This conversation sure has gone to a lot of different topics....
On a note, it's pointless to say anything about non-nuclear climate change on a forum, a few lines of text aren't going to change someone's mind either way. Most that reply along those lines probably won't read this far though...
On the other hand, reconsidering the mechanics of using nuclear weapons from a gameplay perspective, and the benefits or disadvantages of different solutions is something I agree
can be discussed.
Calling it "global warming" is just a hangover from the days when Pollution from industry caused the effect. Nuclear weapons also spread pollution, but now with the industry version gone all that's left is fallout.
Following along the lines of -

/+

ideas: Would it be more realistic to hit only the big cities with problems, or have a universal effect that hits both small and large cities equally hard? What I'm thinking of is that +

would only have a significant effect on fully-developed cities (as smaller ones would not have reached their health caps).
A food penalty, on the other hand would have to be very carefully balanced... consider a city in the tundra/ice on the coast that is surviving entirely off the 2

per water tile. What would happen exactly if you give it -2

? One population point would die off, which reduces the food supply by another 2, killing off another, and so on down to the original 2 food on the city tile... yet it would still not have enough to support that population unit. What happens to a city if it's at population 1 and is still starving? In previous versions of Civ that would cause the city to be destroyed, I have no idea how Civ 4 handles it, and either way it would mean all your polar cities would be starved to death. While possibly realistic, it might be game-killing anyways. It probably would be best to go without food penalties.
Then you have another complicating factor to throw into the situation. Corporations... with Sid's Sushi in my current game, it gives every city it's in +18

.
Adding unhealthiness to every city might be the way to go. I also thought of changing the terrain-to-desert mechanic to adding random tiles of fallout, but that would only penalize the civs that have not researched Ecology yet, and would just be the whack-a-pollution effect again.
What would you think of a system closely paralleling war weariness, but with

?
A nuclear winter point tally would increase with every ICBM detonated, and to a lesser extent tactical nukes. The tally would gradually reduce every turn. The value would be used in a random dice roll at the start of every turn to see if winter occurs; the higher it gets, the more likely it will be triggered. If it happens, the current point tally would be used to determine a +

modifier in all cities. Nuclear winter actually taking place would "burn off" the points more quickly, causing the effect to mostly take place all at once. Further detonations during winter would increase the tally even worse, like war weariness.
If it doesn't already, the AI should also carefully weigh the cost of using nuclear weapons and only use them in reasonable situations. What would you think of the following system?
The AI would only use nuclear weapons at all if one of these is the case:
1) Nuclear weapons are used against it
2) It's dramatically outmatched on the Power rating
3) It's close to capitulation
If one of these occurred it considers the option open, otherwise it does not use them and tries to continue to win through conventional warfare. This should limit it enough to a series of checks to be programmable.
If it's #1 (retaliation) the AI would only use about the same number of nukes that have been launched against it. In a tactical sense, it would immediately detonate enough to bring it's total up to 3/4 of it's opponent, then save the remaining 1/4 for situations which are extraordinarily dangerous on a small scale, such as a massive stack of units that presents a major threat.
If it's #2 (rogue state, or losing the conventional war) the AI should use any and all weapons whenever the opportunity provides, against targets that will hurt the most. It should try and position its tactical nukes in good places for launch, and detonate each missile whenever it gets in place.
If it's #3 (a last ditch effort) it should detonate all weapons immediately on the best possible targets that turn.
It'd be even better if an additional diplomacy option could be added for 2 and 3: Give us what we want (in this case it would offer a ceasefire) or we launch. Similar to the "...Or we will attack" option in Rome: Total War, something that would be nice in Civ to threaten opponents.
This second part, altering the AI, would be what I don't know how to do. The first part, simulating nuclear winter I think I can do.