Role of atmospheric oxidation in recent methane growth

Abstract

The growth in global methane (CH₄) concentration, which had been ongoing since the industrial revolution, stalled around the year 2000 before resuming globally in 2007. We evaluate the role of the hydroxyl radical (OH), the major CH₄ sink, in the recent CH₄ growth. We also examine the influence of systematic uncertainties in OH concentrations on CH₄ emissions inferred from atmospheric observations. We use observations of 1,1,1-trichloroethane (CH₃CCl₃), which is lost primarily through reaction with OH, to estimate OH levels as well as CH₃CC₃ emissions, which have uncertainty that previously limited the accuracy of OH estimates. We find a 64-70% probability that a decline in OH has contributed to the post-2007 methane rise. Our median solution suggests that CH₄ emissions increased relatively steadily during the late 1990s and early 2000s, after which growth was more modest. This solution obviates the need for a sudden statistically significant change in total CH₄ emissions around the year 2007 to explain the atmospheric observations and can explain some of the decline in the atmospheric ¹³CH₄/¹²CH₄ ratio and the recent growth in C₂H₆ Our approach indicates that significant OH-related uncertainties in the CH₄ budget remain, and we find that it is not possible to implicate, with a high degree of confidence, rapid global CH₄ emissions changes as the primary driver of recent trends when our inferred OH trends and these uncertainties are considered.

Keywords: 1,1,1-trichloroethane; hydroxyl; inversion; methane; methyl chloroform.

Fig. 1.

(Top) NOAA observations of CH₄. (Middle) INSTAAR observations of δ¹³C-CH₄. (Bottom) The AGAGE observations of CH³CCl₃. Each plot shows the northern hemisphere (NH) and southern hemisphere (SH) means, and shading indicates the assumed 1-sigma model and measurement uncertainty as defined in SI Materials and Methods.

Fig. S1.

(Upper) High-frequency AGAGE Medusa observations at the baseline stations used in the global inversion in addition to; Scripps Institution of Oceanography, La Jolla, CA (SIO); and Gosan, South Korea (GSN). (Lower) Annual SDs relative to the annual mean mole fraction. CGO, Cape Grim, Tasmania; MHD, Mace Head, Ireland; RPB, Ragged Point, Barbados; SMO, Cape Matatula, Samoa; THD, Trinidad Head, CA.

Fig. S2.

Percentage difference in the AGAGE and NOAA baseline monthly means at four colocated sites for (A) CH₃CCl₃ and (B) CH₄. CGO, Cape Grim, Tasmania; MHD, Mace Head, Ireland; SMO, Cape Matatula, Samoa; THD, Trinidad Head, CA.

Fig. 2.

(Row 1) Inferred tropospheric annual mean OH concentration. (Row 2) Global CH₃CCl₃ emissions. (Row 3) Global CH₄ emissions. (Row 4) Global ¹³C/¹²C source isotope ratio of CH₄. The blue lines and shading show quantities inferred when AGAGE CH₃CCl₃ data were used, and the red lines and shading show those inferred using NOAA CH₃CCl₃ data. Lines indicate the medians, and the shading shows the 16th to 84th percentiles (∼$\pm$1 sigma). The green and gray lines in rows 1 and 2 show estimates from previous studies that used the same observations but different methodologies and emissions (13, 24). Inset in row 2 zooms in on the CH₃CCl₃ emissions from 2000 to 2014. The black lines in rows 3 and 4 show the methane and isotopologue changes inferred when interannually repeating OH was used. The gray shading shows the approximate start and end of the methane pause. Numerical values of the quantities in this figure are available in Dataset S1.

Fig. S3.

Percentage difference between modeled and observed mole fractions (model/data) for CH₃CCl₃, NOAA CH₄, and INSTAAR ${\delta }^{13}$C-CH₄ for (A) the AGAGE CH₃CCl₃ inversion and (B) the NOAA CH₃CCl₃ inversion. Gray shading indicates the diagonal elements of the model representation uncertainty covariance matrix (1 sigma). Blue lines show the median northern hemisphere (NH) mole fraction, and green lines show the southern hemisphere (SH) mole fraction. The blue and green shading shows the 1-sigma uncertainty in the a posteriori mole fractions. A seasonal cycle is apparent in the CH₄ and ${\delta }^{13}$C-CH₄ residual. This finding is because we have assumed that CH₄ emissions have no seasonal cycle, which is a clear oversimplification but one that we do not expect to have any impact on our results concerning quantities derived on timescales of at least 1 y. The figure also shows a (nonstatistically significant) trend in the CH₃CCl₃ residual, which is not inconsistent with the observations, because we have included off-diagonal terms in our representation uncertainty covariance that allow for multiyear trends of the order of a few percent. Therefore, the a posteriori uncertainties in OH and other quantities will be consistent with potential drifts in the atmospheric CH₃CCl₃ data, such as those shown in the figure. Fig. S4 shows the outcome of an inversion performed with this correlation-length scale set to zero and that it has little major impact on the median solution. In this solution, the “drift” between the observations and the posterior model is negligible.

Fig. S4.

Outcome of inversion using CH₃CCl₃ data alone from either the AGAGE network (blue) or NOAA network (red). Upper shows the derived OH concentration, and Lower shows the derived CH₃CCl₃ emissions. Shading indicates the $16\mathrm{t}\mathrm{h}$ and $84\mathrm{t}\mathrm{h}$ percentile range, and the solid lines show the medians. The black lines show the AGAGE-only inversion with an assumed zero temporal correlation-length scale in the model mismatch uncertainty, which removes the long-term drift in the model data residual (Fig. S3). Inset shows CH₃CCl₃ emissions from 2000 to 2014.

Fig. S5.

Correlation matrices for three annually derived quantities: (A) OH concentrations, (B) CH₄ emissions, and (C) ¹³C/¹²C source ratios. The figure shows relatively small autocorrelation for OH and CH₄ but significant off-diagonal elements for the source signature, indicating that estimates of the source signature in any 1 y are significantly correlated with surrounding years.

Fig. S6.

Simulation of the change in global ${\delta }^{13}$C-CH₄ relative to 2006 based on varying OH concentrations from the AGAGE and NOAA inversions (blue and red, respectively), with total CH₄ emissions and its source ¹³C/¹²C ratio held fixed and varying CH₄ emissions from the “constant OH” inversion with the source ¹³C/¹²C ratio held constant (dashed line). The black points indicate the observed global mean ${\delta }^{13}$C-CH₄. Each of these signals is a transient response in the model. Over timescales on the order of decades, ${\delta }^{13}$C-CH₄ returns to the values before any perturbations in either the OH concentration or the bulk emissions [similar behavior and response timescales were noted previously (29, 61)].

Fig. S7.

Constant emission simulation of northern hemisphere (NH) and southern hemisphere (SH) ethane (blue and green, respectively) based on OH concentrations from the AGAGE CH₃CCl₃ simulation (blue). The gray data points are previously published deseasonalized column-averaged observations from Zugspitze (45° N, 11° E) and Lauder (45° S, 170° E) (5).