The ongoing rise in atmospheric methane challenges our efforts to limit global warming [1]. Identifying the main cause or causes is a top scientific and societal priority. Conceptually the increase can be caused by increasing emissions, or by reducing sinks, or both. CH₄ is emitted from multiple natural and anthropogenic sources and it is hard to distinguish them. Several studies have attributed most of the increase since 2007 to biogenic emissions, but changes in fossil sources and the OH-based sink have also been suggested [2-7]. Carbon monoxide (CO) is the first stable product from CH₄ oxidation and isotope measurements of CO can be used to constrain the CH₄ removal [8-10]. This approach can constrain both the major OH sink of CH₄ [11], and is particularly sensitive to the reaction of CH₄ with chlorine (Cl) radicals in the atmosphere. The Cl pathway constitutes only a small fraction of CH₄ removal, but has a disproportionate effect on its isotopic composition because of an extraordinarily strong isotope effect [12]. The atmospheric abundance of Cl is so low that it cannot be measured directly but must be inferred from indirect measurements.

There is an intense scientific debate on whether the oxidation capacity of the atmosphere, mainly OH radicals, is increasing, decreasing or staying constant over multi-year to decadal time scales. Online atmospheric chemistry models tend to agree on an increase of OH, whereas indirect methods looking at compounds that are removed by OH do not seem to agree. This is illustrated in Figure 1 from ref [6]. Also, it has recently been suggested that trends in tropospheric Cl could systematically affect previous source apportionment attempts [9]. It is urgent to resolve this discrepancy since trends in OH will directly (inversely) compensate for trends in emissions. The key variables that impact success are the number of CO isotope observations that can be generated, to constrain the isotopic budget as accurately as possible. Furthermore, it is necessary to develop more accurate models that can then answer questions on the OH sink of methane.

Resolving the conundrum on the trend in the natural removal processes of atmospheric CH₄ will strongly boost our ability to attribute the ongoing CH₄ increase to the responsible source sectors. In fact, the uncertainty in the Cl-based sink has been identified as one of the two key limitations regarding the improvement of this source partitioning [2]. Also, better knowledge of natural CH₄ removal will prepare a solid basis for the potential application of techniques aiming at increasing the removal. Any intervention will need to be based on thorough scientific understanding of the chemical system, and this understanding is also necessary for predicting potential negative side effects. Being able to characterize side effects is particularly important for such an important parameter as the oxidative capacity of the atmosphere, the process that keeps the atmosphere clean.

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