I know that in Civ4, using ICBMs trigger global warming, but IMO this is wrong. If anything they should cause nuclear winter, which is the opposite of global warming (wouldn't it be ironic if the solution to global warming was nuclear winter?). Perhaps there could be a kind of "Doomday Clock" (like in Empire Earth, which would begin to count down if nukes were used excessively) that would countdown to a nuclear winter that would effect the planet for at least 10 years, depending on how many nukes were used. The Nuclear Winter would effect the entire planet and either temporarily penalize production, food, wealth, etc. or it might destroy terrain improvements such as farms, which would have to be rebuilt afterwards. I think that global warming should begin to occur after Industrialization is discovered, and then only if there are a lot of cities with more pollution than health. That way global warming is more of an issue, not merely a penalty for using nukes. Also, there'd be even more incentive to adopt Environmentalism (especially passing the UN's Environmentalism legislation, so all civs adopt it) and construct Recycling Centres and Health+ buildings, which would prevent or stop global warming. Also, perhaps the Kyoto Protocol could be added as a new UN legislation? It would reduce pollution but it would also penalize production by perhaps 25%.