Sara Place, a research director at the association, said that cattle-related emissions dropped only 6 percent over that time period and only because of a decline in the number of cattle.
"One of the major things that's happened in the beef industry is we're producing more beef with fewer animals," Place explained. "From 1975 to today, it takes a third fewer beef cattle to produce the same amount of beef."
NRDC also looked at food that increased diet-related greenhouse gas emissions. Americans ate more dairy products, including cheese, yogurt, and butter, between 2005 and 2014. Like beef, production of dairy products is resource intensive.
Products from livestock, whether beef or dairy, have come under increasing scrutiny in recent years for their emissions. Raising cattle requires large amounts of land and feed, mostly corn and soy, which is heavily fertilized. Processing and applying fertilizer releases nitrous oxide, a gas with nearly 300 times the warming potential of carbon dioxide. Meanwhile, methane is emitted from cattle through their digestive systems and from manure disposal and it has at least 25 times the warming potential of carbon dioxide over a 100-year period.
Bergen warned against drawing any nutritional conclusions from the study, but noted that nutritionists and federal dietary guidelines have long encouraged more consumption of vegetables and less saturated fat.
"We do know that red meat and dairy happen to be high in saturated fats," Bergen said, "and there's a very high correlation between health benefits and environmental benefits."