Global warming is not a civilization-ending threat. Even nuclear war is not a civilization-ending threat, outside a few places in the Northern hemisphere. I could be wrong about both of these claims. But you could be as well, and you're taking an extremely pessimistic view based on what you know, which is not the whole picture.
We also know that it causes an extinction event, since it was one of the effects of the K-t extinction event 60 million years ago. It is the one that killed the dinosaurs, and many more: A huge number of species, including almost any large land animals, dies. It is unlikely that homo sapiens would survive this time.
The worst case scenario of global worming would not kill everybody. People living north of 40o will survive.
Then Saddam lit thousands of oil wells on fire as a big fuck-you to the miltary coalition arrayed against him, more oil than would be set on fire by any nuclear war. The cooling effect was detectable, but hardly serious.