Methane is an important greenhouse gas that is emitted from multiple natural and anthropogenic sources. Atmospheric methane concentrations have varied on a number of timescales in the past, but what has caused these variations is not always well understood1,2,3,4,5,6,7,8. The different sources and sinks of methane have specific isotopic signatures, and the isotopic composition of methane can therefore help to identify the environmental drivers of variations in atmospheric methane concentrations9. Here we present high-resolution carbon isotope data (δ13C content) for methane from two ice cores from Greenland for the past two millennia. We find that the δ13C content underwent pronounced centennial-scale variations between 100 bc and ad 1600. With the help of two-box model calculations, we show that the centennial-scale variations in isotope ratios can be attributed to changes in pyrogenic and biogenic sources. We find correlations between these source changes and both natural climate variability—such as the Medieval Climate Anomaly and the Little Ice Age—and changes in human population and land use, such as the decline of the Roman empire and the Han dynasty, and the population expansion during the medieval period.
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We thank M. Bigler for sharing NEEM CFA dust and ion data; the NEEM community for providing us with ice core samples; and O. Pechony, J. Marlon and Z. Wang for sharing data on fire activity and the charcoal index. This project was supported by the Dutch Science Foundation (NOW; projects 851.30.020 and 865.07.001). NEEM is directed and organized by the Center of Ice and Climate at the Niels Bohr Institute and the US NSF, Office of Polar Programs; it is supported by funding agencies and institutions in Belgium (FNRS-CFB and FWO), Canada (NRCan/GSC), China (CAS), Denmark (FIST), France (IPEV, CNRS/INSU, CEA and ANR), Germany (AWI), Iceland (RannIs), Japan (NIPR), Korea (KOPRI), The Netherlands (NWO/ALW), Sweden (VR), Switzerland (SNF), the UK (NERC) and the USA (US NSF, Office of Polar Programs, ARC 0806407). J.O.K. and K.M.K. were supported by the Swiss National Science Foundation (grant PP0022_119049) and FIRB project CASTANEA (RBID08LNFJ).
The authors declare no competing financial interests.
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Sapart, C., Monteil, G., Prokopiou, M. et al. Natural and anthropogenic variations in methane sources during the past two millennia. Nature 490, 85–88 (2012) doi:10.1038/nature11461
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