What percentage of Atmospheric CO2 is derived from methane degradation/reduction ?

Question

Wouldn't this make the Global Warming Potential of methane confusing/skewed?

Answer 1

Not much. CO2 is 385 ppmv, and CH4 is <2 ppmv, so the amount of oxidized methane contributing to CO2 levels is pretty small.

Answer 2

Where the "nonlinear" effects of methane oxidation come into play are in terms of stratospheric chemistry and clouds.  Nearly all water vapor in the stratosphere is due to oxidation of methane.  So the ice clouds that form there are dependent on methane.  Cirrus clouds provide a positive radiative forcing, though their effect is small.

Because of what Keith says (there is little methane compared to CO2), and because the other radiative impact of methane (cloud formation in the stratosphere) is also small, the major impact of methane on global warming is through its direct affect on longwave radiative transfer.  Therefore, its GWP is very accurately estimated by neglecting these second-order effects.

Edit:  Keith answered part 1, I answered part 2.

edit 2:  Wait, I think I get what you are asking.  You are upset because anaerobic respiration produces both CO2 and CH4 in a 1:1 ratio, so that you actually get twice the CO2 out of CH4 since you got one molecule of CO2 when you got one molecule of CH4 in the first place right?  

First off, only about 2/3 of anthropogenic CH4 sources are biological, so it's not strictly a 2:1 thing.  Secondly, even if it is 2:1, the total amount of methane produced biologically is around 600 tera-grams CO2 equivalent.  In contrast, the anthropogenic CO2 production is measured in several hundreds of peta-grams, or a factor of 500 larger.  There just isn't enough CO2 produced by production of CH4 to make a dent in the CO2 concentration.  In fact, the uncertainty in the CO2 budget is a factor of 10 larger than the total CH4 production.