So You Want An Apocalypse/Cataclysm In Your Plot?

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Disease

Various works of fiction use a devastating disease to explain why the whole world's population has been dramatically reduced - but often as not, the disease explanation falls apart under just a little scrutiny. For example, in Fictionland you often see super-deadly plagues where people show symptoms in a matter of hours or even minutes after exposure and die a day or two later. In reality, this would severely hamper the spread of the disease, as those who contracted it would sicken and die so fast that they'd never have a chance to spread it very far. In the real world, the best-spread diseases are the ones where people can be contagious before realizing they're sick, and then after are still likely to live long enough to spread the disease to a few other people.

Outbreaks of other super-deadly illnesses in nations with fairly decent hospitals aren't as likely as people often imagine, either. US hospitals, for example, have procedures and protocols in place designed precisely to stop horrifically awful diseases from spreading willy-nilly. Among other things, hospitals will put the infected person into an isolation unit with a separate and contained air system, and interview and quarantine people the person has been in contact with recently. Hospitals will also report outbreaks like these to higher authorities, so it won't be long before everyone's on the alert. Yes, there will be hiccups and slipups in the system, but it's not as if people would be caught completely unprepared for such an event.

Insofar as outbreaks in other areas go, those with the ability to give aid will likely do so if for no other reason than because it's in their own best interests - EG, it decreases the chance of the disease reaching and spreading in their own areas, or reduces the chance that so many people will die in places they are ultimately economically dependent on that their own economies will end up be hurt.

Some works use airlines as a disease vector. While this is a potential way to spread disease, it's not as simple as some think it is. Airlines have a right to keep people who are obviously sick from boarding in the first place, as a sick person could mean having to make an emergency stop somewhere, thereby wasting passengers' time and the airline's money. In any scenario where people are aware that a devastating disease is running loose, they're probably going to err to the side of caution as well. Also, airlines have the right to deny passengers who look like they're going to be trouble in general. This means that an angry businessman with pale skin, watery eyes, and a runny nose who tries to shove his way onto a plane because he has to attend that meeting is actually pretty likely to find himself grounded.

One weak point with airlines is that many stewards and pilots work while ill. Even though they're advised not to fly while sick, they'll still be disciplined and could even lose their jobs if they actually follow this advice. However, planes can be diverted if someone needs obvious medical attention, and then the abovementioned procedures and protocols would come into play. Furthermore, after 9/11 commercial jet planes were designed so that the cockpit was completely sealed off from the rest of the plane, thus making any contact between the passengers and pilots impossible.

Massive War

The trouble with war is that it's messy, expensive, and entering it will mean sustaining personal losses. Many countries, for all of their bluster, couldn't sustain an extended war effort for long and/or stand far too much to lose to risk actually entering a major war. Furthermore, war is typically an unpopular move with people who don't feel there's a huge necessity for it (IE, that they aren't in imminent danger of being destroyed).

Let's look at World War II: It wasn't started simply because Germany's leadership took a whim to conquer the world one day. Germany was a country in dire straits, bankrupt and economically destroyed by the Treaty of Versailles. (The treaty essentially required that Germany pay for all of the damage wrought by World War I.) This would have resulted in many people feeling like they had very little to lose - when it comes down between going into war and possibly dying but potentially winning a better future for your country or staying home and starving while nothing gets better for anyone, which seems like the better choice?

Not only was Germany in dire financial straits, but its leader was a delusionally xenophobic man who felt like he was backed into a corner. He was convinced that his people were being sabotaged and dragged down by "inferior" races and that if he didn't strike back hard and quick, he and his people would perish. He also had the support of enough like-minded people that he had been able to overtake the government and set up a system of propaganda to try to convince the citizens thus as well. Most political figures out there, angry at other governments as they may be, aren't that desperate yet or at least have enough opposition from other politicians and pundits to prevent them from getting too many people worked up into thinking that launching a war that huge would be a good idea.

Global Warming

Warming the planet even by a small margin will have potentially catastrophic consequences: weather systems become more extreme (EG, more powerful hurricanes), rainfall will change (some places will become wetter and some will become drier, which will affect farmers and life dependent on certain amounts of water), increased temperatures will be harmful to life sensitive to temperature change or reliant on conditions only available at certain temperatures, melting ice will cause an increase in sea levels that will make many coastal areas uninhabitable, and more.

However, if your story is going to use global warming in a believable manner, the important thing to remember is that none of the changes and consequences it will bring could happen overnight and people would have time to adapt and adjust as these changes occurred somewhat. For example, coastal cities might end up flooded as sea levels rose, but as it would happen over a course of decades, there would be time for many to move elsewhere. (Which would not necessarily be without difficulty or consequence - for example, the cost of living space would increase as availability decreased, and food shortages might be compounded if space that was used for growing food had to be developed into living space for people - but at the same time, people would be trying to implement more space-efficient food growing systems.)

Aliens

Wherein aliens come to Earth to wreak havoc on us for one reason or another. One common reason is to raid us for food, supplies, and maybe even slaves. However, this typically ends up creating a lot of plotholes that become readily apparent with just a small amount of thought.

First, if the aliens are so advanced that they can cross those vast interstellar distances in the first place, surely they'd be smart enough to know that minerals and water can be found literally floating around all over the galaxy. If it's slaves they're after, surely it would be a lot easier and a lot cheaper to build robots to do their work. If it's food, if they can build environment-controlled spaceships to get them all the way to Earth, then why can't they build environment-controlled orbital farms above or near their planet? If their air and water are so very polluted back home yet they can build the air and water renewal systems that would be necessary to keep a ship livable during the trip to another planet, then why can't they just use them to clean up their own world? And if they're trying to eat humans, why humans? They breed and mature at such a slow rate and are so peskily prone to planning revolts when mistreated that they'd make one of the worst possible organisms to choose as livestock. And then there's the fuel costs - the energy required to get a ship to Earth and back could probably be used to power the aforementioned robots and orbital farms for years.

Another reason that's popped up in a few works is aliens deliberately sabotaging humans to restore Earth's "natural balance" or something. Depending on how it's done, this can raise some questions - like, why aliens are so invested in Earth (do they really have nothing better to do than to be interplanetary busybodies?), and why they consider what humanity, a species that developed on Earth, is doing to the planet to be less a natural part of its development and evolution than any of the other catastrophic events to hit the planet?

The Biblical Apocalypse

An apocalypse involving the forces of Earth and the cosmos is certainly one way to make an epic story, and ancient prophecies predicting such lend an air of credence to whatever is going on. The Book of Revelation provides both - it's an ancient prophecy that promises massive chaos and an epic battle between the forces of Good and Evil.

Just one wee issue: writers often use Revelation to lend credence to what's happening in the story, but they ignore huge swaths of the very prophecy their story claims is coming true. A good example of this is the Four Horsemen of the Apocalypse: in fiction, they're sometimes portrayed as hero-bothering bugbears to be defeated, but in Revelation, they're more impersonal forces of politics and nature than anything else. They don't attack individuals or even cast their curses on small towns; they affect things on a worldwide scale. Also, they're released on God's authority as part of a master plan that ends up with evil eternally defeated and good eternally prevailing. And then there's the thing where after the Four Horsemen are released, the stars start falling from the sky (which rolls up like a scroll), every mountain and island is moved out of its place, and then "hail and fire mingled with blood" burns up a third of the trees and all of the green grass, and... things just get worse from there. Also, the author of Revelation was pretty adamant that all of these things would happen, as evidenced by the curse at the end of the book warning that anyone who removed anything from the book would have xir name stricken from the Book of Life - IE, lose salvation. So whether the things described in the book happen literally or metaphorically, it's all supposed to happen.

(If you're thinking about using Biblical themes in your works, Things That Show Up In Christianity-Inspired Fiction That Aren't In The Bible might interest you.)

What could make a more believable apocalyptic scenario

A more believable natural/mundane scenario might not rely in a sudden single cataclysmic event that causes an immediate change, but might rely on a combination of causes and effects that take place over a period of years, possibly even generations. For example, rising global temperatures might result in shortage of food and living space, which might trigger civil unrest, which might spark result in war and revolution breaking out in different times and places. With food in scarce supply, it would cost so much that people might not be able to afford food and medical care, which would lend to the proliferation and spread of mundane yet still potentially serious diseases like the flu, pertussis, and other things usually kept at a minimum by vaccination, sanitation, and having warm places to rest in. In such a scenario, illnesses formerly considered trivial could easily become deadly - even a common cold can become pneumonia if people can't rest up and stay warm. Of course these illnesses wouldn't take out staggeringly huge amounts of the population with every go, but if even a small portion of people were taken out by illness every year combined with deaths from malnutrition and war, the population could be whittled down significantly over time. And on top of that, you might even see outbreaks of old plagues like the Black Death as sanitation and medical supplies became scarcer. Wealthier people and places would fare better for awhile, but eventually poorer people might revolt against them and/or their wealth might dry up.

If a sudden catastrophic event with a natural cause behind it is really necessary, then a large (IE, several miles across) comet (not an asteroid) might be plausible, as comets (unlike asteroids) are nigh impossible to detect until they're close enough to the sun to start melting and leave a tail. An impact from such an object would kill everything nearby from the shockwave alone, and then could throw up enough dust into the air to block out part of the sun's light. As a result, Earth's temperature would drop enough to start killing off temperature-sensitive lifeforms, which would result in (among other things) worldwide crop failures. The colder temperatures could potentially stay thus for years. Likewise, an eruption of a supervolcano would have much the same effect - the shockwave would kill off everything nearby, and the ash would block out sunlight as well as kill anything that inhaled enough of it and kill off any plant life it covered.

So far as overtly supernatural apocalypses or cataclysms that rely on prophecy go (IE, where the fantastic things described in prophecy aren't just supposed to be metaphors), something that doesn't rip only select bits from a real text or can account for the difference between the text and what's happening in the story would not be amiss. Alternatively, you could simply drop the ancient prophecy thing altogether.

As for aliens, if they're coming to Earth in need of something then it needs to be something on that they can't do or find in their own galactic back yards for a fraction of the effort. If they're coming to be meddlesome busybodies, it should be done in such a way that people aren't left asking why they don't have more important things to worry about, or why they'd care. (Parasyte does this well - the identity of the alien or aliens responsible for the release of the human-eating creatures on Earth is left murky, thus avoiding setting up a scenario that only raises more questions than it answers.)

So in essence, whatever your scenario is, do some research into the way things actually work and work that into your story, and put a little thought into whatever you're doing to try to spot potential plot holes and poorly thought-out scenarios.