Livestock
According to a 2006 United Nations report, livestock is responsible for some 18% of the world's greenhouse gas emissions as measured in CO2 equivalents. However this also includes the net effect of deforestation in order to create grazing land, as well as livestock natural methane gas emissions.
It also includes 65% of human-induced nitrous oxide (which has 296 times the global warming potential of CO2) and 37% of all human-induced methane (which has 23 times the global warming potential of CO2).
In the Amazon Rainforest, 70% of deforestation is specifically carried out to make way for grazing land, and so is a major factor in the 2006 UN FAO report; the first agricultural report to factor in land usage change and radiative forcing in regard to the influence of livestock production.
Cattle, Methane, and Climate Change