It is all so complex.....

Cut your carbon footprint … take the car
NEIL REYNOLDS
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May 7, 2008 at 6:00 AM EDT
British environmentalist Chris Goodall asserted last year, in his provocative book How to Live a Low-Carbon Life, that driving your car to the supermarket could be better for the environment than walking there.
It all depended, he said, on the food you use to supply the energy for the stroll. He cited a 1.5-mile (2.4-kilometre) jaunt as an example. Consume 100 grams of beef and your car – “a typical car” – would be four times better for the environment. Drink a pint of milk and it would still be better. Eat a potato, though, and you could walk with a clear conscience – assuming you cooked the potato efficiently.
For Mr. Goodall, this calculation was simple science.
“It makes more sense to drive than to walk if walking means that you need to eat more to replace the energy you have lost,” he said. “Walking is not zero-emission. We need food energy to move ourselves from place to place. Food production creates carbon emissions.”
Some people found the proposition absurd. (University of Michigan economist Mark Perry proposed that all exercise be considered eligible for trade in carbon offsets.) Yet the Goodall heresy continues to garner support.
University of Chicago economist Steven Levitt, co-author of the best-selling and contrarian Freakonomics, has written sympathetically of Mr. Goodall's argument. (“When it comes to saving the environment, things are often not as simple as they seem at first blush.”) And New York Times science columnist John Tierney, on his blog, has described Mr. Goodall's argument as “an interesting challenge” to conventional wisdom. (“Mr. Goodall takes into account something that a lot of environmentalists don't,” Mr. Tierney says, “[such as] the human energy expended in averting fossil-fuel use.”)
Now the Pacific Institute, an international consulting company based in Seattle, has published a scholarly critique of the Goodall hypothesis, which concludes that, in the end, it all depends.
The institute found that there are too many variables, requiring too many arbitrary assumptions, to conclude that driving is environmentally superior to walking – but found that, in some instances, it can be.
Researchers Michael Cohen and Matthew Heberger, authors of the report, calculated that walking 1.5 miles at a moderate pace (three miles an hour) requires 123 calories – equivalent to the calories in 67 grams (2.4 ounces) of sirloin steak. They calculate further that the calories burned by walking this distance would be equivalent to 1.9 kilograms of greenhouse gas emissions.
The real problem arises with assumptions adopted – a particular feed, a specific fertilizer. On balance, however, the researchers concluded that between 1.3 kg and 2.4 kg of CO{-2}-equivalent emissions are produced “to raise, process, store and transport the sirloin that powers a 1.5-mile walk.”
Further, substituting ground beef for sirloin, they calculated that between 0.79 kg and 1.5 kg of CO{-2}-equivalent emissions would be produced to power the 1.5-mile walk. Substituting eight ounces of 2-per-cent milk, they calculated that between 0.23 kg and 0.66 kg of CO{-2}-equivalent emissions would be produced. Substituting a large apple (237 grams), they calculated that between 0.07 and 0.17 kg of CO{-2}-equivalent emissions would be produced.
“Under these assumptions,” the researchers concluded, “[Mr.] Goodall's numbers pan out. Using Japanese agricultural statistics, a person who eats sirloin would generate double the greenhouse gases by walking compared with driving the 1.5 miles. A person who eats ground beef would generate 30-per-cent more emissions by walking rather than driving. Even using lower British [statistics], the sirloin eater generates 16-per-cent more GHG emissions by walking rather than by driving.”
How do cars compare? Mr. Goodall based his conclusions on the assumption, officially used by the British government, that “a typical car” produces 0.29 kg of CO{-2}-equivalent emissions a mile – roughly comparable to the consumption of two or three apples. The Pacific Institute researchers based their conclusions on the quite different assumption that “a typical car” emits more than 1 kg of CO{-2}-equivalent, roughly the equivalent of the sirloin steak. (They also assumed that all cars get 17.4 miles a gallon.)
Environmentalists have accused Mr. Goodall of ignoring the “life-cycle” costs of driving a car. Mr. Goodall insists that his calculations fully incorporate these costs. Life-cycle costs of cars and gasoline, he says, are not enough to affect even remotely the conclusion “that car travel is less carbon intensive than walking [in all cases in which] the walker replaces lost energy with animal products.”
These calculations miss an important point. Cars are rapidly getting more efficient. Within a few years, “a typical car” will average 35 mpg, or more, on greener energy. You won't dare walk to the supermarket – unless you have bought carbon offsets from your virtuous vegan neighbours. The fuel in your own tank will be as environmentally important as the fuel in your car.