If a hostile group wanted to render Earth or an Earth-like planet completely unfit for human life, what could they do to go about achieving that end?

By uninhabitable, I mean a planet on which humans are unable to survive in the open for the foreseeable future. Centuries in the future, whatever's happening could calm down and maybe allow for habitation again, but after the event, nobody could survive. The group who is rendering the planet uninhabitable wants to salt the earth, so to say.

The two ways I've thought of that this could be done would be to create a Venus-like runaway greenhouse effect, or else to strip away the atmosphere, but I'm not sure how people could actually do either of those things. Other methods that I considered were some sort of biological agent that would be lethal to people or a nuclear war. These both seem like they'd be able to kill most people on earth, but radiation dies down fairly quickly and biological agents can have cures.

I'd like to limit things to solutions that are feasible using modern science, and would also like to avoid the outright destruction of the planet. (So no crashing the moon into the surface.)

What convincing ways could I have such an attack on Earth render it unfit for human habitation?

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    $\begingroup$ That type of thing is called a 'Anthropogenic Global Catastrophic Risk'. I can recommend the book "Global Catastrophic Risks" by Bostrom et al. WP says "The Cambridge Project states that the "greatest threats" to the human species are man-made, they are artificial intelligence, global warming, nuclear war and rogue biotechnology." $\endgroup$ – A E Dec 15 '14 at 19:37
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    $\begingroup$ . . . Are you planning anything for Earth? $\endgroup$ – HDE 226868 Dec 15 '14 at 21:52
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    $\begingroup$ Actually, this sounds like a question well-suited to xkcd what-if $\endgroup$ – KnightOfNi Dec 15 '14 at 22:43
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    $\begingroup$ "strip away the atmosphere, but I'm not sure how people could actually do [that]" i.imgur.com/1kvpLb6.jpg $\endgroup$ – Adam Davis Dec 16 '14 at 17:19
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    $\begingroup$ Nothing. With the direction the earth is going now. The path to rendering the earth uninhabitable would be to do nothing. $\endgroup$ – Magic-Mouse Dec 17 '14 at 11:22

16 Answers 16


I would aim for grey goo. Make a group of nano-scaled robots that alternate between massive self-replication, and consuming specifically human life for resources. They are small enough that they would carry on the wind, and if they had significant downtime during their self-replicating phase, no one would know there was a problem until it had reached pandemic levels. If they leave the body once consumed, it would take ages to figure out what we were facing, and they could be made to eat through whatever containment suits/air scrubbers humans have devised to keep out pathogens.

  • $\begingroup$ Alternately: design nanobots that consume all biomass and convert it into toxins. Photosynthetic nanobots might similarly be able to deplete all of the oxygen in the atmosphere and convert it into CO2. I like this idea :) $\endgroup$ – ckersch Dec 15 '14 at 20:04
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    $\begingroup$ Isn't this Prey? (although here there is no pandemic effect...) $\endgroup$ – Bakuriu Dec 15 '14 at 21:13
  • $\begingroup$ @ckersch I was working under an assumption that we only wanted humans dead. My imaginary robots swarm when they see bipedal movement. Bakuriu Yes, I read Prey, but that was more of a "robots weren't meant to do this but we built them to adapt. Sorry." I'm imagining a small group of dedicated scientists/programmers whose aim is to eradicate humanity alone. $\endgroup$ – IchabodE Dec 15 '14 at 21:58
  • $\begingroup$ Oh, also these are specifically not powered by the sun so if humans were off world until the robots' batteries ran down in a decade's time, they could return safely. $\endgroup$ – IchabodE Dec 15 '14 at 22:00
  • $\begingroup$ Kind of reminds me of the plot in the video game Horizon Zero Dawn. I like it. $\endgroup$ – Apologize and reinstate Monica Dec 19 '18 at 17:42
  • Start by getting all those useful fossil fuels out of the earth to deny them to future generations. Turn one half into non-biodegradable polymers and contaminate oceans with them to damage the food chain at its root. Turn the other half into carbon dioxide and release it into the atmosphere by burning it. The resulting green house effect will lead to a climate catastrophe dealing further damage to the biosphere.

  • Get all low-radioactive uranium isotopes out of the earth and turn it into much more radioactive fission products using nuclear reactors. The resulting waste will bother future generations for millennia. Not just because these elements are very hazardous, but also because some of them can be used to manufacture nuclear weapons.

  • Any liquid biological or chemical waste should be dumped into fresh-water springs to deny clean drinking water.

  • Speaking of drinking water: Ground water can be contaminated by pumping large amounts of hazardous chemicals into the ground in order to hydraulically fracture underground rock formations. This breaks up deposits of natural gas which can add further contamination to ground water. As an additional plus, it can also cause an earthquake or two and brings plenty of contaminated waste water to the surface (see previous point).

  • Drive various species of natural fauna and flora to extinction while overcultivating a few selected others. Those species should be bred for total dependence on humans to make it impossible for them to survive in the wild. The resulting biological imbalance will result in a critically unstable ecosystem which will soon collapse on its own.

You might wonder: How could I convince billions of people to cooperate with such a crazy plan? Here are a few tips:

  • Establish a world-wide economical system which incentivizes short-term gain over long-term sustainability. Design it in a way that those who embody this doctrine become more powerful while those who don't get as few political influence as possible.

  • To prevent the humanity from realizing your plan, distract them from what is happening through a very broad supply of shallow entertainment. Mix it with subliminal messages which manipulate them to crave useless status symbols which can only be obtained by working harder for the destruction of the planets ecosphere.

  • When there are still people who care about politics after all, distract them by putting topics on the public agenda which are highly emotional but don't actually matter much for the survival of the human species, like gay marriage, abortion or sexism in video games. Throw them a bone once in a while by making a public statement in one or the other direction in these regards and watch them fight while you continue your plan.

  • To prevent humanity from allying against you, follow the good old tradition of "divide and rule". Separate humanity into easily distinguishable sub-cultures like race or religion. Then make sure they hate each other and enter pointless wars. Sounds hard? You will be surprised how easy it can be to destabilize a whole region just by supporting the right people. When the wars start, the involved people will not only stop bothering you when they think there is a greater threat, these wars will also motivate even more ruthless exploitation of natural resources and stop them from caring much about any collateral damage they cause to the ecology.

In short: continue doing what we already do.

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    $\begingroup$ Yes, feeling a bit hippie today. $\endgroup$ – Philipp Dec 15 '14 at 22:21
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    $\begingroup$ Don't write a story like that. It will end up being a horrendous scifi cliche that's been done to death and laughed at in 30 years en.wikipedia.org/wiki/Soylent_Green $\endgroup$ – Matthew Lock Dec 15 '14 at 23:56
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    $\begingroup$ Step 4: Boast about plan on StackExchange, just to rub it into their soon-to-be radioactive faces. $\endgroup$ – Serban Tanasa Dec 16 '14 at 2:12
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    $\begingroup$ Regarding your second point: the amount of radioactivity released by fossil fuels is orders of magnitude higher than nuclear plants. It's not likely to destroy the Earth unless somehow spewn out into the atmosphere (nuclear weapons, containment incidents) at much higher rates than the 60s, Chernobyl, Three Mile Island, and Fukushima have given us. Also, the issue is usually the fission products, not the "more dangerous" uranium/plutonium. $\endgroup$ – Nick T Dec 16 '14 at 2:43
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    $\begingroup$ Or: Convince people that everything that results in economic growth and makes life better and more pleasant is inherently dangerous and evil. Tell them that the only hope for the future is to give up their freedom and let an all-powerful government run every aspects of their lives, for their own good, of course. Then proceed to reduce everyone to poverty, sickness, and slavery, all the while assuring them that this is the only way to prevent poverty, sickness, and slavery. In other words, pretty much what lots of people are trying to do today. :-) $\endgroup$ – Jay Dec 16 '14 at 15:58

If you absolutely, positively need to sterilize the surface, you must blast it from orbit. It's the only way to be sure.

The problem you'll quickly run into is that the Earth is big. You can crash 10km sized asteroids into it, and there'll still be some survivors to whine about the injustice of it all. Now, we wouldn't want that, would we? There are two surefire ways.

1. Antimatter Bombardment

No, the only way to be sure is to be thorough. You'll need a fair bit of antimatter, um, let's see...

1 Mt is $4.1\times10^{15} J$ so the Tsar Bomba (in the tested config) at $42MT = 1.72 \times10^{17} J$
2 kg of $E=mc^2 = 1.79 \times 10^{17} J$

So each 2 kg antimatter bomb (as it anihillates with 1kg ordinary matter) would have a blast similar to the Tsar Bomba, the most powerful nuclear device ever detonated in the puny humans' history. That gives you a nice $1,200 km^2$ blast incineration area. Now, if we were sloppy, we would just pepper the land area of $148,300,000 km^2$, so about 1,236 MIEVs (Multiple Independently Targetable Entry Vehicles) with 100 2kg warheads each would do.

But that would miss all the boats, planes and submarines crowding their primitive buoyancy based transportation lanes. With about 100,000 ships out there, that's a lot of survivors. $361,000,000 km^2$ to cover, you'd need another 3,009 MIEVs. You might still miss a submarine or two, but without space launch capabilities, zero industrial capacity, and an all-male crew, you're set anyway.

Pro: Clean blasts, little of that nasty radioactive slag. Also, once in atmosphere, any attempt to shoot down will only damage the containment system and detonate the antimatter.
Con: You need a lotta bombs.

2. A dozen or more relativistic ships.

Thanks to our friends at Wolfram Alpha, I don't even have to do the calculation myself. Take your standard light-crafts, a 3,000 ton craft, push them to a reasonable 0.25 c and crash them on opposite hemispheres (preferably from multiple directions, but you could also do several hours apart). Each will clock in at around $8.8\times10^{21}J$, or about 10,000 times those anti-matter firecrackers from earlier, or about the same as a 10 km asteroid impact. To be really sure, send a few dozen of these.

Pro: Almost impossible to intercept once acceleration is complete.
Con: Your reptilian colonists awaiting in orbit might themselves cook a bit from the impact gamma blasts.

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    $\begingroup$ Added bonus: Anti-matter annihilation leaves little in the way of directly generated radioactive isotopes, so you could land your reptilian colonists on the (crusty and toasty) surface soon after. $\endgroup$ – Serban Tanasa Dec 16 '14 at 2:16
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    $\begingroup$ You simply have to raise the median temperature of earth above 100°C. We are actively trying this. $\endgroup$ – Reinstate Monica - M. Schröder Apr 13 '15 at 20:03

Simply unleash the entire world's nuclear arsenal on the planet. If you aim for population centers, the majority of the population would be vaporized or killed by acute radiation poisoning. This has the added (dis)advantage of destroying or irradiating most of the useful technology (factories etc.) making it difficult for humans to rebuild. The fallout would probably cause global radioactive rain, bad for any remaining survivors. Finally, the resulting nuclear winter would devistate the ecoystem. Without the abundance of plant and animal life we are used to, humans would find it difficult to survive in the decades or centuries it takes the earth to recover.

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    $\begingroup$ Do note this is one of the realistic scenarios in current human development. We only need one Cold War getting a little hot to make our solar system run out of inhabitable planets. $\endgroup$ – Mast Dec 15 '14 at 21:14
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    $\begingroup$ Nuclear winters are not as clear-cut as that, might be less acute than imagined. Moreover, the current arsenals are too small to kill everyone. There'd be millions of survivors. $\endgroup$ – Serban Tanasa Dec 16 '14 at 2:19
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    $\begingroup$ @SerbanTanasa the current arsenals that we know off. $\endgroup$ – ratchet freak Dec 16 '14 at 13:47
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    $\begingroup$ Yes, but the... whole point of the doomsday machine... is lost... if you keep it a secret! $\endgroup$ – Serban Tanasa Dec 16 '14 at 14:00
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    $\begingroup$ @SerbanTanasa but as you know, the Premier loves surprises. $\endgroup$ – 2012rcampion Dec 16 '14 at 19:49

biological's are very hard to be %100 fatal and still spread. If you have a very good designer you might a huge number but there will always be survivors. Chemical poisons, have another issue. Volume and production. It would take a huge amount of resources just to completely wipe out a large city where the people are concentrated.

Nuclear also has it's issues, unless you are able to get all nuclear powers to launch their arsenal around the globe and bring about the holocaust and nuclear winter. That might be pretty close.

I suspect the simplest one would be to deflect a decent sized asteroid to slam directly into the earth. I would guess if you had the ability to aim for the middle of the Pacific Ocean.

If it hits the ocean there will be huge Tsunami's that make the anything we've seen look like a wave pool, maybe a half mile high wave front? More? There will be a huge amount of heat dissipated by vaporized water and the impact would likely cause a lot of volcanic activity, adding a lot more stuff into the air.

I would say a second best would be a biological agent that generates a poison, such as botulism, only it is virulent like the flu or common cold with few symtoms. The agent doesn't kill, but the poison it generates does. This way it can spread quickly and widely long before the poisons start to build up and take effect in the host.


Assuming that this is set far enough in the future that space travel is possible to some degree (I'm guessing it is, otherwise I don't know why anyone would choose to destroy a world) the easiest solution is dropping any mass on the earth. It doesn't have to be a comet, moon, or anything nearly that large. Run your average space ship into the earth, or have simple projectiles that are designed to accelerate towards the planet using whatever propulsion system everyone uses.

Any mass going at a decent speed will be able to create a nuclear winter as easy as our nuclear weapons would. Ideally you would have someone bombard the planet from a few different angles, to throw up dust all over the planet rather then having one area completely saturated with dust and the other side of the planet still being potentially inhabitable. Any space fairing race will find simple mass projectiles to be easy and cost efficient way of rendering the place uninhabitable.

In addition if this isn't planet earth your talking about you could set up another option. Perhaps you have a world that is being terraformed but isn't hospitable to human life yet. If people are living on it using domes and complicated terraforming technologies then those technologies could easily be disabled through virus, sabotage etc etc. The planet it self could easily be so deadly that it doesn't take much to ruin humanities precious grip on it.


Two words: Methanogenic Microbes

Check out this wiki page on "The Great Dying" aka the Permian-Triassic extinction event:

Great Dying

Killed 90% of all marine organisms and 70% of all terrestrials; only known extinction where even insects were affected. Insects!

From the wiki regarding methanogenic bacteria:


Note the massive volcanism, which is thought to have been caused by a bolide impact (the trigger for this cascade of catastrophe).

If that doesn't kill us, the forces of nature might as well give up.

  • $\begingroup$ The introduction of oxygen had a profound effect, but what grew back requires what was originally a poison. The issue is for climate to settle into a new stable equilibrium. See Hal Clement's Half Life and Nitrogen Fix for another such change that gets rid of all the atmospheric oxygen as life evolves a nitrogen metabolism. $\endgroup$ – JDługosz Jan 3 '15 at 8:40

One practical solution that hasn't been mentioned yet is simply to move the Earth a bit.

The Earth orbits the sun in the Goldilocks Zone, where is neither too hot nor too cold for human survival. Bumping it a bit further from the Sun would ensure it becomes totally uninhabitable for humans, with the bonus that it could be undone by simply moving it back again.

Actually moving the Earth is probably easiest to achieve with the gravity assistance method. Using currently available technology, you can simply send robotic spacecraft to attach a means of propulsion to nearby asteroids and accelerate them into a close fly past of the earth. This will affect the trajectory of the asteroid significantly, and the trajectory of the Earth slightly. Repeat this with many asteroids (reusing them if desired) and you can divert the Earths orbit however you wish.


I doubt that it's possible to destroy all life on Earth using present technology. Realistically, I think science fiction stories -- both those published as fiction and those which claim to be fact -- tend to overestimate the power of human technology.

Consider: People very regularly try to kill off large numbers of other people. We call it "war" and "genocide". And yet, there has never been a war in human history that has even significantly slowed the growth of human population. World War 2 resulted in something like 25 million deaths. Certainly a terrible tragedy, but in 1940 world population was about 2.3 billion while in 1945 it was about 2.4 billion. The catastrophe of WW2 didn't reduce population, it just slowed the growth a bit.

Biological weapons can certainly be horrifying. One can certainly imagine a biological weapon that would wipe out all human life. But in practice, there always seem to be some number of people who are naturally immune. Presumably if the U.S. or a European country was attacked with a biological weapon that caused massive deaths they would devote every available resource to finding a cure. Would they find one in time? Who can say?

Similarly, nuclear weapons certainly make it possible to kill many more people than in the past. But even there, you might kill a significant percentage of the population, but 100%? You could certainly cause massive casualties if you dropped one bomb on New York City, but no one has enough bombs to hit every isolated ranch in Montana. Some number of people would hide out in bomb shelters until the fallout and died down. Etc.

In any proposed disaster -- natural or man-made -- one can construct plausible scenarios for how you would kill millions, maybe even hundreds of millions. But even if it's literally possible to kill billions, it would be very very difficult to kill every last person. Surely when the disaster struck, there would be some people in very isolated places, like a remote ranch or mining colony, a research station in the Antarctic, etc. And some number of people would be prepared and would have suitable shelters.

Of course if you set the story in the future, it is at least plausible to suppose that even more advanced technologies of destruction are possible. Maybe people or aliens or whomever will invent an incurable virus, a bomb that can blow up the planet (well, you ruled that out), etc.

For a fiction story, you don't have to present a rigorous academic argument how this technology will work. You just have to come up with something marginally plausible, throw some fast techo-speak past the reader, and boom, you're there.

Personally, I'd suggest that if you want maximum plausibility, you make it a multi-pronged attack. If I was going to try to wipe out all human life, I would launch a nuclear strike AND a biological agent AND an army of robots that will relentlessly hunt down the survivors. But I'm still working on perfecting the design of the robots so it will probably be at least another year or two before I'm ready to take action.

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    $\begingroup$ a research station in the Antarctic good luck with rebuilding a civilization from there. $\endgroup$ – njzk2 Dec 16 '14 at 16:59
  • $\begingroup$ I like the idea of a multi-pronged attack. I may go with heavy bombardment plus a nanobot-based attack to shred what's left of the biosphere while people are too busy dealing with the other problems and threats. If, for example, 90% of the population is killed and massive damage is done to agriculture, and THEN something is released that starts destroying the biosphere while people try to rebuild, the biosphere may pass a tipping point before people have time to react and find a way to stop the nanobots. $\endgroup$ – ckersch Dec 16 '14 at 19:22
  • $\begingroup$ @njzk2 Presumably if 90+% of the worlds population was wiped out, wherever the survivors were, rebuilding civilization would not be easy. But hey, at least a group of researchers in the Antarctic would have the advantage of likely being highly intelligent and able to live and work under extreme conditions. My guess is that they'd have a better chance than, say, a group of fashion designers or politicians. Of course the people with the best chance would probably be a bunch of wacko survivalists. $\endgroup$ – Jay Dec 16 '14 at 21:30
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    $\begingroup$ @ckersch Yeah, good scenario. I think if you really wanted to kill every single human being you would have to end up with hit squads of some sort hunting down people who managed to hide out in shelters, who were immune to the biological attacks, etc. If 99% is good enough, maybe not necessary. Hey, this is such a pleasant conversation. $\endgroup$ – Jay Dec 16 '14 at 21:34
  • $\begingroup$ @Jay: as well as the disadvantage of being stranded in a cold desert with virtually no food or fuel source and no way of reaching any other continent. $\endgroup$ – njzk2 Dec 16 '14 at 21:42

Detonate several Tsar bombs at a high altitude at the same time and you will completely destroy the ozone layer in that part of the world. http://en.wikipedia.org/wiki/Tsar_Bomba

  • $\begingroup$ Destroying the ozone layer was a thought I had as well, but using nuclear weapons for it is a little more fun than CFC. $\endgroup$ – kutschkem Dec 16 '14 at 10:44
  • $\begingroup$ I did not know Centucky Fried Chicken was destroying Earth's ozone! $\endgroup$ – Noctis Skytower Dec 16 '14 at 15:19
  • $\begingroup$ There is no other way. You could "destroy the sky", lets say invent a chemical that stays airborne very very long and cant degrade fast over time and blocks the sun's energy from reaching the ground, seed this chemical over the world, and slowly plants and wildlife will begin to die. It will adapt over time, but during this time a lot of things will die, including us. >>Comment inspired by: Animatrix - The Second Renascence $\endgroup$ – Konstantin Velickovic Dec 17 '14 at 11:19

Actually one way for a hostile group to render the Earth uninhabitable with no repercutions for the group, itself, whatsoever is to simply fund oil companies, city expansion, poaching, hunting of wildlife, damming of rivers (covering them with a dam, not cursing them from God :D ), lumberjacking, etc. while at the same time funding anti-greenpeace actions, anti-hybrid/elecrocar research, basically any kind of research or action that strives to conserve the Earth nature.


I would like to offer an easier solution that is extremely feasible. Take use of Harmful Algal Blooms. By adding iron to certain parts of the earth's oceans and lakes will cause an explosion of algae. While some theorize that iron fertilization can help our planet, some areas contain species of algae that are essentially toxic. Another harmful effect would be population booms of deep sea creatures an other bacteria which would feed on organic bloom detritus. These organisms use oxygen, and a growth in population could lead to an anorexic environment.

We've seen these types of blooms occur before, and they were simply caused by the run-off from industry flowing into rivers, lakes, and oceans. If it were to be done in a concentrated effort, the effects could be catastrophic.

Sustained feeding of iron to the earth's ocean can be done for decades (It doesn't take that much for the effects to show).


I think a lot of people missed an important part of the question: The issue needs to persist for centuries.

Nothing nuclear meets this criteria. Despite the anti-nuke hysteria it's not going to be dangerous that long even if you turn every bit of mined fissionables into dirty weapons.

Biologicals might work but you need something that will persist harmlessly in some host and yet kill humans. That's probably beyond current tech to develop.

Blasting the ozone layer won't persist long enough.

I do see one approach that would work but it would be awfully slow. Bring down enough comets. Turn the Earth into a waterworld.

  • $\begingroup$ Kevin Costner showed waterworld is survivable by floating around. However, if the whole climate heats up into a positive feedback loop and roasts everything... that seems to be happening already, so... $\endgroup$ – Dronz Apr 5 '15 at 4:39
  • $\begingroup$ @Dronz Nothing in Waterworld told us where the materials came from to build those floating things. $\endgroup$ – Loren Pechtel Apr 5 '15 at 18:35
  • $\begingroup$ @LorenPechtel The submerged cities were still there under the water, it was a post apocalypse Earth as we know it. All the artifacts except the blimp were current technology. Not quite believable though $\endgroup$ – KalleMP Apr 12 '15 at 12:01
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    $\begingroup$ @KalleMP Yeah, the cities were down there. They don't have any fancy tech on the surface, though, which means they were breathing simple compressed air when diving. You don't want to go below about 200' on ordinary compressed air--and the cities were much deeper than that. From the ending we can conclude there was much more than 20,000' of extra water--there are no cities that an ordinary human could have dived. $\endgroup$ – Loren Pechtel Apr 12 '15 at 22:15
  • $\begingroup$ @LorenPechtel True, but the earths surface need not have remained at the same equilibrium level as before. Finland is still rising after having lost the glaciers. If all the Antarctic ice melts the land freed would probably rise (causing much further sea level rise) but in other places there could be cities that remain close to the surface and accessible to gills and simple scuba gear. The presence of gills hints at genetic engineering or many generations in Waterworld so not all is consistent. $\endgroup$ – KalleMP Apr 13 '15 at 8:05

Slowing Earth spin will weaken magnetic field. Which will make dry land uninhabitable because of radiation (not oceans tho), and will increase atmospheric loss caused by solar wind (like Mars).

Suggested by answer to question Could we still live if Earth's spin speed slowed down by 90% by @Physicist137

  • $\begingroup$ Earth's magnetic field is based on internal convection currents, not rotation. $\endgroup$ – ckersch Jan 8 '15 at 0:20

You basically want to Venusform Earth, i.e. raise the median temperature of Earth above 100°C. That's quite easy: Just continue burning more and more fossil fuels as we do today. Eventually you will a get positive feedback loop and a runaway climate change.


A method not discussed here could be Snowball Earth - the exact opposite of the greenhouse effect. Lower the amount of greenhouse gasses in the atmosphere and/or block the sun's radiation from reaching the planet (maybe an overzealous attempt at geoengineering) so that the planet starts to cool.

Glaciers, snow and ice would start moving from the poles towards the equator, rising the planet's albedo and thus having even more solar energy reflected back into space without heating the planet. Thus you get a positive feedback loop that cools the planet even further until the whole surface is covered with ice.

Higher lifeforms would die off in the planet-wide arctic conditions because they lack temperature tolerance or by simply starving.

In the end, the planet would be devoid of higher lifeforms, single-cell organisms or microbes may survive on the surface, marine life may degrade, too - the thick ice coating would block out the sunlight, bringing marine algae photosynthesis to a halt, too. Black smokers or volcanic vents on the seafloor may provide energy for complex lifeforms, though.

Humans might survive in biodomes or in closed environments, but the planet as a whole would be unsuitable to sustain higher-order life.

