Somewhere beneath the Siberian tundra, a quantity of carbon three times greater than that stored in every tree on Earth — including the entire Amazon — sits locked inside frozen ground. That number, 1,500 gigatons, is what makes Gustaf Hugelius uneasy. A professor at the Bolin Centre of Climate Research at Stockholm University, Hugelius has spent years studying permafrost: the ground that remains frozen year-round across roughly 20 million square kilometres of Russia, Canada, Alaska and parts of Europe — an area five times the size of the European Union. The mechanism he describes is disarmingly simple and deeply alarming.

As fossil-fuel combustion raises global temperatures, frozen ground that has been stable for thousands of years begins to thaw. Ancient plant remains, locked inside what Hugelius calls "a natural freezer," are suddenly exposed to microbes. Those microbes consume the organic matter and release carbon dioxide and methane into the atmosphere, which drives further warming, which drives further thawing. The feedback loop is self-reinforcing and, critically, it cannot simply be reversed by a period of cooling.

That irreversibility is the detail most likely to be underestimated. When permafrost thaws, the ice beneath the surface melts and drains away, causing the land above to collapse. The result is a scarred landscape of lakes, wetlands and gullies known as thermokarsts — landforms with entirely different drainage patterns and soil temperatures that will continue emitting carbon for decades or centuries. On any human timescale, Hugelius warns, those systems have effectively tipped.