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I have long been wanting to write a story about Earth freezing relatively fast. In the past I have asked some questions here considering a scenario where Earth becomes a rogue planet... But I also want to keep it inhabitable for humans on the long term, and a rogue Earth would kill everyone in months or years.

So I have considered provoking a glaciation instead. Just as in a rogue Earth scenario, something would push Earth into a different orbit, but not enough to cause a solar escape.

I used an online physics calculator to figure that a 10% increase in the semi-major axis of Earth's orbit would increase the length of the year by approximately one month. If the push happens in July, it means the perihelion will be pushed to such a greater altitude that it becomes the new aphelion - and the Earth will be there during winter on the northern hemisphere. Having the northern winter last longer, combined with the increased distance from the sun, should cause global temperatures to drop considerably.

I am aiming for an impact similar to the eruption of the Tambora volcano in 1815, which led 1816 to be known as "the year without a summer", but harsher and permanent. But I don't know how to calculate how much the drop in global temperatures throughout the year would be in this case, nor if this would be such a catastrophe that a post-apocalyptic scenario with humans in it would be beyond optimistic.

Asides from the practically magical push that changes the Earth's orbit, I am aiming for full, scientific realism (though I am restraining myself from using the tag here).

So... Is this longer winter possibly survivable, or would that surely be the last one humanity would see? If the latter, I'll consider scaling the orbital changes down.

Consider that the technological level of humanity when the orbit changes is exactly what we have today. In other questions here in the site we came to the conclusion that the change in orbit would be detected practically instantly, giving humanity some time to prepare for the worst. But we'd lose most if not all artificial satellites (which by itself would already cause a lot of disruption).

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    $\begingroup$ @SeanOConnor If you ever track that question down, those calculators are fantastic and belong in the list of Worldbuilding resources. $\endgroup$
    – JBH
    Commented Nov 6, 2022 at 16:39
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    $\begingroup$ Here is that ? worldbuilding.stackexchange.com/questions/230473/… But the only calculator is in your answer. Maybe an account was deleted? In a fit of pique?? $\endgroup$
    – Willk
    Commented Nov 7, 2022 at 13:32
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    $\begingroup$ @SeanOConnor - is this it: satsig.net/orbit-research/orbit-height-and-speed.htm $\endgroup$
    – Willk
    Commented Nov 7, 2022 at 23:12
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    $\begingroup$ Look at the short story "A Pail of Air" by, I think, Asimov. It doesn't answer your questions but shows people surviving in those conditions. $\endgroup$
    – ShadoCat
    Commented Nov 8, 2022 at 19:43
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    $\begingroup$ @JBH It worked for me. That's how I got the 15ish degrees that I mention below. Just tried it on a different device (Phones both times). The Build button that you need hides under the Wayback Machine banner at the top. $\endgroup$
    – user86462
    Commented Nov 9, 2022 at 1:00

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The tropics will do fine.

Glaciers. Mammoths! The cold, the cold. And we already painted the whole cave. But south of all that it is still pretty nice. It might get nicer. It did during the little Ice Age.

Tracing the effects of the Little Ice Age in the tropical lowlands of eastern Mesoamerica

Our data provide evidence that the densest tropical forest cover and the deepest lake of the last two millennia were coeval with the LIA.... The high tropical pollen accumulation rates limit LIA's winter cooling to a maximum of 2 degrees C. Tropical vegetation expansion during the LIA is best explained by a reduction in the extent of the dry season as a consequence of increased meridional flow leading to higher winter precipitation.

Thicker forest, deeper lakes. Sounds not too bad for human survivors! When average global temperatures cool, it is because high latitudes get really cold. The same is true for global warming - it is the high latitudes driving the change. The tropics do not change that much in either scenario. I think in your superbad little Ice Age scenario, the tropics will stay nice for humans. Maybe they will get nicer! Because a lot of the tropics are pretty freaking hot right now.

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    $\begingroup$ Second this. According to the calculator we found, 10% orbital radius distance = 15 deg C mean temperature. Challenging, but life will go on in the tropics, unless nukes get launched. $\endgroup$
    – user86462
    Commented Nov 8, 2022 at 11:16
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Willk is right. All evidence points to the southern hemisphere being warmer due to many reasons: lower albedo due to less land mass, the fact that it is summer there during aphelion (which I am changing, but alas), currents etc.

Thanks to the comments in the question I was reminded of our list of worldbuilding resources. I played around and found this simulator that I liked: Monash Simple Climate Model (now offline). I am linking directly to a simulation that includes changes in orbital distance.

The simulator is assuming a mostly circular orbit, whereas the one I suggested is thrice as eccentric as our current one. So I think what would happen is somewhere between the results in the simulator and current reality, leaning a bit towards the simulation since the winter will be much longer than the summer. To be fair, I adjusted the simulation to an smaaller global increase instead to 1.06 AU, thinking it will match my model more closely. This is what we have for surface temperatures:

A simulation of global temperatures with an increased distance from the Sun

And this is ice coverage year-round:

Ice cover on the planet with an increased distance from the Sun

North and Central America, as well as most of Eurasia would be covered in ice year-round. US and China in particular would be under glaciers. It would be the end of agriculture (at least as we know it) in those places, causing huge famines and mass migration from the populations to more equatorial regions.

But South America and Africa would have huge spans where it would be a livable ~14 C (57F) year round. It is possible to grow crops in those regions, and our ancestors of dozens of millennia ago have fared well against worse odds (not considering the potential for nuclear annihilation).

Oddly enough, Australia would be on the path to enact a polar version of Mad Max.

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