Changes in length of day at millennial timescales
Variations in the length of day (LOD) reconstructed from ancient records of
eclipses suggest that the LOD is currently increasing at a steady secular rate of approximately 1.7 milliseconds(ms) per century (cy) (see Figure 1 below). This results from the combined affect of tidal friction and post-glacial rebound. Recent climate change during the past century has lead to ice melt in polar regions and to an associated increase in sea level; this effect, by itself, is causing the LOD to increase.
The eclipse record also suggest the presence of fluctuations of the order of a few ms over a few millennia (Figure 1). Although these observed changes are not well constrained by the eclipse record, we expect LOD changes of this magnitude as a result of angular momentum exchange between the core and the mantle. Indeed, longitudinal motions of magnetic field features, with a similar millennial timescale, are observed and can be connected to fluctuations in the zonal core flow (Figure 2).
Figure 1: Changes in the Length of day reconstructed from Solar and Lunar eclipse data. This figure appeared in a paper by F. R. Stephenson, in Astronomy and Astrophysics, 2003. |
Figure 2: Eastward and Westward drift of magnetic field features over millenial timescale at latitude 60N on the Core-Mantle boundary. The x-axis is longitude, the y-axis is time. Red (Blue) represent the intensity of the outward (inward) radial magnetic field (the axial dipole part has been subtracted). Figure taken from Dumberry & Finlay, EPSL, 2007. |
Some of my papers on this topic
Dumberry, M., Gerick, F. and Gillet, N., 2025, Millennial axially symmetric Magneto-Coriolis modes in Earth's fluid core, Geophysical Journal International, 240, 2076-2090.
Suttie, N., Nilsson, A., Gillet, N. and Dumberry, M., 2025, Large-scale palaeoflow at the top of Earth's core, Earth and Planetary Science Letters, 652, 119185.
Kiani Shahvandi, M., Adhikari, S., Dumberry, M., Mishra S. and Soja, B., 2024, The increasingly dominant role of climate change on length of day variations, Proceeding of the National Academy of Sciences, 121, e2406930121.
Dumberry, M., 2018, Earth Rotation, Excitation, Core, In: Grafarend E. (eds) Encyclopedia of Geodesy , Encyclopedia of Earth Sciences Series, Springer, Cham.
Mitrovica, J. X., Hay, C.C., Morrow, E., Kopp, R. E., Dumberry, M. and Stanley, S., 2015, Reconciling past changes in Earth's rotation with 20th century global sea-level rise: resolving Munk's enigma, Science Advances, 1, e1500679.
Dumberry, M. and Finlay, C. C., 2007, Eastward and westward drift of the Earth's magnetic field for the last three millennia, Earth Planet. Sci. Lett., 254, 146-157.
Dumberry, M. and Bloxham, J., 2006, Azimuthal flows in the Earth's core and changes in the length of day at millennial timescales, Geophys. J. Int., 165, 32-46.