Unexpected acceleration of the Earth's rotation

[SergUpstart]

On June 29, the Earth unexpectedly made a revolution around the axis 1.59 ms faster than 24 hours, and this became its fastest revolution since atomic clocks began tracking such data with great accuracy in the 1960s.
In recent years, the Earth's rotation has been gradually accelerating, but no one knows why this is happening. Since 2020, the planet has already broken the rotation speed record a couple dozen times, despite the fact that nothing like this had happened for decades before.
The Earth is not a perfect ball, so its rotation is constantly fluctuating, being subject to a variety of factors, including its internal structure, the tidal effect of the Moon, climatic changes. One team of scientists suggested that the acceleration of rotation may be due to fluctuations in the geographical poles of the Earth. This phenomenon was discovered in the XIX century by the American astronomer Seth Carlo Chandler, and is called the "Chandler oscillation of the poles."
According to Matt King, a professor at the University of Tasmania, a specialist in observing the behavior of the planet, this is really a strange phenomenon. Clearly something has changed, and it has changed in a way that has not manifested itself since the advent of precision radio astronomy in the 1970s.
If the days continue to shorten, scientists will have to subtract a second from the readings of the atomic clock. This will be the first case of a negative correction of the arrows in history. Since 1972, the scientific world has adopted the practice of adding an additional second, which is added to the coordinated Universal time (UTC) scale to align it with the average solar time UT1. So far we have only had to add these seconds, but not subtract them.
At the same time, scientists previously calculated that due to changes in the interaction of the Earth with the Moon in 6.7 million years, the day on Earth should become one minute longer.

It can be assumed that the core of the Earth cools and contracts, and then its rotation accelerates due to the law of conservation of angular momentum.

[swansont]

Shortest recorded in the last ~50 years, but reconstructions show it was faster in the past, and we’ve gone to negative excess length of day in the atomic clock era. In the early 2000s ELOD was negative during part of the year. No leap seconds were inserted for 7 years. Then the days got longer again.

https://www.ststworld.com/understanding-day-length-fluctuations-what-they-are-and-what-causes-them/

Even with the fluctuations, you can see the long-term trend is toward longer days, and that we’ve had negative ELOD in the 1930s and a much larger dip from ~1860-1900

https://geodesy.geology.ohio-state.edu/course/refpapers/dLOD_1800-2000.pdf

Earth rotation rates have geologic- and climate-related contributions, so focusing on very short time scales is a tad misguided 

 

[studiot]

2 hours ago, SergUpstart said:

Unexpected acceleration of the Earth's rotation

 

Thanks to swansont, +1, for pointing out the long term and statistical nature of the rotation.

Statistical fluctuations are, by definition, 'unexpected' in any specific fluctuation timing or quantity.

 

[swansont]

Press release. Pretty sure some of my colleagues were tired of fielding calls on the topic

https://www.cnmoc.usff.navy.mil/Portals/49/Short day press release_ngs_grc_220809_1.pdf

In addition to the dip I mentioned (~1860-1900) there was an even bigger one back around 1660

[studiot]

I note that swansont's press release refers to milliseconds of variation.

 

Set that against the addition of leap seconds (last one in 2016, two in 1972) to the clocks to balance out variations in the rotation speed.

[joigus]

I once heard claims that even such subtle effect as changes in the moment of inertia of the Earth due to seasonal leaf shedding in deciduous forests can have a measurable effect in the Earth's rotation. I don't know how much truth is behind such claims, or whether it's in the milliseconds. 

Could that be true?

It seems like small potatoes in comparison with motions in the mantle, for example.

[studiot]

9 minutes ago, joigus said:

I once heard claims that even such subtle effect as changes in the moment of inertia of the Earth due to seasonal leaf shedding in deciduous forests can have a measurable effect in the Earth's rotation. I don't know how much truth is behind such claims, or whether it's in the milliseconds. 

Could that be true?

It seems like small potatoes in comparison with motions in the mantle, for example.

Seems like small potatoes compared to the mass of eroded material brought down from the mountains by water or just gravity to me.

I haven't measured it but not only is the mass small but the average leaf fall from trees is about 30m whereas rivers of say the Himalya drop severalthousand metres.

[joigus]

30 minutes ago, studiot said:

Seems like small potatoes compared to the mass of eroded material brought down from the mountains by water or just gravity to me.

I haven't measured it but not only is the mass small but the average leaf fall from trees is about 30m whereas rivers of say the Himalya drop severalthousand metres.

Thank you. It seemed too far-fetched to me too. You're right, material from erosion must be orders of magnitude more sizeable for both reasons you point out.

[mistermack]

2 hours ago, joigus said:

Thank you. It seemed too far-fetched to me too. You're right, material from erosion must be orders of magnitude more sizeable for both reasons you point out.

The thing is though, some of the Earth's biggest mountains are still growing. And they obviously grow from the bottom, which is a gigantic area. The Himalayas grow about 1cm per year, which doesn't sound a lot, but if you imagine the entire mass of the himalayas, raised 1 cm, it will knock hit the leaf fall and erosion into the weeds. I would imagine that most of the variation is down to the movements of plates, indirectly, with some input from hotspot volcanoes as well, both underwater and on land.

[swansont]

13 hours ago, joigus said:

I once heard claims that even such subtle effect as changes in the moment of inertia of the Earth due to seasonal leaf shedding in deciduous forests can have a measurable effect in the Earth's rotation. I don't know how much truth is behind such claims, or whether it's in the milliseconds. 

Could that be true?

It seems like small potatoes in comparison with motions in the mantle, for example.

 

9 hours ago, mistermack said:

The thing is though, some of the Earth's biggest mountains are still growing. And they obviously grow from the bottom, which is a gigantic area. The Himalayas grow about 1cm per year, which doesn't sound a lot, but if you imagine the entire mass of the himalayas, raised 1 cm, it will knock hit the leaf fall and erosion into the weeds. I would imagine that most of the variation is down to the movements of plates, indirectly, with some input from hotspot volcanoes as well, both underwater and on land.

These things can be estimated. I once calculated that a large number of people (a million?) driving in the same direction would change the rotation rate by a tiny amount.

Leaf growth would slow the rotation down in the spring and speed it up in the fall, with a larger effect in the northern hemisphere owing to the larger land area.

The mountain growth has to come from mass moving from some other area; as the tectonic plates push up on the mountains, there is subduction somewhere else.

I = 2/5 MR^2 for a sphere of uniform density, and since we're going for an order-of-magnitude estimate, we can use this. 

2/5 * 6e24*6.4e6^2 = 10^38 kgm^2 and we know that Iw is going to be constant; the angular speed compensate for changes in the moment of inertia

(I'm doing this without the morning caffeine having taken effect, so check my math)

There are ~3 trillion trees in the world, and some don't shed leaves. Let's say 10^12 participate (north vs southern hemisphere) and they drop 10 kg of leaves 10m, on average. dI/dr is 2 * r dr, or ~10^8, and we have 10 kg, so our moment changes by ~10^9, meaning a part in 10^29 reduction in the rotation rate. Not measurable.

The mass of mountains is much higher, but the change in elevation is smaller. Not sure what the area in question is, but keep in mind that people estimate the change in rotation rates after earthquakes, which move a lot of mass around, and these estimates are of order a few microseconds per day.

https://www.jpl.nasa.gov/news/nasa-details-earthquake-effects-on-the-earth

They also found the earthquake decreased the length of day by 2.68 microseconds.
...
To make a comparison about the mass that was shifted as a result of the earthquake, and how it affected the Earth, Chao compares it to the great Three-Gorge reservoir of China. If filled, the gorge would hold 40 cubic kilometers (10 trillion gallons) of water. That shift of mass would increase the length of day by only 0.06 microseconds and make the Earth only very slightly more round in the middle and flat on the top. It would shift the pole position by about two centimeters (0.8 inch).

Consider snowfall storing mass in higher latitudes for a few months and then melting. Droughts and floods likely have a bigger effect than the Three Gorges dam mentioned here.

A bigger effect is possible if a mass is rotating, since that mass will have angular momentum. A hurricane/typhoon for example. ~200 million tons of water but rotating, some parts much faster than the earth. Any angular momentum it has has been traded with the earth's rotation.

[mistermack]

3 minutes ago, swansont said:

The mountain growth has to come from mass moving from some other area; as the tectonic plates push up on the mountains, there is subduction somewhere else.

Yes. One thing that complicates it still further, is where that subduction occurs. If it's at the poles, it won't cancel out the slowing, but if it's at the equator, it will more than cancel it. Most probably it's averaged out around the globe though.

I would think that Greenland's shrinking ice cap will be having some sort of effect.

[swansont]

28 minutes ago, mistermack said:

Yes. One thing that complicates it still further, is where that subduction occurs. If it's at the poles, it won't cancel out the slowing, but if it's at the equator, it will more than cancel it. Most probably it's averaged out around the globe though.

I would think that Greenland's shrinking ice cap will be having some sort of effect.

The ice melt is accompanied by the solid earth rebounding since it's no longer being compressed by as much mass. The earth becomes less oblate, so it spins faster. But the water tends to spread out, so there is more mass near the equator, which would tend to slow it down. The current thinking is that the effect of the shape change is the larger one.

[studiot]

 

I think all this consideration is small compared to the fact that different parts of the Earth are rotating at different (angular) speeds and possibly in different directions.