International rules allowing the burial of greenhouse gases beneath the seabed have come into force in what could be a step toward fighting global warming, if the costs of the technology is reduced and the risk of leaks can be averted.

The new rules will mean carbon dioxide emitted from power stations and industrial plants can be entombed offshore. This would slow warming while allowing continued use of fossil fuels.

"Storage of CO2 under the seabed will be allowed from 10 February 2007 under amendments to an international agreement governing the dumping of wastes at sea," said the UN's International Maritime Organisation (IMO) in a statement.

The new rules, agreed upon in November, amend the UN's London Convention on dumping at sea. Its text had been unclear about whether CO2 was classified as a pollutant.

The changes apply to oceans worldwide and could clear the way to more investment in sub-sea carbon storage by governments and companies. "This removes a lack of clarity and doubt for investors," said Tore Torp, CO2 storage adviser at Norwegian oil group Statoil, which opened the world's first commercial store of CO2 under the North Sea in 1996.

Renewable energy
However, Greenpeace, which has previously branded sub-sea storage as illegal dumping, said the revisions were too hasty. "We think the London Convention has not taken objections seriously – such as who will be responsible for leaks, who will oversee the storage, who will clean up," said a spokesperson.

Green groups generally argue that preventing CO2 emissions in the first place, via renewable energy sources and efficiency measures, is a better solution than trying to deal with the gas after it has been produced.

CO2 is not toxic but can lead to acidification of sea water, making it hard for creatures to build shells. In heavy concentrations above ground it can displace air and so asphyxiate people, animals and plants. This occurred as a result of volcanic activity in the 1980s in Cameroon.

Overwhelming, uncertain
The new amendments allow for carbon storage in "sub-seabed geological formations" and say the gases injected must consist "overwhelmingly" of CO2 with no added waste.

Torp said there was uncertainty about what "overwhelmingly" meant – emissions from a coal-fired power plant, for example, might include some toxic sulphur dioxide.

Statoil has injected about 9 million tonnes of CO2 in rocks below its Sleipner gas field, with no leaks, Torp said. Following Sleipner, two other big carbon storage sites are in operation in Canada and Algeria and more are planned.

But a 2005 UN report concluded that the costs of such storage meant it would only be widely implemented if the cost of emitting CO2 to the atmosphere was $25 to $30 a tonne, far above the current prices in the European Union market.