Interestingly, methane from livestock is a large factor.

>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.)