Ambiguity in the causes for decadal trends in atmospheric methane and hydroxyl

Abstract

Methane is the second strongest anthropogenic greenhouse gas and its atmospheric burden has more than doubled since 1850. Methane concentrations stabilized in the early 2000s and began increasing again in 2007. Neither the stabilization nor the recent growth are well understood, as evidenced by multiple competing hypotheses in recent literature. Here we use a multispecies two-box model inversion to jointly constrain 36 y of methane sources and sinks, using ground-based measurements of methane, methyl chloroform, and the C¹³/C¹² ratio in atmospheric methane (δ¹³CH₄) from 1983 through 2015. We find that the problem, as currently formulated, is underdetermined and solutions obtained in previous work are strongly dependent on prior assumptions. Based on our analysis, the mathematically most likely explanation for the renewed growth in atmospheric methane, counterintuitively, involves a 25-Tg/y decrease in methane emissions from 2003 to 2016 that is offset by a 7% decrease in global mean hydroxyl (OH) concentrations, the primary sink for atmospheric methane, over the same period. However, we are still able to fit the observations if we assume that OH concentrations are time invariant (as much of the previous work has assumed) and we then find solutions that are largely consistent with other proposed hypotheses for the renewed growth of atmospheric methane since 2007. We conclude that the current surface observing system does not allow unambiguous attribution of the decadal trends in methane without robust constraints on OH variability, which currently rely purely on methyl chloroform data and its uncertain emissions estimates.

Keywords: hydroxyl; methane; oxidative capacity; renewed growth; troposphere.

Fig. 1.

Schematic of the two-box model. Inputs are annual hemispheric OH anomalies, methyl chloroform emissions, methane emissions, and δ¹³CH₄ for the methane emissions. Outputs are annual hemispheric concentrations of methyl chloroform, methane, and the δ¹³CH₄ of atmospheric methane. Interhemispheric exchange time is 1 y.

Fig. 2.

Most likely solution. Left column shows observed (black triangles) and modeled (solid lines) concentrations of atmospheric CH₄ (Top), ${\delta }^{13}$CH₄ (Middle), and methyl chloroform (Bottom). The Northern Hemisphere is yellow and the Southern Hemisphere is blue. Right column shows the methane emissions (plotted as a deviation from the constant prior emissions; Top), the isotopic composition of the methane emissions (Middle), and the OH anomaly relative to a global mean concentration of $1\times {10}^6$ molecules (molec)/cm³ (Bottom).

Fig. 3.

Analysis of OH anomalies and the methane lifetime from the most likely solution. Top is the same as Fig. 2, Bottom Right but includes the OH anomalies from Montzka et al. (31), Rigby et al. (33), and McNorton et al. (21) (black lines). OH anomalies from Montzka et al. (31), Rigby et al. (33), and McNorton et al. (21) are offset such that their mean matches the mean 1997–2007 anomaly found here. Middle is the ratio of Northern to Southern Hemispheric OH and the black line is from Patra et al. (34). Bottom is the methane lifetime in our two-box model and the black line is the lifetime from Prather et al. (32). OH is the only sink included in our two-box model so the methane lifetime shown here is more representative of the actual methane lifetime, not a lifetime due to OH loss.

Fig. 4.

Sensitivity test with fixed OH concentrations. Details are the same as in Fig. 2. Dashed lines are from the most likely solution (Fig. 2).

Fig. 5.

Sensitivity test with fixed methane emissions. Details are the same as in Fig. 2. Dashed lines are from the most likely solution (Fig. 2).