For the report, published Tuesday in the journal Environmental Research Letters, scientists built a database that evaluated the environmental impacts of producing more than 300 types of food. They then linked the database to responses from a nationally representative dietary survey of more than 16,000 American adults, which allowed them to estimate the impact of food choices on greenhouse gas emissions in the United States.
The team ranked the diets according to their associated greenhouse gas emissions and divided them into five equal groups, or quintiles. They found that the quintile with the greatest carbon footprint represented about 46 percent of total diet-related greenhouse emissions.
That quintile was responsible for about eight times more emissions than the lowest-impact group. Beef consumption accounted for 72 percent of the emissions difference between those two groups, according to the findings.
“A big take-home message for me is the fact that high-impact diets are such a large part of the overall contribution to food-related greenhouse gases,” said first author Martin Heller, a researcher at the University of Michigan.
The team only assessed the emissions related to food production. Emissions associated with cooking, refrigeration, distribution, processing and packing were not evaluated for the study but would likely raise emissions by at least 30 percent, Heller said.
“Reducing the impact of our diets – by eating fewer calories and less animal-based foods – could achieve significant reductions in greenhouse gas emissions in the U.S.,” said Heller.
If members of the highest-impact quantile adjusted their diets to match the U.S. average by consuming fewer calories and less meat, the one-day reduction in greenhouse gas emissions would be equivalent to eliminating 661 million passenger-vehicle miles, the team found.
If such a shift were implemented every day for a year and accompanied by equivalent shifts in domestic food production, the U.S. would achieve nearly 10 percent of the emissions reductions needed for it meet its targets under the 2015 Paris climate agreement, the team notes. While President Donald Trump has announced that he will withdraw the U.S. from the accord, many states and municipalities are still attempting to meet the emissions targets.
In 2010, food production in the U.S. accounted for about 8 percent of the nation’s heat-trapping greenhouse gas emissions. Animal-based foods generally produce more greenhouse gas emissions per pound than plant-based foods. The production of both dairy cows and beef cattle is associated with particularly high emissions levels.
For one, cows do not convert plant-based feed into milk or muscle efficiently, so they must consume a lot of it. Growing that feed often involves the use of fertilizers and other substances that are manufactured through energy-intensive operations.
Cows also burp lots of methane, which is also released by their manure.
“Previous studies of diet-related greenhouse gas emissions have focused mainly on the average diet in a given country,” said principal investigator Diego Rose, a professor of nutrition and food security at Tulane University.
“This study is the first in the U.S. to look instead at self-reported dietary choices of a nationally representative sample of thousands of Americans.”
By connecting their database to the self-reported diets, the team was able to estimate the distribution of diet-related impacts across the entire U.S. population on a given day.
The scientists found that the highest-impact group consumed more than twice as many calories on a given day than Americans in the lowest quintile, 2,984 versus 1,323. Even after adjusting for calorie intake, the highest-impact quintile still accounted for five times more emissions than the bottom group.
Meat represented 70 percent of the food-related emissions in the top group, but only 27 percent in the lowest-impact quintile.
The report was funded by a grant from the Wellcome Trust, a biochemical research charity based in London that supports studies on human and animal health.