China, India and other countries have ramped up burning of coal this decade to power their growth and brake their dependence on expensive imported oil.

By doing so, they have also cranked emissions of carbon dioxide (CO2), the heat-trapping byproduct of this dirty, low-energy fuel.

A powerful lobby has emerged, sustained by the coal industry itself, to argue that with financial help and innovation, "clean coal" lies just on the horizon.

This term refers to a basket of technologies, the biggest of which is carbon capture and storage, or CCS.

Under CCS, power stations would burn coal but siphon off the CO2 at source before pumping the unwanted gas into deep chambers underground, such as disused gas or oilfields.

The CO2 would be stored there indefinitely rather than be disgorged into the atmosphere to add to the greenhouse effect.

CCS has surged up the agenda in the past few years as the United States, Australia and other big coal producers struggle to address their greenhouse-gas emissions yet also keep using their biggest sources of indigenous energy.

But ecologists and geologists have sounded a loud word of caution: what are the guarantees that these chambers are leakproof? A breach would surrender the man-made CO2 to the air, amplifying global warming.

In a study published on Wednesday in the journal Nature, a British team give a qualified "yes" to that question.

So far, attempts to investigate the leak issue have been limited to computer simulation and a few small-scale pilot studies in the North Sea and the United States, which have only been going on for a few years.

The British scientists have taken a different tack.

They pored over chemical signatures from gas fields in North America, China and Europe that are rich in natural CO2.

"We looked at nine CO2 fields, ranging from 10,000 years to about 42 million years old, and they have all stored CO2 for this length of time without obvious leakage signs," said Stuart Gilfillan, of the Scottish Centre for Carbon Storage at the University of Edinburgh.

"Basically, if you store it in the right location, the CO2 should be contained on that sort of time scale."

What traps the CO2 is, quite simply, underground water, Gilfillan and colleagues believe.

They looked at the ratio between isotopes of carbon dioxide and isotopes of a group of so-called noble gases, including helium.

These signatures indicate that highly soluble CO2 dissolves in the groundwater -- just like sparkling mineral water -- and does not leak to the surface, said Gilfillan.

The team believe that just a small amount -- just 18 percent or less -- of the CO2 reacts with minerals in the rock to form a new carbonate shell in the chamber. This is the other theoretical way by which CO can be sequestrated.

Gilfillan said the outcome is good news for "clean coal" but not an automatic green light for it.

Any potential storage site had to be thoroughly prospected to see it was geologically sound and ensure that its groundwater did not leak to the surface, he said.

"If you're talking about an old gas field or an old oil field and you've got no evidence of [gas] leakage up to the surface or oily water coming up, then that would be a strong candidate for storage."

Another shadow hanging over CCS is the cost of stripping out the CO2 and pumping it to the nearest suitable underground chamber.

Critics say the price could be higher than switching to a mix of solar, wind, fuel cells and other renewables that would resolve the problems of fossil fuels once and for all.

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