The Arctic tundra represents a vast store of carbon that may be broken down by microbial activity into greenhouse gases such as CO₂ and CH₄. Though microbes are less active in winter, the long duration of the cold season makes this period very important for carbon cycling. We show that, under conditions of warmer winter air temperatures and greater snowfall, deeper soils can remain warm enough to sustain significantly enhanced CH₄ emission. This could have large implications for future climates.