Image caption Getting gas from rocks is relatively easy, concerns about cost and the environment are emerging

The world may have twice as much natural gas than previously thought, according to the rich nations' think tank the International Energy Agency (IEA).

The world may have 250 years of gas usage at current levels thanks to "unconventional gas" from shale and coal beds, Anne-Sophie Corbeau, senior gas expert at the IEA told BBC News.

Estimates may even be revised upwards.

Studies are underway into newly-recoverable sources, Ms Corbeau said.

But she stressed that the totals were highly uncertain, and depended on price, technology and the accessibility of supplies.

"The gas story is huge," she told BBC News.

"A few years ago the United States was ready to import gas. In 2009 it had become the world's biggest gas producer. This is phenomenal, unbelievable."

'Huge resources'

The US achieved the change through a technological breakthrough in which firms found a way of using tiny explosions to free gas previously trapped in a common rock - shale.

Not all of this will be recoverable Anne-Sophie Corbeau, IEA Can Europe benefit from shale gas?

Miss Corbeau said other nations were now rushing to replicate the US success by exploiting gas currently trapped in various types of rock where it was thought to be impossible to access.

She said conventional natural gas supplies were assured for 60 years - with maybe a further 60 years if engineers could get to other supplies.

She admitted there is great uncertainly about how much unconventional gas is possible to exploit, but said the best estimate is that new sources will stretch gas supplies to 250 years at current levels.

"The resources are really huge," she said.

"We probably have 920 trillion cubic metres - that is more than 300 times the current annual demand for gas.

"Not all of this will be recoverable, but any country that develops new gas supplies will have a global impact on gas availability and price, as gas markets are all inter-connected."

Some sceptics believe that unconventional gas is being "talked up".

Firms wishing to exploit shale gas are already finding opposition from people living near gas deposits, some of which are in urban areas.

Shale gas needs many more drilling sites than conventional gas, which is obtained from porous rocks that allow the gas to flow into the bore without the need for explosive persuasion.

The revolution in unconventional gas is likely to affect climate change policy.

The IEA's chief economist Fatih Birol told BBC News that natural gas, which has only half the carbon emissions of coal, is already competing for priority with clean renewable energy.

Gas companies in Europe also insist it offers a cost-effective solution.

The IEA said Australia is taking the lead on unconventional gas with China, India and Indonesia close behind.

Firms in rich nations are pressing governments to allow them the same level of subsidies as renewables if they agree to capture and store their carbon emissions in rocks underground to prevent them warming the planet.

Today the World Meteorological Organisation said climate change was firmly underway.

It confirmed that 2010 had equalled the hottest year on record, with record low Arctic ice for December.