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I have a first-contact scenario where I would like an ice age to be occurring rapidly. All the things that could change to make a fast ice age more credible: orbital period (403.42 local days), length of day (21.79 Terran hours), and surface gravity .67G. I haven't figured out how far that is from the star or what kind of star it is. My protagonist muses that the small molten iron core was part of the reason for the rapid freeze, as I already said the Van Allen belt is much weaker than Earth. I know this is ridiculously specific but can anyone point me to reference material I would like that - or pontificate, everything is welcome.

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    $\begingroup$ An ice age occurring rapidly is a rapid change. The orbital period, the length of the day, and the surface gravity are permanent constants: they do not change, or change very slowly. Stuff which does not change cannot induce a change, let alone a rapid one. $\endgroup$
    – AlexP
    Nov 12, 2022 at 20:10
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    $\begingroup$ Something that would suddenly cause cool summers so the so doesn't melt which increases albedo, etc. more effective then colder winters. $\endgroup$ Nov 14, 2022 at 19:34

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Albedo change.

A runaway snowball earth effect causes your rapid ice age.

https://news.mit.edu/2020/sunlight-triggered-snowball-earths-ice-ages-0729#:~:text=A%20runaway%20snowball&text=As%20more%20ice%20covers%20the,resulting%20in%20a%20global%20glaciation.

Regardless of the particular processes that triggered past glaciations, scientists generally agree that Snowball Earths arose from a “runaway” effect involving an ice-albedo feedback: As incoming sunlight is reduced, ice expands from the poles to the equator. As more ice covers the globe, the planet becomes more reflective, or higher in albedo, which further cools the surface for more ice to expand. Eventually, if the ice reaches a certain extent, this becomes a runaway process, resulting in a global glaciation.

Your world was mostly rainy before and there was not much axial tilt, so minimal seasons. There is a lot of land and less open water. Snow was rare and it did not stick. A volcanic winter event cooled down the poles enough that snow did stick and the albedo went up - more light was reflected and so the globe cooled. The more it cooled the more snow sticks. Oceans freeze and snow sticks there too. Higher and higher the albedo goes. Runaway snowball.

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  • $\begingroup$ Thanks, folks - this was enormously helpful! $\endgroup$
    – dsanford0
    Nov 14, 2022 at 1:58

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