[benj111]

Wouldn't you then get stuck in a local maxima?

Your tomato example probably depends on the time of year, your location, the farm. And even then how are you storing it? Fridge? Cupboard? So probably not generally answerable.

I know Tesco (a uk supermarket) started labelling with embodied CO2, but gave up.

I do agree with you though. Even some marketing regs, so when someone claims something is greener, it actually is.