National Geographic , 17 August 2006

Ocean Floor Could Be Greenhouse Gas "Dump," Scientists Say

The ocean floor could provide storage for more excess carbon dioxide than the world can ever produce, scientists say.

A team of researchers proposes disposing of the carbon dioxide produced from burning fossil fuels by injecting it into seabed sediments nearly two miles (three and a quarter kilometers) below the ocean's surface

There the combination of cold temperatures and extreme pressure will trap the gas for millions of years, says the study's lead author, Kurt Zenz House, a graduate student in geoscience at Harvard University in Cambridge, Massachusetts.

House's team published its proposal last week in the Proceedings of the National Academy of Sciences.

The accumulation of carbon dioxide (CO2) in the atmosphere from fossil fuel use is believed to be a leading contributor to global warming.

One way to reduce global warming without drastic reductions in energy consumption is by removing carbon dioxide from power-plant emissions and putting it where it cannot enter the atmosphere.

In the past scientists have proposed storing CO2 in geologic formations such as depleted natural gas fields.

"Those storage options make sense and can be used to large degree," House said.

But they don't eliminate the possibility that the gas could make its way back to the surface, he adds.

"If it can find a conduit through a well or a fault, it has the potential to escape," House said.

Under the Sea

Undersea storage is different, House says.

At depths below 8,900 feet (2,700 meters) the pressure is great enough that CO2 is compressed into a liquid that is denser than water. Therefore it can't bubble back to the surface.

But simply putting carbon dioxide on the surface of the seabed would wreak ecological havoc, House explains.

Not only would the carbon dioxide smother life on the ocean floor, but it would also react with seawater to form acid.

House proposes to bypass this problem by injecting the carbon dioxide into sediments below the seabed.

This would take advantage of the unique changes in temperature and pressure found beneath the deep ocean, he says.

About 1,000 feet (300 to 400 meters) deep in the sediment, the Earth is warm enough that the carbon dioxide will no longer be denser than the seawater in the rock's pores.

That's a good thing, House says, because as the gas percolates up through the waterlogged sediment, it will react with seawater to produce a type of ice called a hydrate.

This ice will block the pores in the sediments, House says, creating a cap that keeps additional carbon dioxide from moving upward.

"Enormous" Capacity

The seabed of the U.S.'s territorial waters alone has enough suitable sediments to hold thousands of years' worth of CO2 produced from fossil fuel use, House says.

"The storage capacity is enormous," he said.

The prospects are even greater for a country like Japan, where there are very few depleted gas fields or other land-based options for storing CO2.

"If Japan wanted to do something with their carbon dioxide, this might be their best option," House said.

But the technology would come with a big price tag.

House estimates that it would cost about U.S. $35 to $75 per ton of CO2—about the amount produced by burning two barrels of oil.

At the moment the new idea is a proposal, not a mature technology.

"This is one of those ideas that you have to try out to see if it works," said David Goodstein, vice provost of California Institute of Technology in Pasadena and author of Out of Gas: The End of the Age of Oil.

But that doesn't mean that innovative ideas for getting rid of carbon dioxide aren't important, he says.

"We are doing an uncontrolled experiment on the climate of the only planet we have," Goodstein said, regarding carbon dioxide emissions from fossil fuels. "That is very foolish."