THE INTERPLAY BETWEEN SOURCES OF METHANE AND BIOGENIC VOCS IN GLACIAL-INTERGLACIAL FLUCTUATIONS IN ATMOSPHERIC GREENHOUSE GAS CONCENTRATIONS

Recent analyses of ice core methane concentrations have suggested that methane emissions from wetlands were the primary driver for prehistoric changes in atmospheric methane. But these data conflict as to the location of wetlands, magnitude of emissions, and the environmental controls on methane oxidation. The flux of other reactive trace gases to the atmosphere also controls apparent atmospheric methane concentrations because these compounds compete for the hydroxyl radical, which is the primary sink for methane. In a series of coupled biosphere-atmosphere chemistry-climate modelling experiments, we simulate the methane and biogenic volatile organic compound emissions from the terrestrial biosphere from the Last Glacial Maximum to present. Using the chemistry-climate model, we simulate the atmospheric concentrations of methane, the hydroxyl radical, and numerous other reactive trace gas species. Over the past 21000 years methane emissions from wetlands increased slightly to the end of the Pleistocene, but then decreased again, reaching levels at the preindustrial Holocene that were similar to the LGM. Global wetland area decreased 14% from LGM to preindustrial. However, emissions of biogenic volatile organic compounds nearly doubled over this time period. Atmospheric OH burdens and methane concentrations were affected by this major change in BVOC emissions, with methane lifetimes increasing 4-6 years from LGM to present. This would represent a change in methane concentration of ca. 250 ppb. Thus glacial-interglacial changes in atmospheric methane concentrations would have been modulated by BVOC emissions. In addition, the increase in atmospheric methane concentrations since the mid-Holocene are captured in our results without invoking the hypothesized major increase in anthropogenic emissions over this period. The sustained increase in total BVOC emissions since the LGM would have had a major impact on the global carbon cycle as a whole. While the interplay between BVOC and wetland methane emissions since the LGM cannot explain the entire record of ice core methane concentrations, consideration of BVOC source dynamics is central to understanding ice core CH₄ and CO₂. Rapid changes in atmospheric methane concentrations, also observed in ice-cores, require further study.