The last decade or so has brought ample evidence that Americans are gradually changing their diets, driven by health concerns and other factors.
But a new study points to one change that is starker than many have thought: Americans cut their beef consumption by 19 percent — nearly one-fifth — in the years from 2005 to 2014, according to research to be released on Wednesday by the Natural Resources Defense Council.
The environmental group found that consumption of chicken and pork fell as well, though less drastically, as Americans ate more cheese, butter and leafy greens.
The council is hailing the plummeting popularity of beef as a victory in the fight against climate change, because greenhouse gases are produced when cattle are raised. The group estimates that the resulting reduction in pollution would equal the emissions of 39 million cars, or about one-sixth of the number of cars registered in the United States in 2015. (Some of those environmental benefits, the group says, were erased by increased consumption of other foods that also create emissions.)
The research, which is based on data from the Agriculture Department and calculations using the same methodology as the Environmental Protection Agency, found that changes in the overall American diet reduced emissions by the equivalent of pollution from 57 million cars — despite population growth of about 9 percent.
“Whether we realize it or not, Americans have been fighting greenhouse gas emissions with their forks,” said Sujatha Bergen, a policy specialist in the Natural Resources Defense Council’s food and agriculture program.
The council did not ask Americans why they were eating differently. But Ms. Bergen said that greater attention to how food affected health and well-being — rather than awareness of the environmental impact — was probably driving the decline in beef consumption.
Asked what prompted them to eat less beef, 37 percent of consumers surveyed cited its price as the No. 1 reason in research published in January by Mintel, a consumer research firm. Thirty-five percent of the respondents said they were eating more protein from other sources, like chicken or tofu. But more than a quarter ascribed the change to their concern about cholesterol and saturated fats.
The National Cattlemen’s Beef Association, a trade group for beef producers, said the main reason for beef’s decline was a reduced supply of meat, because of increased exports: The United States was a net exporter of beef from 2010 to 2013.
Another reason was increased competition from other meats like chicken and pork, the group said. Droughts during the period covered by the defense council report also cut into the beef supply, said Daren Williams, a spokesman for the organization. Facing higher feed prices, ranchers cut their production, and the price of beef rose.
It takes two years on average for a cow to reach slaughter size, which explains why the nation’s cattle stock is just now showing recovery, the association said. The industry group predicts a recovery in beef consumption, as does the Agriculture Department.
The beef association said it was “fallacious” to draw a link between beef consumption and automobile emissions. Sara Place, the group’s senior director of sustainable beef production research, said consumers could do more to reduce carbon emissions by throwing away less food, particularly fruits and vegetables, than by eating less beef.
The beef industry has reduced its carbon footprint, Dr. Place said, with improvements in cattle genetics and production methods. One-third fewer cows were needed in 2015 to produce the same amount of beef produced in 1975, she said.
“Our cattle numbers have gone down over a 40-year period, reducing carbon emissions and our carbon footprint,” she said.
Consumption of all meats, taken together, has been falling from a peak in 2007, according to the Agriculture Department. Per capita consumption of beef topped out in 1976, but it remained America’s favorite meat until the mid-1990s, when it was surpassed by chicken.
The new report shows declines in other food, like orange juice and frozen potatoes, that have notable carbon footprints.
But of all the foods Americans eat, beef has by far the biggest footprint. The feeds given to cattle are grown with petroleum-based fertilizers, and the animals’ digestive systems produce methane, a pollutant 25 percent more damaging to the environment than carbon dioxide. The manure, which is spread on fields or collected in large tanks, also emits greenhouse gases, said Ms. Bergen of the Natural Resources Defense Council.
A drop in orange juice consumption generated the second-largest reduction in greenhouse gases linked to a food, eliminating the equivalent of some 63 million megatons of carbon dioxide from the environment — or roughly 10 percent of the reduction attributable to eating less beef.