Soils could also be giving up their carbon stores: evidence emerged in 2005 that a vast expanse of western Siberia was undergoing an unprecedented thaw. The region, the largest frozen peat bog in the world, had begun to melt for the first time since it formed 11,000 years ago. Scientists believe the bog could begin to release billions of tonnes of methane locked up in the soils, a greenhouse gas 20 times more potent than carbon dioxide. The World Meteorological Organisation recently reported the largest annual rise of methane levels in the atmosphere for a decade.