Abstract
Abstract. The hydroxyl radical (OH), which is the dominant sink of methane (CH4), plays a key role in closing the global methane budget. Current top-down estimates of the global and regional CH4 budget using 3D models usually apply prescribed OH fields and attribute model–observation mismatches almost exclusively to CH4 emissions, leaving the uncertainties due to prescribed OH fields less quantified. Here, using a variational Bayesian inversion framework and the 3D chemical transport model LMDz, combined with 10 different OH fields derived from chemistry–climate models (Chemistry–Climate Model Initiative, or CCMI, experiment), we evaluate the influence of OH burden, spatial distribution, and temporal variations on the global and regional CH4 budget. The global tropospheric mean CH4-reaction-weighted [OH] ([OH]GM-CH4) ranges 10.3–16.3×105 molec cm−3 across 10 OH fields during the early 2000s, resulting in inversion-based global CH4 emissions between 518 and 757 Tg yr−1. The uncertainties in CH4 inversions induced by the different OH fields are similar to the CH4 emission range estimated by previous bottom-up syntheses and larger than the range reported by the top-down studies. The uncertainties in emissions induced by OH are largest over South America, corresponding to large inter-model differences of [OH] in this region. From the early to the late 2000s, the optimized CH4 emissions increased by 22±6 Tg yr−1 (17–30 Tg yr−1), of which ∼25 % (on average) offsets the 0.7 % (on average) increase in OH burden. If the CCMI models represent the OH trend properly over the 2000s, our results show that a higher increasing trend of CH4 emissions is needed to match the CH4 observations compared to the CH4 emission trend derived using constant OH. This study strengthens the importance of reaching a better representation of OH burden and of OH spatial and temporal distributions to reduce the uncertainties in the global and regional CH4 budgets.
Highlights
Methane (CH4) plays an important role in both climate change and air quality as a major greenhouse gas and tropospheric ozone precursor (Ciais et al, 2013)
We test the 10 OH fields presented in by Zhao et al (2019), including 7 OH fields simulated by chemistry– transport and chemistry–climate models from Phase 1 of the Chemistry–Climate Model Initiative (CCMI) (Hegglin and Lamarque, 2015; Morgenstern et al, 2017), 2 OH fields simulated by the Interaction with Chemistry and Aerosols (INCA) model coupled to the general circulation model of the Laboratoire de Météorologie Dynamique (LMD) model (Hauglustaine et al, 2004; Szopa et al, 2013), and 1 OH field from the TransCom-CH4 intercomparison exercise (Patra et al, 2011) (Table 1)
Based on the ensemble of the 10 different OH fields listed in Table 1, global total emissions inverted by our system in Inv1 vary from 518 to 757 Tg CH4 yr−1 during the early 2000s (July 2000–June 2002)
Summary
Methane (CH4) plays an important role in both climate change and air quality as a major greenhouse gas and tropospheric ozone precursor (Ciais et al, 2013). CH4 is emitted from various anthropogenic sources including agriculture, waste, fossil fuel, and biomass burning, as well as nat-. Y. Zhao et al.: Influences of hydroxyl radicals ural sources including wetlands and other freshwater systems, geological sources, termites, and wild animals. CH4 is removed from the atmosphere mainly by reaction with the hydroxyl radical (OH) (Saunois et al, 2016, 2017). Tropospheric CH4 levels have more than doubled between the 1850s and the present day (Etheridge et al, 1998) in response to anthropogenic emissions and climate variabilities, leading to about 0.62 W m−2 of radiative forcing (Etminan et al, 2016) and increases in tropospheric ozone levels of ∼ 5 ppbv (Fiore et al, 2008). The global CH4 atmospheric mixing ratio stabilized in the early 2000s but resumed growing at a rate of ∼ 5 ppbv yr−1 or more starting in 2007 (Dlugokencky, NOAA/ESRL, 2019)
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