The idea is to store the carbon dioxide produced by the burning of fossil fuels under the sea, stopping it from getting into the atmosphere and adding to the greenhouse effect.

In theory, this would reduce the rate of global warming, but now a report in the journal Science has cast doubt on whether the "burial" would be effective.

The high pressures and low temperatures on the seabed should force liquid carbon dioxide to form a solid known as gas hydrate - an icy mixture of water and carbon dioxide. Gas hydrates occur naturally in many parts of the world.

"Burying" carbon dioxide could help reduce its atmospheric concentration

Scientists hoped that carbon dioxide converted into gas hydrate would remain safely locked away and not reach the air.

Tests carried out in the laboratory showed enough promise for researchers from the Monterey Bay Aquarium Research Institute in the United States to carry out an experiment 3.5km below the Pacific, using a remote-controlled submarine.

This took flasks of carbon dioxide down to the sea floor, let it leak out and then watched what happened using a video camera.

The gas did react with the seawater to form hydrates, but these expanded rapidly and started to dissolve.

At shallower depths of around one kilometre, these rose up towards the surface, meaning they would quickly return to the surface. At 3.5km the gas hydrate drifted off and dissolved, but did not sit stably on the sea floor.

The liquid carbon dioxide spills over the beaker

This dissolved carbon dioxide could remain in deep ocean waters. If so, it would not return to the atmosphere for up to several hundred years, but further tests will be needed.

In Europe, a group of researchers and oil companies have for several years been looking at the feasibility of placing carbon dioxide into exhausted oil and gas reservoirs under the North Sea.

This appears to hold better prospects of locking up the carbon dioxide for long periods but many questions remain to be answered.

Carbon dioxide in the atmosphere acts like a blanket over the Earth, trapping heat in, causing the planet to warm up.