Well the ideal system
Not Global Warming but (Pollution)
Cause=Unhealth + Nukes [5000 per 9 tile explosions or 1000 for meltdowns/1 tile explosions (meltdowns and tac nukes should only have a 1 tile effect)]
Each turn
(Unhealth of the World + Nukes Points - #Land Tiles-#Forests+Jungles)*Total tiles/ (Land Tiles*'Pollution Effect')
(minimum 0)
would be added to the Pollution for the world
If Pollution >= 1, then a number of tiles would have the 'Blight' feature placed on them
The number of tiles of Blight would be
Pollution/200 (round off randomly; 2.3= 30% chance for 3 and a 70% chance for 2)
Subtract Number of Tiles that get Blight from Pollution
The tiles that get Blight couild be ANY tiles, Land or Water. If a tile has Forest or Jungle already, then the Forest/Jungle is removed (and no Blight appears).
'Pollution Effect' would be adjusted for Gamespeed (probably 24 on normal)
Blight would then have the following effects
1. -1 Food, +1 Unhealth
2. Cannot be 'removed' by workers
3. Blocks Access to Certain resources (Biological ones: Fish, Clams, Crabs, Whale, Wheat, Corn, Rice, Silk, Sugar, Wine, Fruit, Incense, Spices, Dye, Deer, Fur, Ivory, Horses, Cows, Pigs, Sheep)
The Blight would take 'Pollution Effect'*5 points to be removed, it would get 1 point each turn from each of the 4 tiles next to it If they are un-Blighted and 4 points each turn if it is controlled by a Civ that has Ecology [if tile with Blight got Blighted again it would just add more to the Recovery time]
The Environmentalism Civic would then be changed to
High Maintenance
+25% Corporation Maintenance
No Population+Terrain Unhealth
No Resource Unhealth with Public Transportation*
+0.5 Unhappy from each Unhealth
+0.25 Happy from each Health
So that you could eliminate Pollution by
1. Not setting off any more Nukes
2. Building a safe source of Power [Nuclear Plant Meltdown needs to be an Event.. something you can Handle+Prevent]
3. Building Recycling Centers
4. Going Environmental
5. Going Environmental and Building Public Transportations
The best a developed nonEnvironmentalist City could get is Population+2 (Power) + 3 (Resources) Which means that if all land tiles were worked by population... and no specialists, it would be producing net 4 Unhealth/turn enough to generate 1 Blight every 6 turns on Land. Each Blight would take 15 turns to remove, so you would have about 2.5 Blight per city if the whole world was like this.
Even the worst Environmentalist city would have 4 (Power)+6(Resources)+7 (Buildings)=16 Unhealth still probably less than the land tiles per city. (and they would have a real reason to cleanup with the unhappiness)
Then just wait it out. (a long time because of the 200 lag)
* Change Public Transportation to 1/2 Resource Unhealth, 0 Resource Unhealth with Environmentalism
That way it is reversible, and still has an Impact. And gives you a reason to want someone else to turn Environmentalist... by turning Environmentalist they help You, not just themselves (making the UN Resolution a good one to pass once Blight is all over the map) And Environmentalism would help yourself slightly if Blight is all over, you would avoid the unhealth Problems that it would be causing.
I would also change Forest/Jungel Preserves fairly significantly
1. allow the "Preserve" to be placed on ANY Grassland/Plains/?Tundra? tile
2. only if the Preserve has a Forest/Jungle does it give the benefits (happy)
3. If the Preserve is on the Jungle, it should give the additional benefits of +2 Food and +3 commerce [with the right techs ie Ecology, Medicine, Genetics]
4. Forest/Jungle should be able to spread into a preserve tile (preserves should not block Forest/Jungle Spread)
5. If you are Working a Preserve Tile with no Forest/Jungle/Fallout/Blight AND it is adjacent to a Forest, then in 50 turns it will become a Forest Preserve
6. If you are Working a Grassland Preserve Tile with no Forest/Jungle/Fallout/Blight AND it is adjacent to a Jungle, then in 30 turns it will become a Jungle Preserve
#5+6 would work similar to Cottages and the Rate would double with discovering Ecology.