There are approximately 1.5 trillion tons of organic carbon locked in Arctic permafrost — roughly twice the amount currently in the atmosphere. As the Arctic warms four times faster than the global average, that carbon is thawing and decomposing. A new synthesis of soil measurement data from 47 Arctic field sites suggests climate models have been systematically underestimating this feedback by a factor of two to three. The mechanism, it turns out, involves microbial communities that behave very differently under intermittent freeze-thaw cycles than under steady warming.