Two Sides to CO₂ Rise

Not all of the carbon dioxide that flies out of our tailpipes and industrial smokestacks stays in the atmosphere. A sweeping study released last week shows that roughly half of the CO₂ released from burning fossil fuels over the past 200 years has ended up in the ocean. And there, researchers report, it may harm marine organisms [Science, 305, 367 and 362 (2004)].

Scientists have long known that some fraction of atmospheric CO₂ is taken up by oceans and forests. But pinning down how much has been difficult. In the 1990s, international teams collaborated to measure the oceans' inorganic carbon concentration. A group led by Christopher L. Sabine and Richard A. Feely at the National Oceanic & Atmospheric Administration evaluated the numbers.

They conclude that, of the total CO₂ released by fossil-fuel burning and cement-making between 1800 and 1994, the ocean took up about half: 1.2 x 10¹⁷ g. Over the same time period, the world's forested areas actually released CO₂. That's not surprising, says Jorge L. Sarmiento, professor of atmospheric and oceanic sciences at Princeton University. He calls the bubble of CO₂ released to the atmosphere as forests were cleared for farmland during the 1800s and 1900s the "pioneer effect." Today it is different. Many areas are being reforested, and the trees are now taking up CO₂.

But the oceans, Sabine says, have been the only consistent CO₂ sink, and they are by no means full. Top to bottom, ocean water mixes slowly. Half of anthropogenic CO₂ is still in the upper 10% of the water. "As the ocean continues to mix CO₂ down, it will continue to take up anthropogenic CO₂ for thousands of years," Sabine points out.

That's good news, Feely says, when considering CO₂'s atmospheric greenhouse effect. The other side of the coin, however, is that increasing concentrations of CO₂ lower the pH of ocean water, lower carbonate ion concentrations, and change the environment for sensitive sea creatures.

Some marine organisms, including coral reefs and plants called coccolithophorids, take up carbonate ion to form calcium carbonate shells. When the shells are discarded, they dissolve, releasing carbonate. Feely has shown that shell dissolution is now occurring at shallower depths than it did in 1800, which actually helps the ocean take up even more CO₂. But the uptake of CO₂ makes it more difficult for marine organisms to form their shells.

Laboratory experiments show that, if ocean CO₂ concentrations triple, as they could by 2100, some organisms won't form their shells properly.

"We're changing ocean chemistry," Sarmiento says. "It's one thing about CO₂ emissions that worries me a lot." Sabine and Feely stress that the effect on coral reefs and shells is just one factor they've studied. They don't yet know how rising ocean CO₂ levels will impact ocean biology overall.