Why aren't environmentalists focussing their wrath at fido and mittens?

Gregory Okin is quick to point out that he does not hate dogs and cats. Although he shares his home with neither - he is allergic, so his pets are fish - he thinks it is fine if you do. But if you do, he would like you to consider what their meat-heavy kibble and canned food are doing to the planet.

Okin, a geographer at UCLA, recently did that, and the numbers he crunched led to some astonishing conclusions. America's 180 million or so Rovers and Fluffies gulp down about 25 percent of all the animal-derived calories consumed in the United States each year, according to Okin's calculations. If these pets established a sovereign nation, it would rank fifth in global meat consumption.

Needless to say, producing that meat - which requires more land, water and energy and pollutes more than plant-based food - creates a lot of greenhouse gases: as many as 64 million tons annually, or about the equivalent of driving more than 12 million cars around for a year. That doesn't mean pet-keeping must be eschewed for the sake of the planet, but "neither is it an unalloyed good," Okin wrote in a study published this week in PLOS One.

"If you are worried about the environment, then in the same way you might consider what kind of car you buy . . . this is something that might be on your radar," Okin said in an interview. "But it's not necessarily something you want to feel terrible about. "

This research was a departure for Okin, who typically travels the globe to study deserts - things such as wind erosion, dust production and plant-soil interactions. But he said the backyard chicken trend in Los Angeles got him thinking about "how cool it is" that pet chickens make protein, while dogs and cats eat protein. And he discovered that even as interest grows in the environmental impact of our own meat consumption, there has been almost no effort to quantify the part our most common pets play.

To do that, Okin turned to dog and cat population estimates from the pet industry, average animal weights, and ingredient lists in popular pet foods. The country's dogs and cats, he determined, consume about 19 percent as many calories as the human population, or about as much as 62 million American people. But because their diets are higher in protein, the pets' total animal-derived calorie intake amounts to about 33 percent of that of humans.

Okin's numbers are estimates, but they do "a good job of giving us some numbers that we can talk about," said Cailin Heinze, a veterinary nutritionist at Tufts University's Cummings School of Veterinary Medicine who has written about the environmental impact of pet food. "They bring up a really interesting discussion."

Okin warns that the situation isn't likely to improve any time soon. Pet ownership is on the rise in developing countries such as China, which means the demand for meaty pet food is, too. And in the United States, the growing idea of pets as furry children has led to an expanding market of expensive, gourmet foods that sound like Blue Apron meals. That means not just kale and sweet potato in the ingredient list, but grain-free and "human-grade" concoctions that emphasize their use of high-quality meat rather than the leftover "byproducts" that have traditionally made up much of our pets' food.

"The trend is that people will be looking for more good cuts of meat for their animals and more high-protein foods for their animals," Okin said.

What to do about this? That's the hard part. Heinze said one place to start is by passing on the high-protein or human-grade foods. Dogs and cats do need protein, she said - especially cats, which are obligate carnivores, unlike their more omnivorous canine housemates. But the idea that they need the equivalent of prime rib and lots of it comes from what she calls "the pet food fake news machine." There's no need to be turned off by some plant-based proteins in a food's ingredients, she said, and dog owners in particular can look for foods with lower percentages of protein.

Human-grade, Heinze said, doesn't even have a regulatory definition, but it does suggest that a product might be using protein that humans would eat. Meat byproducts - all the organs and other animal parts that don't end up at the supermarket - are perfectly fine, she said. "Dogs and cats happily eat organ meat," Heizne said. "Americans do not."

Okin has some thoughts about that. The argument that pet foods' use of byproducts is an "efficiency" in meat production is based on the premise that offal and organs are gross, he says. (Look no further than the collective gag over a finely textured beef product known as "pink slime.") But if we would reconsider that, his study found, about one-quarter of all the animal-derived calories in pet food would be sufficient for all the people of Colorado.

"I've traveled around the world and I'm cognizant that what is considered human edible is culture-specific," he said. "Maybe we need to have a conversation about what we will eat."

In the meantime, Okin suggests that people thinking about getting a dog might consider a smaller one - a terrier rather than a Great Dane, say. Or, if you think a hamster might fulfill your pet desires, go that route.

Heinze, for her part, sometimes offers the same counsel to vegetarian or vegan clients who want their pets to go meat-free. They are typically motivated by animal welfare concerns, not environmental ones, she said, but such diets are not always best for dogs, and they never are for cats.

"There have been a few times in my career where I've honestly said to my clients, 'We need to find a new home for your pet,'" she said, "'and you need to get a rabbit or a guinea pig or something like that.'"