I think it would make sense if mines added -1 healthy to a city for each one within its radius. Historically (as is true today) mines are very unhealthy places to work, and alot of people died working in them, and they often affect the land in bad ways. I think this might help promote keeping trees for hammers, rather than cutting them down for mines.
Also, maybe we can have -1 health in cities for certain techs, like combustion, industrialization, plastics, for example.
So, so counter all these unhealthies, I think we should change recycling plants to give +3 health instead of removing city improvement unhealthiness, and then we can give environmentalism the ability to remove all unhealthiness from mines and buildings and the 'dirty' techs, and make it a high upkeep civic (enforcing environmental standards is expensive).
Btw, I have to bring this up as someone said trees don't regulate weather, but they do have an effect. The evaporation of trees keeps surface tempuratures cooler than they normally would be without them, and the evaporation also feeds into the air to form clouds and rain. The amazon rain forests almost generates its own rain, in fact, and keeps the climate relatively stable (albeit warm); contrast that to deserts, where the temperatures can vary hugely between night and day.