Q: I'm an environmentalist with no kids, no pets, no dependents. If I drive or fly 5,000 miles a year, how does my carbon footprint compare with that of my hypothetical twin sister who owns a 50-pound dog but rarely takes trips? 

—Anne-Marie in Salida, Colorado 

A: A few years ago, claims that a dog had a bigger environmental footprint than an SUV were bouncing around social media. It was easy to prove that this canine slander was the result of prodigious mathematical fudging. But putting your hypothetical sister's dog—let's call him Reginald—up against air travel is tougher, because a plane can move a person 50 miles on a gallon of jet fuel, whereas only hybrid or electric cars can beat that. (Of course, if you have a passenger in your car, it can then equal or beat the plane because you cut the gallons per person in half—a basic truth that has yet to dawn on the millions who don't carpool.) 

To make the case for the hypothetical Reggie, I put him on an equally hypothetical luxury diet of hamburger and enough carbs to meet his caloric requirements. Even so, his annual carbon footprint would be only three-quarters that of your 5,000-mile flight, and possibly only half, because high-altitude emissions have a greater global warming effect than those at ground level. 

Most real dogs, however, dine on hamburger only in their dreams. Because most pet-food makers use innards, mystery parts, and other stuff deemed unfit for human consumption, the carbon footprint of a dog-food diet is much lower than posited above. So Reggie's paw print is smaller.