Seven Seas Sop Up CO 2

Balancing the pH budget on the backs of shellfish and corals

3 min read

In mid-July, Science magazine published two landmark papers reporting on a 15-year investigation into the role of carbon dioxide in the Earth's oceans. The major findings are, first, that nearly half of the carbon dioxide that humans have pumped into the atmosphere over the last 200 years has been absorbed by the oceans, and, second, that the rising CO 2 concentrations could start to have serious adverse effects on some marine life.

The background to the reports is essentially this: since 1957-1958, when as an activity of the International Geophysical Year instruments were placed on Hawaii's Mauna Loa volcano, precise measurements have confirmed that CO 2 is rising sharply and steadily in the atmosphere. But an anomaly also appeared. Only about half the CO 2 estimated to be coming from fossil fuel combustion has been showing up in the atmosphere. Oceanographers and atmospheric scientists were pretty sure that most of the missing CO 2 must have been taken up by the oceans, but it is only with the completion of the latest study that this belief is confirmed by solid empirical evidence.

"What is new is not so much what they are telling us," explains Inez Y. Fung, the director of the Berkeley Atmospheric Sciences Center at the University of California at Berkeley, "but rather their establishing what they are telling us by means of global, precise observations." With information assembled from a great many locations and analyzed by means of much better instruments, "now we have high-precision data that permits eking out [that is, identifying] the anthropogenic CO 2 in the ocean," she says.

According to Christopher L. Sabine, an oceanographer at the Pacific Marine Environmental Laboratory in Seattle and the main author of the first Science article, the last global survey of oceanic CO 2 was performed in the 1970s. At that time, roughly 6000 carbon samples were analyzed, and scientists had to contend with an absurdly large error range nearly half the signal strength. In the new survey, based on 72 000 samples worldwide, the error bars were cut by a factor of 10. It took 10 years to do the actual survey and another five to pull all the data together.

The key conclusion , empirically filling a link in the general model of where CO 2 originates and where it ends up, is that between 1800 and 1994, the ocean absorbed 48 percent of total CO 2 emissions. Boasting that their study "permits us, for the first time, to place observational constraints on the anthropogenic CO 2 budget" for the human era, Sabine and his collaborators concluded that the "ocean has constituted the only true net sink for anthropogenic CO 2 over the past 200 years" [see chart, " "Where Has All the CO2 Gone?"].

In recent decades, however, with CO 2 levels in the atmosphere rising exponentially, they found that the terrestrial biosphere has been much more of a sink, consuming as much as 20 percent of CO 2 emissions while the oceans took up just 30 percent.

Looking decades and centuries ahead, they speculate that ocean-atmosphere feedbacks are more likely to be unfavorable than favorable, so that the oceans may become a less efficient CO 2 sink. If that turns out to be right, stabilizing atmospheric levels of CO 2 , which are already nearly double what they were at the dawn of the industrial revolution, will be all the harder.

Even more unsettling, arguably, are the probable effects of rising CO 2 levels on marine life. The second Science report, whose lead author is Richard A. Feely, a marine chemist at the Pacific Marine Environmental Laboratory, explores precisely how ocean chemistry is being affected. The basic underlying principle, familiar from high school, is that when a chemical reaction is in equilibrium--that is, has settled into a particular rate going in both directions--it will strive to get back to equilibrium if it is perturbed.

In the case of the oceans, increasing the CO 2 concentration boosts acidity, which is neutralized when CO 2 reacts with calcium carbonate and water to form ions of HCO 3 and calcium. And the reverse is true as well, with the ions reacting to form CO 2 , water, and calcium carbonate.

In the face of increased acidity from rising CO 2 levels, the ocean tries to maintain its equilibrium by increasing the rate of reaction in the direction of making more ions. But that means consuming more calcium carbonate, which unfortunately for ocean life is the stuff of bone, shell, and coral [see photo, " "Endangered Reefs"].

The impacts vary at different depths, making for a complex picture, but the net effect isn't good for marine life. Observes Berkeley's Fung, you might as well be "dunking critters in lemon juice."

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