I have no clue how you came to the conclusion that self driving cars would be more harmful to the environment than human driven ones, but even if the number of cars on the road remained the sane (it would actually drastically decrease), and even if all self driving cars were ICEs, vehicle emissions would still drastically stop.
If every car was self driving, then every car would know the position of every other car around it and be able to communicate to every other car. Traffic jams would cease to exist, and potentially even stopping at intersections would go away too.
But, the reality is that once self driving cars reach that critical mass, owning a car won’t be a thing anymore. You’ll pay a service to shuttle you around. Hell, if the service was a public utility then it would be trivial to set up ride sharing for a reduced rate. That would be what drastically decreases the amount of cars on the road.