Living in Oxford, there used to be a hedgerow outside my window where cats hid. When birds landed on the lawn, they were swiftly overcome. Oddly, the presence of random feathers everywhere was never an adequare warning for them.

I’m not so sure this is fair to the dog; they are carnivorous animals, much more so than humans. From their teeth, to their digestive tract, to the fact they can even synthesis their own vitamin C (they don’t eat a lot of citrus naturally), they are designed for meat.

It’s not a perfect method, since other things would be changing at the same time, but it still seems pretty good.

For the car, the amount of land it would take to grow enough biomass to produce the liquid fuel to run it.

For the dog, the amount of land to grow the plants to feed to the animals that the dog eats.

I am not sure if that is the methodology those running the study used. Both are effectively about sunlight, and energy return on investment when converting primary products (plants) into final fuels (meat or liquid fuel).

I guess this is fair, because the amount they exhale will be taken out of the atmosphere in next years crop.

Still, regarding how much energy could be produced by an amount of land, how is this figured out? Presumably to use the land to fuel a Land Cruiser, one would have to make biodiesel. In growing the feedstock for biodiesel, there would be a significant energy input, as well. How much land would be needed to produce fuel for the SUV, as well as for the farm machinery that farms the feedstock in the first place?

If this isn’t the method they used to compare cats and cars, is it more hypothetical? For instance, is it something like “an equivalent of 55 gigajoules of sunlight (the energy it take to run a Land Cruiser) falls on 0.41ha of land a year,” disregarding the fact said vehicle doesn’t run on sunlight?

How much they exhale doesn’t really matter, since it was biomass before and presumably will be again soon. What matters is the fossil fuel inputs to agriculture: natural gas for fertilizer, oil for machinery, energy for refrigeration, etc.

Not having to try and sort all this stuff out is one good argument in favour of a carbon tax imposed at the highest possible level (such as where hydrocarbons are produced and imported). From there, the costs would flow appropriately through the economy.

A hectare of land can produce that much energy in what capacity? Liquid fuel that could drive the aforementioned Toyota Land Cruiser?

In thinking about this issue more, I’m becoming a bit skeptical of it. Which isn’t to say that I don’t believe pets have an impact, just that I find it hard to believe a dog is more than a Land Cruiser. Also, we’re talking in units of energy, not in units of CO2… Does a dog, and the chickens that feed it, exhale more CO2 that an SUV?

“SHOULD owning a great dane make you as much of an eco-outcast as an SUV driver? Yes it should, say Robert and Brenda Vale, two architects who specialise in sustainable living at Victoria University of Wellington in New Zealand. In their new book, Time to Eat the Dog: The real guide to sustainable living, they compare the ecological footprints of a menagerie of popular pets with those of various other lifestyle choices – and the critters do not fare well.”