I believe I used a fairly small number for civilizational collapse and extinction, on the order of ten to fifteen percent. I just don't find such doomsday scenarios that likely or plausible.

It may be that my background and upbringing have inured me to it - I've seen the end of the world not happen far too many times in my lifetime:

• Communists failed to conquer everyone
• There was no nuclear war/nuclear winter with russia
• The UN new world order didn't enslave everyone
• The end times due to the second coming of christ haven't happened at least a dozen times
• y2k didn't cause problems
• 2012 was just retarded
• There has been no superflu
• The stock market has become more stable over time, not less
• Peak oil happened and nobody even noticed
• There was no hard AI takeoff

There's probably more if I stop to think about it.

At the moment, I find biotech to be the most likely existential threat, with general civilization collapse and strong AI the next two major candidates.