It has been taken into account. Radiative forcing (global warming) of Methane is 12-20% of that of CO2. That's not to say that Methane is not also a contributor to the problem but clearly man made CO2 is the big gorilla and as we continue with CO2 emissions and to melt the ice caps, more methane will be released which would cause even more ice caps to melt and more methane to be released etc.. etc.... It's all in the article I sent you.
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