I think we know that there have been bouts of global warming that was preceded by release of methane into the atmosphere, followed by 6 to 9 degrees of warming over a long period of time. It fucked with the ecosystem but mammals managed to survive at least two of them.
This appears to be a man-made beginning to one of these warming periods, though it's happening at a much faster rate than ever by orders of magnitude.
http://www.wunderground.com/climate/PETM.asp
So, I think what's scary is that this looks like a massive global extinction event that hasn't happened in 25 ~ 50 million years, and it's starting now, and it's happening faster than it ever has. I dunno, doesn't that scare you a little?
It looks like if we're not unlucky we'll manage to survive (mammals have done it before). Probably scariest is the political turmoil that will result. That's the part that will affect us in our lifetime.