Early anthropogenic CH₄ emissions and the variation of CH₄ and ¹³CH₄ over the last millennium

This study presents a new hypothesis to explain the observed variation of CH₄ and δ¹³C-CH₄ over the last millennium. It was originally proposed that the observed minimum of δ¹³C-CH₄ prior to the start of industrialization is caused by a large shift in biomass burning emissions between 1400 and 1700 A.D. According to our new hypothesis, however, the δ¹³C-CH₄ minimum is the first sign of the global rise of anthropogenic CH₄ emissions. The main idea is that emissions of isotopically depleted CH₄, from, for example, rice cultivation, domestic ruminants, and waste treatment started increasing earlier than the isotopically enriched emissions from fossil fuel, which started with the start of industrialization. However, because the observed increase of atmospheric methane only started around 1750 A.D., these preindustrial anthropogenic emissions must have been accompanied by a net reduction of natural CH₄ sources during the Little Ice Age (LIA) compensating for the increase of anthropogenic emissions during that period. Results of transient box model simulations for the last millennium show that under the new hypothesis a close agreement can be obtained between model and measurements. Prior to 1400 A.D., low emissions from anthropogenic biomass burning require a sizable contribution of methane emissions from vegetation to explain the observed high level of δ¹³C-CH₄. During the Little Ice Age, a larger than expected reduction of natural sources is needed, which calls for further verification using a more sophisticated modeling approach and additional constraints from ice core measurements.