Permafrost contains about 50% of the global soil carbon(1). It is thought that the thawing of permafrost can lead to a loss of soil carbon in the form of methane and carbon dioxide emissions(2,3). The magnitude of the resulting positive climate feedback of such greenhouse gas emissions is still unknown(3) and may to a large extent depend on the poorly understood role of microbial community composition in regulating the metabolic processes that drive such ecosystem-scale greenhouse gas fluxes. Here we show that changes in vegetation and increasing methane emissions with permafrost thaw are associated with a switch from hydrogenotrophic to partly acetoclastic methanogenesis, resulting in a large shift in the delta C-13 signature (1015 parts per thousand) of emitted methane. We used a natural landscape gradient of permafrost thaw in northern Sweden(4,5) as a model to investigate the role of microbial communities in regulating methane cycling, and to test whether a knowledge of community dynamics could improve predictions of carbon emissions under loss of permafrost. Abundance of the methanogen Candidatus Methanoflorens stordalenmirensis(6) is a key predictor of the shifts in methane isotopes, which in turn predicts the proportions of carbon emitted as methane and as carbon dioxide, an important factor for simulating the climate feedback associated with permafrost thaw in global models(3,7). By showing that the abundance of key microbial lineages can be used to predict atmospherically relevant patterns in methane isotopes and the proportion of carbon metabolized to methane during permafrost thaw, we establish a basis for scaling changing microbial communities to ecosystem isotope dynamics. Our findings indicate that microbial ecology may be important in ecosystem-scale responses to global change.