Global Warming will start appearing in your game in the Industrial Ages (around the time of factories, coal plants, hydro plants, etc - there's a list of pollution producing improvements in the editor). Global Warming is represented by a sun in the info box in the bottom right of your screen. The brighter it is, the more global warming there is.
Global warming turns grassland into plains, plains into desert, and I think (?) jungle/marsh into grassland. Basically, it turns good terrain into worse terrain...and yes, nukes can increase the chances of more global warming, as do factories, and other pollution improvements, as well as a high population. To combat global warming, you can build a recycling plant and another improvement I can't think of right now (maybe someone else knows) to lower the chances of it. That, and having a lower population, not building pollution causing buildings, and not using nukes.