Amid continuing warnings that greenhouse gases are driving up global temperatures and raising sea levels, a handful of industries and a small army of scientists are trying to drive those gases underground, where they can't do much harm.
They're using an emerging technology called "carbon capture and sequestration" that has already produced promising results.
In Algeria, for example, gas producers are pumping carbon dioxide back into the Earth, preventing a million tons a year from venting into the air.
In Canada, CO2 piped from a coal gasification plant is being injected into an aging oil and gas field to boost production. Over its lifetime, the operation will keep 20 million tons of CO2 out of the atmosphere.
And in Maryland, scientists are determining how quickly restored marshland can soak up CO2 from the atmosphere and lock it up as organic matter.
Scientists see carbon capture and storage as part of a "portfolio" of solutions to cut emissions and avoid the worst consequences of global warming. Others include conservation, alternative fuels, nuclear and renewable energy and cleaner combustion technologies.
It may be decades before the necessary laws, technologies and infrastructure - such as pipelines and injection wells - are in place. But proponents say carbon storage will expand as a growing global market in "carbon credits" and the expectation of U.S. emissions curbs provide financial incentives.
"It's not a matter of 'if.' It's a matter of 'when,'" said Bill Lyons, president of climate change and technology development at AES, a global power generator and distributor based in Arlington, Va.
AES has committed $10 billion over the years to develop alternative energy technologies for its industrial customers.
Not everyone is a booster. Some environmental groups see only a limited use for carbon storage. Others see it as political cover for the coal industry.
"Our priorities are to reduce dependence on fossil fuels," said Daniel A. Lashof, of the Natural Resources Defense Council in Washington. "But to the extent we continue to use fossil fuels, we need to capture the CO2 that's produced."
Greenpeace is less accommodating.
"It's being used to greenwash the coal industry in a very despicable way," said John Coequyt, an energy policy specialist with the environmental group. "We should be focusing on energy efficiency and renewables alone to address global warming."
The Energy Department says there's enough storage capacity in the U.S. to handle 1,000 years of CO2 emissions. Meanwhile, large quantities of CO2 already vented to the atmosphere can be soaked up by plant life and stored in the soil, scientists say.
Studies suggest that changes in agricultural practices, along with restoration and replanting of degraded land can enable the soil and plant life to absorb even more.
Carbon storage "really does look like an important technology," said Jim Dooley, a senior scientist with the Joint Global Change Research Institute at the University of Maryland, College Park. It's "important in the sense of helping society reduce the costs of addressing climate change, and important in the sense of its broad-scale applicability."
A by-product of combustion, CO2 is, after water vapor, the most abundant "greenhouse" gas in the atmosphere, a family of compounds that trap solar heat.
Methane, a constituent of natural gas and a byproduct of organic decomposition, is 20 times as potent as a greenhouse gas, but less abundant than CO2.
The "greenhouse effect" is critical in keeping the Earth warm enough to be habitable. But most scientists agree that when humans freed large amounts of ancient carbon by burning fossil fuels, they produced too much of a good thing.
Worldwide emissions of CO2 have grown from insignificant levels at the start of the Industrial Revolution to more than 25 billion tons a year, scientists say.
The result has been unwanted climate change - warming global temperatures, ecological changes and rising sea levels. Continued economic expansion, especially in the developing world, will accelerate the trend.
In combating the problem, power plants equipped for carbon capture would be more costly to operate than current facilities.
But the Intergovernmental Panel on Climate Change, which produced a sobering report on global warming last week, has said that carbon storage, spurred by a system of government carbon "permitting" and financial incentives such as "carbon credit" trading, could reduce power plants' CO2 emissions by 80 percent to 90 percent.
Carbon markets are still voluntary in the U.S., which has not signed the Kyoto treaty on climate change. It sets up a system by which major polluters earn credits by cutting emissions or buy them from cleaner operators.
Although the Bush administration has refused to join the international effort, the government does have a carbon sequestration program. The administration hopes the program will yield viable technologies by 2012, and commercial applications by 2020, that add no more than 10 percent to the costs of using fossil fuels.
The world's largest carbon sequestration projects include the Weyburn Project in Canada, where oil and gas producer EnCana is using CO2 piped 205 miles from a coal gasification plant in North Dakota to pressurize an old oil field and boost production.
EnCana believes the project will permanently sequester 20 million tons of CO2 during its lifetime and add 122 million barrels to the field's oil production. The Dakota Gasification Co. earns $30 million a year from the CO2 it sells EnCana.
Norwegian natural gas producers are capturing CO2 from wells in the North Sea's Sleipner field and reinjecting it into a deep saline aquifers at a rate of a million tons annually, simultaneously saving $160,000 a day in Norwegian carbon emissions taxes.
There are no commercial sequestration projects on that scale in the United States. But one coal-fired power plant is capturing CO2 from its waste stream and selling it.
AES's 180-megawatt Warrior Run power plant near Cumberland scrubs 3 percent to 4 percent of the CO2 from its flue gas and produces 45,000 tons of beverage-grade liquid CO2 annually. It ends up as bubbles in Coke, Pepsi and Coors beer, according to Ed Giugliano, the plant's technical leader.
The gas escapes to the atmosphere when the drinks are poured, of course. But Guigliano said it's a more efficient use of the plant's energy and thus reduces CO2 emissions indirectly.
The chemistry of capturing CO2 from oil and gas fields, power plant smokestacks and industrial waste is well understood. But it can cost $6 to $60 per ton of CO2, according to a 2006 report by Battelle Science and Technology. So researchers are looking for new ways to make the old processes more efficient.
Once deep underground, liquefied CO2 is generally trapped or dissolved in water. It may also combine with coal and other minerals, rich in organics, or, over thousands of years, combine chemically with other rocks to form stable carbonates. And, thick, largely impermeable "cap rock" layers should block the CO2 from rising to the surface.
Seepage from deep storage into the atmosphere could set back efforts to control global warming. And major leaks, especially from pipelines, could pose a suffocation hazard if people were exposed to concentrations in excess of 7 percent to 10 percent, the IPCC says.
The climate panel called those risks no higher than those posed by more familiar oil and gas pipelines. But few communities have had to confront the issue. "I am completely unwilling to say the [public] reaction will be bad," said Dooley. "I think it's a dialogue that needs to happen, but it will happen when we start to deploy these things commercially."
Field tests are already underway, under federal sponsorship.
Maryland is one of seven states in the Midwest Regional Carbon Sequestration Partnership (MRCSP). The research consortium of government, academia and industry is one of seven formed by the Energy Department in 2003. Power stations in the region emit nearly 600 million tons of CO2 annually.
Research by the MRCSP in the region has identified layers of porous sandstone and brine thousands of feet below the surface - capable of holding up to 475 billion metric tons of CO2. Depleted oil and gas fields, along with unmineable coal and shale seams could hold another 1.75 billion, officials say.
Drilling is underway in Michigan, Kentucky and Ohio to learn more about the deep geology of the region, and effective ways to monitor the gas after it's pumped into the deep rock formations.
Elsewhere, scientists have inventoried eroded croplands, degraded and abandoned mine property, marshes and wetlands that could be restored with trees and other vegetation to soak up more CO2 from the atmosphere.
If it were all restored, "we estimated you could ... sequester 20 percent of current emissions from large point sources" in the region, said Dave Ball, program manager for the partnership.
Plants convert atmospheric CO2 into carbon for their tissues, which remain in the soil as plant residue after harvest. But on farmed land, repeated plowing allows carbon from the decomposing plant matter to combine with oxygen and return to the air as carbon dioxide.
MRCSP scientists want to know how the carbon uptake in tilled soils differs from acreage farmed with no-till techniques designed to retain plant residue in the soil.
Similarly, scientists in West Virginia, eastern Ohio and southwestern Pennsylvania are studying how carbon is recaptured when old mining properties are replanted.
"We're trying to get a handle on the effectiveness of different reclamation methods, and the rate at which they're able to move carbon back into the soil after it's been depleted," Ball said.
In Maryland's Blackwater refuge, Brian Needelman is studying how fast the natural marsh can accumulate carbon in the form of new organic matter. Needelman, a soil scientist at the University of Maryland, College Park, is comparing it with marshland that was reclaimed with dredge sediment in 2003 and replanted.
At Blackwater, the Army Corps of Engineers and the U.S. Fish and Wildlife Service are using dredge spoil from the Baltimore Harbor channel to restore 20,000 acres of eroded marsh.
If estimates from earlier work prove accurate, Needelman said, the new marsh could soak up between 67,000 and 152,000 metric tons of additional CO2 every year.
"Marshes have among the highest [net organic productivity] of any ecosystem," Needelman said. "It is the ideal ecosystem for carbon sequestration."