Carbon-13 Evidence of the Causes of the Atmospheric Methane Concentration Increase Since 1700AD and of a Varying Methane Budget Beforehand

The causes of the major increase in the concentration of atmospheric methane since 1700 AD are investigated. We measured d13CH4 in air extracted from Antarctic ice cores and firn and in air archived from Cape Grim, Tasmania. This was done using a newly developed GC-IRMS technique that analyses very small air samples with high precision. The results show that d13CH4 increased over the past century, consistent with the addition of isotopically enriched sources from anthropogenic activity. However a large increase (about 1.5 per mil) in d13CH4 from 1700AD back to about 1500AD was also observed. This was quite unexpected as it was previously assumed, based on the small observed changes in methane concentration, that the pre-anthropogenic methane budget was approximately in balance. Explanations for the cause of this d13CH4 increase invoke changes in enriched sources such as biomass burning with compensating changes in biogenic (mainly wetland) sources, or changes in a strongly fractionating sink.