S.M. Miller, A.M. Michalak, and S.C. Wofsy
In this letter to PNAS, we describe the inherent differences between “bottom-up” inventory estimates of methane emissions and “top-down” estimates that derive information directly from atmospheric observations. Top-down estimates provide the most natural measure of total emissions over relatively large geographic regions, while inventories are the natural avenue for cataloguing various types of emissions at small scales. As a result, we argue that careful, detailed assessments are needed to reconcile the emissions clearly visible from atmospheric observations with bottom-up emissions inventories.
Hristov et al. (1) argue that our study “provides a comprehensive, quantitative analysis of anthropogenic methane sources,” but that the conclusion “that US EPA [US Environmental Protection Agency] estimates for livestock methane emissions are grossly underestimated appears to be unsubstantiated by … [a] ‘bottom-up’ approach” outlined in their letter. In this reply, we discuss the information provided by atmospheric methane data about methane emissions, and comment on the challenge of connecting “bottom-up” and “top-down” estimates, a conclusion shared Hristov et al. (1).
Miller S.M., A.M. Michalak, S.C. Wofsy (2014) “Reply to Hristov et al.: Linking methane emissions inventories with atmospheric observations”, Proceedings of the National Academy of Sciences, 111:14, E1321, doi: 10.1073/pnas.1401703111.