There's a new report out that asks the question, "If everyone became vegan and so ate only fruit and vegetables, what would happen to our greenhouse emissions?" Interestingly, the report states that if this occurred, and mankind suddenly decided to eat as vegans do, then there would be a mere 7% reduction in the earth's greenhouse emissions.
The widespread adoption of vegetarianism would have even less impact, while organic food production actually leads to a net increase in greenhouse gas emissions. Those are the conclusions of this research paper that was published in the journal Progress in Industrial Ecology.
I found it fascinating, and disturbing, that the authors (Helmi Risku-Norja and Sirpa Kurppa of MTT Agrifood Research Finland) determined that the cultivation of soil for whatever purpose, whether growing crops or raising livestock, is the primary source of greenhouse gas emissions in food production. In other words, it's not the fertilizer production, animal husbandry, or agricultural energy requirements that cause the problem.
Using the example of Finland, the authors explain that for current average food consumption, emissions from soil represent 62% of the total emissions. Greenhouses gases released by cows and sheep account for 24%, and energy consumption and fertilizer manufacturing about 8% each. The problem with extensive organic production (despite this approach to farming being considered the “green” option) is that there is such a lower efficiency. It requires the cultivation of greater areas of soil, which counteracts many of the benefits of the organic approach.