A reminder for everyone: on this day in 1983, Stanislav Petrov saved the world.
It occurs to me this time around that there's an interesting relationship here - 9/26 is forgotten, while 9/11 is remembered. Do something charitable, and not patriotic, sometime today.
I can see how the tropical forests may become tropical deserts, but I don't see why now-frozen huge territories in Canada and Siberia won't become available for agriculture as temperatures rise.
Worst case scenario: We can devour the flesh of 90% of humanity, and we'd still be 9% better than in the thermonuclear war scenario you mentioned.
When scenario 1 begins with the death of 99% of humanity, and scenario 2 does not begin with any deaths, I think the burden of proof is on you to explain how the hypothetical dangers of scenario 2 could possibly be worse than the given deaths of scenario 1...
Voted up for thinking numerately.