Ieuan Compton (@welshboy) asked me what I thought of an article that suggested that cyclists following a Paleo diet were travelling in a worse way for the planet than those driving cars.
The answer is really that the Paleo diet is the problem, not the cycling.
Cycling a kilometre uses an additional 30-50 calories (actually kilocalories but calories that dieters count). These calories are additional to the normal metabolic energy use that will happen whether we are active or not. The way that we obtain these, say, 50 calories can have big implications for the carbon impact of cycling (or any other activity that uses 50 additional calories, like running at the gym).
One of the lowest carbon impact ways of fuelling activity is with sugar. British Sugar calculated the carbon footprint of sugar as 600 g using the PAS 2050 standard. 1 kg of sugar has about 3800 calories so you can get 50 calories of energy for 8 grams of CO2.
Walkers Crisps calculated the carbon footprint of a packet of crisps as 80 g of CO2 for a 23 gram, 130 calorie packet. So if you us crisps to fuel your 50 calories there will be 31 g CO2.
But beef has a footprint of around 13300 g CO2 per kg, with a calorie content of about 2000 per kg. this means 50 calories has 266 g CO2 associated with it.
So you can see that there are plenty of low impact ways to cycle. But how does this compare to driving. The average petrol car produces about 240 g of CO2 per km, accounting for direct emissions from the vehicle and the emissions associated with producing the fuel. But an efficient, small diesel car could have much lower carbon emissions. As the driver is not active in the car, he has no additional calories to fuel so it is right that a Paleo cyclist would have a similar impact to a typical car travelling the same distance. But if the driver used the gym to get his exercise after driving to work say, whereas the Paleo cyclist cycled to work but didn’t go to the gym, then it would be pretty equal if the driver used sugar. But if he too was Paleo with a big car then driving would be terrible.
And if the cyclist used anything other than beef then cycling would be less damaging, with sugar being close to zero impact compared to driving.