I believe in the little Ice Age theory myself,i t will be caused by global warming.
I seen it a few times on the History channel,
When enough of the polar caps melt and flood the gulf stream with fresh water, it will cause it to shut down. Hence, triggering an ice age.
Either way, it won't be good. Life as we know it will change for everyone.
Edit, The movie, The Day After Tomorow was based on this theory!