The reason why ruminant meat has such a high carbon footprint is the methane emissions of ruminant digestive systems, so a ruminant being alive means it's emitting.
Apparently the carbon footprint of dairy products is actually not too bad - I guess the caloric output is high enough to offset the emissions.
Also, I don't CW food itself, or discussion of foods, but this is about what someone should eat for political/environmental reasons, and that shit gets incredibly preachy.
The source I'm using on the emissions, BTW, is https://earthscience.stackexchange.com/a/10177 - which used the Climate Working Group's Meat Eater's Guide's numbers for embodied CO2 per kg of food product, and FDA numbers for calories per kg of food.
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