What do we know about the global methane budget? Results from four decades of atmospheric CH(4) observations and the way forward

Atmospheric CH(4) is arguably the most interesting of the anthropogenically influenced, long-lived greenhouse gases. It has a diverse suite of sources, each presenting its own challenges in quantifying emissions, and while its main sink, atmospheric oxidation initiated by reaction with hydroxyl radical (OH), is well-known, determining the magnitude and trend in this and other smaller sinks remains challenging. Here, we provide an overview of the state of knowledge of the dynamic atmospheric CH(4) budget of sources and sinks determined from measurements of CH(4) and δ(13)C(CH4) in air samples collected predominantly at background air sampling sites. While nearly four decades of direct measurements provide a strong foundation of understanding, large uncertainties in some aspects of the global CH(4) budget still remain. More complete understanding of the global CH(4) budget requires significantly more observations, not just of CH(4) itself, but other parameters to better constrain key, but still uncertain, processes like wetlands and sinks. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 1)'.