Methane Feedback on Atmospheric Chemistry: Methods, Models, and Mechanisms

The atmospheric methane (CH 4 ) chemical feedback is a key process for understanding the behavior of atmospheric CH 4 and its environmental impact. This work reviews how the feedback is defined and used, then examines the meteorological, chemical, and emission factors that control the feedback strength. Geographical and temporal variations in the feedback are described and explained by HO x (HO x = OH + HO 2 ) production and partitioning. Different CH 4 boundary conditions used by models, however, make no meaningful difference to the feedback calculation. The strength of the CH 4 feedback depends on atmospheric composition, particularly the atmospheric CH 4 burden, and is therefore not constant. Sensitivity tests show that the feedback depends very weakly on temperature, insolation, water vapor, and emissions of NO. While the feedback strength has likely remained within 10% of its present value over the industrial era and likely will over the twenty‐first century, neglecting these changes biases our understanding of CH 4 impacts. Most environmental consequences per kg of CH 4 emissions, including its global warming potential (GWP), scale with the perturbation time, which may have grown as much as 40% over the industrial era and continues to rise.