Superman is environmentally friendlier than Iron Man
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https://www.thecooldown.com/green-business/robert-downey-jr-doctor-doom-mcu/
Superman is solar powered, and produces no emission while flying (except perhaps a small amount of CO2 and the occasional methane).
As for Iron Man, it would produce 100,000 pounds of CO2 just to make the suite before it even leaves the ground.
Plus Robert Downey Jr. flies a lot in private jets, and Christopher Reeves not so much.
