This differs from gases like methane which do have a «real» lifetime due to reactions with hydroxyl radicals, where for any given steady state anthropogenic methane emissions are, a stabilized concentration value will eventually be reached. (realclimate.org)
However, if we assume, just for the sake of getting a ballpark estimate, that the permafrost fossil methane «capacitor» has been holding back the gas flow for 10,000 years and will discharge over 1000 years of future warming, we could look forward to an additional average transient flux of 90 Tg of methane per year (a 25 % increase over current anthropogenic methane emissions). (critical-angle.net)
The United Nations» Food and Agriculture Organization estimates that livestock production accounts for some 2.2 billion tons of carbon dioxide equivalent greenhouse gases annually, or about 35 percent of total anthropogenic methane emissions. (thinkprogress.org)