I wasn't trying to give a real number, I was saying that 'too hot, planetwide' is rather unrealistic. The initial ass-number was '6', why does your argument rely on numbers an order of magnitude larger?

Because there is no predictable upper limit, and any quick fix is at immediate risk of not being enough shortly later.

I think it's perfectly reasonable to postpone worrying about 50+ degrees until it's already shifted at least 5. If it turns out 5 is terrible, we'll have hundreds of years to implement plans to stop 50.

If you think 5 degrees is catastrophic, then sure worry about that in 2013. But it won't make the planet uninhabitable.