Is it dangerous to store carbon dioxide underground?

It's worked for millions of years. The question for Elliot Morley, the environment minister who reignited the debate about carbon sequestration this week, is whether giant bubbles of gas pumped down there from power stations will be as stable as natural reservoirs.

Morley was seeking support for a plan to reduce greenhouse gas emissions by storing millions of tonnes of polluting carbon dioxide in disused oil fields and underground water sources. "Our priority is to reduce emissions but as an interim move carbon sequestration is an option we should be exploring," he said.

The move follows a report prepared for the government last year which warned that carbon sequestration - or capture and storage - might be the only way for Britain to reach ambitious targets to reduce carbon dioxide emissions. "With the 60% reduction target for carbon dioxide emissions by 2050, large-scale deployment of carbon capture and storage may be needed for electricity generation from about 2020," the report said.

The UK is well placed to develop the technology because it could exploit the oil and gas drilling infrastructure in the North Sea, where many oilfields are beginning to run dry. The oil industry has long pumped carbon dioxide from natural sources into oilfields to squeeze out remaining fuel reserves, a technique known as enhanced oil recovery.

Environmental groups and politicians want guarantees that the stored gas will not leak out over time. The world's only large-scale investigation of carbon sequestration at sea is being led by the Norwegian company Statoil, also in the North Sea. Since 1996, the company has been pumping carbon dioxide into a sandstone layer about half a mile below the seabed. The resulting bubble now contains more than 6m tonnes.

Andy Chadwick, of the British Geological Survey, is part of the team monitoring the project. He says there is no sign of the gas escaping, though the seismic surveys used to check are only sensitive enough to spot leaks of several hundred tonnes.

More sensitive monitoring has just begun at a much smaller scale onshore project in Texas.

Scientists from the US National Energy Technology Laboratory in Pittsburgh will tell the American Geophysical Union annual meeting in December how they laced the several thousand tonnes of carbon dioxide injected into a saltwater formation nearly a mile underground last month with fluorocarbon tracer molecules. They will now monitor the soil above to see if the stored gas makes its way back to the surface.