B043-07
Improved constraints on global methane emissions and sinks using δ13C-CH4

Wednesday, 9 December 2020: 19:24
Virtual
Xin Lan1,2, Sourish Basu3,4, Stefan Schwietzke5, Lori Bruhwiler2, Edward J Dlugokencky2, Sylvia Englund Michel6, Owen Sherwood7, Pieter P Tans2, Kirk W Thoning2, Giuseppe Etiope8, Qianlai Zhuang9, Licheng LIU9, Youmi Oh9, John B Miller2, Gabrielle Petron1,2, Bruce H Vaughn6 and Monica Crippa10, (1)University of Colorado at Boulder, CIRES, Boulder, CO, United States, (2)NOAA, Global Monitoring Laboratory, Boulder, CO, United States, (3)NASA Goddard Space Flight Center, Global Modeling and Assimilation Office, Greenbelt, MD, United States, (4)University of Maryland College Park, College Park, MD, United States, (5)Environmental Defense Fund, Berlin, Germany, (6)University of Colorado, Institute of Arctic and Alpine Research, Boulder, CO, United States, (7)Dalhousie University, Department of Earth and Environmental Sciences, Halifax, NS, Canada, (8)INGV National Institute of Geophysics and Volcanology, Rome, Italy, (9)Purdue University, Department of Earth, Atmospheric, and Planetary Sciences, West Lafayette, IN, United States, (10)European Commission Joint Research Centre, Directorate for Energy, Transport and Climate, Ispra, Italy
Recorded Presentation
Abstract:
The atmospheric CH4 burden has been increasing since 2007 after a relatively stable period from 1999-2006. The reasons behind this increase are still debated, because CH4 sources and sinks are grossly under-constrained by existing atmospheric CH4 observations. Here, we examine the stable carbon isotope ratio of atmospheric CH4, 13C/12C (reported as δ13C-CH4), for additional constraints on CH4 emissions and sinks, since different CH4 sources have distinct δ13C-CH4 signatures over large spatial scales and different CH4 sinks have different preference for oxidation of 12C over 13C, i.e. the kinetic isotopic effect. While the global mass balance requirements for CH4 and δ13C-CH4 suggest a much larger fossil emission magnitude than those estimated in most emission inventories (e.g. EDGAR 4.3.2), the CH4 source attribution is also sensitive to assumptions about CH4 sinks. We constructed 11 candidate emission scenarios based on proposed hypotheses explaining CH4 emissions in the literature, and simulated them in the TM5 chemical transport model with 3 different sink scenarios for the period 1984-2016. We find that a proposed decreasing trend in the abundance of hydroxyl radicals (OH) cannot explain the observed decrease in global mean δ13C-CH4. That is because total sink-weighted fractionation becomes stronger if the OH sink contribution weakens relative to the other sinks, and atmospheric δ13C-CH4 increases as a result. Instead, a decreasing trend in destruction of CH4 by soil microbes, which has much stronger isotopic fractionation factor than the OH sink, can produce a decreasing trend in δ13C-CH4. We also investigate the uncertainty introduced by the CH4 reaction with tropospheric chlorine, a CH4 sink whose abundance and temporal changes remain uncertain but has large isotopic fractionation. We find that including or excluding tropospheric Cl (~ 13 Tg/yr CH4 sink in our model) changes the magnitude of estimated fossil emissions by ~20 %. While many CH4 budget analyses, with or without using δ13C-CH4 as an additional constraint, have mostly focused on estimated emissions, we also recommend comparing different sink assumptions due to the large uncertainties associated with them.
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