Someone in my public speaking class several years ago, during a persuasion speech, actually tried to dismiss CO2, CFCs, et al as a cause of global warming in favor of methane from cow farts (bovinepogenic global warming?)
>According to a new report published by the United Nations Food and Agriculture Organization, the livestock sector generates more greenhouse gas emissions as measured in CO2 equivalent – 18 percent – than transport.
http://www.fao.org/newsroom/en/news/2006/1000448/index.html
(Though it's worth noting that that report is a few years old at this point.)