There are signs up around my workplace inviting people to walk/cycle to work on some particular day- not sure which one it is- as a token gesture to reduce CO2 emissions. I was thinking about whether this was sensible or not.
I live too far away to propel myself to work without spending 4 hours a day commuting, but I will assume for the purposes of argument that I live only 5 km away. I get around about 500 km for a 50 litre tank of petrol, so driving 10 km I would use a litre of petrol- about 750 g of stuff that gets converted into CO2 (2.3 kg) and water (1.1 kg).
Now, if I am going to walk or cycle to work indefinitely, and not just as a token gesture, I will need to increase my food intake, or I will waste away and die. My gee-whiz exercise bike says it takes about 250 kcal to bicycle 10 km. When I convert this into rice, it comes out at only 69 g that I convert into about 100 g of CO2 and 20g of water. So it looks pretty good!
But... My rice has been grown somewhere with diesel-powered tractors and fertilizers produced using fossil fuels, and irrigation water probably pumped with fossil-fuel derived energy. It is packaged in plastic bags made from fossil fuels by machines run on fossil fuels. Then, the somewhere it has been grown is a long way away. The closest somewhere is around Griffith, from which it has probably been carried in a diesel-fuel powered vehicle to a central depot in Sydney, and then carried here the same way. But the packet actually says it is from Thailand: so it has been trucked from somewhere in Thailand to somewhere else in Thailand, loaded onto a fossil-fuel-powered ship, and taken to Sydney first. How much does that all add up to? I have no idea. The internet factoids that purport to answer this sort of question are all from end-of-cheap-oil doom and gloom people who want to make things sound as bad as possible, but I did find some reasonable looking data from a Centre for Sustainable Systems at the University of Michigan estimating that in the United States 7.3 units of energy are consumed for every unit of food energy produced, on average. In another part of the same article they have some specific calculations for a tin of sweet corn (8.1:1) and grain-fed beef (36:1) and estimate ratios of about 5:1 for a vegetarian diet and 10:1 for an omnivorous one. They also calculate that on average, 1.8 units of food energy are produced for every unit consumed. Things here are probably not too different from the United States- if anything, we probably have a larger proportion of food transported by road instead of rail or internal waterways.
So in getting to work on my bicycle, I am probably (indirectly, statistically) burning enough fossil fuel to make 3375 kcal of energy. How much is this?
Looking in the back of my 1st year chemistry textbook, I calculate that burning 1 mole (114 g) of octane should produce 1066 kcal of energy. Thus I am using about 360 g of fossil fuel in cycling to work: this is only about half the amount I would burn in the car, so voila, cycling to work is a good idea! Especially if we were to repeat the exercise and take the whole supply chain for producing and distributing the petrol into account.
Except... my 1066 kcal was the theoretical maximum, and I have no idea whether the University of Michigan study uses the actual energy produced by burning the fossil fuel, or the theoretical energy content. If it is the first, then cycling and driving are becoming pretty much of a muchness. And if my diet consists of meat from non-native animals and fresh fruit and vegetables airfreighted from exotic locations, than I am doing the environment a grave disservice if I don’t drive to work...