There are reasonable people predicting that type of mass extinction will occur within our lifetime. This blog makes a pretty good argument it will happen: http://arctic-news.blogspot.com/p/extinction.html?m=1
In my opinion, to think that human caused global warming and sea levels rising by 1.5mm per year will lead to global extinction event seems a bit far fetched when you compare it to what caused the last global extinction. The current climate changes are minuscule compared to huge climate changes (not caused by any human activity!) that happened quite recently.
Sure, we're going to have a smaller change in climate but we're also a lot more precariously placed than they were (and about as incapable of dealing with it.)
You are right that our civilization is very vulnerable because of our agriculture and food logistics being very fragile. A single really bad draught on a global scale would cause massive problems in feeding the large population we have.
I would argue that in this respect overpopulation is a bigger problem than climate change though. Instead of focusing on emissions and renewable energy there should be more focus on decreasing world population from current unsustainable level.
Oh, no, sorry, I wasn't - I was just saying that it was already quite wobbly and even a small push from something like climate change could topple it.
Have they got Paul Erlich on board?