Disappearing Arctic ice, which gets most of the attention from climate scientists, is an effect of humanmade climate change. By contrast, the melting of frozen soil, or permafrost, can drive warming. As it thaws, microbes devour carbon previously locked inside, unleashing carbon dioxide—a potent greenhouse gas—in the process. The carbon dioxide amplifies the warming power of carbon pollution in a vicious feedback loop.

Scientists have struggled, however, to quantify this threat. Permafrost occurs on a quarter of the Northern Hemisphere landmass—from Alaska to Canada and across Siberia—but researchers have taken far too few readings to feel very confident about the risk. "We are working on really large landmasses with limited data," says physical geographer Gustaf Hugelius of Stockholm University. Measuring the carbon content of permafrost requires muddy field work with heavy drilling machinery, operated in remote areas, and satellite data help little.

Those logistical constraints have largely limited researchers' previous estimates of carbon to the top meter of permafrost. But scientists think that carbon down to a depth of 3 meters is susceptible to thawing and release as well. An influential 2009 estimate that Arctic permafrost held 1.6 trillion metric tons of carbon included only 45 field sites analyzed down to that depth. In the new study, Hugelius added 405 new analyses of field sites that went to 3 meters, some through new field work he and colleagues performed, some from archived data. Adding up the carbon found in different soil types he says, yields an initial new estimate of 1894 billion metric tons of carbon locked into permafrost across the Arctic, 13% more than the previous estimate.