Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 7 8 2007 10.5194/acp-7-2119-2007 http://www.atmos-chem-phys.net/7/2119/2007/ http://www.atmos-chem-phys.net/7/2119/2007/acp-7-2119-2007.html http://www.atmos-chem-phys.net/7/2119/2007/acp-7-2119-2007.pdf 2119 2139 2007-05-02 Centennial evolution of the atmospheric methane budget: what do the carbon isotopes tell us? K. R. Lassey k.lassey@niwa.co.nz D. M. Etheridge D. C. Lowe A. M. Smith D. F. Ferretti National Institute of Water and Atmospheric Research, P.O. Box 14-901, Wellington, New Zealand CSIRO Marine and Atmospheric Research, PMB 1, Aspendale Vic. 3195, Australia Australian Nuclear Science and Technology Organisation, PMB 1, Menai NSW 2234, Australia Institute of Arctic and Alpine Research, University of Colorado, Boulder, Campus Box 450, CO 80309, USA Little is known about how the methane source inventory and sinks have evolved over recent centuries. New and detailed records of methane mixing ratio and isotopic composition (¹²CH₄, ¹³CH₄ and ¹⁴CH₄) from analyses of air trapped in polar ice and firn can enhance this knowledge. We use existing bottom-up constructions of the source history, including "EDGAR"-based constructions, as inputs to a model of the evolving global budget for methane and for its carbon isotope composition through the 20th century. By matching such budgets to atmospheric data, we examine the constraints imposed by isotope information on those budget evolutions. Reconciling both ¹²CH₄ and ¹³CH₄ budgets with EDGAR-based source histories requires a combination of: a greater proportion of emissions from biomass burning and/or of fossil methane than EDGAR constructions suggest; a greater contribution from natural such emissions than is commonly supposed; and/or a significant role for active chlorine or other highly-fractionating tropospheric sink as has been independently proposed. Examining a companion budget evolution for ¹⁴CH₄ exposes uncertainties in inferring the fossil-methane source from atmospheric ¹⁴CH₄ data. Specifically, methane evolution during the nuclear era is sensitive to the cycling dynamics of "bomb ¹⁴C" (originating from atmospheric weapons tests) through the biosphere. In addition, since ca. 1970, direct production and release of ¹⁴CH₄ from nuclear-power facilities is influential but poorly quantified. Atmospheric ¹⁴CH₄ determinations in the nuclear era have the potential to better characterize both biospheric carbon cycling, from photosynthesis to methane synthesis, and the nuclear-power source. Allan, W., Lowe, D. C., and Cainey, J. M.: Active chlorine in the remote marine boundary layer: Modeling anomalous measurements of $\delta ^13$C in methane, Geophys. Res. Lett., 28, 3239–3242, 2001a. Allan, W., Manning, M. R., Lassey, K. R., Lowe, D. C., and Gomez, A. J.: Modeling the variation of $\delta ^13$C in atmospheric methane: Phase ellipses and the kinetic isotope effect, Global Biogeochem. Cycles, 15, 467–481, 2001b. Allan, W., Lowe, D. C., Gomez, A. J., Struthers, H., and Brailsford, G. W.: Interannual variation of $^13$C in tropospheric methane: Implications for a possible atomic chlorine sink in the marine boundary layer, J. Geophys. Res., 110, D11306, doi:10.1029/2004JD005650, 2005. Allan, W., Struthers, H., and Lowe, D. C.: Methane carbon isotope effects caused by atomic chlorine in the marine boundary layer: Global model results compared with southern hemisphere measurements, J. Geophys. Res., 112, D04306, doi:10.1029/2006JD007369, 2007. Andreae, M. O. and Merlet, P.: Emission of trace gases and aerosols from biomass burning, Global Biogeochem. Cycles, 15, 955–966, 2001. Bainbridge, A. E., Suess, H. E., and Friedman, I.: Isotopic composition of atmospheric hydrogen and methane, Nature, 192, 648–649, 1961. Bellisario, L. M., Bubier, J. L., Moore, T. R., and Chanton, J. P.: Controls on CH₄ emissions from a northern peatland, Global Biogeochem. Cycles, 13, 81–91, 1999. Bergamaschi, P., Brühl, C., Brenninkmeijer, C. A. M., Saueressig, G., Crowley, J. N., Grooß, J. U., Fischer, H., and Crutzen, P. J.: Implications of the large carbon kinetic isotope effect in the reaction CH₄ + Cl for the $^13$C/$^12$C ratio of stratospheric CH₄, Geophys. Res. Lett., 23, 2227–2230, 1996. Bergamaschi, P., Bräunlich, M., Marik, T., and Brenninkmeijer, C. A. M.: Measurements of the carbon and hydrogen isotopes of atmospheric methane at Izaña, Tenerife: Seasonal cycles and synoptic-scale variations, J. Geophys. Res., 105, 14 531–14 546, 2000. Bousquet, P., Ciais, P., Miller, J. B., Dlugokencky, E. J., Hauglustaine, D. A., Prigent, C., van der Werf, G. R., Peylin, P., Brunke, E.-G., Carouge, C., Langenfelds, R. L., Lathière, J., Papa, F., Ramonet, M., Schmidt, M., Steele, L. P., Tyler, S. C., and White, J.: Contribution of anthropogenic and natural sources to atmospheric methane variability, Nature, 443, 439–443, 2006. Brenninkmeijer, C. A. M., Lowe, D. C., Manning, M. R., Sparks, R. J., and van Velthoven, P. F. J.: The $^13$C, $^14$C, and $^18$O isotopic composition of CO, CH₄, and CO₂ in the higher southern latitudes lower stratosphere, J. Geophys. Res., 100, 26 163–26 172, 1995. Cantrell, C. A., Shetter, R. E., McDaniel, A. H., Calvert, J. G., Davidson, J. A., Lowe, D. C., Tyler, S. C., Cicerone, R. J., and Greenberg, J. P.: Carbon kinetic isotope effect in the oxidation of methane by the hydroxyl radical, J. Geophys. Res., 95, 22 455–22 462, 1990. Chanton, J. P., Rutkowski, C. M., Schwartz, C. C., Ward, D. E., and Boring, L.: Factors influencing the stable carbon isotopic signature of methane from combustion and biomass burning, J. Geophys. Res., 105, 1867–1877, 2000. Chanton, J. P., Bauer, J. E., Glaser, P. A., Siegel, D. I., Kelley, C. A., Tyler, S. C., Romanowicz, E. H., and Lazrus, A.: Radiocarbon evidence for the substrates supporting methane formation within northern Minnesota peatlands, Geochim. Cosmochim. Acta, 59, 3663–3668, 1995. Conny, J. M. and Currie, L. A.: The isotopic characterization of methane, non-methane hydrocarbons and formaldehyde in the troposphere, Atmos. Environ., 30, 621–638, 1996. Craig, H.: The geochemistry of stable carbon isotopes, Geochim. Cosmochim. Acta, 3, 53–92, 1953. Craig, H.: Isotopic standards for carbon and oxygen and correction factors for mass spectrometric analysis of carbon dioxide, Geochim. Cosmochim. Acta, 12, 133–149, 1957. Craig, H., Chou, C. C., Welhan, J. A., Stevens, C. M., and Engelkemeir, A.: The isotopic composition of methane in polar ice cores, Science, 242, 1535–1539, 1988. Crowley, J. N., Saueressig, G., Bergamaschi, P., Fischer, H., and Harris, G. W.: Carbon kinetic isotope effect in the reaction CH₄+Cl: a relative rate study using FTIR spectroscopy, Chem. Phys. Lett., 303, 268–274, 1999. Cunnold, D. M., Steele, L. P., Fraser, P. J., Simmonds, P. G., Prinn, R. G., Weiss, R. F., Porter, L. W., O'Doherty, S., Langenfelds, R. L., Krummel, P. B., Wang, H. J., Emmons, L., Tie, X. X., and Dlugokencky, E. J.: In situ measurements of atmospheric methane at GAGE/AGAGE sites during 1985–2000 and resulting source inferences, J. Geophys. Res., 107, doi:10.1029/2001JD001226, 2002. Dentener, F., Peters, W., Krol, M., van Weele, M., Bergamaschi, P., and Lelieveld, J.: Interannual variability and trend of CH₄ lifetime as a measure for OH changes in the 1979–1993 time period, J. Geophys. Res., 108, 4442, doi:10.1029/2002JD002916, 2003. Dlugokencky, E. J., Masarie, K. A., Lang, P. M., and Tans, P. P.: Continuing decline in the growth rate of the atmospheric methane burden, Nature, 393, 447–450, 1998. Dlugokencky, E. J., Myers, R. C., Lang, P. M., Masarie, K. A., Crotwell, A. M., Thoning, K. W., Hall, B. D., Elkins, J. W., and Steele, L. P.: Conversion of NOAA atmospheric dry air CH₄ mole fractions to a gravimetrically prepared standard scale, J. Geophys. Res., 110, D18306, doi:10.1029/2005JD006035, 2005. Ehhalt, D. H.: The atmospheric cycle of methane, Tellus, 26, 58–70, 1974. Eisma, R., Vermeulen, A. T., and van der Borg, K.: $^14$CH₄ emissions from nuclear power plants in northwestern Europe, Radiocarbon, 37, 475–483, 1995. Etheridge, D. M., Steele, L. P., Francey, R. J., and Langenfelds, R. L.: Atmospheric methane between 1000 A.D. and present: Evidence of anthropogenic emissions and climate variability, J. Geophys. Res., 103, 15 979–15 993, 1998. Etheridge, D. M., Smith, A. M., Lowe, D. C., Trudinger, C. M., Langenfelds, R. L., Steele, L. P., Lassey, K. R., Levchenko, V. A., and Manning, M. R.: Sources of atmospheric methane during the 20th century from methane isotopic measurements in Antarctic firn air, in: 8$^th$ Scientific Assembly of IAMAS, Innsbruck, Austria, pp. 106, 2001. Etiope, G.: New directions: GEM – Geologic emissions of methane, the missing source in the atmospheric methane budget, Atmos. Environ., 38, 3099–3100, 2004. Etiope, G. and Klusman, R. W.: Geologic emissions of methane to the atmosphere, Chemosphere, 49, 777–789, 2002. Ferretti, D. F., Miller, J. B., White, J. W. C., Etheridge, D. M., Lassey, K. R., Lowe, D. C., MacFarling Meure, C. M., Dreier, M. F., Trudinger, C. M., van Ommen, T. D., and Langenfelds, R. L.: Unexpected changes to the global methane budget over the past 2000 years, Science, 309, 1714–1717, 2005. Ferretti, D. F., Miller, J. B., White, J. W. C., Lassey, K. R., Lowe, D. C., and Etheridge, D. M.: Stable isotopes provide revised global limits of aerobic methane emissions from plants, Atmos. Chem. Phys., 7, 237–241, 2007. Fiore, A. M., Horowitz, L. W., Dlugokencky, E. J., and West, J. J.: Impact of meteorology and emissions on methane trends, 1990–2004, Geophys. Res. Lett., 33, L12809, doi:10.1029/2006GL026199, 2006. Francey, R. J., Manning, M. R., Allison, C. E., Coram, S. A., Etheridge, D. M., Langenfelds, R. L., Lowe, D. C., and Steele, L. P.: A history of $\delta ^13$C in atmospheric CH₄ from the Cape Grim Air Archive and Antarctic firn air, J. Geophys. Res., 104, 23 631–23 643, 1999. Fung, I., John, J., Lerner, J., Matthews, E., Prather, M., Steele, L. P., and Fraser, P. J.: Three-dimensional model synthesis of the global methane cycle, J. Geophys. Res., 96, 13 033–13 065, 1991. Hein, R., Crutzen, P. J., and Heimann, M.: An inverse modeling approach to investigate the global atmospheric methane cycle, Global Biogeochem. Cycles, 11, 43–76, 1997. Houweling, S., Dentener, F., and Lelieveld, J.: Simulation of preindustrial methane to constrain the global source strength of natural wetlands, J. Geophys. Res., 105, 17 243–17 255, 2000. Houweling, S., Kaminski, T., Dentener, F., Lelieveld, J., and Heimann, M.: Inverse modeling of methane sources and sinks using the adjoint of a global transport model, J. Geophys. Res., 104, 26 137–26 160, 1999. Houweling, S., Röckmann, T., Aben, I., Keppler, F., Krol, M., Meirink, J. F., Dlugokencky, E. J., and Frankenberg, C.: Atmospheric constraints on global emissions of methane from plants, Geophys. Res. Lett., 33, L15821, doi:10.1029/2006GL026162, 2006. Hua, Q. and Barbetti, M.: Review of tropospheric bomb $^14$C data for carbon cycle modeling and age calibration studies, Radiocarbon, 46, 1273–1298, 2004. Kammen, D. M. and Marino, B. D.: On the origin and magnitude of pre-industrial anthropogenic CO₂ and CH₄ emissions, Chemosphere, 26, 69–86, 1993. Karlsdóttir, S. and Isaksen, I. S. A.: Changing methane lifetime: Possible cause for reduced growth, Geophys. Res. Lett., 27, 93–96, 2000. Kasischke, E. S. and Penner, J. E.: Improving global estimates of atmospheric emissions from biomass burning, J. Geophys. Res., 109, D14S01, doi:10.1029/2004JD004972, 2004. Keppler, F., Hamilton, J. T. G., Braß, M., and Röckmann, T.: Methane emissions from terrestrial plants under aerobic conditions, Nature, 439, 187–191, 2006. Kirchgessner, D. A., Piccot, S. D., and Masemore, S. S.: An improved inventory of methane emissions from coal mining in the United States, J. Air Waste Manage. Assoc., 50, 1904–1919, 2000. Kirschbaum, M. U. F., Bruhn, D., Etheridge, D. M., Evans, J. R., Farquhar, G. D., Gifford, R. M., Paul, K. I., and Winters, A. J.: A comment on the quantitative significance of aerobic methane release by plants, Funct. Plant Biol., 33, 521–530, 2006. Kunz, C.: Carbon-14 discharge at three light-water reactors, Health Phys., 49, 25–35, 1985. Lacroix, A. V.: Unaccounted-for sources of fossil and isotopically-enriched methane and their contribution to the emissions inventory: A review and synthesis, Chemosphere, 26, 505–557, 1993. Lassey, K. R., Lowe, D. C., and Manning, M. R.: The trend in atmospheric methane $\delta ^13$C and implications for isotopic constraints on the global methane budget, Global Biogeochem. Cycles, 14, 41–49, 2000. Lassey, K. R., Scheehle, E. A., and Kruger, D.: Towards reconciling national emission inventories for methane with the global budget, Environ. Sci., 2, 193–204, 2005. Lassey, K. R., Lowe, D. C., and Smith, A. M.: The atmospheric cycling of radiomethane and the "fossil fraction" of the methane source, Atmos. Chem. Phys., 7, 2141–2149, 2007. Lelieveld, J., Crutzen, P. J., and Dentener, F. J.: Changing concentration, lifetime and climate forcing of atmospheric methane, Tellus, 50B, 128–150, 1998. Lelieveld, J., Peters, W., Dentener, F. J., and Krol, M. C.: Stability of tropospheric hydroxyl chemistry, J. Geophys. Res., 107, 4715, doi:10.1029/2002JD002272, 2002. Levin, I. and Kromer, B.: Twenty years of high precision atmospheric $^14$CO₂ observations at Schauinsland station, Germany, Radiocarbon, 39, 205–218, 1997. Levin, I. and Kromer, B.: The tropospheric $^14$CO₂ level in mid-latitudes of the Northern Hemisphere (1959–2003), Radiocarbon, 46, 1261–1272, 2004. Lowe, D., Manning, M., Levin, I., Wahlen, M., Tyler, S., Etheridge, D., and Lassey, K.: Radiocarbon content of atmospheric methane and the `fossil fraction' of emissions, in: 8th Scientific Assembly of IAMAS, Innsbruck, Austria, pp. 106, 2001. Lowe, D. C., Brenninkmeijer, C. A. M., Tyler, S. C., and Dlugokencky, E. J.: Determination of the isotopic composition of atmospheric methane and its application in the Antarctic, J. Geophys. Res., 96, 15 445–15 467, 1991. Lowe, D. C., Manning, M. R., Brailsford, G. W., and Bromley, A. M.: The 1991–1992 atmospheric methane anomaly: Southern Hemisphere $^13$C decrease and growth rate fluctuations, Geophys. Res. Lett., 24, 857–860, 1997. Lowe, D. C., Brenninkmeijer, C. A. M., Manning, M. R., Sparks, R. J., and Wallace, G.: Radiocarbon determination of atmospheric methane at Baring Head, New Zealand, Nature, 332, 522–525, 1988. Lowe, D. C., Koshy, K., Bromley, T., Allan, W., Struthers, H., Mani, F., and Maata, M.: Seasonal cycles of mixing ratio and $^13$C in atmospheric methane at Suva, Fiji, J. Geophys. Res., 109, D23308, doi:10.1029/2004JD005166, 2004. Lowe, D. C., Allan, W., Manning, M. R., Bromley, A., Brailsford, G., Ferretti, D., Gomez, A., Knobben, R., Martin, R., Mei, Z., Moss, R., Koshy, K., and Maata, M.: Shipboard determinations of the distribution of $^13$C in atmospheric methane in the Pacific, J. Geophys. Res., 104, 26 125–26 135, 1999. MacFarling Meure, C., Etheridge, D., Trudinger, C., Steele, P., Langenfelds, R., van Ommen, T., Smith, A., and Elkins, J.: Law Dome CO₂, CH₄ and N₂O ice core records extended to 2000 years BP, Geophys. Res. Lett., 33, L14810, doi:10.1029/2006GL026152, 2006. Mak, J. E., Manning, M. R., and Lowe, D. C.: Aircraft observations of $\delta^13$C of atmospheric methane over the Pacific in August 1991 and 1993: Evidence of an enrichment in $^13$CH₄ in the Southern Hemisphere, J. Geophys. Res., 105, 1329–1335, 2000. Manning, M. R., Lowe, D. C., Moss, R. C., Bodeker, G. E., and Allan, W.: Short term variations in the oxidizing power of the atmosphere, Nature, 436, 1001–1004, 2005. Manning, M. R., Lowe, D. C., Melhuish, W. H., Sparks, R. J., Wallace, G., Brenninkmeijer, C. A. M., and McGill, R. C.: The use of radiocarbon measurements in atmospheric studies, Radiocarbon, 32, 37–58, 1990. Martens, C. S., Kelley, C. A., Chanton, J. P., and Showers, W. J.: Carbon and hydrogen isotopic characterization of methane from wetlands and lakes of the Yukon-Kuskokwim Delta, Western Alaska, J. Geophys. Res., 97, 16 689–16 701, 1992. McCarthy, M. C., Connell, P., and Boering, K. A.: Isotopic fractionation of methane in the stratosphere and its effect on free tropospheric isotopic compositions, Geophys. Res. Lett., 28, 3657–3660, 2001. Mikaloff Fletcher, S. E., Tans, P. P., Bruhwiler, L. M., Miller, J. B., and Heimann, M.: CH₄ sources estimated from atmospheric observations of CH₄ and its $^13$C/$^12$C isotopic ratios: 2. Inverse modeling of CH₄ fluxes from geographical regions, Global Biogeochem. Cycles, 18, GB4005, doi:10.1029/2004GB002224, 2004. Miller, J. B., Mack, K. A., Dissly, R., White, J. W. C., Dlugokencky, E. J., and Tans, P. P.: Development of analytical methods and measurements of $^13$C/$^12$C in atmospheric CH₄ from NOAA Climate Monitoring and Diagnostics Laboratory Global Air Sampling Network, J. Geophys. Res., 107, doi:10.1029/2001JD000630, 2002. Mook, W. G. and van der Plicht, J.: Reporting $^14$C activities and concentrations, Radiocarbon, 41, 227–239, 1999. Mouillot, F., Narasimha, A., Balkanski, Y., Lamarque, J.-F., and Field, C. B.: Global carbon emissions from biomass burning in the 20th century, Geophys. Res. Lett., 33, L01801, doi:10.1029/2005GL024707, 2006. Nakagawa, F., Yoshida, N., Sugimoto, A., Wada, E., Yoshioka, T., Ueda, S., and Vijarnsorn, P.: Stable isotope and radiocarbon compositions of methane emitted from tropical rice paddies and swamps in Southern Thailand, Biogeochem., 61, 1–19, 2002. Nydal, R. and Lövseth, K.: Tracing bomb $^14$C in the atmosphere 1962–1980, J. Geophys. Res., 88, 3621–3642, 1983. Olivier, J. G. J.: On the quality of global emission inventories, PhD thesis, Utrecht University, Utrecht, Netherlands, 167 pp, 2002. Olivier, J. G. J. and Berdowski, J. J. M.: Global emission sources and sinks, in: The Climate System, edited by: Berdowski, J., Guicherit, R., and Heij, B. J., A. A. Balkema Publishers/Swets & Zeitlinger Publishers, Lisse, The Netherlands, 33–78, 2001. Parsons, A. J., Newton, P. C. D., Clark, H., and Kelliher, F. M.: Scaling methane emissions from vegetation, TREE, 21, 423–424, 2006. Platt, U., Allan, W., and Lowe, D.: Hemispheric average Cl atom concentration from $^13$C/$^12$C ratios in atmospheric methane, Atmos. Chem. Phys., 4, 2393–2399, 2004. Povinec, P., Chud\'y, M., and Sivo, A.: Anthropogenic radiocarbon: past, present, and future, Radiocarbon, 28, 668–672, 1986. Prather, M., Ehhalt, D., Dentener, F., Derwent, R., Dlugokencky, E., Holland, E., Isaksen, I., Katima, J., Kirchhoff, V., Matson, P., Midgley, P., and Wang, M.: Atmospheric chemistry and greenhouse gases, in: Climate Change 2001: The Scientific Basis, Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, edited by: Houghton, J. T., Ding, Y., Griggs, D. J., Nogeur, M., van der Linden, P. J., Dai, X., Maskell, K., and Johnson, C. A., Cambridge University Press, Cambridge, UK, 239–287, 2001. Prinn, R. G., Huang, J., Weiss, R. F., Cunnold, D. M., Fraser, P. J., Simmonds, P. G., McCulloch, A., Harth, C., Reimann, S., Salameh, P., O'Doherty, S., Wang, R. H. J., Porter, L. W., Miller, B. R., and Krummel, P. B.: Evidence for variability of atmospheric hydroxyl radicals over the past quarter century, Geophys. Res. Lett., 32, L07809, doi:10.1029/2004GL022228, 2005. Quay, P. D., Stutsman, J., Wilbur, D., Snover, A., Dlugokencky, E. J., and Brown, T.: The isotopic composition of atmospheric methane, Global Biogeochem. Cycles, 13, 445–461, 1999. Quay, P. D., King, S. L., Stutsman, J., Wilbur, D. O., Steele, L. P., Fung, I., Gammon, R. H., Brown, T. A., Farwell, G. W., Grootes, P. M., and Schmidt, F. H.: Carbon isotopic composition of atmospheric CH₄: fossil and biomass burning source strengths, Global Biogeochem. Cycles, 5, 25–47, 1991. Robbins, R. C., Cavanagh, L. A., Salas, L. J., and Robinson, E.: Analysis of ancient atmospheres, J. Geophys. Res., 78, 5341–5344, 1973. Rosenlof, K. H. and Holton, J. R.: Estimates of stratospheric residual circulation using the downward control principle, J. Geophys. Res., 98, 10 465–10 479, 1993. Ruddiman, W. F. and Thomson, J. S.: The case for human causes of increased atmospheric CH₄ over the last 5000 years, Quaternary Science Reviews, 20, 1769–1777, 2001. Saueressig, G., Bergamaschi, P., Crowley, J. N., Fischer, H., and Harris, G. W.: Carbon kinetic isotope effect in the reaction of CH₄ with Cl atoms, Geophys. Res. Lett., 22, 1225–1228, 1995. Saueressig, G., Crowley, J. N., Bergamaschi, P., Brühl, C., Brenninkmeijer, C. A. M., and Fischer, H.: Carbon 13 and D kinetic isotope effects in the reaction of CH₄ with O($^1$D) and OH: New laboratory measurements and their implications for the isotopic composition of stratospheric methane, J. Geophys. Res., 106, 23 127–23 138, 2001. Schimel, D., Alves, D., Enting, I. G., Heimann, M., Joos, F., Raynaud, D., Wigley, T. M. L., Prather, M., Derwent, R., Ehhalt, D., Fraser, P., Sanhueza, E., Zhou, X., Jonas, P., Charlson, R., Rodhe, H., Sadasivan, S., Shine, K., Fouquart, Y., Ramaswamy, V., Solomon, S., Srinivasan, J., Albritton, D., Derwent, R., Isaksen, I., Lal, M., and Wuebbles, D.: Radiative forcing of climate change. In: Climate Change 1995: The Science of Climate Change, edited by: Houghton, J. T., Meira Filho, L. G., Callander, B. A., Harris, N., Kattenberg, A., and Maskell, K., Cambridge University Press, Cambridge, UK, 65–131, 1996. Snover, A. K. and Quay, P. D.: Hydrogen and carbon kinetic effects during soil uptake of atmospheric methane, Global Biogeochem. Cycles, 14, 25–39, 2000. Sowers, T., Bernard, S., Aballain, O., Chappellaz, J., Barnola, J.-M., and Marik, T.: Records of the $\delta ^13$C of atmospheric CH₄ over the last 2 centuries as recorded in Antarctic snow and ice, Global Biogeochem. Cycles, 19, GB2002, doi:10.1029/2004GB002408, 2005. Stern, D. I. and Kaufmann, R. K.: Estimates of global anthropogenic methane emissions 1860–1993, Chemosphere, 33, 159–176, 1996. Struthers, H., Allan, W., Lowe, D. C., and Bhaskaran, B.: A comparison of the transport of long lived atmospheric trace gas species from two advection schemes incorporated into an atmospheric general circulation model, Tellus, accepted, 2007. Stuiver, M.: Workshop on $^14$C data reporting, Radiocarbon, 22, 964–966, 1980. Stuiver, M. and Polach, H. A.: Reporting of $^14$C data, Radiocarbon, 19, 355–363, 1977. Stuiver, M., Reimer, P. J., and Braziunas, T. F.: High-precision radiocarbon age calibration for terrestrial and marine samples, Radiocarbon, 40, 1127–1151, 1998. Subak, S.: Methane from the house of Tudor and the Ming Dynasty: Anthropogenic emissions in the sixteenth century, Chemosphere, 29, 843–854, 1994. Sugawara, S., Nakazawa, T., Shirakawa, Y., Kawamura, K., and Aoki, S.: Vertical profile of the carbon isotope ratio of stratospheric methane over Japan, Geophys. Res. Lett., 24, 2989–2992, 1997. Tans, P. P.: A note on isotope ratios and the global atmospheric methane budget, Global Biogeochem. Cycles, 11, 77–81, 1997. Trudinger, C. M., Etheridge, D. M., Rayner, P. J., Enting, I. G., Sturrock, G. A., and Langenfelds, R. L.: Reconstructing atmospheric histories from measurements of air composition in firn, J. Geophys. Res., 107, 4780, doi:10.1029/2002JD002545, 2002. Tyler, S. C., Crill, P. M., and Brailsford, G. W.: $^13$C/$^12$C fractionation of methane during oxidation in a temperate forested soil, Geochim. Cosmochim. Acta, 58, 1625–1633, 1994a. Tyler, S. C., Brailsford, G. W., Yagi, K., Minami, K., and Cicerone, R. J.: Seasonal variations in methane flux and $\delta ^13$CH₄ values for rice paddies in Japan and their implications, Global Biogeochem. Cycles, 8, 1–12, 1994b. Tyler, S. C., Ajie, H. O., Rice, A. L., Cicerone, R. J., and Tuazon, E. C.: Experimentally determined kinetic isotope effects in the reaction of CH₄ with Cl: Implications for atmospheric CH₄, Geophys. Res. Lett., 27, 1715–1718, 2000. van Aardenne, J. A., Dentener, F. J., Olivier, J. G. J., Klein Goldewijk, C. G. M., and Lelieveld, J.: A 1$^\circ\times$1° resolution data set of historical anthropogenic trace gas emissions for the period 1890–1990, Global Biogeochem. Cycles, 15, 909–928, 2001. Veres, M., Hertelendi, E., Uchrin, G., Csaba, E., Barnabás, I., Ormai, P., Volent, G., and Futó, I.: Concentration of radiocarbon and its chemical forms in gaseous effluents, environmental air, nuclear waste and primary water of a pressurized water reactor power plant in Hungary, Radiocarbon, 37, 497–504, 1995. Wahlen, M., Tanaka, N., Henry, R., Deck, B., Zeglen, J., Vogel, J. S., Southon, J., Shemesh, A., Fairbanks, R., and Broecker, W.: Carbon-14 in methane sources and in atmospheric methane: The contribution from fossil carbon, Science, 245, 286–290, 1989. Wang, J. S., McElroy, M. B., Spivakovsky, C. M., and Jones, D. B. A.: On the contribution of anthropogenic Cl to the increase in $\delta ^13$C of atmospheric methane, Global Biogeochem. Cycles, 16, 1047, doi:10.1029/2001GB001572, 2002. Wang, J. S., Logan, J. A., McElroy, M. B., Duncan, B. N., Megretskaia, I. A., and Yantosca, R. M.: A 3-D model analysis of the slowdown and interannual variability in the methane growth rate from 1988 to 1997, Global Biogeochem. Cycles, 18, GB3011, doi:10.1029/2003GB002180, 2004. Wang, Y. and Jacob, D. J.: Anthropogenic forcing on tropospheric ozone and OH since preindustrial times, J. Geophys. Res., 103, 31 123–31 135, 1998. Warwick, N. J., Bekki, S., Law, K. S., Nisbet, E. G., and Pyle, J. A.: The impact of meteorology on the interannual growth rate of atmospheric methane, Geophys. Res. Lett., 29, 1947, doi:10.1029/2002GL015282, 2002. Waugh, D. W. and Hall, T. M.: Age of stratospheric air: theory, observations, and models, Rev. Geophys., 40, 1010, doi:10.1029/2000RG000101, 2002.