Human extinction via climate change is off the table.
The end of human civilization? Depends on what one means by it. If it means we'll go back to being hunter gatherers it's ridiculous. Some civilizational wide regression is not impossible, but even the direst IUPAC scenario projects only a one digit drop in global GDP (of course, that's the average, it will be much worse in some regions).
Indeed, but even going back to the 19th century would have dire consequences, given our current global dependence on the industrial fixing of nitrogen.
If civilization were to collapse (and I don't think it will, other than from a near-extinction event), I doubt it would be like going back to any earlier time.
I would still bet against going back to 19th century. Worst case scenario for me is hundreds of millions dying and a big regression in global standards of living, but keeping our level of civilization. Which is horrible, it would be the worst thing that ever happened.
I don't think IUPAC (International Union of Pure and Applied Chemistry) has come out with views on impact of global warming :)