Pseudonym has little scientific ground to stand on here. Here are some of the major studies evaluating the impact of diet on the environment.
1) Lucas Reijnders and Sam Soret (2003). "
Quantification of the environmental impact of different dietary protein choices" in
The American Journal of Clinical Nutrition (Note: this paper became one of the classic studies on the subject and has been cited hundreds of times in the literature)
Their major conclusion:
Assessment suggests that on average the complete life cycle environmental impact of nonvegetarian meals may be roughly a factor 1.5–2 higher than the effect of vegetarian meals in which meat has been replaced by vegetable protein. Although on average vegetarian diets may well have an environmental advantage, exceptions may also occur. Long-distance air transport, deep-freezing, and some horticultural practices may lead to environmental burdens for vegetarian foods exceeding those for locally produced organic meat.
2) Baroni et al. (2006). "
Evaluating the environmental impact of various dietary patterns combined with different food production systems" in
European Journal of Clinical Nutrition (Note: this is also a landmark paper in the literature)
Their major conclusion:
As a consequence, independently from the perspective
selected, the ‘normal’ diet based on products from chemical–
conventional agriculture and conventional farming (NORMINT)
turns out to have the greatest environmental impact,
whereas the vegan diet based on organic products (VEGANBIO)
turns out to have the smallest environmental impact.
3) Rosi et al. (2017). "
Environmental impact of omnivorous, ovo-lacto-vegetarian, and vegan diet" in
Nature (Note: I personally think this is the most important study here for the simple reason that it actually uses the individual diets of real people in Italy, instead of fancy mathematical assumptions about what different diets should look like)
Their major conclusion:
The omnivorous choice generated worse carbon, water and ecological footprints than other diets. No differences were found for the environmental impacts of ovo-lacto-vegetarians and vegans, which also had diets more adherent to the Mediterranean pattern. A high inter-individual variability was observed through principal component analysis, showing that some vegetarians and vegans have higher environmental impacts than those of some omnivores. Thus, regardless of the environmental benefits of plant-based diets, there is a need for thinking in terms of individual dietary habits.
4) Robin White and Mary Hall (2017). "
Nutritional and greenhouse gas impacts of removing animals from US agriculture" in
Proceedings of the National Academy of Sciences. (Note: this paper was one of the more critical ones to look at a mass transition to veganism, but still found reductions in emissions)
Their major conclusion:
US agriculture was modeled to determine impacts of removing farmed animals on food supply adequacy and greenhouse gas (GHG) emissions. The modeled system without animals increased total food production (23%), altered foods available for domestic consumption, and decreased agricultural US GHGs (28%), but only reduced total US GHG by 2.6 percentage units.
Obviously not all ethical and scientific conclusions are perfectly "cut and dried," as Pseudonym puts it. But at this point it's full-on pseudoscience to suggest that veganism is not a better choice for the environment than other diets. It may have some drawbacks in certain contexts, which I have detailed in this thread and which some of the papers above also mention, but on the whole it's the far superior dietary choice, if you care about the long-term viability of global civilization and if you can manage to make the transition (also if you care about improving, you know, this thing called your health).
I do, however, applaud Pseudonym for stating the following:
I'm not arguing against Veganism, I think it's a perfectly reasonable response to the situation we find ourselves in and a perfectly ethical position. What I'm arguing against (and I think I can say this for everyone who's contributed to this thread) is this overly simplistic notion that it is the only ethical position.
This is very much a sensible position, hardly much different than the one I hold as a vegan myself.