Check out this article showing the effect on climate for twenty different types of protein, from lentils to lamb: A Climate Hazard, Medium Rare, Please
The calculations include the energy costs of production, processing,transportation, cooking, and disposal. For lentils, a kilogram consumed generated less than a kilogram of emissions, while the animal proteins analyzed range from 4.8 kg emissions/kg consumed for eggs to 39.2 kg emissions/kg consumed for lamb. I was sorry to learn that lamb has the highest level of emissions of any food assessed for both production and post farm-gate emissions, since sheep are generally raised in better conditions than conventional cattle, pork, and poultry. Sheep, however, are extremely inefficient in their feed conversion and as ruminants generate methane as a natural part of their digestive process. Pork is a relatively efficient animal protein, definitely the best of the mammals, generating 12.1 kg emissions/kg consumed, in part because pigs are monogastric and do not generate methane the way ruminants do.
Stay tuned for my next post - a discussion of the feed conversion rates for pigs raised in confinement (CAFOs) and nonconfinement (with bedding, space, access to the out-of-doors).