A Vaughn, The methane mystery, NewScientist, 22 May 2021, p 16

Globally, the concentration of methane in the atmosphere is rising since 2007, and the causes are not yet known. Methane is shorter-lived than CO2 but its warming effect is 28 times greater. The main sources are likely to be the burning of fossil and wood fuels, wetlands and cattle. Also, the fall during lockdown in NOx may have contributed since this reduces the hydroxyl radicals that remove atmospheric methane.