Abstract
Abstract. 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 hypothetical 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 4.0, 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‰ 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.
Highlights
Methane is the second most important anthropogenic greenhouse gas in terms of radiative forcing (Denman et al, 2007)
We investigate in detail the three main sources, and the main sink process
Anthropogenic emissions were specified according to the EDGAR4.0 emission inventory, and natural sources as well as the methane lifetime were assumed to be the same each year
Summary
Methane is the second most important anthropogenic greenhouse gas in terms of radiative forcing (Denman et al, 2007). Its concentration increased from approximately 700 ppb (nmol/mol) during the pre-industrial period to about 1800 ppb today, with an increase of 1000 ppb during the 20th century (Ferretti et al, 2005). 30 to 40% of methane emissions are from natural origin, of which the largest fraction is from wetlands (100 to 231 Tg CH4 yr−1 (Wuebbles and Hayhoe, 2002; Mikaloff Fletcher et al, 2004a). Removal of CH4 from the atmosphere is primarily due to oxidation by OH, in the troposphere and in the stratosphere, resulting in an atmospheric lifetime of approximately 9 yr (Dentener et al, 2003). A small fraction of the methane is removed by oxidation in soils, and by reactions with chlorine and O(1D) in the stratosphere. Since 2007, measurements suggest that concentrations started are rising again (Dlugokencky et al, 2003, 2009; Rigby et al, 2008)
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