Understanding the causes of Earth rotation variations is important for constraining a large number of geophysical models in fields spanning from climate to mantle and core dynamics. With the advancements in modelling and availability of more accurate data, it is possible to analyze the variations in Earth rotation more robustly. Kiani Shahvandi, et al. (2024a,b) have performed extensive analyses regarding both components of the Earth rotation: movement of the rotation axis with respect to the crust, termed polar motion, and variations in the rate of rotation, measured in terms of the so-called length of day.

For polar motion,  Kiani Shahvandi, et al. (2024a) focused on periods much longer than that of the Chandler wobble and analyzed all the processes perceived to contribute to this long-period polar motion, namely, the influence of climatic variations, seismic activities, and mantle and core dynamics. By analyzing the mentioned geophysical processes in a unified framework based on state-of-the-art machine learning algorithms called physics-informed neural networks, the causes of interannual, multidecadal, and secular variations in the long-period polar motion record are robustly determined. The quasi-period oscillations on interannual and multidecadal timescales are primarily caused by the climatic variations (measured in terms of barystatic processes) and more specifically, the variations in terrestrial water storage. A secondary contribution arises from the core processes, which appear to be systematically anticorrelated with the barystatic processes, thus potentially pointing towards a dynamic link between surface and interior processes. The secular trend observed in the polar motion record is caused primarily by mantle dynamics, namely, the influence of glacial isostatic adjustment and convective processes in the mantle. However, a small yet nonnegligible contribution comes from the core processes, which are most probably connected to the topographic torque acting on a bumpy core mantle boundary. Finally, seismic processes are of little influence on long-period polar motion, since the coseismic and interseismic processes would probably cancel each other out to a considerable degree.

For length of day, Kiani Shahvandi, et al. (2024b) analyzed the influence of ongoing climate change since 1900 on variations in the length of day. They showed that these variations ranged from 0.3 to 1 ms/cy (milliseconds per century) in the last century. However, the accelerating global climate change has resulted this value to increase to around 1.3 ms/cy since 2000. Adding to this, if the ongoing climate change continues, by 2100 the rate of increase in length of day would be as large as 2.6 ms/cy, which is larger than the influence of the tidal friction induced by the gravitational pulling of the Moon on the Earth. These findings have considerable implications for precise timekeeping, and satellite and space navigation.

References

Kiani Shahvandi, M., Adhikari, S., Dumberry, M., Modiri, S., Heinkelmann, R., Schuh, H., Mishra, S., Soja, B. (2024a). Contributions of core, mantle and climatological processes to Earth’s polar motion. Nature Geoscience, 17: 705-710, https://doi.org/10.1038/s41561-024-01478-2

Kiani Shahvandi, M., Adhikari, S., Dumberry, M., Mishra, S., Soja, B.  (2024b). The increasingly dominant role of climate change on length of day variations. Proceedings of the National Academy of Sciences, 121: e2406930121, https://doi.org/10.1073/pnas.2406930121