Page 28 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 1. General introduction
oxidizing microorganisms that are controlled by local biotic and abiotic ecosystem parameters. Many factors within the CH4 pathway thus determine the fraction that is eventually released into the atmosphere.
Another key control on CH4 concentrations is its atmospheric removal rate. The chemical reaction of CH4 with the hydroxyl radical (•OH) leads to the stepwise oxidation of CH4 to CO2. This chemical reaction does approximately, but not perfectly, balance CH4 emissions (Fig. 4). However, the atmospheric sink capacity is subject to changes in the atmospheric composition. For example, variations in the oxidation capacity (primarily by the availability of •OH) have a direct impact on atmospheric CH4 levels (McNorton et al. 2016; Rigby et al. 2017; Turner et al. 2017). The resulting fluctuations in atmospheric CH4 concentrations can form strong positive feedbacks to the current and future climate.
Figure 4. Major global CH4 sources (left) and sinks (right). The size of the arrow indicates the relative contribution of the sources and sinks. The scale symbol indicates the current disbalance between global sources and sinks. Picture taken from Saunois et al. (2019).
CH4 also affects the abundance of other GHGs due to its important role in atmospheric chemistry (Thompson and Cicerone 1986; Fiore et al. 2002) (Fig. 3). First, CH4 oxidation leads to the photochemical formation of the greenhouse gas ozone (O3). Second, in the stratosphere,
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