Whether you buy into Al Gore's spiel or not (admittedly, I'm one of those who can't see how anyone can deny the global warming issue exists), here's an interesting point to consider, which comes to me by way of a coworker: Here in Colorado it will approach 100 degrees this week. This type of heat was uncommon in this area as recently as a decade ago, as demonstrated by the fact that houses built in this area more than ten years ago tend to not come equipped with central air conditioning. Nowadays it is ridiculous to think of buying a house without central air.
I find nothing online to back up this observation with hard data, but it is interesting anecdotal evidence.
Of course, the fact that using something like central air conditioning contributes to the problem is not lost on me...