Permafrost: a climate time bomb?

The Earth's vast tracts of permafrost hold billions of tonnes of planet-heating greenhouse gases that scientists warn will be released by global warming, along with diseases long locked into the ice. Permafrost -- soil that is frozen, although not necessarily permanently as it name implies -- is found mostly in the Northern Hemisphere, where it covers about a quarter of exposed land and is generally thousands of years old. It covers a wide belt between the Arctic Circle and boreal forests, spanning Alaska, Canada, northern Europe and Russia.