Billions of tonnes of the greenhouse gas methane are trapped just below the surface of the East Siberian Arctic shelf. Melting means the area is poised to deliver a giant gaseous belch at any moment  - one that could bring global warming forward 35 years and cost the equivalent of almost a year's global GDP.

These are the conclusions of the first systematic analysis of the economic cost of Arctic melting, which delivers a sobering antidote to other, more upbeat assessments that say melting in this area would improve access to minerals on the ocean bed, increase fishing and create ice-free shipping lanes.

Previous work has estimated that more than a trillion tonnes of methane lie under the shelf, trapped inside lattices of ice known as hydrates, at depths as shallow as 20 metres.