Rising methane: is there a methane emergency? [Preface]

These two volumes of the Royal Society's Philosophical Transactions record the ‘state of methane’ in 2021. The atmospheric methane burden rose rapidly in 2020: more rapidly than at any previous time in the observational record. The causes of this rise are complex and not well understood. It is likely much of the growth is driven by increased emissions from biological sources, such as natural wetlands, agriculture and landfills, especially in the Tropics and sub-Tropics. Other processes such as declining methane sinks may also be contributing. The methane budget is not closed. In the overall estimates by Saunois et al. (2020), there are wide uncertainty margins in each sub-category and huge discrepancies between Top-Down and Bottom-Up assessments. Moreover, in seeking to track methane, we are chasing a very fast-changing target—the global methane budget in 2021 is very different from the budget in 2010.