A note on the relationship between ice core methane concentrations and insolation

We re-examine the link between July 30°N insolation and methane in the Vostok ice core. Based on this link, Ruddiman [2003] suggested that an anthropogenic source of methane must have been present after 5 kyr BP in order to prevent concentrations from declining as insolation decreased through the Holocene. We conclude however, that since precessional forcing does not explain the large glacial-interglacial excursions, the component of methane variability associated with precession is significantly smaller than assumed by Ruddiman [2003]. The implied decrease from 10 kyr BP to the present associated with precession alone is 60 ppbv or less. We argue that increased emissions controlled by northern wetlands and river delta development likely contributed to the observed increase, consistent with similar stable CH₄ levels at MIS 11 in the Vostok and Dome C records. Therefore a significant anthropogenic input is not obviously required to explain the late Holocene record.