The Arctic tundra now emits more carbon dioxide and other greenhouse gases than it absorbs. Rising temperatures due to climate change have shifted the ecosystem’s effect on the planet – it has moved from helping to cool Earth to having a warming effect.

“It’s a really serious change,” says Twila Moon at the National Snow and Ice Data Center in Colorado. “Human-caused warming is now causing warming from nature. It is irreversible on a thousands-of-years timescale.”…