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Livestock, including cattle, are a significant source of greenhouse gas emissions.

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What would happen if all Americans went vegan?

Consider the hamburger. Producing this staple of the U.S. diet takes 25 kilograms of animal feed, 25 square meters of land, and about 220 liters of water—all for four patties. Statistics like those have persuaded some scientists and environmental activists that eating less meat could have a big impact on carbon and other greenhouse gas emissions. But what would happen if every American made the switch to a plant-only diet? According to a new study, a nation of 320 million vegans would reduce greenhouse gas emissions from agriculture by some 28%, far less than the amount now produced by the livestock industry. The authors claim the switch could also lead to deficiencies in key nutrients—including calcium and several vitamins.

“Our logic was to start at the extreme scenario [and work backward from that],” says Robin White, the study’s lead author and an animal sciences researcher at Virginia Polytechnic Institute and State University in Blacksburg. She and fellow animal sciences researcher Mary Beth Hall, of the U.S. Department of Agriculture’s Dairy Forage Research Center in Madison, began by estimating the impact of converting all land now used by the livestock industry to cropland for human food. That would increase the amount of agricultural waste—corn stalks, potato waste, and other inedibles now fed to livestock—and eliminate the animals that now eat much of it. Burning the excess waste would add some 2 million tons of carbon to the atmosphere, they estimate. Fertilizer demands would also go up while the supply of animal manure dwindled. That would mean making more artificial fertilizer, adding another 23 million tons of carbon emissions per year.

As a result, although animals now make up some 49% of agricultural emissions in the United States, a vegan nation would eliminate far less than that. Annual emissions would drop from 623 million tons to 446 million tons a year, the team reports today in the Proceedings of the National Academy of Sciences.

Looking at the nutritional content of crops now produced, the team also found that a plant-only system wouldn’t be able to meet the U.S. population’s requirements for calcium, vitamins A and B12, and a few key fatty acids. “With carefully balanced rations, you can meet all of your nutrient requirements with a vegetarian diet,” White says. “But the types of foods that seem to do that, we don’t currently produce in sufficient quantities to make it a sustainable diet for the entire population.”

Some researchers take issue with the study’s assumptions—for example that fruit and vegetable production wouldn’t make up a bigger part of the pie in a plant-only agricultural system. “[We] could yield a better nutrient profile if we do restructure the land use,” noted Joan Sabate, a nutritionist at Loma Linda University in California.

Mario Herrero, an agricultural researcher at the Commonwealth Scientific and Industrial Research Organisation in St. Lucia, Australia, also thinks the team’s estimate of reductions in greenhouse gas emissions may be low. That’s because the study didn’t take into account how animal-free diets would affect imports, which make up a large part of the U.S. meat market. If Americans stop importing meat, it could lead to a reduction in greenhouse gas emissions in the countries that produce it, like Brazil, he says. “The full repercussions of doing something like this are rarely complete,” Herrero adds. “But [this study] is a valiant attempt.”