Centennial evolution of the atmospheric methane budget: what do the carbon isotopes tell us?

[摘要] Little is known about how the methane source inventory and sinks have evolvedover recent centuries. New and detailed records of methane mixing ratio andisotopic composition (¹²CH₄, ¹³CH₄ and ¹⁴CH₄)from analyses of air trapped in polar ice and firn can enhance thisknowledge. We use existing bottom-up constructions of the source history,including "EDGAR"-based constructions, as inputs to a model of the evolvingglobal budget for methane and for its carbon isotope composition through the20th century. By matching such budgets to atmospheric data, we examine theconstraints imposed by isotope information on those budget evolutions.Reconciling both ¹²CH₄ and ¹³CH₄ budgets with EDGAR-basedsource histories requires a combination of: a greater proportion of emissionsfrom biomass burning and/or of fossil methane than EDGAR constructionssuggest; a greater contribution from natural such emissions than is commonlysupposed; and/or a significant role for active chlorine or otherhighly-fractionating tropospheric sink as has been independently proposed.Examining a companion budget evolution for ¹⁴CH₄ exposesuncertainties in inferring the fossil-methane source from atmospheric¹⁴CH₄ data. Specifically, methane evolution during the nuclear erais sensitive to the cycling dynamics of "bomb ¹⁴C" (originating fromatmospheric weapons tests) through the biosphere. In addition, since ca. 1970,direct production and release of ¹⁴CH₄ from nuclear-powerfacilities is influential but poorly quantified. Atmospheric ¹⁴CH₄determinations in the nuclear era have the potential to better characterizeboth biospheric carbon cycling, from photosynthesis to methane synthesis, andthe nuclear-power source.

[学科分类] 大气科学