Methane is not stable in the atmosphere, and is converted into carbon dioxide in 8-9 years on average (it's also vulnerable to UV radiation, which is why its discovery on Mars is interesting):
http://www.giss.nasa.gov/research/fe...00409_methane/In the lower part of the atmosphere, below about 10-12 km (the troposphere), the key cycles are mediated above all by the presence of what are called OH radicals — colloquially known as the atmospheric detergent. All hydrocarbon chemical species that are emitted can be eventually broken down (or oxidized) by these radicals to CO2 and H2O, and methane is no exception. An average molecule of CH4 lasts around eight to nine years before it gets oxidized. This is a long time compared to most atmospheric chemicals but is fast enough so that there can be significant year-to-year variability.
Thus, the direct effect of this methane should not last long as it comes from a storage that will deplete.
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