And, to round out the above:
2. According to the CivII MGE manual, global warming can occur whenever there are 6 or more polluted squares on the map, although in practice, it doesn't usually happen until pollution gets really out of hand (I'd say 10 squares or more). Of course, once it does happen, you're SOL. If you're not keen on counting the number of polluted squares manually, once you start getting polluted squares, a little yellowish circle thingy shows up on your city screen (next to the science beaker thing). Once that circle starts getting bigger, time to bust out the Engineers...