Off the top of my head, I'd say the earth running out of water would have to be the biggest. We as a species would be done unless something drastic happened. However, that is only one. There are so many possibilities.
Crime thrillers are my favorite kind of read. The suspense and nail-biting page turners reel me in and keep me in a another world that I escape into. While I'm there, I experience terror and chills and thrills, but the cool thing is I can always bring myself back into the safer real world (sometimes).
I think the key to a great thriller is the disaster that each one looks at. They come in all kinds.
I am recently reading a novel where a billionaire sets in motion events to destroy the current U.S. Economy. Definitely a frightening thought. Without a total economic collapse of one of the wealthiest nations in the world, chaos is bound to ensue. Those who are not fortunate enough to be on the right side of the system will surely be hit hard and may not even survive.
I have read others about total devastation to this country as well as nuclear weapons being set off in the middle east and of course, Cormac McCarthy's The Road paints a very dark picture of the future where nuclear war has killed virtually everything in the United States(the plants won't even grow anymore) except a few fortunate(if you can call them that) survivors.
However these are large scale thrillers with dire stakes for a whole country or the entire world. Can a thriller have a smaller scale?
Stephen Hunter's Pale Horse Coming deals with the town of Thebes where a prison is run by big, brutish racist guards who keep the mostly black prison population in horrible circumstances. Only that town is affected but hey, if the protagonist does not stop them, they will only continue on with their reign of cruelty.
My own novel Jack Little deals with the stakes being high for Will Hodge who will lose his house and everything he has worked for if he does not steal to support his family. While these stakes are only concerning Hodge primarily, if we put ourselves in his shoes, we definitely see where his motivation comes from. The down side to it all is he could lose his family if he keeps up with his cold, distant mindset.
So what's the secret?
We as the readers, have to care.
Plain and simple.
If the world is abut to end in a novel and no one cares, then what's the point? The real substance is in the characters. If the world ends, well, we don't want that. But we really do not want the characters' world to end. We have taken a journey with them and we empathize with them to the point to where if they fail, the world won't just end. We the readers will be crushed.
The stakes in thrillers must be high but high for the characters concerned. From what I have read, the world savers are generally in a line of work that would put them in such a position to save the world. They could be a spy, a counter-terrorism operative or some sort of ex-military man/woman. Either way, it's hard for us to see a mild-mannered newspaper reporter saving the world.
Eh. Bad example, maybe.
In a small scale story, the protagonist does tend to be more normal as in Donald Westlake's The Ax. Burke Devore is no spy. He is merely a former manager for a paper company who has decided to start killing his competition(literally). The funny thing about this type of story is that we find ourselves rooting for this man is clearly committing amoral acts. He nearly convinces us he is completely justified in his actions, citing society as the reason for his type being pushed out into the wilderness with only one viable solution- murder. It is small scale and yet, we are drawn in.
Which ever you prefer, small scale or large, I find it's always fun to picture the craziest large scale disaster you can imagine.
So what about you?
What is the worst thing that can possible happen? What kind of protagonist would be required to stop it? Can you think up a problem so big that you have no idea how it could be solved or if it's even possible? I'd love to hear your thoughts(and no, I'm not going to steal them, although I might make an offer on a good one, wink wink).