Earth's climate can change gear very quickly, either sharply warming or fiercely cooling (1). Past shifts of this kind were massive, and some took place within a few years (2). About 11,600 years ago, at the end of the Younger Dryas cold period, the planet warmed very suddenly, with strong increases in atmospheric greenhouse gases, especially methane. On page 506 of this issue, Petrenko et al. use radiocarbon (14C) data to identify the sources of the additional methane (3).