Atmospheric methane (CH4) increased through much of the twentieth century, but this trend gradually weakened until a stable state was temporarily reached around the turn of the millennium1,2, after which levels increased once more3. The reasons for the slowdown are incompletely understood, with past work identifying changes in fossil fuel, wetland and agricultural sources and hydroxyl (OH) sinks as important causal factors1,4,5,6,7,8. Here we show that the late-twentieth-century changes in the CH4 growth rates are best explained by reduced microbial sources in the Northern Hemisphere. Our results, based on synchronous time series of atmospheric CH4 mixing and 13C/12C ratios and a two-box atmospheric model, indicate that the evolution of the mixing ratio requires no significant change in Southern Hemisphere sources between 1984 and 2005. Observed changes in the interhemispheric difference of 13C effectively exclude reduced fossil fuel emissions as the primary cause of the slowdown. The 13C observations are consistent with long-term reductions in agricultural emissions or another microbial source within the Northern Hemisphere. Approximately half (51 ± 18%) of the decrease in Northern Hemisphere CH4 emissions can be explained by reduced emissions from rice agriculture in Asia over the past three decades associated with increases in fertilizer application9 and reductions in water use10,11.
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We thank the many researchers associated with the laboratories referred to as NOAA, NIWA, UW and UCI who took part in collecting and measuring many thousands of air samples over the years. We also thank P. Bousquet for detailed comments on earlier drafts that improved the manuscript. This work has been funded by NASA grants to S.C.T. (NGT5-30409) and J.T.R. (NNX08AF64G), NSF grants to S.C.T. (ATM 9871077) and J.T.R. (ATM-0628637 and AGS-1021776), and additional support from the W. M. Keck Foundation.
The authors declare no competing financial interests.
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Kai, F., Tyler, S., Randerson, J. et al. Reduced methane growth rate explained by decreased Northern Hemisphere microbial sources. Nature 476, 194–197 (2011). https://doi.org/10.1038/nature10259
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