I was reading up on Ireland’s new CO2 motor tax system (https://www.motortax.ie/OMT/pdf/co2_emissions_rates_2009_en.pdf warning PDF) and it got me wondering.
By the same measurements, how efficient are humans in general?
Are we really compatible with car engines? We exhaust CO2 all the time, not only when we are moving.
I looked at a few web sites and they seem to agree that walking burns somewhere between 250 and 300 calories per hour. I assume that’s actually the extra calories burned, compared to when you’re not doing anything. If we assume 300 calories/hour at 5 km/h, that’s 60 calories per km. Carbohydrates contain about 4 calories per gram, so that’s 15 grams of carbohydrates consumed per km. If we assume it’s all from glucose, the resulting CO2 weighs about 50% more than the glucose, or 25 grams/km.
But this is really a meaningless comparison. The reason we want to limit CO2 from cars is because cars are powered by fossil fuels, which are dug up from underground and released into the atmosphere. It’s adding new CO2 into the ecosystem. Whereas the carbon consumed by humans come from the air, converted into food by plants. The carbon footprint is zero.
Unless of course you factor in the energy used for manufacturing fertilizer, operating the farm equipment, transporting the food to you, etc. These do come from fossil fuels, and can be a significant amount.
Yet another complicating factor is whether we actually eat more when we exercise. I suspect most Americans eat enough already that they could walk a dozen miles a day without any extra food.