Every 202,500 Years, Earth Wanders in a New Direction | Hacker News Reader

Every 202,500 Years, Earth Wanders in a New Direction

nytimes.com

130 pointsmontrose51 comments

The change in orbital shape was already known as one of the Milankovitch cycles [1]. This study seems to give us the most precise measurement.

In the context of climate change, when Earth's orbit becomes more eccentric (our path over the next 200k years) seasonal changes increase in magnitude.

[1] https://en.wikipedia.org/wiki/Milankovitch_cycles#Orbital_sh...

> In the context of climate change, when Earth's orbit becomes more eccentric (our path over the next 200k years) seasonal changes increase in magnitude.

Isn't that only so on one of the hemispheres? Right now, earth is slightly further away from the sun when its summer on the northern hemisphere which means that our summers are some 5-7 days longer than our winters and our seasons are actually less intense here.

It’s more complex than either case. The eccentricity of earth’s orbit precesses, as well as earth’s axial tilt.

In a circular orbit, the seasonal variation is driven solely by the axial tilt and relative hemispherical insolation bought about by earth’s own shadow, and angle of the sun to terrain.

With a variable distance from the sun, the seasons become erratic, as these two variables precess out of sync - you end up with winters in one hemisphere where the earth is close to the sun, making for mild winter and scorching summer in the other hemisphere. Equally you end up with the earth far from the sun, and bitter winters and cool summers. Finally, you have the precessions 180 degrees out of sync, and you end up with almost no seasonal variation.

So - it makes the overall swings far more extreme, but in some phases can result in milder seasons.

Where are we now and where are we headed?

We're headed to the end of the current interglacial, returning to a drastically cooler climate.

The last interglacial (the Eemian [1]) lasted 15k years and was warmer than what we experience today; sea level at peak was probably 6 to 9 m (20 to 30 feet) higher than today; Scandinavia was an island. Then the glaciation returned for 100k years with mile-thick ice sheets covering huge parts of Europe and North America, until the current interglacial got going almost 12,000 years ago. [2]

"The axial tilt varies between 22.1° and 24.5°, over a cycle of about 41,000 years. The current tilt is 23.44°, roughly halfway between its extreme values. The tilt last reached its maximum in 8,700 BCE. It is now in the decreasing phase of its cycle, and will reach its minimum around the year 11,800 CE. ... decreasing tilt may encourage the onset of an ice age ..." [3]

[1] https://en.wikipedia.org/wiki/Eemian

[2] https://en.wikipedia.org/wiki/Last_glacial_period

[3] https://en.wikipedia.org/wiki/Milankovitch_cycles#Axial_tilt...

The simple truth is: No one really knows.

How do you determine the exact position in a cycle of 400000 years?

Better article with more details and a link to the relevant paper https://www.universetoday.com/139198/jupiter-and-venus-chang...

I expect this is the danger quote "“The climate cycles are directly related to how Earth orbits the sun and slight variations in sunlight reaching Earth lead to climate and ecological changes,” said Kent, who studies Earth’s magnetic field. “The Earth’s orbit changes from close to perfectly circular to about 5 percent elongated especially every 405,000 years.” which comes from here (https://news.rutgers.edu/earth%E2%80%99s-orbital-changes-hav...)

Based on where we are in the Milankovitch cycle, should we be getting warmer or colder?

(I keep checking the comments of this article hoping that someone knowledgable can explain how this impacts the climate.)

NASA to the rescue: https://earthobservatory.nasa.gov/Features/Milankovitch/

Basically it's not measurable within a lifetime and the signal of human interaction with the climate is magnitudes higher.

It is extravagant and ludicrous to claim that the signal of human influence (CO2 ppm?) on the climate is magnitudes higher than that of the Milankovitch cycles -- which have been driving the regular glaciations and interglacials over the last 3 million years.

That said, we do use ice cores from periods that would cross multiple of these cycles. If these cycles change how we estimate those values, then the statistical models we’ve built based on them will need to adjust accordingly. So, it’s not that it doesn’t have an impact, just that the impact probably doesn’t change the trend line.

Do the math...

There are multiple cycles in that theory, not just one, so the influence on our climate depends on the periods of all these cycles, and involves potential amplification due to their harmonies. The largest observed signal in the geological record of the last 3 million years is a ~100k cycle of interglacials within an Ice Age.

Accordingly we should expect the current interglacial to end "soon" and enormously thick ice sheets shall grow again -- we're 12k years into the Holocene; the last interglacial lasted 15k years.

PBS space time did an episode about the cycle

https://www.youtube.com/watch?v=ztninkgZ0ws

I made a thing. To wrap my head around the Milankovitch cycles. https://i.imgur.com/V3RIKL5.png

TLDR; Scientists believe the Earth's orbit oscillates between circular and more elliptical due to the gravitational pull from Jupiter (big) and Venus (close). It takes 405k years to complete a cycle.

Exactly, but what is new here? The duration of the complete cycle?

Well, that number is new, to me at least, but the principle was well known before, I assume?

I don't get it.. don't they teach basic astronomy in schools anymore?

>don't they teach basic astronomy in schools anymore?

Not in Ontario, Canada, they don't...

I had to get accepted into a university and then apply for first year astronomy to get any basic astronomy instruction whatsoever.

Many astronomers don't study astronomy until grad school! Physics is fundamental.

What do you mean by fundamental here? For studying astronomy at an university? Then yes, agreed, but I am wondering about the very basic understanding of the principles at all, are you implying that kids today don't learn this anymore? You know, what our solar system is, what the earth is, and it's, well, shape..

This particular cycle was not taught in the Astronomy PhD courses where I attended. Your comment near the top of this thread appears to claim that it's a part of 'basic astronomy'. That is not correct. The precession of the equinoxes is basic astronomy.

(Ha. Firefox's spellcheck doesn't have "precession" in it.)

I did well in Physics in High School, and my instructor was massively interested in Astronomy, but he taught us very little about it.

Wait.. are you serious?

Guess I should consider myself lucky that I had a physics teacher who taught us about the thing that we stand on and live on, what it is, where it is, what's around and how our solar system works.

Ha, I am really shocked now. Honestly, I can't believe this. I should probably not dare to ask someone on HN if they ever heard about the Milk Way or something.

The Milky Way is a candy bar, everyone knows that!

By your nick, I'm not surprised you know about Milankovitch cycles.

Or at least don't do it in such a dismissive way.

You think this is equivalent to knowing about the Milky Way? You seem to have put some major significance on something that really has very little baring on our day to day lives. Im sure most countries teach a good deal about the solar system and the planets, It's taught at a very young age in the UK and its refined through the years of school to the point where you should have a good understanding of it all. But we never put any emphasis on this. You know why? Its irrelevant at that level. Unless you are studying for a degree or even higher this won't have any baring on what you are learning.

It mentiones why this is useful:

> it will give scientists a much more accurate method of dating prehistorical events — the dates of fossils, for example.

> “The dream is have a framework independent of the fossils that you can plug the fossils into and see more interesting things — the coexistence of disparate forms, or of similar forms widely separated in location. Now we can place things more accurately in time rather than depending on the fossils to tell us what the time is.”

It's also fascinating to see how it was done:

>By comparing the amount of decay of uranium to that of lead trapped in zircon, the layers in the Arizona core can be dated quite accurately.

Paywall articles should be banned here

The result of which is ... ?

Does this lead to some occurrence such as a mass extinction or something of that nature?

Seems interesting.

> Does this lead to some occurrence such as a mass extinction or something of that nature?

I have no doubt there will be dozens of astrology videos about this on YouTube within a few days.

it would be interesting to see how this correlates with known glaciation periods.

As the orbit elongates we experience greater variation in distance from the sun. At the extremes, we get colder winters and hotter summers.

Perhaps another explanation for climate change.

Why would more eccentricity lead to colder winters and hotter summers in each hemisphere? How would that work?

yeah makes little sense since the earth is closer to the sun in january. if anything higher eccentricity should reduce seasonal differences. (seasonal differences being driven mostly by earth tilt, not distance from sun)

At times of higher eccentricity you get more pronounced differences between summer and winter. If this coincides with a glacial period (and it has) that probably means that the glacial period sets in faster / is colder than when the orbital eccentricity is low -- but it also means that interglacials set in faster and are warmer.

> At times of higher eccentricity you get more pronounced differences between summer and winter.

Why would this be the case?

Because eventually perigee coincides with summer or winter in either the Northern or Southern hemisphere (and then the apogee will coincide with the other hemisphere's opposite season, naturally), so one of them will get more sunlight (heat). BUT! The Northern hemisphere has much more landmass than the Southern one, which accentuates the differences between seasons because land heats up faster in the summer and cools down faster than the oceans in the winter, and also land provides a platform for ice accumulation. This is the reason that glacial periods involve much more ice in the Northern hemisphere than in the Southern hemisphere.

It is generally believed that glacial periods set in when the Northern summer coincides with the furthest point from the Sun on Earth's orbit, while interglacials set in when the Northern summer coincides with the point of closest approach to the Sun. Summer is the key because once ice lasts past the summer then summer cannot heat up the Northern hemisphere as much as usual: ice reflects a lot of the Sun's energy. Conversely, if the summer is warm enough to melt more snow and ice than was accumulated during winter, then you can't have a glacial period.

It is important to note that there are other orbital cycles that matter here, especially the precession of the equinoxes, which is the cycle that relates which hemisphere gets its summer closest to the Sun. Earth's axis of rotation itself rotates, very very slowly. The precession of the equinoxes is a 25ky cycle.

Another cycle is the angle of the Earth's axis of rotation to the ecliptic. This varies a fair bit over the years, and this one is (IIRC) the most chaotic of the orbital cycles.

All of these cycles, known as the Mylankovitch cycles[0], have winter/summer difference accentuation/attenuation effects. The Mylankovitch cycle theory is that when these coincide in certain ways you get glacial or interglacial periods.

[0] https://en.wikipedia.org/wiki/Milankovitch_cycles

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