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.
Permafrost exists to a lesser degree in the Southern Hemisphere where there is less ground to freeze, including in the South American Andes and below Antarctica.
It can vary in depth from a few metres to more than 100.Locked into the permafrost is an estimated 1.7 trillion tonnes of carbon in the form of frozen organic matter — the remains of rotted plants and long- dead animals trapped in sediment and later covered by ice sheets.
When permafrost thaws, this matter warms up and decomposes, eventually releasing the carbon that it holds as carbon dioxide (CO2) and methane, gases which have a greenhouse warming effect on the planet.
Permafrost soils contain roughly twice as much carbon — mainly in the form of methane and CO2 — as Earth’s atmosphere.
Most of the carbon stocks are thought to reside fairly close to the surface.