Some of my friends have been recently "enlightened" by articles concerning Enhanced Greenhouse Warming (EGW) effects from anthropogenic sources of methane (CH4), primarily enteric production from cattle. This entry seeks to address some of these issues and to shed some light on this topic.
A greenhouse gas is a gas that absorbs bands of infrared radiation that are emitted from (primarily) the earth. All molecules that have more than two atoms: water, co2, ch4 are have absorption bands in this range. Oxygen, Nitrogen, Argon, NO do not. The "enhanced" in EGW implies that there is a positive feedback of the warming effects of one gas on the warming effects of the primary greenhouse gas: water. The amplification factor is for the most part a guess in real life, because we cannot accurately model the effect of clouds.
The amount energy absorbed in a column of gas is a logarthmic as a function of gas concentration. This is because additional molecules do not absorb energy that has already been absorbed. From this reference, the EGW effect of CH4 is about 8%. Thus a doubling of CH4 would result in an increase of CH4's warming effect to 13%.
Molecules do not stay in the atmosphere forever. They have an average lifetime, or residence time. Molecules with a short residence time do not stay in the system for very long, for various reasons. Residence time of methane is about 10 years. Carbon dioxide is about 100 years. See here, here, or here.
The primary mechanism for methane removal is OH reduction and by microbial processes in soil. Increases in temperature or methane concentration will increase the rate at which these processes occur. This will result in a reduced residence time if average temperatures increase.
Methane is a trace gas. In the last 200 years, it has increase from 0.7 parts per million to 1.7 parts per million. CO2 is about 200 times more abundant. See reference. There are thousands of water molecules out there for every free-floating methane.
So I think ultimately we have a problem here. CH4 does, in theory add to EGW. The question is does it really matter? Are the effects even measurable outside of the errors of say, clouds.
If we had an atmosphere with no water vapor, I would worry a lot more. But we don't.
Finally, even though CH4 does contribute to EGW, the amount that it does is only measureable under laboratory conditions. Throw in clouds, and the MASSIVE energy fluxes that result from the condensation and evaporation of water, the effects of CH4 adsorption are effectively lost in the noise.