Ice cores taken from Greenland indicate that during the last deglaciation, approximately 10,000 years ago, increases in temperature occurred at the same times as increases in atmospheric methane, a potent greenhouse gas. The source of the increase in atmospheric methane is still debated, but it has been suggested that thermokarst lakes, which form from thawing permafrost, contributed to the increased atmospheric methane. By establishing pathways of regionally varying hydrogen isotopes found in permafrost ground ice to methane produced in thermokarst lakes, Brosius et al. reconciled bottom‐up estimates of emission of methane from thermokarst lakes in various Arctic regions with isotope constraints from ice core records; they show that thermokarst lakes were indeed an important source of atmospheric methane during the deglacial period. (Journal of Geophysical Research‐Biogeosciences, doi:10.1029/2011JG001810, 2012)