Methane global warming potential

[Charles123]

I understand that CH4 absorbs more long-wave radiation, on a molecule-per-molecule basis, than CO2. I also understand that "four indirect radiative effects of CH4 emissions have been identified (see Prather et al., 2001; Ramaswamy et al., 2001). Methane enhances its own lifetime through changes in the OH concentration: it leads to changes in tropospheric ozone, enhances stratospheric water vapour levels, and produces CO2" (http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-10-3.html). All this combined leads to CH4 being defined as having a higher Global Warming Potential than CO2 over 100 years, even with a much lower atmospheric lifetime (around 12 years).
Looking only to the fact that methane absorbs more long-wave radiation, this has to do with the characteristics of the two molecules (methane and carbon dioxide), particularly its chemical bounds. But, looking at the absorptivity of both molecules at different frequencies and the of Earth emitted long wave radiation intensity also at different frequencies is quite difficult to see this. Check the figures in the following links for example:
http://www.meteor.iastate.edu/gccourse/forcing/images/image7.gif
http://www.theresilientearth.com/?q=content/why-i-am-global-warming-skeptic
Although the second figure does not show methane, N2O could indicate approximately the methane behavior.
Can you please help me understand why this figures apparently don’t show what I said in the first paragraph?
Thank you
Best regards

 

[haruspex]

I believe the main reason methane is such a threat is that the parts of the IR spectrum it blocks are not nearly as saturated, so a small increase in gas has a large effect. See e.g. http://zebu.uoregon.edu/2004/es399/lec02.html

 

[ZapperZ]

This is one of our banned topics.

Zz.