How can I kill off 75-90% of humanity by a non-biological weapon, but leave structures standing?

Question

I'm making a world where approximately 75-90% of humanity was taken out by a non-biological weapon (i.e., no viruses or bacteria), but the majority of major structures, such as tall buildings, exist almost untouched (just normal wear and tear over about 150 years or so).

What kind of weapon would be able to do this? It is not necessary that the weapon be conventional/nuclear (could be genetic experiments or some such), but

1. majority of human population died and
2. the agent had completely run its course within 5 years and stopped existing either simultaneously (preferred) or within another 25 years or so (increasingly not preferred).

I don't need valid scientific processes (they would be preferred), but I do need to be able to give a very logical explanation for why it had the effect it did. Science partially survived, once people managed to get power back and get the computers and such on (75 years ago), so BS would have a flashing red light brighter than Times Square at night.

Answer 1

1. Some kind of radioactive weapon that doesn't explode, but instead leaks massive doses of radiation into the environment. Plants, humans, and animals would die, but human structures would last until they eventually collapse from lack of human repair.

2. An ice age. Whether it is caused by a supervolcano, a mini nuclear war, or some kind of environmental factor, a global Ice Age would collapse our food supply. Those who didn't die of starvation would likely die in the wars over the last bits of food. While some of your cities might be destroyed by war or whatever cause your ice age, a majority of would still be intact.

3. Global electro-magnetic pulse wipes out all electricity. This might not kill 75-90 percent of your population, but I think it could easily make 50 percent from the starvation and wars that would follow this event.

4. Something nanite related. Here we get more into science fiction. Part cybernetic, part biological, they can be programmed to multiply and kill off humanity but leave human structures alone.

Answer 2

While I personally like the answer provided by @BryanMcClure, I feel like I should at least weigh in with this possible answer of simplicity.

Based on what you've said you wanted the effects of the weapon to be, it sounds like a neutron bomb fits your need almost perfectly.1

The variable here that might need to be manipulated in order to support the full range of your scenario would be the half-life of the radiation released. Since it's your narrative, you could always come up with a convincing reason that the half-life involved was either brought under control somehow (i.e. reduced timeline), or was of a desired half-life to begin with (for your premises to be supported).

Pertinent excepts from the web source (provided above)

"The tactical neutron bomb is a nuclear weapon that maximizes damage to people but minimizes damage to buildings and equipment. It is also called an enhanced radiation warhead. The neutron bomb is a specialized thermonuclear weapon that produces a minimal blast but releases large amounts of lethal radiation..."

"The idea of the neutron warhead has been hotly debated since its inception. At the time of its introduction, some felt that its relatively small initial blast and fallout was ideal for use in densely populated areas, like Europe."

Answer 3

Use Neutron Bombs

These are very effective for the problem posed.

Scope

Killing 75 to 90% of the population means eliminating humans from Asia, Europe and Africa. The population of the America's isn't quite 1 billion so we'll leave them alone (for now). Since the question explicitly states "weapon", the cause of death must be a man-made device. Super massive coronal mass ejections, rocks thrown at Earth, or other fun one-shot events aren't in scope either.

Kill everyone with Neutrons

There's a type of nuclear weapon called a neutron bomb that releases a significant portion of the bomb's energy in the form of gamma radiation and neutrons. There are still thermal and blast effects but these are relatively slight compared to the gamma and neutron emissions. Also, neutron bombs are relatively low yield, <15KT. In larger weapons, the thermal and blast effects dominate the neutron emissions, so that isn't useful for our purposes.

Neutron bombs won't be able to kill everyone directly. That's just too many bombs over too large an area. However, neutron bombs make excellent area denial weapons and that will be our primary use of them. The primary focus of the attacks will be urban population centers and food distribution points. Some destruction of tall urban buildings is inevitable with this approach though the blasts could be planned to minimize this damage. Area denial from a properly designed neutron bomb lasts about a year.

Targeting Priorities

By irradiating the population centers and food distribution points, billions of people will die immediately. This is preferable to attacking just the food distribution network because starving people will fight for food, thus destroying more buildings in the process.

The secondary effects of denying access to food distrubition and urban centers is that the economy of attacked countries go into massive depressions and widespread starvation in surviving urban and suburban areas.

Building enough weapons

This is tricky. Because neutron bombs have a comparatively light yield, this attack will require tens of thousands of weapons. Across Africa and Asia, there are 2813 large cities covering 171,984 km^2 according to this spreadsheet from the Lincoln Institute. With that many weapons, the attack will need to account for duds. The industrial infrastructure to build that many weapons would exceed or equal the combined nuclear weapons production capacity of the US and the USSR during the 1960s. That kind of activity is going to be noticed by someone but , it just works.

Answer 4

So, I think I've figured out a good way to do this:

A government gray-hat genetic research element went rogue when their department was about to be downsized because public perception of their program became incredibly negative, and proceeded to use their remaining time and funding creating their revenge-the rabids.

The rabids, intended to be release in select locations around the world to cause havoc and only this lab would be able to stop them, thus forcing the government to reinstate and substantially increase funding and positive publicity.

However, the rabids escaped containment en route, being far more clever than their creators had anticipated, which meant that the locations carefully selected by psychologists to make people more willing to help genetic research were not targeted nearly as precisely as had been hoped.

This breach in containment therefore caused mass panic, with looting and vandalism, though nearly all buildings survived at least mostly intact. The rabids spread quickly across every continent, petering out in the far cold reaches, but raging ahead through:

• deserts
• jungles
• forests
• plains
• mountains
• and were nearly unstoppable

One small batch, having been marked for destruction as "failures," escaped before they could be terminated and destroyed the facility, trashing the place (intentionally) so that no one could find a way to stop them.

Soon, however, the remaining few governments and world leaders began launching nuclear strikes at the rabids (who preferred to remain in dense clusters away from cities, only coming in because killing humans was a strong biological need built in) irradiating much of the earth and causing a huge swath of destruction, while leaving cities mostly intact.

The surviving rabids eventually managed to root out all but the hardiest and most remote survivors, including all world leaders and militaries. But a problem soon emerged: they had been built with a genetic "stop-clock" that would cause the rabids' cells to mutate and cease normal separation and functions after a number of years, effectively killing them.

This took place slowly enough that, over several years, the rabids became increasingly stupid, rotting creatures, that would eventually become so mindless that they only followed their basest biological impulse: kill humans.

Eventually, without a source of food, even these despicable creatures began to die off, becoming almost completely extinct by 2091 (apocalypse began in late 2082, nuclear strikes were early 2085). Having no methods of reproduction, they had no options left; they just died.

Now, in 2267, my story takes place. Humanity began emerging from hiding in the early 2250s, and the population soon began to rapidly grow once again. Each group thinks that they are the only survivors, except for the groups in Antarctica and Patagonia (now ironically called Southgate), who had managed to establish electronic contact with each other due to the relative distance and electrical/engineering skills of both locations.

In addition, there is also a group in Russia (community called Kraques-settled in a glacial ravine), one in Iceland (still Iceland, population completely untouched entirely but the distance was too great for communication with the Antarcticans and Southgaters), and a group in Canada (called the Inyukon community).

It is approximately 1 year before the arrival of the experimental ship Colombia 9.8, equipped with a phantom-field generator, is due to return from its first long-distance voyage and the final test of the long-term deep space survival ability of the ship. The Antarcticans, and by extension the Southgaters, know of this.

In no other place does anyone remember that humanity once made it outside the solar system (the return of the ship was supposed to be received with tons of fanfare and everything-this project was what took the funding from the lab for the final development and deployment). The vehicle launched in the year 2069, and was due to return exactly 200 years after they engaged the drive.

The Antarcticans barely even remembered this until they were reminded in 2267 by a preset and unknown alarm of the impending arrival, and only because the facility there was primarily scientists, researchers, and astronomers, who were there to begin the building of an indefinitely self-sufficient and sustainable community.

I know this is long and convoluted, but it is basically my prologue and 1-5 chapters summed up.

Answer 5

I think you have painted yourself into a corner. Let's consider what doesn't work ...

• It is supposed to be a weapon, so a natural disaster like the gamma ray burst (otherwise an excellent suggestion) or an ice age don't work.
• Cities are supposed to remain standing, so megadeaths from megaton explosions are out, too.
• You excluded chemical weapons. They would not work, anyway, because there are no delivery systems to spread the stockpiles wide enough to kill 90% of mankind. (You can't just divide the worldwide production by the lethal dose. For every droplet which kills somebody, many more fall to the ground unused.)
• You excluded classic biological weapons. Biologicals include biologically-produced toxins, with similar problems to chemical toxins, and plagues which could spread.
• A genocidal pogrom on this scale would probably escalate to destroy the cities as people fight back.

Interestingly, your question does allow genetic manipulation. So perhaps an archvillain bent on world destruction created a GM insect which is pesticide-proof and produces a toxin. After five years, the survivors are those with resistance and/or developed a new pesticide. But this explanation seems to stay within the letter of your question while dodging the intent.

Next idea, in the not so distant future household robots become common enough that they can be found all over the world, not just in the industrialized countries. Of course their programming has elaborate safeguards to prevent injury to humans, but a paranoid intelligence agency installed backdoors. A hacker group detects this backdoor and decides to use it for something (ransomware?). While doing so, they disable the safeguards and order the robots to go on a rampage. The malware spreads better than expected and there are not enough bitcoins in the world to pay all the demands. Lots and lots of robots kill their masters.

Answer 6

Famine.

You could have food become nearly impossible. Something that wipes out crops wouldn't be too hard to do since a lot of crops are very very similar genetically speaking.

Massive plankton die-off.

Kill the oceans and the ability for the planet to keep sustaining so much stuff starts to dwindle. It wouldn't be instant, and it wouldn't kill everything, but it could surely cut down on how many people are still around to survive it..

Answer 7

If you want to kill the vast majority of people quickly without damaging most of the man made or technological infrastructure, then you are looking at a cosmic event.

The most probable event would be a gamma ray burst that floods the solar system with massive amounts of gamma radiation. Since this is thought to be caused by something like the collision between two neutron stars, it is not likely to be from somewhere nearby (neutron stars have distinctive signatures in the radio spectrum, and two rapidly orbiting neutron stars spiralling inwards would be very distinct if they were somewhere in the immediate [i.e. several LY] neighbourhood). So a colossal gamma ray burst from several hundred or more LY distant would flood the solar system for hours or days, meaning the entire planet would be irradiated. Only people deep underground in mines or bunkers would survive the immediate strike, and the people on the opposite side of the planet from the burst's point of origin would have 12 hours at most to seek shelter deep underground.

Even more violent cosmic catastrophes might be imagined, for example, black holes often have highly concentrated jets of energetic particles emerging from the polar regions (these are actually from the accretion disc surrounding the black hole, and not from the hole itself). If the black hole is pointed in the right direction, energetic particles moving at velocities close to the speed of light will be entering the solar system. In some ways, this might be more devastating than a gamma ray burst, since there would be all kinds of energetic reactions between the incoming particle stream, the solar wind, the Earth's atmosphere and so on. Imagine radiation, violent EMP and even the physical erosion of the Earth's atmosphere, depending on the magnitude of the incoming jet. Once again, deeply sheltered people might survive, and people on the side of the Earth facing away will have a maximum of 12hr to reach shelter.

Finally, there could be some sort of technological apocalypse. The movie "The Quiet Earth" (New Zealand, 1985 http://www.imdb.com/title/tt0089869/) has scientists around the world accidentally trigger an apocalypse by generating a global "energy grid" (theoretically able to power aircraft in flight and ships at sea). Project Flashlight catapults the Earth into an alternate dimension, but the vast majority of the people on Earth don't come along for the ride (the lead character is one of the scientists from the Flashlight project who survives, and discovers the changes in physical constants). This answer requires a large amount of handwavium, but can be tailored to meet your exact specifications in terms of setting the stage for the story.

Answer 8

A new psychoactive drug is discovered. It has the euphoriant effect of cocaine but none of the downsides. It is as potent as LSD so it is impossible for the authorities to stop it being sold worldwide. Also it is popular and pretty much harmless. It becomes a global craze. "Everybody" is using it. Harmless... not.

It had the side effect of triggering a protein reconfiguration in its users' brains. One similar to BSE or CJD. One dose was enough. It takes ten years to develop but is inevitably and rapidly fatal when it does.

Who is left? A few adults who never indulged even once, and a generation of mostly under-tens. About 15% of the population? With a very skewed age distribution and psych profile that might help your story.

Sorry ... just realized, not a weapon. Will an own goal suffice? Otherwise you will have to invoke an evil genius behind it.

Answer 9

Well, since Neutron Bombs and Gamma Ray bursts are already taken...

Radioactive dust.

If the dust is highly radioactive, it will have a short half life. In the air, it can be breathed in, in the water, it can be drunk, and on food crops, it can be eaten.

This would take a lot of dust but if it is deployed in the gulf streams, it will spread over much of the world.

The 5 year to safe requirement can be met with a highly radioactive substance. Though certain areas of dust concentration might be dangerous longer.

Answer 10

If the story is to take place 150 years after the event, isn't the actual weapon used irrelevant to the story line and also myth to the children of the children of the grandchildren of the survivors ?

Even any people on the incoming ship may not know of any weapon capable of the effects you're describing or could be familiar with several advance weapon systems that could produce the global readjustment in population count and dispersal without property damage you're describing, but unable to identify the specific one after such a long time.

Sometimes leaving a backstory event where the how isn't as important to the plot as the fact it happened to the readers' imagination is better then using several pages to explain something that in the end adds little to the story.

Unless the how is going to be important to your plot I'd skip that part and just work on the effects of the now unknown mega weapon.

Answer 11

As long as you find a way to do this on a global scale, any of these methods would work :

Anything that attacks the breathing : oxygen removal, toxic gas... Here we're talking about a chemical weapon. Oxygen would reform quite fast thanks to photosynthesis. And toxic gas can be made to slowly turn into a non-lethal form. The main issue however is justifying what organisation can have the ressources and logistics to deploy such huge quantities of chemichals all around the world.

"reasonable" heat : almost all non-biological materials resist to temperatures around 70-100°C. Living beings almsot certainly die. The trick is finding a way to do this in an "uniform" way to prevent having places with peaks of heat that whould affect structures or places of low temperature where people would survive. I would suggest releasing in the athmosphere huge quantities of compounds that amplify the greenhouse effect.

Attacking the senses : If you can find a way to render most humans blind, I syspect an immediate high casuality rate similar to what you're aiming for (70-80% in a matter of a few weeks). Here again a chemichal agent can be used. Another way to pull this off is blinding bombs. Something similar to what's commonly called flashbang, but instead of being visible light aimed at temporarily overwhelm the eye, it would emit light in a waveband that would cause permanent damage. The trick here would be managing to get a huge percentage of the population to look at those bombs.

Answer 12

You become Samuel L. Jackson. Or rather, a variation of one of his characters.

In the 2014 film Kingsman: Secret Service,

the main villain - Valentine - designs a SIM card which he gives for free to as many people he can over the world.

Essentially he created a product/service that everyone uses and wants to keep using, except without any costs for the user (almost guaranteeing that people would adhere to it).

However, it had a catch, the cards were able to make people become uncontrollably violent, which would make people try to kill everyone around them (Hopefully while Freebird is playing in the background).

This is the perfect method because you can virtually get to decide who is affected or not.

This could however be prejudicial to buildings, and as such you wouldn't design the SIM Card (or any other device that could "change minds") to turn people into Agressive Primates (because they could just as well destroy a building in their fights). Instead you'd make them suicidal.

By making them suicidal not only you can control exactly how many people will die, but you also assure that they're not even going to destroy anything else with them. They'll just... die.

Answer 13

I concur with @Gawainuk in that this event is a backstory event with little real relevance.

You could handwave it as

It was a freak cosmic accident, that had even been foreseen more than once. They had thought of cosmic poison cloud swamping the Earth¹, or noxious gases being spewed out by volcanoes in a purple cloud suffocating the planet. In the end, the gas did come from space, but it was colorless, odorless and not even directly toxic; the only warning it gave was a sudden lowering of one's voice. But it was markedly heavier than air, and in a few breaths all the oxygen would bubble upwards, out of the victim's lungs. The winds in the stratosphere had already diffused the gas over the whole world when the first traces reached ground level. Very soon, large invisible clouds rolled down over mountains and hills, flowing downward and finally reaching the sea; most areas were not affected for more than a few hours. But for too many people, those few hours were enough; those caught in the open only had minutes. Over half the human population died in the first forty-eight hours together with most cattle and all internal combustion engines. The survivors came out of cellars, landed from the surviving planes, discarded their emergency oxygen bottles and makeshift rebreather kits, and found themselves facing a civilization largely ground to a halt and four billion corpses. Five years later, another two billion people had succumbed to famine, sickness, despair and violence, and the lowlands nearer the seas remained uninhabitable and lethally dangerous for more than fifty years.

^{(1) This is Robert F. Young's The Rachel Carson Effect, which I believe might have been published in The Worlds of Rober F. Young.}

(Okay, no. There is no known mechanism that would lead to the formation of so much sulphur hexafluoride in space, and it would not be a cloud - it would be a frozen solid, burning in the atmosphere just as an ice meteor would).

Answer 14

Entertainment.

Something new, irresistible and addictive well beyond our current smartphones.

People just neglect their lives, stop showing up at their jobs. In some cases, they starve with food still in the fridge, too engaged to bother eating.

When governments begin taking action, too many devices have been distributed. People will flee, hide, and fight tooth and nail to keep their fix.

Civilization collapses. The devices are now impossible to produce, maintain, or even recharge. Many still die from accumulated neglect. Quite a few commit suicide. But many others recover gradually over weeks, and quite a few never used it in the first place.