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It's 2070, global warming is a terrifying reality. Coastal cities such as New York and Shanghai has already fallen to rising sea levels and extreme weather. In an effort to stem and perhaps counteract this global phenomenon, the brightest minds of Earth have developed a substance called Cryion. The Cryion was released into the air, but it worked too well. The pendulum swung the other way, causing the Earth to rapidly cool.

How Severe must the coming Ice Age be to cause an Apocalyptic Scenario?

Such a Scenario is defined by:

  • All or almost all above ground human settlements are destroyed or rendered uninhabitable.
  • Most of humanity is wiped out, with less than 100,000 surviving across the globe in perhaps small, mostly underground, communities.
  • The ice age hasn't made the world completely barren or uninhabitable, and mass extinction didn't mean total extinction
  • What communities that survive can still build a semi-sustainable living
  • Most technologies are rendered useless, or at least their construction and maintenance mostly lost to time

I'm looking mostly for kinds of weather conditions, how cold it'd have to be, how quickly the Ice Age happens, and any other things that come to mind.

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    $\begingroup$ If at least parts of nature remain, i.e. liveable for flora and fauna, then in those regions humanity will have no problem surviving. It might even be a really pleasant place. But that means most of your other requirements don't fit. $\endgroup$ – Burki Jun 28 '17 at 15:10
  • $\begingroup$ We are currently living in an ice age, known as the Quaternary Glaciation. (Earth does not normally have permanent ice. A timespan when Earth has permanent ice is geologically rare and is called an ice age.) What you mean is probably a glacial period. $\endgroup$ – AlexP Jun 28 '17 at 16:04
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A global glaciation would probably be apocalyptic.

The past few ice ages the Earth has experienced have seen glaciers advance substantially closer to the equator than they currently are. Areas in the northern US, such as Wisconsin, were largely covered in ice.

That amount of glaciation, while problematic, probably wouldn't be apocalyptic. Global food supplies would be interrupted, and widespread famine would dominate for a few years, causing chaos, but the survivors would likely still number in the billions, with plenty of arable land close to the equator, extending even into the mid latitudes, to sustain them.

However, the Earth has had substantially colder, longer lasting, and icier ice ages further back in its history. The Cryogenian era was marked not just by glaciers extending further south than they do today, but covering most or all of the globe. There is some debate as to whether the Earth was completely frozen, or if there was an open patch of water near the equator, but either scenario could be described as apocalyptic. The vast majority of the world, at least, would be locked under the ice, and the remaining areas would be devastated by changing climate. Native crops, and most other plants, would die, due to the unsuitability of their new climate. Some areas would likely be farmable, but only by those with the resources to cultivate plants from colder climates. In the short term, most of the global population would perish. A complete glaciation, of course, would render the entire surface of the Earth unsuitable for farming.

Even a complete glaciation, rendering the surface of the Earth completely unsuitable for farming, wouldn't end all life, though. Portions of the ocean would likely remain at least seasonably open, sustaining populations of marine life, as well as a suitable habitat for terrestrial animals like seals and penguins that feed only on marine food sources.

NASA released a paper recently estimating that the average global temperature during the Cryogenian was around -12$^{\circ}$ C, which is about 26 $^{\circ}$ C colder than the Earth today.

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Not much worse that the Last Glacial Maximum

The key to causing this apocalypse isn't necessarily extent of glaciation, but speed. What if temperatures proceeded to those of the last glacial maximum within a year or two? Lets assume your Cryion can act that quickly.

Well, the ice sheets would not form right away, but the basis for the glaciers is that more snow is falling than can be melted. This would quickly spread long persistent snowfall that would ruin spring planting in some of the most productive agricultural regions of the Earth (Ukraine, the Midwest, northern France). If the changes happened that quickly, the agricultural knowledge base in those regions would be useless, and there would be several years with reduced or non-existent crop yields.

Now look at the vegetation map during the last glacial maximum. Several other very productive agricultural regions will be too dry to farm as before, such as the Pampas of Argentina, and most of China north of the Yangtze. Finally, lets say that the climate change disrupts the monsoon cycle; India is already a desert.

With half of China and the United State, and most of India, Europe, Brazil, and Russia agriculturally disrupted, we have lost, at least temporarily, about 50-75% of the world's arable land. Can you imagine the chaos that follows? What do you, a rich world suburb dweller, eat when no more food is being shipped in?

The resulting mass starvation would lead to mass violence and general societal collapse. Keep in mind that the three biggest food exporters in the world (US, Canada, Brazil) are no longer even agriculturally self-sufficient, much less exporting. The fact that ice sheets are then about to cover a good chunk of the northern hemisphere isn't going to help.

At this point, the collapse of systems in advanced countries leads to trickle down collapses. Who is going to make farm equipment if Deere and Caterpillar collapse in violence? Who is going to keep the high yield seeds once Monsanto and Syngenta are gone? Even when agriculture is restored after a few years, the systems that kept it operating at high production may have already failed. No gas can be delivered across continents to run tractors in Brazil, and those Brazillian farmers have no experience with non-mechanized agriculture.

The result of all this is the apocalypse you want.

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  • $\begingroup$ The chaos would be severe, but I don't think severe enough to drop world population down to 1 million, let alone 100,000. That many people can sustain on stored food in US alone, weather through any kind of civil unrest, and after things quiet down, move down south. $\endgroup$ – Alexander Jun 28 '17 at 19:59
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To get the Apocalypse you want, you would need a "Snowball Earth" to happen. This would make nearly all land life extinct, and marine life severely affected. Remaining people would still sustain themselves on some marine animals and would be burning fossil fuels making their life palatable.

Anything less than that, and you would have many more survivors, with at least some land suitable for agriculture.

Anything more than that, and marine life would drop to an unsustainable level, with people eventually going extinct too.

However, if the most interesting part of Apocalypse for you is the initial chaos and destruction, you would get a plenty of that if Cryion would cause a rapid cooling.

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