I liked this article from 1989: http://www.straightdope.com/columns/read/832/do-cow-and-term...

> Methane breaks down in the atmosphere to form carbon dioxide, ozone, and water, all of which absorb heat.

Almost every current article about cow burps cites a 2006 U.N. study saying that livestock accounted for 18% of methane pollution. That study was later called into question as being slightly exaggerated because it included pollution from farming infrastructure itself, not just the animals.

http://www.telegraph.co.uk/news/earth/environment/climatecha...

> The meat figure had been reached by adding all greenhouse-gas emissions associated with meat production, including fertiliser production, land clearance, methane emissions and vehicle use on farms, whereas the transport figure had only included the burning of fossil fuels.