That sounds unfortunate, to say it lightly. So an actual model would predict almost immediate doom? Is there any climate model taking these things into account?
No it isn't, it's around (under, really) 10%. CO2 from US transportation alone is 6.7 billion tonnes, vs 4.8 for all tropical deforestation (which is the place land use for livestock takes place). It is a common misconception that land use or agriculture are significant drivers of climate change. They are of course tremendously damaging in their own particular ways (eg water/runoff/biodiversity), but globally energy, and fairly close behind it transportation, are by far the largest causes of increased greenhouse gas emissions.