Aliens; it's worse than I thought.

The world hasn't ended, yet. But it certainly could. At any moment, with no warning, and in a whole bunch of creative ways. Consider the following.

1. Enormous geomagnetic storm
2. Geomagnetic polarity reversal
3. Gamma ray event
4. Methane clathrate release
5. Rogue black hole

Those are five ways that Earth or life-on-Earth-as-we-know-it could end in the blink of an eye with no warning. Some of them are hypothetical---no one has ever observed a black hole to exist, but they are highly likely---and some of them are real. All of them would be disastrous, and there is absolutely nothing we can do about it. Take geomagnetic storms for instance. In late August and early September 1859, during the Carrington event, Earth was bombarded by energetic particles from the sun. The bombardment caused wild fluctuations in the Earth's magnetic field, which in turn induced electrical currents to flow. The currents caused telegraph offices to catch fire, electrical transformers to explode, and generally wreaked havoc on anything that could function as a current path. If a geomagnetic storm the size of the Carrington event happened now, pretty much everything we use would break. Our power grids are synchronized using GPS signals, but those satellites would get blasted early in the flare, so our power grid synchronization would collapse over the next few minutes. A large part of the world's electrical power grid would turn off, melt, or explode. That would be pretty bad for a lot of the people in the world.

A really terrifying possibility is a gamma ray event. If there were an intense gamma ray burst in the local stellar neghborhood---or even in our galaxy, it could cook half the planet's atmosphere. Maybe nobody would die from the gamma ray exposure, but since this would destroy the ozone, you might just die as a result of the cascading mass extinctions. And if that didn't get you, the cancer caused by the gamma rays would eventually catch up to you.

In addition to the five things listed above, there are numerous other ways that the world could end with different degrees of probability and different amounts of warning. Plague, global war, ecological collapse, asteroid impact, and a host of human-caused possibilities.

Or, of course, aliens could invade.