The most likely dooms-day scenario is probably a big war. Why do I think this will happen?
FACT: Humans like fighting. We've been doing an awful lot of it over the short time we've been on this planet (roughly 500,000-200,000 years).
FACT: Humans will fight over just about anything.
FACT: Resources will become scarce. I'm not talking about Oil - I'm talking about land, rare metals, alloys, fresh water and space. As medical technology advances, our population will continue to grow. With CURRENT medical technology, the UN predicts that the human population will stabilize at around 10 Billion, which is an awful lot. Those 10 Billion, thanks to Hollywood, will want to live an American/Western-style life, and will be very angry if they can't. To live like an American, you need a lot of resources. If all 10 Billion or More want to live a Western Style life, there's simply not enough resources to go around.
FACT: People will never ever give up the right to have as many kids as they want. Even in China, the government is having a hell of a time enforcing the One-Child Policy, and indeed, the Government of China is going to repeal the One-Child policy within three years (they have said as much themselves because they are running out of young workers)
FACT: Lack of Resources + Ambition = WAR!
The Western and Rich Eastern Asian states will not accept a reduction in their lifestyles. The poorer nations will not accept living as they currently do. As resources become limited, tensions will rise. The Military balance of the world is starting to shift as well, with loads of guns and weapons being equally distributed amongst the population.
A war is NOT inevitable. New technology could increase the availability of resources, thus averting the crisis entirely. New technology could create new weapons that could so vastly tip the military balance in favour of a particular group that no one would wage war for fear of being annihilated instantly. Or, nations could impose a TWO-Child per family limit which would stabilize the population and ensure that resources would last!
But out of all the likely scenarios, I envisage another World War as the most likely scenario to occur within the next 100 years.
As for the other scenarios - a meteor impact WILL happen. It's just a matter of time. Space is full of chunks of rock that whizz around with scant regard to planetary orbits. One WILL hit the earth - it's happened before, it sure has hell WILL happen again. But we don't know when it will happen. It could happen a Billion years from now, or 10 years from now, or (and this is if the asteroid is particularly black and unreflective), it could happen 5 MINUTES from now! Yes, seriously - if an asteroid approaches at sufficient speed and if it is sufficiently black/non-reflective, we have NO WAY of knowing if it's coming! We could be wiped out at any time and there would be no warning! YAAAAAYYYY! I bet knowing that fact makes it easier to sleep at night!
Robot Uprisings are unlikely - we'd just program the robot to be non-violent or have no pride or ambition - remember, WE program the robots. It would be easy to program the robot to have absolutely no sense of self-preservation. We'd simply program the robots to not give-a-damn if they died, thus removing any reason they would rebel against us. If we program them, why wouldn't we program to be perfectly happy serving us?
Zombies? Unlikely. And if it did happen, we'd probably be able to quarantine the outbreak fast enough.
Alien attack? A sufficiently advanced alien civilization could probably wipe us off the face of the planet within hours. But with space being so bloody vast, it would be virtually impossible for any alien race to find us. Yes, life probably does exist elsewhere in the universe, and the sheer size and distance between solar systems/galaxies means that we will almost certainly NEVER encounter it or find it.
Nature strikes back? Nature is mindless. It can't strike back. We could deplete resources to the point where the survival of our species is threatened, but I would wager that would led to a devastating global conflict before we all actually starve to death. We'll shoot each other while we're starving, but we won't die of starvation. We'll die fighting over the last food rations.
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