Accounting for the natural 40% reduction in methane during the Last Glacial Maximum (LGM) represents one of the most challenging problems posed by polar ice core studies. Inverse modelling, constrained by observations, suggest it is driven by a decline in wetland sources, but this is not supported by forward modelling. Using a comprehensive Earth system model, we show that closure of the methane budget requires reduced emissions and an amplified sink caused by reduction in emissions of volatile organic compounds (VOCs) from LGM forests. Our study emphasizes a key role for the terrestrial biosphere in the methane budget during the Quaternary.