Food production comprises a huge slice of the global greenhouse gas pie. Despite centuries of efforts aimed at efficiency, the global food system is estimated to generate more than a third of global emissions.
However, not all calories are created equal; 14% of emissions from the food system come from agricultural products not used as food or feed (like cotton or rubber), and 29% of emissions come from plant-based foods raised for human consumption. The other 57% comes from the production of animal-based foods like beef and poultry, including growing crops to feed livestock and pastures for grazing.
So about two-thirds of the emissions from the human-oriented parts of our food system come from raising animals for human consumption. But think about this– do two-thirds of your calories come from animal products?
Unless you're a glutton for meat and dairy, the answer is almost certainly no. On average, about a third of a typical person's calories come from animal-based foods.
Diving further, not all animal-based calories are equal. Think about all the different animal-based foods you can buy at the grocery store, from eggs to milk to pork to chicken to beef. Each of these foods have stark differences in how much land and water they require plus how much greenhouse gas they generate.
As the World Resources Institute noted in an analysis of sustainable diets, many of those animal-based foods are somewhat more intensive to produce than plant-based equivalents but not excessively more.
"Beef production requires 20 times more land and emits 20 times more greenhouse gas emissions per unit of edible protein than common plant-based protein sources such as beans, peas, and lentils." statement by the World Resources Institute.
Relative to other herbivores that humans consume, ruminants consume large quantities of calories while providing few calories for human consumption– making them inefficient sources of protein and calories in comparison. Ruminants are versatile eaters who can subsist on a wide range of plants and grains and transform them into high-quality meat and milk largely because of their rumens, which are specialized stomachs housing a variety of microorganisms that can help the animal break down nutrients from tough and fibrous material like grasses. This specialized digestive process demands that they consume a lot of food, which means ruminant animal production is often resource-intensive.