Analysts say the search for a suitable technology could become a $150 billion-plus market. But a big worry is that gases may leak from badly chosen underground sites, perhaps jolted open by an earthquake.

Such leaks could be deadly and would stoke climate change.

Part of the answer could be to petrify or liquefy gases like carbon dioxide – emitted for example from power plants and factories run on coal, oil or natural gas – if costs are not too high.

"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

In theory, carbon dioxide reacts with porous basalt and turns into a mineral, but no one knows how long that takes.

Other researchers say pumping the gas into sediments below the seabed at depths of around 3 000 metres (9,800 ft) would expose it to enough pressure to turn it into a viscous liquid, like honey or treacle.

In the worst known example of a leak, a natural volcanic eruption of carbon dioxide from Lake Nyos in Cameroon in 1986 killed more than 1 700 people. Carbon dioxide is not toxic but can cause asphyxia in high concentrations.

Heatwaves, storms
The U.N. Climate Panel said in a 2005 report that carbon storage could be one of the main ways to offset global warming, which could cause more powerful storms and heatwaves, and melt glaciers from the Himalayas to the Andes.

Costs are a big problem: the Climate Panel estimates the penalty for emitting carbon dioxide would have to be stable at $25-$30 a tonne to make carbon storage viable – pushing up the cost of everything from electricity to steel as the price of slowing climate change.

Carbon emissions' prices in a European Union market of more than 11 000 industrial sites are now above the trigger level, at 24.55 euros ($37.97) a tonne. But the volatile market has often been far below $25 since trading began in 2005.

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.

"There have been no leaks," said Tore Torp, project leader at operator StatoilHydro. The gas, under pressure, is in a "supercritical" state between liquid and gas and would stay put even in the event of a cataclysmic earthquake, he said.

The project is commercial because Norway, outside the EU, imposed taxes on carbon emissions in 1991. The natural gas at Sleipner has an unusually high percentage of carbon dioxide.

"In theory, if you had (a giant earthquake) a crack would be filled by the North Sea in seconds and would maintain the pressure down there," Torp said.

"The most important thing for safety is selecting the site. We would avoid the San Andreas fault" in California, he said.

Plans to mix carbon dioxide into the world's oceans, perhaps by dumping thousands of tonnes of iron filings that would encourage the growth of carbon-absorbing algae, are in doubt because of worries they would disrupt marine life.

"All such projects are on hold," said Rene Coenen, office head for the London Convention which oversees dumping at sea. "Our official advice is: 'don't start with it. Wait until we have more knowledge from the scientific side'."

U.S. group Planktos has indefinitely postponed a plan to seed the Pacific Ocean with iron filings. Silicon Valley startup Climos is still studying the idea.

Shellfish
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."

Costs might be even higher for the more exotic solutions.

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.

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% 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.