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"...an erstwhile PETA member, says, 'I wouldn't look to PETA for science.'" This has risen to the top of my daily "no shit" comment list. (Or "no manure and nasty methane," if you like.) PETA faces a marketing choice similar to that of toothpaste manufacturers and sports agents: best to make exaggerated and even counter-factual statements in order to impress, or best to stick to the facts in order to preserve integrity? PETA, obviously, opts for the former. "Number one cause of global warming!" is more memorable than "meat consumption is the cause of 6% of America's carbon emissions!" regardless of whether it's true. And being memorable is the primary goal of all PETA campaigns.
But there's an ideological point at work that also deserves observation. Let us imagine that meat consumption actually reduced global warming and that a vegan diet increased it. PETA would still advocate veganism because their principles are not fundamentally about the environment or even good health. Rather, they accord other sentient life equal value to human life and cannot, therefore, admit to a middle ground. It is a position much closer to religion than science, and it's a shame because it means that all of the exposure PETA generates will always remain fringe. If, for example, they took the chicken idea and ran with it--"Eat white, not red!"--they might have a greater affect. But they can't do that. Chickens have the same rights as cows and thus it's vegan or nothing.