I kind of like the idea of taking air pollution

yuck

and (de)forestation into account. And like you said - it is a more refined mechanism with somewhat mod-/use-able tweaking variables...
But i just finished a game, and i really did not like the effect.
Even with nukes baned (and no meltdowns i know of) there was no way to prevent GW. You can't influence AI city building, Ecology and recycling centers seem not to be too high on AI priority as well...
And with the AI chopping everything they get they hands on, keeping enought worldwide forest coverage for a significant amount of GW_DEFENSE is utterly impossible (Let alone hitting the green threshold...).
I think it needs a different valuation/policy for AI terrain Improvements.
And the effect is still completely irreversible of course... Be it feature removal (even with Forest Preserves the Regrowth is laughably slow) or straight desertification...