As I mentioned before, cattle belches are a source of GHG emissions and therefore global warming, but there are other culprits for methane, nitrous oxide and Co. associated with our food. Translated in CO2 eq (methane has 25 times and nitrous oxide 298 times more global-warming potential than carbon dioxide), this source provides some interesting numbers:
In 2011, the GHG emissions of agriculture on the whole amounted to 5.3 Gt CO2 eq, of which are for instance
- enteric fermentation (belching of mostly beef cattle and dairy cows) 2,071 Mt CO2 eq,
- manure left on pasture (nitrous oxide and methane due to decomposition of the manure left by grazing animals) 824 Mt CO2 eq,
- synthetic fertilizers (nitrous oxide) 725 Mt CO2 eq,
- rice cultivation (methane, emitted by anaerobic decomposition) 522 Mt CO2 eq and
- manure applied to soils (nitrous oxide and methane due to slurry from cattle and swine as organic fertilizer) 185 Mt CO2 eq.
In comparison, in 2010 the deforestation (mainly rain forest converted to cropland in South America or to palm farms in Indonesia) is to blame for 3.7 Gt CO2 eq, and the energy use in agriculture (mainly carbon dioxide produced by fossil fuel burning for agricultural machinery, power irrigation and fishing vessels) amounted to 785 Mt CO2 eq.
But this does not include the transport of feed, agricultural products or ingredients (as illustrated for a bottle of ketchup here), so I add that in 2012 all fossil fuel burning contributed to 31.6 GtCO2 eq (how much of which is due to shipping, driving or flying livestock feed like soybeans and corn from South America or meat, fruit and aquaculture fish from Asia or ingredients for processed food across the planet I can only guess).