For every unit of food, there is a unit of oil to produce food. I'm not sure what the ratio would be, but it surely must be there.
I suppose walking in a foreign city is not accounted because agricultural production, transport, hospitality etc. already is accounted elsewhere.
There's surely a need to join up all the accounting up though.
For example, imagine that I am taking an intercontinental flight in 8 hours:
- I spend 8 hours flying from London to Mumbai. The majority of my CO2 emissions come from the plane jet engines.
- I spend a month walking between London and Mumbai. The majority of my CO2 emissions come from the food I consume.
Note if I DID fly I would not have spent that 1 month eating, BNBing, using Air Con for a month. Which means that is the CO2 cost of my month-length walk.