The secular trends observed in the historical records of polar motion and length of day represent the role of geophysical processes that operate on geological timescales. The causes of these secular trends are disputed. However, the two recent studies Kiani Shahvandi, et al. (2024a,b) delve deep into this problem and suggest a complete explanation of the aforementioned secular trends.

For polar motion, Kiani Shahvandi, et al. (2024a) show that the secular trend is primarily controlled by the dynamics of the Earth's mantle, namely, Glacial Isostatic Adjustment (GIA) and mantle convection. When constraints are put on the mantle convection through the analysis of subduction history and backward advecting density anomalies, it is possible to separate the effect of GIA and mantle convection. The value derived for GIA (around +1.9 mas/year towards 77 West longitude) is consistent with an independent estimate based on the contemporary geoid rate of change. Furthermore, the direction is towards the Hudson Bay area, a region that was covered with large continental icesheets during the last ice age. Since the disappearance of the massive icesheets after the termination of last ice age, the rebound of the Earth causes the Earth's rotation axis to move towards this region to preserve the Earth's angular momentum. These provide strong basis to suggest that the main causes of the secular trend in polar motion are GIA and mantle convection processes. However, Kiani Shahvandi, et al. (2024a) also point out the contribution of barystatic and core processes to the secular trend. The latter processes arise most probably as a consequence of a topographic torque acting on a bumpy core mantle boundary. This contribution might not be truly secular, but would have a very long period that would be seen as a secular trend in the range 1900-2018.     

For length of day, Kiani Shahvandi, et al. (2024b) concluded that the secular trend can be explained entirely by the sum of tidal friction and GIA. They showed that the independent reconstructions for length of day in the range 1976-2023 can be converted to the coefficient of ellipticity (J2) and compared with the satellite-laser-ranging-derived J2. When the aforementioned converted values are subtracted from the observed values, the remaining signal features a prominent negative secular trend of -0.8  ms/cy. When this value is added to the rate of tidal friction (+2.4 ms/cy), the resulting value of  explains-within the uncertainty limits-the observed secular trend of  1.72 ms/cy derived from length of day observations in the past three millennia.

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