The acid bath

What carbon dioxide does to the oceans

The oceans are the ultimate sink for most of the heat from the sun and also for most of the greenhouse gases we are pouring into the atmosphere. The atmosphere may be the place in which we live and breathe, but for long-term planetary systems it is just a holding bay. At any one time, there is fifty times as much carbon dioxide dissolved in ocean waters as there is in the atmosphere. Given time, the oceans can absorb most of what we can throw into the atmosphere. But time is what we do not have, and the oceans' patience with our activities may be limited.

Carbon dioxide moves constantly between the oceans' surface and the atmosphere, as the two environments share out the gas. And, because of ever-rising concentrations in the atmosphere, the oceans currently absorb in excess of 2 billion tons more a year than they release. Much of that surplus eventually finds its way to the ocean floor after being absorbed by growing marine organisms—a process often called the biological pump. Sometimes there are so many skeletons falling to the depths that biologists call it marine snow.

Though they are the ultimate sink for most carbon dioxide, the oceans do not simply absorb any spare carbon dioxide left in the atmosphere. The relationship is much more dynamic—and much less reliable. In the long run, carbon dioxide seems to seesaw between the oceans on the one hand and the atmosphere and land vegetation on the other. Plants on land generally prefer things warm. Certainly the carbon "stock" on land is greater during warm interglacial eras like our own, and less during ice ages. By contrast, ocean surfaces absorb more carbon dioxide when the waters are cold. This seems to be partly because the plankton that form the basis of life in the oceans prefer cold waters, and partly because when the land is cold and dry, dust storms transport large amounts of minerals that fertilize the oceans.

During the last ice age, some 220 billion tons of carbon moved from the land and atmosphere to the oceans. This process didn't cause the ice ages, but it was a very powerful positive feedback driving the cooling. And that is a worry. For if the ice-age pattern holds, future generations can expect the oceans' biological pump to decline as the world warms. The story of the oceans' exchanges of carbon dioxide with the atmosphere may turn out to be rather like that of the carbon sink on land. In the short term, the extra carbon dioxide in the air has fertilized the biological pump and encouraged greater uptake. But in the longer term, warmer oceans are likely to weaken the biological pump and release large amounts of carbon dioxide into the air.

Is something of the sort likely? Very much so, said Paul Falkowski, of Rutgers University, in New Jersey, in a long review of the carbon cycle in Science. "If our current understanding of the ocean carbon cycle is borne out, the sink strength of the ocean will weaken, leaving a larger fraction of anthropogenically produced carbon dioxide in the atmosphere." With tens of millions of tons of carbon moving back and forth between the atmosphere and the oceans each year, it would take only a small change to turn the oceans from a carbon sink into a potentially very large carbon source. This may already be happening. In 2003, the NASA scientist Watson Gregg published satellite measurements suggesting that the biological productivity of the oceans may have fallen by 6 percent since the 1980s. It could be part of a natural cycle, he said, but it could also be an early sign that the biological pump is slowing as ocean temperatures rise.

So far, since the beginning of the Industrial Revolution, the oceans have absorbed from the atmosphere something like 130 billion tons of carbon resulting from human activities. While much of it has fallen to the seabed, a considerable amount remains dissolved in ocean waters—with a singular and rather remarkable effect: it is making the oceans more acid.

The carbonic acid produced by dissolving carbon dioxide is corrosive and especially damaging to organisms that need calcium carbonate for their shells or skeletons. These include coral, sea urchins, starfish, many shellfish, and some plankton. Besides eating away at the organisms, the acid reduces the concentration of carbonate in the water, depriving them of the chemicals they need to grow.

Acidity, measured as the amount of hydrogen ions in the water, is already up by 30 percent. To put it another way, the pH has dropped by 0.1 points, from 8.2 to about 8.1. If the oceans continue to absorb large amounts of the atmosphere's excess carbon dioxide, acidification will have more than tripled by the second half of this century, badly damaging ocean ecosystems. The most vulnerable oceans are probably the remote waters of the Southern Ocean and the South Pacific. They are distant from land, and so are already short of carbonate—in particular a form known as aragonite, which seems to be the most critical.

"Corals could be rare on the tropical and sub-tropic reefs such as the Great Barrier Reef by 2050," warned a report from Britain's Royal Society. "This will have major ramifications for hundreds of thousands of other species that dwell in the reefs and the people that depend on them." Other species may suffocate or die for want of energy. High-energy marine creatures like squid need lots of oxygen, but the heavy concentrations of carbon dioxide will make it harder for them to extract oxygen from seawater.

"It is early days," says Carol Turley, of the Plymouth Marine Laboratory, a world authority in this suddenly uncovered field of research. "The experiments are really only getting under way." But one set of results is already in. James Orr, of the Laboratoire des Sciences du Climat et de l'Environnement, in France, put tiny sea snails called pteropods into an aquarium and exposed them to the kind of ocean chemistry expected later in this century. These creatures turn up all around the world and are vital to many ecosystems. They are the most abundant species in some waters around Antarctica, where a thousand individuals can live in 300 gallons of seawater. As well as being a major source of food for everything from fish to whales, pteropods are the biggest players in the biological pump there.

Orr found that within hours, the acid pitted the pteropods' shells. Within two days, the shells began to peel, exposing the soft flesh beneath. In the real world, predators would break through the weakened shells. "The snails would not survive," he concluded. The demise of the pteropods would cause a "major reduction in the biological pump," the Royal Society agreed. Within a few decades, it could leave the oceans more acid than at any time for 300 million years.

Whatever the outcome, we are seeing the start of an unexpected and frightening side effect of rising atmospheric carbon dioxide levels. Perhaps the nearest parallel to the current situation was 5 5 million years ago—the last time a major slug of carbon was released into the atmosphere over a short period ...

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