We should tackle the worst offenders first. A global shift to zero emission transportation would be a game changer.
Except, of course, that transportation isn't "the worst offender"; globally it's about 14% of total GHG emissions, and only a fraction of that can reasonably be switched to "zero emission". The "worst offenders" are industry, heating, electricity, and agriculture.
True, (but or and,) guess what contributes to that agriculture section? Cattle have to eat, and they eat a lot, compared to to all our other protein sources. They contribute double all the other animals combined.
http://www.fao.org/docrep/018/...
So, besides just the farts (aka eructation), here's some sources of greenhouse gasses produced through the creation of food used to feed livestock:
Feed production:
Direct and indirect N2O from:
Application of synthetic N
Application of manure
Direct deposition of manure by grazing and scavenging animals
Crop residue management
Non-feed production:
Energy use in field operations
Energy use in feed transport and processing
Fertilizer manufacture
Feed blending
Production of non-crop feedstuff (fishmeal, lime and synthetic amino acids)
CH4 from flooded rice cultivation
Land-use change related to soybean cultivation