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Gidon Eshel is mostly vegan because he thinks that is best for the environment.
http://www.satyamag.com/feb07/eshel.html
Here is a quote of his from the above article:
The bottom line is if you eat the mean American diet, then you are responsible for the emissions of an extra ton and a half of CO2 equivalent per person per year, as compared to a vegan who eats the same number of calories but derived only from plants.
Huh?