[njb311]

While it is absolutely true that cycling and running produce CO2 emissions, the figures I found for running "at a very challenging pace" equate to 54g per mile above normal respiration [1], so even with two people in a car (especially driving in urban areas) running would still produce less than most cars on the road today. Of course, there are other factors involved.

Stated tailpipe/windpipe emissions don't include the carbon cost of production for either fuel or food. That will depend on your diet, where in the world you live, where food or fuels are grown/extracted, where/how they are processed and transported, and so on. The environmental impact of production can also cause CO2 emissions long after production through land use change, land remediation, etc most of which is not accounted for when making these comparisons even if a "full" LCA is undertaken.

The carbon cost of manufacturing a car is rarely quantified but I remember one maker estimated it was the equivalent emissions of driving 40-50,000 miles. A bicycle is far less and lasts far longer (I've had my bike for 23 years and it's fine). And of course, that's just the manufacture. Car maintenance also has a much greater carbon impact than a bicycle or replacing the soles for your shoes.

We also have a fairly crazy situation where people drive to work or other places, but then go for a run or go to the gym to burn calories and 'get fit' – perhaps even running on a treadmill which itself consumes energy. Just the idea that you have excess calories that you need to undertake an activity specifically to burn them is a counter to the argument that walking or cycling more would lead to increased food intake. Of course, I know that some people go to the gym to do resistance training targeting physiological benefits.

But then again, the obesity epidemic also demonstrates that excess calories (exacerbated by shifts in the make-up of diet) is an existing problem.

[1] https://www.livestrong.com/article/524672-what-effect-does-e...