The issue here is that for local farms to exist, factory farms also need to exist. If you removed every factory farm, there would suddenly be a massive shortage of supply to meet the demand, and we would need to replace these industrial gaps with more local farms -- effectively ceasing them to be [i]local[/i] farms. So, the same amount of greenhouse gas emissions -- or at least in the same vicinity, since the largest contributors are the land usage, enteric fermentation and manure, and fossil fuels.