Greentext

This seems like something Chat Gippity might actually be of some value:

The CO₂ exhaled by a person riding a bike is tiny compared to a car’s emissions. Let's break it down:

Human CO₂ Emissions from Biking

• A resting human exhales about 0.7 kg of CO₂ per day (~29 g per hour).
• Moderate cycling increases breathing rate, and a cyclist might exhale around 4x more CO₂ than at rest.
• This comes to ~116 g of CO₂ per hour.
• A cyclist at 20 km/h would emit ~5.8 g of CO₂ per km.

Car CO₂ Emissions

• The average gasoline car emits ~200-250 g of CO₂ per km.
• Even efficient cars emit over 100 g of CO₂ per km, which is still far more than a cyclist.

Comparison

• A cyclist exhales ~5.8 g CO₂ per km.
• A typical gasoline car emits ~200 g CO₂ per km.
• A cyclist produces ~30-40 times less CO₂ than a car per km.

Even when considering food production emissions for fueling the cyclist, biking is vastly cleaner than driving.

"carbon footprint of the diet needed to fuel said travel"

This only works under the assumption that people would only eat as much as they need to, looking at the increasing overweight problems in developed countries this is clearly not the case, most people would have eaten that much annyway.

You do need to take into account however that biking might reduce the need for other form of exercise which would counteract the increased emissions. But either way I'd bet that per km biking is vastly more efficient, as in orders of magnitude more efficient.

This is an argument for ebikes, though not the strongest one, I think. The carbon output per mile traveled on an ebike is actually a bit lower than a regular bike because the food you eat has a carbon output. Yes, this includes charging the ebike from a coal power plant.

It does output more CO2 during initial manufacturing, though. Never does quite catch up with a regular bike over their expected lifetime. Both are better than cars and it's not even close.

The only important carbon part is the carbon used in transporting and growing the food in the form of fuel. We're not releasing trapped carbon when we're eating food as the only way we would save carbon in that situation is if we grew the food and buried it. Worrying about raw energy efficiency gets nonsensical because soon you'll be factoring in the solar energy conversion efficiency between growing and eating plants vs growing and burying plants to turn them into oil.

The better point is just that a bicycle is an incredibly efficient machine for moving a person in terms of energy input to work done compared to the hunks of metal cars are.