I'm hoping for alien invasion myself
I really don't think the end of humanity will be man made. It seems more likely that there will be a NEO that would hit us. NASA is tracking several objects that have the potential to intersect the planet. I think the likely cause for extinction would be either biological in nature(perhaps even a wierd disease) or astrological in nature. A large enough solar flare could impact the Earth. Almost anything else we would have time to adapt our environment. There are safe locations in the US where an entire city's worth of personnel would be able to survive for a significant amount of time. Something like a solar flare would make even those uninhabitable.