Global Warming sounds more possible to me. People will find out some sort of temporary or premanent solution to the up-coming problems. Or they will simply announce: "OK, fellas. We did it... Um, I hope you all know how to swim, right?"
I rather not think about it at all. It's going to happen when it's time, whatever it is, ocean-level rise or new ice age, we have to face it.
This reminds me of a Hungarian dramatic poem which was first published in 1861. It's called: The Tragedy of Man. It starts from the Creation and goes on different historical sceenes to the end of humanity. I think it's interesting, I liked it when I had to read it for Literature lesson. If someone's interested, here's a link to an English version:
The Tragedy of Man