Scientists tally the environmental impact of feeding meat to our cats and dogs. It's huge

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You've heard about the carbon footprint, but what about the carbon paw-print? According to a new study, U.S. cats' and dogs' eating patterns have as big an effect as driving 13.6 million cars for a year.

The findings, published in the journal PLOS ONE, reveals how our furry, four-legged companions' consumption of meat and other animal products adds a sizable, and largely overlooked, climate cost.

When it comes to environmental effects, meat-eating takes the cake. A 2014 study in the Proceedings of the National Academy of Sciences found that producing a kilogram of chicken results in about 3.7 kilograms of carbon dioxide, while a kilogram of pork comes with 24 kilograms of carbon dioxide. The same amount of beef, however, can be responsible for up to 1,000 kilograms of CO2 - a worrisome figure given that this greenhouse gas is largely responsible for the significant warming of the Earth's climate. That's not even counting the livestock's water usage footprint, which dwarfs that of agricultural crops.

It's a growing concern given that developed countries such as the U.S. consume lots of animal protein, and that developing countries that are economically on the rise seem to be increasing their share of meat consumption too.

But one sleepless night about five years ago, UCLA geographer Gregory Orkin realized something: Those environmental assessments rarely if ever took into account the consumption by dogs and cats. The thought gave him pause - perhaps even paws.

"Because I couldn't sleep, I got up and just kind of started throwing some numbers together," he said. "It's evolved a lot since then."

He calculated the likely number of calories needed by the United States' pet dogs and cats, who number around 163 million, and examined the ingredients in pet food and tallied up which ones were derived from animals.