The United States signed a charter on June 25 with the European Union's executive Commission and 12 countries including Russia, China, Japan, Canada and Brazil to research the technology in a "Carbon Sequestration Leadership Forum".

But some environmentalists say the idea is costly and like trying to sweep one of the planet's greatest problems under the carpet.

"The storage potential is enormous," said Tore Torp of the Norwegian oil company Statoil, which has the world's first commercial store of carbon dioxide (CO2) in sandstone 1,000 metres (3,300 feet) under the North Sea.

"We believe the CO2 will stay there for many thousands of years. There is no sign of leakage," Torp said of the project he leads at the Sleipner gas and condensate field. Tests began at Sleipner to filter out and bury CO2 in 1996.

CO2 is the main gas blamed for blanketing the planet and driving up temperatures, disrupting the climate with more frequent floods, droughts and storms that could trigger everything from desertification to higher sea levels.

In a U.S. project, drills near a coal-fired power plant run by American Electric Power in West Virginia have reached about 1,200 metres (4,000 feet) in a search for CO2 storage sites in sandstone as deep as 3,000 metres (10,000 feet) below the Ohio River Valley.

"We'll collect rock samples to learn as much as we can about the local geology before deciding," said Neeraj Gupta, project manager at research group Battelle which is working with groups including BP and Schlumberger.

Under the scheme, CO2 would be filtered from American Electric's 1,300 MW Mountaineer power plant and buried. CO2 might be injected into nearby oilfields where it could help raise the amount of oil pumped out.

PROLONG OIL DEPENDENCE?

Many environmentalists see CO2 storage as a distraction from shifting to clean, renewable energy like wind or solar power and away from dirty fossil fuels like coal and oil.

"Our view is that this is illegal," said Truls Gulowsen at the environmental group Greenpeace, referring to Statoil's Sleipner scheme. "Dumping of industrial waste at sea or beneath the sea is banned."

"It's more important to build windmills in China than to show that you can bury CO2," he said. "No one can know if it will not leak over thousands of years. And who'll check?" CO2 might corrode concrete plugs meant to seal wells.

Jason Anderson, energy specialist at the Climate Action Network in Brussels, which represents 75 non-governmental organisations, said European environmentalists were generally more sceptical than U.S. groups about carbon storage.

President George W. Bush pulled the United States out of the Kyoto pact on limiting greenhouse gas emissions, arguing it was too costly and wrongly excluded developing countries.

Anderson said CO2 storage could prolong dependence on fossil fuels.

"And it costs more fuel to capture CO2," he said. "Rather than use one unit of coal you use 1.3 units for the same power. Everything gets worse."

Statoil now pumps about one million tonnes a year of non-toxic CO2, produced by everything from living organisms to car exhausts, into rocks below Sleipner. A million tonnes is the rough equivalent of emissions by a U.S. city of 50,000 people.

Statoil reckoned the technology was cheaper than paying a CO2 tax of about $40 a tonne on CO2 emissions in the 1990s. Many other countries are considering similar taxes to force industry to meet goals under the Kyoto climate pact.

Torp rejected Greenpeace's suggestion that CO2 injection was illegal, noting that it was legally sold and exported -- it is the ingredient that puts fizz in beer or soft drinks.

"If it was industrial waste it could not be transported across borders," he said. "There are no such restrictions."

U.S. officials estimate that up to 250 billion tonnes of carbon could be captured from the atmosphere and stored underground or captured in the soil in the United States. The United States generates about 5.8 billion tonnes of CO2 a year.

Another solution is to inject CO2 into oil wells. It makes oil flow more freely and loosens it from surrounding rocks, perhaps raising output by 5-15 percent.