My answer:
I thought in terms of "what could kill every last human?"
At first my impulse was to lean towards environmental (climate change) like catastrophe, but then i considered recorded history doesnt even go back to the last time the desert belt (Sahara, Arabia, etc) was green, 7000-4000 BCE. At worst a melt off and ice age await, population bottleneck does not equal extinction. In my opinion nothing we can do would kill all humans, not even a nuclear winter of every nuke being deployed.
Volcanic Winter? Survived Toba.
Then I thought of biological warfare. Like an airborne super-AIDS that infects the entire planet. But even that I dont think could end all life.
What could kill off man kind?
I think we are even adept enough as a species to survive even KT extinction repeat.
Then I got it:
The greatest threat is:
If the earth goes into a pole reversal, and is hit at the point when the magnetic field is momentarily nonexistent. Reversals happen every 200,000 years.
*If* an X-Ray, Gamma Ray burst from a Magnetar or neutron star collision were to hit the earth at the worst possible time then the atmosphere would be INSTANTANEOUSLY stripped away.
All not in a self contained bunker with its own air supply would instantly die like in Total Recall. No reactor Quaid could start could save us.
What about you, what do you think it is?
