More cuddly holiday season news: According to two architects who specialize in sustainable-living solutions, it takes twice as much land-use in a year to keep a medium-sized dog fed as it does to run a Toyota Land Cruiser. A cat uses as much in natural resources as a Volkswagen Golf.

In both instances, it's because cats and dogs have to eat meat that they have such a substantial ecological "footprint'' (meat uses up far more land and fuel per pound than vegetables). So if you ride your bike everywhere and keep a couple of tabbies, you've got no right to look down on the SUV owner next door.

Sustainable living means reconsidering much more in life than your lightbulbs, write the pair, Brenda and Robert Vale. There's a subtle hint of their worldview in the title of their book: Time to Eat the Dog?

Marion Nestle, New York University's formidable scholar of nutrition and food politics, is having none of this. She thinks the Vales' numbers are screwy, overestimating pet impact by a factor of two. Moreover, she points out, pet food is made of by-products from the production of meat for humans. In other words, pets are a disposal system for offal, without which, she and her collaborator Mal Nesheim write, "we would have to find a means of getting rid of it: landfills, burning, fertilizer, or converting it to fuel, all of which have serious environmental consequences." The meat industry's production for people is the big source of climate-forcing gases, they say. Instead of adding dog or cat to the menu, then, we'd be better off subtracting cow, chicken, pig and other beasts.

So it looks to me as if the Vales are wrong. (Full disclosure: I'm biased--that's one of our cats in the photo, and she's not there because we ran out of pastrami. She just likes to jump into pots when the stove is off.)

But give them credit for starting the right kind of discussion. Creating a sustainable way of life will require wrenching changes in people's habits and expectations. The Vales are correct in saying it won't be about convenient changes like switching brands of car or bulb.  We should start getting used to questions like "can people in rich nations justify keeping pets?"