The atmospheric CH4 increase from ∼0.7 to 1.68 p.p.m.v. over about the past 300 years which has been documented from analysis of air trapped in ice cores1-4 and from tropospheric measurements (see ref. 5 for example) is attributed to anthropogenic modifications of the CH4 cycle. The concern about this increase is due to the radiatively and chemically active nature of CH4. Here we present strong evidence from analysis of the Vostok ice core, that CH4 concentrations increased from 0.34 to 0.62 p.p.m.v. between the end of the penultimate ice age and the following interglacial, about 160-120 kyr BP. This CH4 change may be explained by considering the effect of the climatic change on the CH4 cycle. Its contribution (including chemical feedback) to the global climatic warming is estimated to be about 25% of that due to CO2. © 1988 Nature Publishing Group.