Interpreting methane variations in the past two decades using measurements of CH<sub>4</sub> mixing ratio and isotopic composition

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 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‰ 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

Atmospheric methane isotopic record favors fossil sources flat in 1980s and 1990s with recent increase

“…Emissions from ruminant animals are also generally recognized to have risen over the past few decades due to increased populations of livestock (8,31). Increases in both waste emissions and livestock emissions, which contribute to observed trends in δ 13 C and δD, are supported by inversion results here over the 1984-2009 period (19 [15,25] and 9 [4,15] Tg y −1 respectively; Fig. 2 C and D).…”

“…Despite the promise of isotopic CH 4 to help constrain the CH 4 budget, the scientific community is historically data limited; before ∼1998 there were few isotopic CH 4 time series available and none that spanned decadal lengths (12,(14)(15)(16)(17). This is particularly true for δD, for which very little data are available at all before the turn of the century.…”

“…To 15,17). The δD time series from CM were combined with existing data from MDO and NWR (2000-2006; ref.…”

Enhanced methane emissions from tropical wetlands during the 2011 La Niña

“…The mean isotopic signatures of the biogenic category is ∼ −60‰ (includes wetlands, agriculture, waste), for the thermogenic category it is ∼ −37‰ (includes fossil-fuels) and for pyrogenic category it is ∼ −22‰ (includes biomass burning)31. To account for impact of meteorological variability on δ 13 C-CH 4 , a meteorology simulation of δ 13 C-CH 4 was performed using TM532.…”

Changes to the chemical state of the Northern Hemisphere atmosphere during the second half of the twentieth century

“…E.g. decreasing Arctic alkane mixing ratios (Helmig et al, 2014b;Aydin et al, 2011), decreasing Arctic CO mixing ratios (Petrenko et al, 2013), increasing d 13 C of methane (Monteil et al, 2011;Sapart et al, 2013), and decreasing dC 16 O of Arctic CO .…”