No inter-hemispheric δ13CH4 trend observed

Arising from Kai, F. M., Tyler, S. C., Randerson, J. T. & Blake, D. R. Nature 476, 194–197 (2011)10.1038/nature10259 To understand the reasons for global CH4 changes since the 1990s, Kai et al.1 evaluated a combined record of observed tropospheric CH4 mole fractions as well as δ13C-CH4 and δD-CH4 measurements from mid latitudes of both hemispheres. Their data set shows a strongly decreasing inter-hemispheric difference (IHD) in δ13C-CH4 from −0.24 ± 0.11‰ during 1989–1993 to −0.10 ± 0.04‰ during 2001–2005, which they largely attribute to decreasing rice emissions during the last two decades. Here we show that the experimental data used by Kai et al.1 are probably biased because the authors used only one continental mountain station (Niwot Ridge, Colorado, USA) as representative of the entire Northern Hemisphere, thereby incorrectly determining the IHD of δ13C-CH4. Our more comprehensive data set—representing background air in both hemispheres—shows that the IHD of δ13C-CH4 has changed by less than 0.05‰ since 1990; the conclusion drawn by Kai et al.1 of a significant reduction of NH microbial sources is, thus, not constrained by our data.