue, he predicts, reefs will one day survive only in walled-off, acid-controlled refuges.

Massive outbursts of CO2 and other greenhouse gases have acidified the oceans in the geologic past, but equilibrium returned as the oceans stored away excess CO2 in minerals on the seafloor. This time nature may be slow to heal. "Our emissions are huge compared with natural fluxes," Caldeira says. "If you could stop emissions and wait 10,000 years, natural processes would probably take care of most of it." These days we're simply dishing it out faster than the oceans can mop it up.
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