Many nations want a draft U.N. report to tone down prospects for sucking greenhouse gases from the air to help fix global warming, reckoning the technologies are risky, documents seen by Reuters show.

Government officials and scientists are meeting in Berlin this week to edit the report, which says time is running out to keep warming below an agreed ceiling of 2 degrees Celsius (3.6 Fahrenheit) over pre-industrial times.

The study, focused on solutions to climate change, is meant to guide almost 200 governments in preparing a U.N. pact due by the end of 2015 to curb rising emissions and help limit heat waves, floods, droughts and rising seas.

China, the European Union, Japan and Russia were among nations saying the draft, to be published on Sunday, should do more to stress uncertainties about technologies that the report says could be used to extract carbon dioxide from the atmosphere and bury it below ground to limit warming.

Carbon dioxide removal (CDR) "technologies are currently not available and would be associated with high risks and adverse side-effects," the German government said in a comment on the draft by the Intergovernmental Panel on Climate Change (IPCC).

"There are no CDR technologies by now," Russia said. The technologies would go far beyond the traditional focus on cutting emissions from burning coal, oil or natural gas.

Several nations were especially sceptical about the draft's mention of stripping greenhouse gases from electricity-generating facilities burning biomass - wood or other plants - to bury them underground as a way to extract carbon from nature.

Ethanol
Plants soak up carbon as they grow and release it when they rot or burn. Chemicals can extract carbon from the exhaust fumes from burning crop waste, for instance, or from fermentation of corn to make ethanol.

Among projects, Archer Daniels Midland Co has a facility in Illinois to inject 333,000 tonnes of carbon dioxide a year into the ground from a factory producing ethanol from corn. Husky Energy in Canada produces carbon dioxide from ethanol for injection into oil wells.

Many nations said that the draft should do more to mention drawbacks of bioenergy with carbon capture and storage (BECCS), such as the amount of land needed to grow plants and risks that it would compete with food production.

Internal IPCC documents show that China said BECCS "bears great uncertainties". Japan said that "considerations of trade-offs with water, land and biodiversity are crucial to avoid adverse effects" with CDR technologies.

A sub-chapter of the report says that BECCS has the theoretical potential to extract up to 10 billion tonnes a year of carbon dioxide from nature - roughly equivalent to China's carbon emissions - but would cost between $60 and $250 a tonne.

Other methods for extracting greenhouse gases from the atmosphere include simply planting trees or fertilising the oceans to promote the growth of algae, hoping that the tiny carbon-rich plants would fall to the seabed when they die.

Among other debates in Berlin on Tuesday, delegates said that Saudi Arabia, the world's top oil exporter, objected to a line in the report pointing out that fossil fuels were the overwhelming cause of rising emissions in the past decade.

(Reporting By Alister Doyle, Environment Correspondent; Editing by Tom Heneghan)