Almost all U.S. meat comes from animals that are raised or fattened in factory farms known as Concentrated Animal Feeding Operations (CAFOs). These medium- and large-scale industrial operations have grown increasingly intensive in recent decades, packing thousands and sometimes tens of thousands of animals together. The largest 2 percent of facilities now hold more than 40 percent of all U.S. livestock.

The greenhouse gases from feedlots come from the digestive processes of cattle and sheep (enteric fermentation), manure and fuels burned for transportation and by farm equipment. Manure, which is stored in large lagoons or spread as fertilizer on fields, releases both nitrous oxide and methane, whose warming potential is 300 times greater and 25 times greater, respectively, than carbon dioxide’s. Manure is the fastest growing major source of methane in the U.S., up 60 percent from 1990 to 2008.

Pollutants from manure can flow into groundwater, lakes, streams and oceans when storage lagoons leak or too much is spread on farm fields, leading to fish kills, contaminated drinking water and marine “dead zones.” Contaminants include nutrients (particularly nitrogen and phosphorus), organic matter, solids, pathogens such as E. coli and salmonella, salts and heavy metals, along with antibiotics, pesticides and hormones. Decomposing waste also releases dust, odors and toxic gases such as ammonia and hydrogen sulfide that can cause itching, dizziness, and general discomfort among nearby residents and workers.

More than 34,000 miles of rivers and 216,000 acres of lakes and reservoirs have been degraded by waste from confined feeding operations in the U.S., according to the EPA. These numbers likely underestimate the damage since only a quarter of all rivers and under half of all lakes have been assessed.