I still think that we would survive almost anything short of the Earth being totally demolished. Any number of situations would make it difficult to survive, however for total eradication of the human race, we'd have to have some form of astronomical event. The most realistic of the events is a solar flare reaching us, flashing the whole planet. I dunno, maybe Venus could explode and cause Earth to shift it's orbit.
"I would rather be exposed to the inconveniences attending too much liberty than to those attending too small a degree of it." Thomas Jefferson