You may have seen the meme circulating around the internet:  some researchers from Australia New Zealand are claiming that owning a dog has as much impact on the planet as owning an SUV.  I’ll let New Scientist summarize their case:

[A] medium-sized dog…consume[s] 90 grams of meat and 156 grams of cereals daily in its recommended 300-gram portion of dried dog food…So that gives him a footprint of 0.84 hectares…

Meanwhile, an SUV…driven a modest 10,000 kilometres a year, uses 55.1 gigajoules, which includes the energy required both to fuel and to build it. One hectare of land can produce approximately 135 gigajoules of energy per year, so the Land Cruiser’s eco-footprint is about 0.41 hectares – less than half that of a medium-sized dog.

It’s just the sort of counter-intuitive claim that gets lots of attention on the brave new internet era.  So interesting!  So science-y!  So Twitter-able!

And yet, so false!  Once you sniff around the numbers, it quickly becomes apparent that those researchers are barking up the wrong tree.

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  • Let’s get one thing out of the way:  I’m not a dog owner.  Much to my kids’ dismay, I don’t even want a pet.  Nor do I own an SUV.  So, in theory, I…er…don’t have a dog in this fight.  Still, this claim struck me as so wrong that it made the hair on my neck stand up.  And I’d hate to have someone catch scent of this meme and conclude that buying an SUV is no big deal—“It’s not like I’m buying a dog or anything”—if the real numbers don’t support that conclusion.  (That’s the risk of bad information: it can lead us to make choices that are in stark conflict with our values.)

    So let’s paws for a moment, and see if this sleeping dog is actually a lie.

    First, let’s look at that SUV.  The calculations behind the internet meme say that it’s driven about 6,200 miles per year (10,000 km).  And yet, according to the US Department of Energy, a real SUV in the US is driven an average of 13,700 miles annually.  Already, the internet meme is off by a factor of roughly 2.2.  I haven’t checked whether the 10,000 km figure is reasonable for Australia New Zealand—but when compared with typical US driving habits, their mileage assumption certainly skews the numbers in favor of SUVs, and against dogs.

    And then there’s the total energy estimates.  The pet-pessimists estimate that an SUV (in their calculations, a 4.6 liter Toyota Land Cruiser driven about 6,200 miles) consumes 55.1 gigajoules of energy in both fuel and amortized manufacturing energy every year.  That, too, is low.  A Land Cruiser gets about 15.25 mpg in combined city/highway driving—meaning that if it’s driven 10,000 km, it consumes about 407 gallons of gas, or 53.6 gigajoules worth of energy.  But once I add in the energy used to produce that gas, along with what’s likely a low-ball estimate of the “embodied” energy from vehicle manufacturing, I get get about 74.9 gigajoules—44 percent more than the authors estimate.  Yet again, they’ve low-balled the impacts of the SUV in a way that makes dogs look worse by comparison.  (Here, I’m drawing from the data collection and calculations I did for our CO2-by-transportation-mode charts. And I’m looking only at energy, not at the additional climate and pollution impacts of emissions from tailpipes and smokestacks.)

    So even before you start to look at dogs, the authors have underestimated the environmental impacts of SUVs by a factor of at least 3.  And that’s not including the indirect impacts of SUVs—the parking spaces we build for them; the roads and bridges they drive on; the impacts of insurance and licensing operations; etc., etc., ad nauseum.

    Then there’s flip side:  the authors’ claims about the impact of feeding pets.  The anti-doggists estimate it takes .84 hectares—or about 2.1 acres of cropland—to meet a a pooch’s food needs for a year. There are a little over 70 million dogs in the US (the Humane Society says 74.8 million, the veterinarians say 72.1 million, and the pet food industry says 66.3 million, for an average of 71.1 million canines).  So by the authors’ estimates it must take about 150 million acres of US farmland to feed our dogs.  In all, there are 440 million acres of cropland in the US—suggesting that the equivalent of one-third of all US cropland is devoted to producing dog food.  [EDIT, Feb 17 2010:  I’ve been informed that this paragraph is not accurate:  the .84 hectares represents an estimate of the ecological footprint of a dog—which is a very different thing than the amount of land required to raise food for a dog.  So for the time being, consider the cropland comparison moot.]

    We use the equivalent of a third of all US cropland to feed dogs?  That’s barking mad!

    To see why it’s wrong, you can look from the bottom up, at the foods that dogs eat.  Or you can look from the top down, at the aggregate sales of dog food vs. the entire agricultural economy.  I’ll do both.

    First from the bottom up:  what, exactly, do dogs eat?  The anti-pet-ites seem do a good job of calculating dogs’ calorie requirements.  Canines wolf down a lot of food:  a mid-sized dog consumes roughly 30 calories per pound of body weight per day.  (Smaller dogs eat as many as 40 calories per pound of body weight, while larger dogs eat as few as 20 calories per pound.  Call it the yapping-to-napping spread.)  I couldn’t find the average weight of dogs in the US, but the median dog breed listed here has an adult weight of 47 pounds.  If that’s representative of US dogs, then the average dog will eat 1,410 calories today, give or take—which, as I read it, is roughly what the authors’ figures imply.

    So the real problem with the authors’ calculations isn’t with their estimates of how much each pet eats.  It’s with this statement:

    [A] medium-sized dog…consume[s] 90 grams of meat and 156 grams of cereals daily

    Strike that: most dogs DO NOT eat meat and cereals.  With a few exceptions, they eat “meat” and “cereals.”  The “meat,” in particular, tends to be byproducts—things that people in the US simply won’t eat, even in hot dogs. Here’s one description of the ingredients in pet food:

    The protein used in pet food comes from a variety of sources. When cattle, swine, chickens, lambs, or other animals are slaughtered, the choice cuts such as lean muscle tissue are trimmed away from the carcass for human consumption. However, about 50% of every food-producing animal does not get used in human foods. Whatever remains of the carcass—bones, blood, intestines, lungs, ligaments, and almost all the other parts not generally consumed by humans—is used in pet food, animal feed, and other products. These “other parts” are known as “by-products,” “meat-and-bone-meal,” or similar names on pet food labels.

    Even the cereals dogs eat are often deemed unfit for human consumption. I’m not trying to gross you out here, or encourage you to feed choice cuts to your pooch.  Instead, I think it’s probably a good thing that dogs eat things that humans won’t—since otherwise they really would be eating people food, which really would increase their environmental impact.  But since most dogs get their calories and protein from the waste products of people food, the idea that the environmental impact of dog food is additional to the impact of human food is simply wrong.

    Of course, that’s not to say that dog food has no environmental impact.  Dog food, and meat byproducts generally, provide some financial contribution to the meat industry, and hence to the overall planetary impact of meat production.  Dog food also also requires energy for processing, packaging, and transportation.

    Yet when you look at pet food from a macro-economic perspective—that is, from the top down, rather than the bottom up —dog food is little more than a rounding error.  Total retail food sales in the US topped $1.1 trillion in the US in 2008 (see table 36 from the USDA’s Agricultural Outlook statistics.)  But according to the pet food industry, retail dog food sales totaled just $11 billion in 2008.  By that measure, dog food represents about one percent of the total food economy.

    Looking more narrowly at the economics of meat byproducts, I found these USDA estimates of meat “price spreads”, which show that meat byproducts are worth somewhere between 4 and 15 percent of the total value of livestock, depending on the year and the kind of animal.  And obviously, dog food is only one of many uses of those byproducts—there’s also food for other pets, and a variety of industrial uses as well. So based on the economics, there’s just no way to attribute much of the impact of agriculture on our dogs.

    In short, whether you go by the macro-economics, or by the actual constituent parts of dog food, there’s simply no principled way to say that the dog food has the same impact as human food.  I’d be very surprised if ANY principled life-cycle assessment found that dog food has more than a small fraction of the overall environmental impact of US agriculture. My guess is that dog food accounts for a maximum of 5 percent of all US crop production, and possibly as little as 1 percent.  That’s a far cry from the one-third that the authors imply.

    Of course, dogs have indirect environmental impacts, just as SUVs do:  veterinarians, energy for heating and cooling, the food calories that humans use while walking their dogs, etc.  I won’t even try to tally them up, because there’s no real point.  Just looking at the numbers so far—combining the underestimates of SUV impacts with the overestimates of dog food impacts—the anti-doggites are off by a factor of at least 18, and probably more.

    But because I’m doggedly persistent, I’ll mention one final issue.  The authors of the original meme estimate that:

    One hectare of land can produce approximately 135 gigajoules of energy per year

    I haven’t looked at the original book, so I have no real idea what this means.  A well-located solar power installation can produce roughly 10 times that much energy per acre per year.  Perhaps it’s got something to do with biofuels—maybe the net annual production of corn ethanol per hectare, after accounting for the energy for fertilizer, tractor fuel, and distilling.  Yet having run the numbers before, I’ve concluded that there’s absolutely no way run the US SUV fleet—roughly the size of our dog population—on corn ethanol alone.  There’s just not enough cropland in the country to do it. But obviously, we power our fleet of dogs (and cats and people and horses, etc.–and even some cars) fairly easily with the cropland we’ve got.

    Let’s be clear—I’m not claiming that we should ignore the environmental impact of dogs.  That’s one of reasons that I, personally, am reluctant to own one!  But I think that making an empirical claim without doing solid research does a grave disservice to public discourse.  Being wrong can have consequences—including, potentially, encouraging people to make the wrong choices, even if their heart is in exactly the right place.

    So I say to the folks who made the original claim:  Bad Researchers!  Fur Shame!!!  And to the rest of you: let’s consider the “dogs are worse than SUVs” meme debunked:  buried in the back yard, put to sleep, and whatever other bad dog pun comes to mind.

    Photo courtesy of Flickr user Mil under a Creative Commons license.