By Paul Rincon BBC News Online science staff

Commercial exploitation of hydrates may still be possible

Gas hydrates contain huge quantities of natural gases - mainly methane - and are tipped as a future energy source.

Estimates of hydrate-bound gas fell steadily in the last 30 years due to growing knowledge of their distribution and concentration in sediments.

The findings are presented in the academic journal Earth Science Reviews.

Gas hydrates are deposits of ice-like crystals that trap natural gas under conditions of high pressure and low temperature such as those found in sea-floor sediments or in permafrost.

Limited reservoir

One widely cited estimate proposes that 10,000 gigatonnes (Gt) of methane carbon is bound up as hydrate on the ocean floor.

The worldwide numbers are important but they don't have much to do with the resource issue Tim Collett, US Geological Survey

The scientist reviewed previous data on the area of hydrate-bearing sediments and the gas yield of sediments to arrive at his conclusions.

Many researchers believe gas hydrates play an important role in the carbon cycle - one of the most important ecological processes on our planet.

For example, bursts of methane from these hydrate reservoirs have been implicated in several rapid warmings in the Earth's climate record.

Professor Gerald Dickens of Rice University, US, argues that a a sharp rise in global temperatures about 55 million years ago was caused by just such a burst.

'Profitable future'

Based on his own findings, Dr Milkov proposes the amount of methane released must have been an order of magnitude lower than was needed to cause this warming event.

Gas hydrates contain twice the methane of other fuel deposits

Hydrate accumulations supposedly contain about twice the methane carbon present in other fuel deposits.

"The worldwide numbers are important but they don't have much to do with the resource issue," said Tim Collett of the US Geological Survey in Denver.

"No matter if you take the lowest or biggest number, it still tells you there's a lot of gas out there."

In 1999, the Japan National Oil Company began drilling in the Nankai Trough, off Japan's coast.

There are also three drilling projects under way in the US, one in the Gulf of Mexico and two in Alaska.

But gas hydrate recovery carries a considerable risk. Destabilisation of hydrate reserves could cause sudden, massive releases of gas from the seafloor, threatening drilling platforms and ships.

"Drilling gas hydrates is estimated to be six times more expensive than exploitation of oil and other gas sources," said Prof Bahman Tohidi, director of the Centre for Gas Hydrate Research in Edinburgh.

Burning methane is supposedly "cleaner" than other fossil fuels. But the prospect of exploiting new large resources of fossil fuels alarms environmentalists who advocate cuts in carbon dioxide emissions to the atmosphere.