I'd like to see a poll of NationStates' personal favorite end of the world scenarios.
Never mind. That's been asked a million times already and OP didn't specify anyway.
My personal favorite is this:
Climate change is usually being pointed at as the possible cause of the world ending. Usually Venus is pointed at as a cautionary example of runaway climate change, with our methane-holding ice caps melting and baking everything extra crispy. However, there's more to that which you might not be aware of and makes the problem of the seas warming up more than just sea levels rising, a common theme when talking global warming.
As the seas warm up, their ability to bind oxygen decreases and so anoxic events start to happen more. This in itself is a problem with much of marine life pretty much choking to death. The dead life is then broken down by bacteria through anaerobic digestion, releasing hydrogen sulfide. This is normal and happens pretty much all the time in places like sewers, swamps and most familiarly inside your bum. This is the smell no one admits to emitting after the funniest sound on Earth is made. They told me to never trust a fart and this is about as literal as that advice is gonna get, because that may well be the smell of the worst extinction event in Earth's history (some 57% of all biological families and 83% of all genera becoming extinct) which started the dinosaur age. Back to the sea.
Now normally, the seas binding oxygen into hydrogen sulfide forms sulfate, which the anaerobic bacteria break into hydrogen sulfide again to be bound into oxygen again. Circle of life, yo, except when the oxygen decreases due to the seas warming up, increasing dead matter and its digestion, increasing hydrogen sulfide which can't bind to oxygen that isn't there. Not only that, but the sulfate itself also is digested, releasing more free hydrogen sulfide. We start smelling it in very small amounts of it so don't worry, you won't accidentally fart yourself to death even though it is a VERY POISONOUS gas.
However, all that hydrogen sulfide out of all the digested dead matter entering the atmosphere as gas bubbles from the sea, having nowhere else to go, might not only poison much life on the surface of the Earth to death, plants and animals, but it also binds to the oxygen in ozone, depleting the ozone layer and finishing off the rest with deadly UV rays from the sun.
TL;DR: oceans make poot, mankind kaputt. It's called euxinia. Look it up, it's pretty cool.
