It's extremely, extremely tiny compared to the impact of the thousands of commercial airplane flights that happen every day. Plus, methalox burns very cleanly, second only to the much more dangerous and finicky hydrolox in terms of environmental friendliness.
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).
