Aren't all the scenarios listed at the beginning of this just pulled from the documentary "Last Days on Earth?"
In any case:
-Global warming: it's not called that anymore, it's called climate change. Why? Because, according to ocean conveyor theory, we could make it so hot that natural ocean salinity cycles will cease, actually initiating a new ice age in the process. I actually think it's a mix of both hot and cold; I've lived in Washington State for seven years, and I've been noticing (perhaps in error) that at high solar angles in the summer it gets hotter and hotter as the CO2 in the atmosphere traps sun rays, while at lower solar angles it gets colder and colder as the CO2 deflects them.
I'm all for hydrogen fuel. The problem is electrolysis: it takes more energy to derive hydrogen from water than can actually fuel the car with that hydrogen. Hopefully advances in solar and wind power will help matters in this instance. Or if I'm lucky, I'll figure out that overunity device I've been working on, and we can render all forms of fuel obsolete.
-Plague: If you've played Pandemic II, you know that generally speaking Madagascar and/or Japan will survive any outbreak. :P But in all seriousness, avian flu is to be taken seriously. Then again, such "superpathogens" may trigger the next evolution in the species, and adapt the human immune system in to an even more versatile and reactive defense system than it already is. There's no way to know.
A deliberately set outbreak will of couse happen to quickly for the species to evolve its way around it. But don't worry, my crazy little friend isn't planning on killing off people in the billions with one virus...she merely intends to sterilize 99% of the population.
-Nuclear war: Plausible, if Palin gets elected in 2012. But don't worry; Alaska will be our refuge, even if it is the most likely target of any Russian, Chinese or Korean missile. God said so. :P
Getting back to my actual answer, how a nuclear holocaust will affect us is questionable. Chernobyl has rejunvenated itself in the space of about two decades. Then again, a planetwide nuclear event doesn't leave much room for the isotopes to dissipate. If we're lucky, they'll just settle on the ground after a few years under their own weight, and make the majority of land infertile and water undrinkable.
-Comet: We're working on countermeasures. Hopefully it's not too little too late. Although a recent NatGeo documentary postulates a meteor impact didn't kill off the dinos, at least not on its own, so we might be safe from total extinction.
-Supervolcano: Plausible, especially considering Yellowstone is overdue. But we have answers to this that the last supervolcano survivors didn't, in the form of self-contained environments. We can filter outside air and water, grow food in hydroponics labs, and manufacture medicine as usual, and overall we would be okay.