The most polluting engine since the Saturn 5. It burns methane, the worst greenhouse gas of them all.
But compared to even a small player in CO2 emissions, like the airline industry, rockets currently do not even compare at all! We would need several orders of magnitude more launches to even begin to need to factor in their contributions compared to other industries.
Fun fact: the space shuttle RS25 engines still being used by NASA to power the new launch system produce nothing but water vapor. They have almost exactly the same power and thrust as the Raptor engines only they cost more.
He got cheap. That's all there is to it.
Hydrogen is perhaps the cleanest burning fuel. When you burn hydrogen with oxygen, you literally just get water vapor.
But there is a trace amount of nitrogen oxides aka NOx while the vehicle is in the lower atmosphere otherwise known as the troposphere as an after-burning effect of the hot flame coming in contact with the air. Literally all rocket engines will do this to a certain degree when in our troposphere, which is primarily composed of nitrogen."
Water vapor is actually a much more powerful greenhouse gas than CO2. You can think of CO2 as more of the thermostat and water vapor as heater, kind of.
I just wished they spent a fraction of their time and money fixing real fucking problems rather than adding to an already untenable pollution problem for the sole sake of selling space flights to millionaires.