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Having learned about the Milankovitch cycles, I've found two entirely separate explanations from different sources on how the combination of precession and eccentricity changes affect glaciation vs deglaciation. Some sources I've found state that the combination of high eccentricity and Northern Hemisphere winter at aphelion (and hence summer at perihelion) leads to glaciation in the Northern Hemisphere, since the winters are particularly cold (both because high eccentricity means the distance from the sun at aphelion is particularly high, and because aphelion winters would be longer compared to the corresponding perihelion summers, since the earth moves slower around the sun at aphelion compared to the perihelion). However, other sources state that it is the amount of sunlight during summer solstice in the Northern Hemisphere summer that drives deglaciation. Namely, high eccentricity in combination with a perihelion Northern Hemisphere summer would now lead to deglaciation, not glaciation. This is because with high eccentricity, the distance to the sun at summer would be particularly low, hence sunlight intensity would be particularly high during the summers. On the other hand, all sources I've seen agree that high obliquity favors deglaciation.

So which is it? Does high eccentricity in combination with perihelion Northern Hemisphere summers (and hence aphelion winters) lead to deglaciation or glaciation? I'd love a direct answer together with an explanation.

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Does high eccentricity in combination with perihelion Northern Hemisphere summers (and hence aphelion winters) lead to deglaciation or glaciation?

Paradoxically, it's the other way around, at least with regard to when glaciations start. The bellwether metric used by many is the mean July temperature at 65° north latitude. When that drops below 0° C is a key trigger for the start of a glaciation. The conditions that lead to this are a relatively high eccentricity and aphelion in June or July. It doesn't matter that the summers are long. What matters is that summers have become mild enough so that snow can last over the summer, and thus accumulate over multiple years.

The last glaciation started its end about 13000 years ago, and was pretty much complete 11500 years ago. That was when aphelion was in January (making for a long, cold winter) and perihelion was in July (making for a fierce but short summer). A long, cold winter is exactly what is not wanted for the onset of a glaciation. It doesn't snow in the deepest parts of winter in the far north; -40° C or colder is too cold for the atmosphere to hold much moisture. Keep in mind that much of Antarctica and much of the very far north are technically deserts. Snowfall in the far north occurs mostly in autumn and spring. Those short but fierce summers 13000 to 11500 years ago made July temperatures in the far north exceed 0° C; that winters were long and cold back then was a bit irrelevant. This is why the July temperature at 65° north latitude is such a key metric.

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  • $\begingroup$ Thank you. This is the kind of clear answer that I wanted. Though I wonder why I've seen other sources that said something quite different. $\endgroup$
    – User3141
    Apr 25, 2023 at 13:05

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