"The unknowns about the amount and rate of methane release from the thawing Arctic makes it a wild card when considering climate change risks," UNEP head Achim Steiner said in an annual report with a special section on Arctic methane.

UNEP said that global methane emissions from all sources, both natural and caused by human activities, were estimated at 500-600 million tonnes a year. A quarter to a third was emitted from the wet Arctic soils, where microbes produce methane.

Arctic methane emissions were projected to at least double during the 21st century, partly because of an increase in wetlands caused by thawing of permafrost, it said.

"The balance of evidence suggests that Arctic feedbacks that amplify warming, globally and regionally, will dominate during the next 50 to 100 years," it said.

Vast amounts of methane entering the atmosphere "would lead to abrupt changes in the climate that would likely be irreversible," UNEP said. "We must not cross that threshold."

Global warming, blamed mainly on human burning of fossil fuels that produce heat-trapping carbon dioxide, is projected to bring more floods, droughts and to raise world sea levels. Methane is the number two greenhouse gas from human activity.

"A potentially very large Arctic source of methane to the atmosphere is the decay of organic matter in the form of dead plant, animal and microbial remains that have been frozen in shallow permafrost for tens of thousands of years," it said.

"This important source of atmospheric methane is not currently considered in modelled projections," it said.

Another vast source of methane is in icy deposits known as methane hydrates, often in sediments deep under the world's oceans. Such hydrates store more carbon than all the proven reserves of coal, oil and gas and could also thaw, UNEP said.

Temperatures in the Arctic are rising by about twice the global average -- darker ground and seawater, when exposed, soak up far more heat than reflective ice and snow.

(For Reuters latest environment blogs go to: http://blogs.reuters.com/environment/)

(Reporting by Gerard Wynn, writing by Alister Doyle in Oslo, Editing by Stephen Weeks)