MONTREAL (AFP) - More than a third of annual emissions of carbon dioxide (CO2), the principal gas blamed for global warming, could be stored underground by 2050, according to a report issued by a top UN scientific panel.

By 2100, between 220 billion and 2,200 billion tonnes of CO2 could be stored in the Earth's crust, amounting to between 15 and 55 percent of global pollution by the end of the century.

CO2 is the biggest of the greenhouse gases that trap the sun's heat and cause Earth's temperature to rise.

The pollution results from burning fossil fuels such as oil, gas and coal.

Curbing it entails a cost in fuel efficiency and in the need to invest in cleaner energy sources -- and this is a tab that many countries, led by the United States, find dauntingly high.

Carbon storage is thus being explored as a means of tackling some of the pollution while also easing the cost.

Under it, CO2 would be taken at source from power stations, oil refineries, glassworks, cement factories and other big emitters.

Instead of being unleashed into the atmosphere, the gas would be trapped, compressed and then piped deep underground, into disused hydrocarbon fields beneath the sea, secure rock chambers on land, or out into the sea.

Three pilot schemes have been running successfully in the North Sea, Canada and Algeria, but they have failed to convince green activists.

They worry especially about what would happen if an earthquake caused a chamber to rupture, sending hundreds or billions of tonnes of CO2 into the sky and dramatically worsening the greenhouse effect.

But the IPCC gave guarded approval to storage, saying it boosted the range of options.

"(It) has the potential to reduce overall mitigation costs and increase flexibility in achieving greenhouse-gas emission reductions," its report said.

It added, though, "the widespread application of (carbon storage) would depend on technical maturity, costs, overall potential and transfer of the technology to developing countries and their capacity to apply the technology, regulatory aspects, environmental issues and public perception."

It also warned the sea had only a limited capacity for absorbing CO2.

Recent studies showed that higher CO2 levels in the oceans, caused by absorption from the atmosphere, were also increasing the seas' acidity, and this spelt a threat to biodiversity.

The report was ordered in 2003. The IPCC is a UN panel tasked with gathering the best scientific opinion about global warming.

Under the UN's Kyoto Protocol, industrial countries that have ratified the treaty are required to trim pollution of carbon gases by a deadline of 2012 as compared with a 1990 benchmark.

But this effort falls far sure of what is need to avoid damaging and potentially calamitous climate change, scientists say.

The deal does not include the world's top polluter, the United States, which by itself accounts for a quarter of global emissions.

Nor does Kyoto require targeted cuts by developing countries. China and India are on track to becoming mega-polluters as their fast-growing economies, driven by huge populations, gobble up fossil fuels.

As for the costs of storage, the IPCC said separating CO2 at source and compressing it would cost between 15-75 dollars per tonne of CO2.

Transporting the CO2 is put at between one and eight dollars per tonne of CO2 for every 250 kms (156 miles) carried. Burial costs would range from 0.5 to 100 dollars per tonne, depending on the method.