Presumably, each molecule of atmospheric methane (CH4) primarily degrades through oxidation to 1 CO2 and 2x H2O - this is basic chemestry.
Although atmospheric losses to space and capture into oceans and subsequent precipitation into hydrates cannot be excluded as potential atmospheric methane sinks, I expect those to be only marginal at the grand scheme of things.
Source? I understood that the long-term radiative forcing of methane was lower than CO2, which is incompatible with this claim.