Nuclear exchanges = aerosols pumped into the stratosphere, courtesy of the bombs themselves, burning cities, and forest fires. This means immediate cooling, perhaps for years.
However, over the long run a warming trend is entirely possible once the dust filters back to earth. This part is much more speculative, since the effect of nuclear explosions on the composition of the atmosphere was never researched terribly well. If I recall, the worry was that the ozone layer would be damaged, allowing photochemical reactions that produce warming gasses to occur near the surface (ironically, tropospheric ozone, a key ingredient in smog, is itself a potent warming gas despite stratospheric ozone's role in preventing warming). There were also hypotheses about the amount of carbon released by all the burning (dust settles to earth, but the gasses continue to extert a warming effect for many years) and the production of nitrous oxides in the explosions and burning.
The nuclear winter research is much better than the nuclear summer research simply because the nuclear summer hypothesis only kicks in after the nuclear winter has passed. It's therefore much more speculative.
If I were to model climate effects of nuclear exchanges in the game, I would track the number of nukes exploded over the last two turns and proportionally cool the earth. 150 nukes should generate immediate and intense cooling, leading to starvation in cities. In such a mod I'd probably also remove the "only 1 pop lost due to starvation per turn" rule, since that would artificially mitigate the effect of a few years' cooling. As for warming, I would base that on how many nukes were exploded in the last 25 years or so, and make the effect much weaker per nuke than the cooling effect.
The result? A large-scale nuclear exchange would lead to immediate mass starvation. After a few turns, the climate would return to its pre-war state and then begin a slow warming which would only end after another 20 years or so.
Question: Are improvements in Civ IV destroyed when climate change renders them unbuildable on their new terrain? Mod-makers would need to decide whether this was a good idea. Perhaps farms should be destroyed if their plains become tundra. Then again, perhaps they merely become inactive or less productive, regaining full productivity after the nuclear winter.
BTW what exactly is "fascist" about saying nuclear war might just have an effect on the environment? It's not THAT implausible to think that burning huges areas of the planet all at once might affect climate. CKHenson reminds me of all those conservatives that condemned the original studies on nuclear winter because it was unpatriotic to say the US couldn't fight and win a nuclear war. Why, if the commies thought that we didn't think we could win a nuclear war, they'd just roll into Western Europe without fear of nuclear escalation! They pretty much shut up about the nuclear winter hypothesis after Reagan publically said, "A nuclear war cannot be won and must never be fought." For some reason that had more effect on conservatives than all the scientific studies that were published. I'm sure it was just Reagan's carefully controlled climate experiments and research methodology that won them over.
