[icytotem]

This may be totally dumb, but if the variation is attributed to the tides, is it possible that global warming affected it? With the ice cap melting, there is more sea water sloshing around instead of just floating at the top.

[mianos]

It's not. They are statistically insignificant compared the rotation of the core mass.

[sigmoid10]

Wrong. For rotation you don't look at mass, but at moment of inertia. That scales like ~M*R^2 or ~rho*R^5 for a density of rho and distance R from the axis of rotation. So even if the core is much denser, it contributes a lot less to the earth's total moment of inertia. Earth's rotation is even measurably affected by plants changing throughout the seasons. Glacial melts and earthquakes also have measurable effects that need to be compensated for. See here for the plot:

https://upload.wikimedia.org/wikipedia/commons/thumb/5/5b/De...

And here for a thorough explanation:

https://www.sciencedirect.com/science/article/pii/S167498471...

[mianos]

That second paper summarises that the changes are quite probably due to deformation which is caused by core movement. We are not debating seasonal variation.

[sigmoid10]

That is one source. But not the source.

[nextaccountic]

M*R^2 is understandable but what's this raised to 5th power stuff in rho*r^5? And is this lowercase r the same as the uppercase R?

[sigmoid10]

Mass scales like density times radius cubed. The first M would technically be a function of position. So you would integrate over slices of varying M not just because of varying density.

[Someone]

It is statistically significant. https://www.nature.com/articles/d41586-024-00850-x:

“Earth’s rotation seems to have accelerated, outpacing the time standard, and raising the possibility that an unprecedented ‘negative’ leap second might soon be required — a daunting prospect in a world reliant on consistent timekeeping.

• Agnew1 reports that human-induced melting of polar ice exerts a slowing effect on Earth’s rotation, effectively delaying a decision on the need for a negative leap second.”

Global warming moved the date that we would need a negative leap second out by 3 years.

[mianos]

This paper was by a cosmologist with heaps of papers in other areas and just a single paper in this area, highly publicised in the climate change news.

All the reports around this were hyperbolic. I'd be waiting for a bit more research in this area. It reminds of other recent paper that said possible close passes by other stars had a huge impact on the environment according to simulations. No mention of the problem predicting orbits over tens of thousands of years.