Could thawing permafrost release carbon frozen since the ice age?

It could, and some of it already is. The big questions are how much, and how fast. Permafrost is ground that stays frozen for at least two years running. It covers only about 9 percent of Earth's land. Yet it holds between a quarter and half of all the carbon stored in soils. The northern permafrost region holds about 1,460 to 1,600 billion tonnes of carbon. That is more than double what is in the whole atmosphere today.

Much of that carbon is thousands of years old. Dead plants froze into the ground before microbes could break them down. As the Arctic warms, most permafrost thaws slowly, centimetre by centimetre. But there is also a fast path, called abrupt thaw. A hillside collapses, or a lake suddenly forms, and old soil is exposed within days.

Siberia's Batagaika crater shows what that looks like. The ground began to sink in the 1960s, after the forest on a hillside was cleared. Today it is a gash a kilometre long and up to 100 metres deep, and its rim keeps sliding in. As it grows, it uncovers ancient forests, pollen, and the carcasses of mammoths and other ice-age animals.

How much carbon will escape is still uncertain. A 2018 study found that abrupt thaw could more than double permafrost emissions by 2100. A 2022 assessment put the extra warming from abrupt thaw at about 0.04 °C for every degree of global warming by 2100. Measurements disagree too: one estimate has the Arctic's permafrost losing carbon each year, while a 2022 study found the region gaining carbon on balance. What is certain is the size of the store, and that warming shrinks it.