Enormous areas of permanently frozen ground across the Arctic have warmed by more than a quarter of a degree Celsius every year between 2007 and 2016. These surface soil and pockets of deeper ground in these fast-thawing permafrost regions are among the largest storehouses of carbon in the world, holding between 1.4 and 1.6 trillion tonnes of the greenhouse gas—equivalent to up to 37 years of current total global emissions.