[garmaine]

A small factor more than aircraft in the short term. Zero in the medium term when launch companies vertically integrate propellant production based on renewables. A net positive once we start relying on non-terrestrial resources instead of poising our planet to extract metals and power.

[gpm]

Ignoring the positive externalities, there is still some environment cost when using synthetic CH4. You're still taking water from the ground and moving it into the upper atmosphere (CO2 + ground H2O -> CH4 + O2 -> CO2 + atmospheric H2O) - which contributes to the greenhouse effect.

It's not worth worrying about, given all the other more impactful ways we can move the needle on global warming, but it's not actually zero.