- Man-Made Bombs
- Super Bugs and Plagues
- Comet or Meteorite Collisions
- Termination by a Higher Authority
Probably the most serious threat is from the fast-mutating microbes and bacteria that share our space. They're getting smarter at a much faster rate than we are. For the other threats, there is a period of grace where we can prepare for the end, at least emotionally. But with bugs, we really have no way of guessing what might be coming at us in the very near future.
The world we have built for ourselves will just have to live with the ever-present threat that some nutter will let loose the massive destructive-power available from nuclear-devices. In recent times, negotiations to limit the spread of undesirable nuclear-usage have gone badly. To my mind, what has been missing from the equation is the concept where "I'll give up mine, if you give up yours". This was the basis for the original nuclear-disarmament treaty, but seems to have disappeared from current diplomatic-rhetoric. Let's hope we can keep the bombs at bay long enough to attempt to address the Super-Bugs.
The probability of a collision with an interplanetary object of significant proportions, has diminished greatly as technology improves the monitoring process. We now have a window to do something about the problem before it happens. We could sleep easier if all threats were reduced to this same level.