The environmental impact of our diet on the planet is well known, but new research shows that the impact of our faithful furry friends and their stomachs is substantial.

Studying the recent trends in pet food, Gregory Okin, a geographer at the University of California, Los Angeles, found that the roughly 163 million pet cats and dogs in the United States eat about a quarter of the meat produced in the country, reports Karin Brulliard for The Washington Post. And all this food comes at a cost. Okin estimates it's responsible for greenhouse gas emissions equivalent to 64 million tons of carbon dioxide. That's about the same as driving 13.6 million cars around for a year, according to a press release.

“I’m not a vegetarian, but eating meat does come at a cost,” Okin says in a statement. “Those of us in favor of eating or serving meat need to be able to have an informed conversation about our choices, and that includes the choices we make for our pets.”

Getting at this estimate required a slew of calculations, reports Alessandra Potenza of The Verge. After estimating the number of pets in America, a metric not tracked by most cities and states in the country, Okin then calculated the average weight of these pets to estimate how much they eat in a year. He then turned to the ingredient labels of the country's most popular pet food brands to tabulate how much meat our furry friends are consuming annually. Okin published the results of this investigation last week in the journal PLOS One.

Importantly, Okin presents his results as CO2 equivalents, which takes into account the differing impacts of various greenhouse gasses. This latest study focuses on methane and nitrous oxide, which are potent greenhouse gasses, contributing to the depletion the ozone layer much more strongly than carbon dioxide.

One driving factor behind the meaty diets of America's pets is the growing investment of pet owners in their beloved companions, reports Brulliard. Sales of pet food have more than doubled since 2000, and a large chunk of that revenue has come from owners going for "premium" food for their animals. This food often uses higher-quality meat that is suitable for human consumption, Okin notes.

As for a solution, Okin stresses that he isn't suggesting making our pets vegetarians. Carnivorous cats, for example, require meat in their diet to stay healthy. But our pets don't need to be eating human-grade meat. And they're often not super picky about where that meat comes from or what it looks like.

Americans, however, often are quite picky, Cailin Heinze, a veterinary nutritionist at Tufts University’s Cummings School of Veterinary Medicine who was not involved in the study, tells Brulliard​. "Dogs and cats happily eat organ meat," says Heinz. “Americans do not.”

If more people used pet food with meat from non-prime cuts, that would free up more of the premium bits for people, and require the raising of fewer food animals—along with reducing greenhouse gas emissions. Moreover, unlike cats, dogs are omnivorous, Okin tells Amina Khan of the Los Angeles Times, so dog owners can and should consider mixing more plant-based products into Fido's food.

"We should consider all the impacts that pets have so we can have an honest conversation about them," Okin said in a statement. "Pets have many benefits, but also a huge environmental impact."