Between 1999 and 2006, a plateau interrupted the otherwise continuous increase of atmospheric methane concentration [CH4] since preindustrial times. Causes could be sink variability or a temporary reduction in industrial or climate-sensitive sources. We reconstructed the global history of [CH4] and its stable carbon isotopes from ice cores, archived air, and a global network of monitoring stations. A box-model analysis suggests that diminishing thermogenic emissions, probably from the fossil-fuel industry, and/or variations in the hydroxyl CH4 sink caused the [CH4] plateau. Thermogenic emissions did not resume to cause the renewed [CH4] rise after 2006, which contradicts emission inventories. Post-2006 source increases are predominantly biogenic, outside the Arctic, and arguably more consistent with agriculture than wetlands. If so, mitigating CH4 emissions must be balanced with the need for food production.

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Abstract. We report results from a worldwide interlaboratory comparison of samples among laboratories that measure (or measured) stable carbon and hydrogen isotope ratios of atmospheric CH 4 (δ 13 C-CH 4 and δD-CH 4 ). The offsets among the laboratories are larger than the measurement reproducibility of individual laboratories. To disentangle plausible measurement offsets, we evaluated and critically assessed a large number of intercomparison results, some of which have been documented previously in the literature. The results indicate significant offsets of δ 13 C-CH 4 and δD-CH 4 measurements among data sets reported from different laboratories; the differences among laboratories at modern atmospheric CH 4 level spread over ranges of 0.5 ‰ for δ 13 C-CH 4 and 13 ‰ for δD-CH 4 . The intercomparison results summarized in this study may be of help in future attempts to harmonize δ 13 C-CH 4 and δD-CH 4 data sets fromPublished by Copernicus Publications on behalf of the European Geosciences Union. 1208T. Umezawa et al.: Interlaboratory comparison of δ 13 C and δD measurements of CH 4 different laboratories in order to jointly incorporate them into modelling studies. However, establishing a merged data set, which includes δ 13 C-CH 4 and δD-CH 4 data from multiple laboratories with desirable compatibility, is still challenging due to differences among laboratories in instrument settings, correction methods, traceability to reference materials and long-term data management. Further efforts are needed to identify causes of the interlaboratory measurement offsets and to decrease those to move towards the best use of available δ 13 C-CH 4 and δD-CH 4 data sets.

Abstract. We report results from intercomparison exercises between laboratories that conduct measurements of stable carbon and hydrogen isotope ratios of atmospheric CH 4 (δ 13 C-CH 4 and δD- The results indicate significant offsets of δ 13 C-CH 4 and δD-CH 4 measurements among datasets reported from different laboratories; the data spread over ranges of 0.5 ‰ for δ 13 C-CH 4 and 13 ‰ for δD-CH 4 .The intercomparison results summarized in this study may be of help when combining isotope datasets 5 from different laboratories in future studies.