Lord Kloo said:
War - Most likely
Global Warming - In the UK anyway we're gonna freeze in the next 50 years when the gulf stream stops..
Places like Lavtia, Estonia and Russia don't have the full effects of the gulf stream and they manage ok. We'll just have to grit our teeth and wear more layers...
Disease - We're overdue but it would have to be a disease immune to antibiotics..
Like every non bacteria infection? The common cold is immune to antibiotics
Resource collapse - Oil's sell-by date is fast approaching
If people like greenpeace didn't flip out over nuclear power we'd be good for at least a few more decades, allowing more time to come up with a proper sustainable energy source. Heck, there are billions of humans out there, why not rig up massive hamster wheel like generators and we walk in them to create power? Would help combat obesity too...
Asteroid, solar flare, super volcano and anything else which can drastically change the climate or has explosive power..
Asteroids and solar flares are the only foreseeable natural events that could severely screw us over. Australia would especially be screwed from a solar flare due to the weakened ozone layer sitting over it. A pretty huge bad ass asteroid would have to be absolutely huge to wipe out all life. Anything that huge would be traveling that much slower and would hopefully be diverted due to gravity from the other planets in our solar system, or be partially blocked by the moon. Although that would cause a heck load of other problems.
Nazi re-invasion in 2018.. wait... what..
Nuke 'em from orbit
Science goes wrong and does something stupid to the gene pool and DNA..
Hopefully we'd have the intelligence to only attempt this on a realistic scale in a controlled environment. Creating a ATP inhibitor would be plain silly and devastating for all life everywhere. Let's hope our scientists behave themselves.
Those little micro-organisms that NASA found the other day that turn phosphorous into arsenic..
Don't know much about these guys, but presumably they would struggle to adapt to other environments.
Generally my bet is with any of the above for short term, for a inter-world human empire I'd say it will be our own in-fighting or vastly superior alien races..
It was a very reasonable list and I can understand the reasoning behind most of them, they just don't stand up to scrutiny. I'll look more into those micro-organisms that nasa found.