Constraining methane flux estimates using observations of atmospheric methane and carbon-13/carbon-12 isotopic ratios in methane

Understanding the budget of CH₄ is crucial to predicting climate change and managing earth's carbon reservoirs. CH₄ is the second most important anthropogenic greenhouse gas, contributing about 20% to the total radiative forcing from long-lived, globally mixed greenhouse gases. In addition, CH₄ has a significant impact on atmospheric chemistry, affecting OH, O₃, and CO mixing ratios in the troposphere and O ₃ and Cl in the stratosphere. At present, many of the sources and sinks of CH₄ are poorly understood due in part to the large spatial and temporal variability of the sources.; Model simulations of CH₄ mixing ratios using bottom-up source estimates typically over-predict the latitudinal gradient of atmospheric CH ₄ relative to the observations; however, the specific source processes responsible for this discrepancy have not been identified definitively. The aim of this work is to employ the differing isotopic signatures of the source processes to add an additional constraint to the CH₄ budget and attribute discrepancies between bottom-up source estimates and observations of CH₄ in the atmosphere to a source process or group of source processes.; To this end, observations of CH₄ and 13C/ 12C isotopic ratios in CH₄ are used in conjunction with an inverse model of the CH₄ budget. Inverse modeling is a top-down approach which uses observations of trace gases in the atmosphere, an estimate of the spatial pattern of trace gas fluxes, and a model of atmospheric transport to estimate the sources and sinks. Global and regional estimates of CH ₄ fluxes are presented that are consistent with the atmospheric observations of CH₄, observations of the 13C/12C isotopic ratios, and process-level understanding of sources using two complimentary approaches to the CH₄ inverse problem. Both approaches imply interesting differences from previous methane budget estimates, most notably a large increase in the tropical biomass burning source and the tropical and southern extra tropical wetland sources and a decrease in emissions from rice cultivation and fossil sources. In several cases, the inverse estimates are supported by recent process studies, known anomalies in climate, and other model-independent information.