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"Researchers who have recently returned from the region found that an area of permafrost spanning a million square kilometres - the size of France and Germany combined - has started to melt for the first time since it formed 11,000 years ago at the end of the last ice age.
The area, which covers the entire sub-Arctic region of western Siberia, is the world's largest frozen peat bog and scientists fear that as it thaws, it will release billions of tonnes of methane, a greenhouse gas 20 times more potent than carbon dioxide, into the atmosphere.
It is a scenario climate scientists have feared since first identifying "tipping points" - delicate thresholds where a slight rise in the Earth's temperature can cause a dramatic change in the environment that itself triggers a far greater increase in global temperatures."