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 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.

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SOURCE: NY Times, Stephanie Strom