"Livestock also produce prodigious quantities of greenhouse gases: methane (from fermentation of food in the largest part of an animal's stomach and from manure storage), nitrous oxide (from denitrification of soil and the crust on manure storage), and carbon (from crop, animal, and microbial respiration as well as fuel combustion and land clearing). Livestock now account for 50% of the emissions from agriculture and land use change. Remarkably, annual emissions from livestock total some 7.1 billion tons (including 2.5 billion tons from clearing land for the animals), accounting for about 14.5% of emissions from human activities. Indeed, a cow/calf pair on a beef farm are ..more GHG emissions in a year than someone driving 8,000 miles in a mid-size car."
Sara J. Scherr and Sajal Sthapit. "Farming and Land Use to Cool the Planet," State of the World 2009. 2009, 39-40.