It's become almost impossible to deny that adding carbon dioxide to the Earth's atmosphere has led to changes in the planet's climate. But it is also increasingly clear that CO₂ emissions are going to increase dramatically in the coming decades regardless of the number of Priuses sold or the compact fluorescent lightbulbs installed. The world economy is too enmeshed with fossil fuel to avoid it.

So a paper from researchers at Harvard and Columbia Universities, and the Massachusetts Institute of Technology suggesting the U.S. has a perfectly good hiding place for all the carbon it could ever emit is certainly welcome. Rather than simply venting carbon dioxide into the atmosphere, the researchers say, gas could be sent to a watery grave.

For every billion tons of carbon let loose into the atmosphere—the equivalent of burning 1.5 billion tons of coal or 12 billion barrels of oil—the concentration of carbon dioxide increases by nearly 0.5 parts per million. Given a half-life in the atmosphere of about a century, any CO₂ added today will be with us for a long time. Indeed, it's the carbon dumped into the skies over the past hundred or so years that has led to an increase in CO₂ levels from 280 parts per million to 380 ppm.

Locking away, or sequestering, carbon dioxide has been seen for some time as a promising route to slowing the rise in greenhouse gases in the atmosphere. Rather than forgo the use of plentiful energy sources such as coal or invest heavily in expensive alternative energy—both of which fly in the face of human nature, many observers say—power plants could be fitted with devices that capture carbon dioxide by-products. The carbon could then be stored in such a way that it doesn't enter the atmosphere.

Just where to stick the carbon has been a point of contention, however. One early suggestion was to pump it to the bottom of deep ocean trenches, where crushing pressure and frigid temperatures would liquefy the gas and keep it pinned to the sea floor. Others have proposed injecting the gas into saline aquifers many thousands of feet below ground. Neither approach has won over skeptics, who worry that CO₂ lakes on the ocean floor could be disturbed and that injected gas could leak out of deep caverns—to catastrophic effect.

Last year, Kurt House, a doctoral student at Harvard, and his colleagues began searching for a more permanent depository. "We wanted to look into a CO₂ storage option that was inherently safe and stable," House said. "We needed a high-pressure, low-temperature subsurface."

The conditions at the ocean floor were close to ideal—conditions there make carbon dioxide about six percent denser than water—but how to keep it stable? House hit upon a novel solution: injecting the liquefied gas a few hundred feet under the bottom of the sea. There, the gas would not be in danger of mixing with deep-ocean water; instead, CO₂ and water in porous rock formations would form an impervious solid hydrate cap.

House and his colleagues admit that there are still details to be worked out. For one, there are yet no firm numbers on how much storage 10,000 feet below the ocean's surface would cost. A 2005 paper by economist Jeffrey Sachs and physicist Klaus Lackner, both of Columbia University, estimate that capturing and sequestering some 17 billion tons of carbon dioxide a year by 2050 could run as high as $800 billion annually in today's dollars. That's about three cents per kilowatt-hour for the projected global power output.

House says that the biggest cost would be in capturing the carbon, not in injecting and storing it. Although
integrated-gasifier combined-cycle plants can be designed so they don't release carbon dioxide, other types of plants will have to be fitted with CO₂ scrubbers, at no small energy penalty and cost. And to deal with other carbon sources, such as vehicles, that are dispersed, there are even proposals to draw CO₂ directly from the air; if done with present-day technology, this would add more than 80 cents a gallon to the cost of gasoline.

Be that as it may, the storage potential of House's proposal is enormous. He estimates that, in seabeds off the coasts of California and the Carolinas, for instance, there is enough capacity to stow a millennium's worth of carbon emissions.

"We could put as much CO₂ as we want into these sediments," House said. "The limiting factor is the will and the economics to do it."

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