Carbon dioxide: where does it all go?

Carbon Dioxide: Where Does It All Go?

Scientists studying carbon dioxide have run into arithmetic problem. A research team reports that the world's oceans do not absorb nearly as much of this greenhouse gas each year as previous work has suggested. Though they can't agree on whether to call it good or bad news, experts say it points out an unnerving ignorance about carbon dioxide.

"We've been overconfident for a long time about our knowledge of the carbon cycle. we've held on to the idea that the oceans were absorbing roughly 40 percent of the [carbon dioxide from] fossilfuel combustion. Now we really think that's wrong," says Pieter P. Tans, an atmospheric scientist with the National Oceanic and Atmospheric Administration in Boulder, Colo.

The burning of coal, oil and natural gas each year spews about 5.3 billion tons of carbon into the atmosphere in the form of carbon dioxide. Because oceans and land can absorb only a fraction of the gas, 3.3 billion tons a year go into the air, causing the carbon dioxide buildup that has concerned the world. Of all the "sinks" that pull carbon dioxide from the atmosphere, scientists assume that oceans -- particularly in the southern Hemisphere -- do most of the work.

But computer simulations and actual measurements dispute this assumption, says Tans, who worked with Inez Y. Fung from NASA's Goddard Institute for Space Studies in New York City and with oceanographer Taro Takahashi of the Lamont-Doherty Geological Observatory in Palisades, N.Y. Fung described their work earlier this month in Toronto at a meeting of the American Institute of Biological Sciences.

About 96 percent of the carbon dioxide from fossil-fuel combustion enters the atmosphere in the Northern Hemisphere. Although winds blow the gas around the world, measurements show that North Pole air contains 3 parts per million (ppm) more carbon dioxide than South Pole air, Fung says. This number plays a key role in the new study.

Using an atmospheric general circulation model, Fung and her colleagues examined carbon dioxide's movement around the globe after its release during fossil-fuel combustion. By adding and subtracting various processes, they tested parts of the carbon cycle.

When they "turned off" the oceans and all other sinks for carbon dioxide, the computer atmosphere generated a north-south gradient of 4.4 ppm. With an ocean sink, working mostly in the south, the gradient increased--a result farther from the real value of 3 ppm. Fung says the simulations indicate the southern ocean cannot possibly account for most of the carbon dioxide absorbed worldwide. Only when the largest sink resides in the Northern Hemisphere does her model match real-world measurements.

Measurements show the northern oceans don't pull in much carbon dioxide, leading the researchers to suggest that land areas in the northern midlatitudes must absorb a staggering amount, between 1 billion and 2.5 billion tons of carbon a year.

If this is true, scientists face a grand mystery. Nobody knows what on land could store all that carbons, says ecologist Richard A. Houghton of the Woods Hole (Mass.) Research Center. Trees or soil might be doing the job, but Houghton and others say researchers should have noticed such conspicuous storage by now.

Lamont-doherty's Wallace S. Broecker speculates that the finding may be good news. If soils currently absorb much of thecarbon dioxide from fossil-fuel combustion, he says, policymakers might consider using this natural process to reduce the greenhouse threat.

Takahashi sees it differently. "If what we're saying is true," he says, "it's more bleak than we used to think." He and others believe land areas cannot store as much carbon dioxide as the oceans, and may reach capacity in the foreseeable future. At that point, carbon dioxide would accumulate much more rapidly in the atmosphere, accelerating a greenhouse warming.