A: Producing meat, as well as other animal products (e.g., dairy and eggs), is inherently wasteful, due to large losses of otherwise edible calories and protein when animal feed is inefficiently “processed” into a retail meat product. That is, when plant calories are consumed by animals, only a small fraction can be stored as biomass, and further losses are incurred when animals are taken to slaughter and hides, bones, and organs are removed prior to retail sale. Animal feed is the single largest end-use of crops grown in the US, requiring vast inputs in terms of (non-renewable) mineral fertilizers, synthetic pesticides, water, fossil energy, but most especially land, which is a finite resource. Habitat destruction (and not global warming), primarily for agriculture, is by far the greatest cause of wildlife loss and biodiversity declines worldwide. At the global scale, a majority of farmland goes towards animal production, but yields a minority of dietary calories and protein. American also consume more meat per capita (around 220 pounds per person per year) than any other society on Earth, and consumption continues to climb. Such an inefficient use of resources with such a great footprint is clearly not sustainable, although some meat can be part of a “sustainable” diet.

It should also be mentioned that animal agriculture has a significant global warming impact: Croplands store less carbon than natural or semi-natural lands, nitrogen fertilizer production is extremely energy-intensive and also leads to nitrous oxide emissions (a major non-CO2 greenhouse gas). Cattle and other ruminants have a unique digestive system that also produces appreciable quantities of methane, a short-lived but potent greenhouse gas, while manure stored in confined animal feeding operations also produces methane and nitrous oxide emissions.

Generally speaking, the per-unit environmental impact of plant-based protein sources is an order of magnitude lower than most meats and animal products. The impact of different meats also varies appreciably, with beef having the highest per-unit impacts in terms of land, feed and other resources, and greenhouse gas emissions. Poultry, being comparatively efficient at converting feed to biomass, has a lower impact, with eggs likely the lowest-impact animal product. However, one should remember than most cows spend much (not all) their lives on pasture, while poultry are condemned to much more nightmarish confinement systems for life.

Coming from such a high baseline average, it seems possible for typical Americans to cut meat consumption appreciably and still enjoy a relatively high meat diet, by historical and global standards. If everyone ate “just” 100 pounds of meat a year (still over a quarter-pound hamburger a day equivalent), this would be more impactful than half the country going strictly vegan, and spare large amounts of land and other agricultural resources. In sum, eating more plants and fewer animals is a necessary part of making diets more efficient on a resource basis, and therefore more sustainable.