Annual fuel consumption = Starship's 1000 t capacity of fuel per launch * 365 day * 24 hour/day / 4 hour per launch = 2 million tons.

Global annual CO2 emissions = 30 billion tons.

If all the fuel became CO2, that's 0.01% of the global emission rate, so worthy sacrifice for a Mars colony, I think.

> If all the fuel became CO2

Methane combustion produces 2.75kg of CO2 for each kg of fuel.

Because of the oxygen which is also part of the 1000 t of "fuel".

However, I think I used the wrong value anyway, for Starship instead of its booster, so x4 or so. Doesn't affect the conclusion.