Do green roofs in cities actually work or is it just greenwashing?

It's not pure greenwashing, it really does work but you shouldn't expect miracles either. They help with thermal insulation, reduce heat islands locally and retain some rainwater, that's concrete stuff. But if you put a little green roof on your building thinking it's gonna solve the city's pollution problem, you're being naive - it's an interesting addition to a broader strategy, not a magic solution.