An analysis of 1516 radiocarbon dates demonstrates that the development of the current circumarctic peatlands began approximately 16.5 thousand years ago (ka) and expanded explosively between 12 and 8 ka in concert with high summer insolation and increasing temperatures. Their rapid development contributed to the sustained peak in CH4 and modest decline of CO2 during the early Holocene and likely contributed to CH4 and CO2 fluctuations during earlier interglacial and interstadial transitions. Given the decreased tempo of peatland initiation in the late Holocene and the transition of many from fens (which generated high levels of CH4) to ombrotrophic bogs, a neoglacial expansion of northern peatlands cannot explain the increase in atmospheric CH4 that occurred after 6 ka.