No. None of these are very likely to destroy the world before we humans do it ourselves. We have already punched a huge whole in the O-zone layer and have killed off countless species of plants and animals to make way for modernization of the globe. Besides, it would take something pretty destructive to bring down an entire planet, and humans are just dumb enough to do it. We've already got enough nukes just lying around to blow up the world a couple billion times over. Not to mention all of these diseases we're creating to scare the bejesus out of people.
With all of these stockpiled weapons, resentment between civilizations, and competition for resources, it is obvious that a country will break down sooner or later and start blowing stuff to hell.
"Look out! Here comes the nukes! And whats that you say? Biological weapons! We totally didn't see this coming! Not like we could have prevented all of this years ago!"
Yep, some little backwater nation is gonna get pissed off at some huge country like America and start launching their stockpile. Then America will get pissed and retaliate. Then the small country's allies will retaliate in response to that, and before you know it, the whole world will have blown itself away beyond the point of recognition. Everything except Austrailia, of course. Everybody loves Austrailia.
Then will come the viruses and diseases. Like V-X gas, you know, that stuff in The Rock? Nicolas Cage describes it all to Sean Connery, it goes something to this effect: You lose all communication from your brain to your spinal collumn within 30 seconds, you break your back and choke so hard you puke your own guts out, then your blood begins to boil, but thats after your skin melts off.
I just hope that little country doesn't get pissed off in my lifetime. But thanks to Bush and his "War on Terrorism", whatever that means, that little country is probably already pissed.