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In my fantasy series of ''Diggoran'' ( pronounced Dia-Ran ) Humans initially came from a continent in the northern hemisphere known as ''Atmoran'' which spans 20.561 million square miles. For over 1.9 million years, Atmoran has been a frozen continent which due to the planet's average tilt of 45-60 degrees.

I wanted to create a story of human evolution on Diggoran in which Atmoran was once a warmer, Africa like continent until a sudden tilt change brought about an ice age that forced humans to migrate south across now frozen over oceans.

Is such an event possible?

I can provide more information if needed.

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    $\begingroup$ I think you probably need to define "sudden." Sudden change for a human could take place in a day or less...sudden change in geological terms...even a change that drastic that happens in 5000 years is very very sudden. If you tell us how long you want the change to take it would help people answering. $\endgroup$
    – James
    Aug 16, 2019 at 18:24
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    $\begingroup$ Additionally, welcome to the site. Check out the help center and tour to familiarize yourself with how things work around here. Good first question, happy world building. $\endgroup$
    – James
    Aug 16, 2019 at 18:25
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    $\begingroup$ See true polar wander on Wikipedia. Money quote: "the speed of rotation (around the axis of lowest inertia) is thought to have been less than about 1° per million years". $\endgroup$
    – AlexP
    Aug 16, 2019 at 18:32
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    $\begingroup$ "dramatic tilt change" would mean a life-ending cataclysm. But there are other ways to alter continent's climate. $\endgroup$
    – Alexander
    Aug 16, 2019 at 18:46
  • $\begingroup$ It is believed that the presence of a big moon is necessary to prevent wild swings of the axis (over cosmic time). $\endgroup$ Aug 17, 2019 at 14:51

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Yes, such an event is possible, whether it is survivable or not is a different question.

You wouldn't even need to tilt the axis by that much, the Earth's ice ages are caused by a 25,000 year cycle of procession around its axis.

If your planet had a close encounter with a large body, it would be possible for it to alter the planet's rotation. A massive cataclysmic earthquake could cause it as well. Both of these are of questionable survivability for anything living on the planet, but it's not impossible.

Perhaps an easier solution is to simply say that your planet, like Earth, has a long term cycle that results in periodic ice ages.

As an entirely different possibility that gives you the same result, you could say that your planet's sun went into a "Deep Grand Minimum" where its output decreases. Such a thing has happened in Earth's past as well and resulted in more severe winters. The most recent was the Dalton Minimum, which resulted in extremely cold winters and summers. This was seen reflected in the works of Charles Dickens, which portrayed a London with a much colder and snowier climate than today, and mentioning the Thames river freezing over, as well as in the USA with the famous painting of George Washington crossing the Delaware river in winter, which portrayed it being so cold that men froze to death and chunks of ice in the river had to be pushed out of the way. Both are climates that are very different from today, as snow is relatively rare in London, and ice in the Delaware river almost never happens.

Before the Dalton minimum was the Maunder Minimum, which was even colder, and resulted in stories like Hansel and Gretel, where parents would often abandon their children because they could not provide food for them due to the poor climate.

In addition, it's even theorized that severe climate changes in Africa forced the evolution of human intelligence, as "becoming smart" was the only way to survive the rapid changes between ice age and tropical climate occurring every few decades. The cause of these rapid changes in Africa's climate are still debated, but a wide variety of reasons have been proposed including "impact winter" due to asteroid impacts, volcanic eruptions, decreases in solar output, supernovae, etc..

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Assuming you don't want a fantasy explanation: get some "helpful" nearby gas giants. In the right configuration, the terrestrial planet can end up experiencing large-scale variations in inclination and orbital eccentricity on timescales of tens of thousands to millions of years. The paper "Effects of Extreme Obliquity Variations on the Habitability of Exoplanets" by J.C. Armstrong et al. (2014) includes several simulations of such systems. The inclination variation will generally be occurring on a different timescale to the changes of the direction of the axis of rotation, so will lead to axial tilt variations (see figure 1 in the paper).

This avoids the need for giant catastrophic impacts which would kill everyone on the planet. You also have the possibility that the climate variation you described is only part of the full cycle, with the potential for even more extreme and unpleasant climates as things progress.

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In Atlantis Found by Clive Cussler, a similar event happens, although the planet's tilt does not change. An asteroid hits Hudson Bay in Canada and brings about a period of major geological upheaval in which Antarctica moves many thousands of miles closer to the South Pole within a few years and is covered by ice. There are also major earthquakes and tsunamis, killing most of humanity and burying the advanced seafaring civilizations on the coast under many meters of silt. The ones that survived were primitive tribes that lived on higher ground and could avoid the miles high waves.

If the tilt change is not an essential part of your story, you could consider something similar to this, where your surviving humans lived on a continent with a high plateau or mountain range which shifted north, and were able to leave south for warmer climates after the tsunamis died down

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    $\begingroup$ Clive Cussler is not the go-to guy on anything that doesn't involve diving. That novel is one long series of geological howlers. Among other things, there is zero evidence the Nastapoka Arc is the result of an impact, and if it was, the crater would have been 450 kilometers in diameter. This wouldn't have "disrupted" civilizations in 7120 BCE, it would have been an extinction level event for the entire planet. Also, even if the Arc really is impact in origin, it would need to be at least 2.5 billion years old. $\endgroup$ Aug 16, 2019 at 19:46

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