Thawing permafrost could inject enough carbon into the atmosphere to cook the planet. But nobody’s quite sure how fast it’s going to happen.

Permafrost is a giant cold-storage compost heap, stuffed full of frozen carbon. Just like you chucked out last night’s potato peelings, the planet has chucked out billions of tonnes of dead plants, trees, mammoths and, yes, polar bears, all of which is now happily interred under the Arctic wastes.

The difference is that while your compost heap ticks over at a nice warm temperature, breaking down the potato peelings into compost, the frozen ground which makes up permafrost stops that organic stew of Arctic flora and fauna from decomposing, safely locking up the carbon stored in it.

I say ‘safely locking up’ because from the point of view of creating human civilisation, permafrost has been pretty handy. While the permafrost has been permanently frozen, we’ve been busy ekeing out human life, discovering fire, developing agriculture, growing our population. While we’ve been busy nurturing the capabilities that ultimately allow the lucky few to participate in Britain’s Got Talent, the planet’s been watching our backs by keeping this massive store of carbon locked up under the frozen parts of the planet’s surface.