No inter-hemispheric δ¹³CH₄ trend observed

Abstract

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.