References

ACPD

Atmospheric Chemistry and Physics Discussions

ACPD

1680-7375

Copernicus GmbH

Göttingen, Germany

10.5194/acpd-11-6771-2011

Interpreting methane variations in the past two decades using measurements of CH4 mixing ratio and isotopic composition

Monteil

¹ ² Houweling

¹ ² Dlugockenky

E. J.

⁴ Maenhout

⁵ Vaughn

B. H.

³ White

J. W. C.

³ Rockmann

Institute of Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht,The Netherlands

Netherlands intitute for space research, Utrecht, The Netherlands

University of Colorado, Boulder, Colorado, USA

Earth System Research Laboratory, Boulder, Colorado, USA

Joint Research Center, Ispra, Italy

25 02 2011

11 2 6771 6803

This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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The availability δ13C-CH4 measurements from atmospheric samples has significantly improved in recent years, which allows the construction of time series spanning up to about 2 decades. We have used these measurements to investigate the cause of the methane growth rate decline since 1980, with a special focus on the period 1998–2006 when the methane growth came to a halt. The constraints provided by the CH4 and δ13C-CH4 measurements are used to construct hypothetic source and sink scenarios, which are translated into corresponding atmospheric concentrations using the atmospheric transport model TM3 for evaluation against the measurements. The base scenario, composed of anthropogenic emissions according to Edgar 4, constant emissions from natural sources, and a constant atmospheric lifetime, overestimates the observed global growth rates of CH4 and δ13C-CH4 by, respectively, 10 ppb yr−1 and 0.02&permil; yr−1 after the year 2000. It proves difficult to repair this inconsistency by modifying trends in emissions only, notably because a temporary reduction of isotopically light sources, such as natural wetlands, leads to a further increase of δ13C-CH4. Furthermore, our results are difficult to reconcile with the estimated increase of 5 Tg CH4 yr−1 in emissions from fossil fuel use in the period 2000–2005. On the other hand, we find that a moderate (less than 5% per decade) increase in the global OH concentration can bring the model in agreement with the measurements for plausible emission scenarios. This study demonstrates the value of global monitoring of methane isotopes, and calls for further investigation into the role OH and anthropogenic emissions to further improve our understanding of methane variations in recent years.

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