Which is more polluting--driving a mile to work or walking that mile? The easy answer is, of course, driving. Cars have tailpipes; people don't. Far more energy is needed to push a 3,000-pound car along the road than is needed to move a 150- to 250-pound body along a sidewalk. Walking seems like the green thing to do.
But appearances can be deceiving, making easy answers dead wrong. That's the case here when the calories expended in walking are replaced.
His reasoning is that a huge amount of energy is lost in the food supply chain. And, even when the food enters our bodies, "Only about 15 percent of the potential energy in food eaten goes into activities such as walking, as well as maintaining all bodily functions."
Put those two factors together and McKenzie concludes:
This means that the energy that the human body actually converts into work is meager percentage-wise--something on the order of 1.3 percent of the fossil fuel energy that is used along the entire length of the food-supply chain.
When it comes to energy use and greenhouse gases emitted, appearances can be grossly deceiving. Granted, people who drive everywhere are energy users and polluters. But walkers also use fossil fuels through the food they eat to replace the calories burned while walking. Of course, driving can be more polluting under some circumstances, such as when large SUVs are the preferred vehicles or when drivers insist on doing wheelies at every stoplight. Bicycling the distance can also be less polluting than driving. [Derek] Dunn-Rankin [who, McKenzie tells us, is "a professor of engineering at the University of California, Irvine and an avid environmentalist] sums up the central, largely counterintuitive, point of this commentary: "Driving a small [or moderate-size] car and not having to replace burned calories saves more energy (and greenhouse gases) than walking when the extra calories expended are replaced."