It is extremely tiny now- but definitely will be something worth thinking about when (here's hoping!) SpaceX and others reach their goals. When we run 100x or 1000x current launch capacity I don't think it's unreasonable to worry about rocket pollution.
Commercial passenger flights departing in the United States produced 179 million metric tons of carbon dioxide emissions in 2019.
1 Ton of Methan produces 3 tons of CO2.
Starship+Booster fully loaded has 1000 tonnes of methane (and three times oxygen).
So a thousand Starship launches a year would be 3 million metric tons of CO2. Not nothing but (surprisingly?) less than 1.5% of commercial US flight industry.
Theoretically one can produce Methane climate neutral through bio gas. I found different prices for liquid methane (1000 to 2000 dollars?), with that it seems the fuel cost are only around 2 million per launch? That is cheap. A 1000 Starships would only cost 1 billion in Methan fuel (disregarding oxygen and heightened demand and economy of scales).
