Analysts say that in the next 20 years carbon storage could become a $150 billion-plus market. A new approach is gaining a lot of attention and it involves turning green house gases in stone.

"If you can convert (the gases) to stone, and it's environmentally benign and permanent, then that's better," said Juerg Matter, a German scientist at Columbia University in New York who is working on a project in Iceland to turn carbon dioxide, the main greenhouse gas, to rock.

In theory, carbon dioxide reacts with porous basalt and turns into a mineral, but no one knows how long that takes. Matter and U.S., French and Icelandic experts plan to inject 50,000 tonnes of the gas into basalt in a test starting in 2009.

Most plans by firms such as BP, Rio Tinto, E.ON, American Electric Power and StatoilHydro focus on capturing greenhouse gases and pumping them into porous rocks in shallower oil and gas reservoirs, or into disused mines or saline aquifers.

Water readily absorbs carbon dioxide -- beer and other fizzy drinks are examples. But carbon makes the seas more acidic and could make it harder for shellfish, corals, crabs or lobsters to build their protective coats.

So carbon burial may be only answer, despite environmentalists' fears that it will encourage nations to keep on burning fossil fuels rather than shift to cleaner renewable energies such as wind or solar power.

"To my mind, if declared safe and acceptable, it is going to be an imperative in terms of an effective solution," said Yvo de Boer, head of the U.N. Climate Change Secretariat.

"If I look at some of the really huge coal-based economies around the world, like China, like India, like South Africa, like Australia, I don't really see how we can come to grips with climate change without using carbon capture and storage."

In Iceland, the theory goes that a chemical reaction in the calcium- and magnesium-rich basalt formations will bind carbon molecules to form calcite and dolomite. Matter said that costs could be comparable to burial in oil or gas fields.

But it is unclear whether the reactions in the rocks are quick enough. "We do not know if these geochemical reactions ... will take 50, 100 or thousands of years," Matter said.

He said suitable basaltic rocks were also found in places such as India and Siberia.

Kurt Zenz House at Harvard reckoned the costs of pumping carbon to depths of more than 3,000 metres (9,800 ft) would be about 25 percent higher than shallower burial. "It's more expensive but the advantage is that it wouldn't move," he said, adding that it would also not interfere with nature.