[Symmetry]

Not too huge. This person[1] did some math and found that a BFR trip across the world should take 40% to 240% as much carbon as a regular plane flight. For short trips this would be worse, though, because a plane's range goes up almost linearly with fuel use while a rocket's goes up much faster than linearly.

Rockets that use solid boosters like the Space Shuttle, SLS, or Atlas V do a good amount of ozone damage as they go up but the ozone damage from cryogenic propellants of the sort that a BFR uses is pretty small.

[1]https://www.quora.com/Elon-Musk-suggested-that-the-SpaceX-BF...

[byw]

Also I'm not sure where SpaceX gets its methane from. If it's from above-surface sources wouldn't its net carbon contribution essentially be zero?

[et2o]

Did a little googling. Methane is primarily obtained from natural gas fields which are below ground. Atmospheric concentrations are relatively low (on order of parts per billion) although it is still an important greenhouse gas (30x more potent than CO2 per ton). There's a lot of ongoing research into generating CH4 from CO2 using renewable energy but it's relatively energy intensive to date.

[Semirhage]

So there’s noise pollution on a grand scale, it depends on natural gas extraction or a breakthrough in electrolysis, and it will produce CO2 when burned in flight. Great. It will also cost a fortune, and even SST isn’t viable, yet somehow this will be?

I could not be more skeptical.