Causes of Earth's spin axis motion unraveled by physics-informed neural networks

Earth’s spin axis exhibits movements relative to its crust over different timescales, commonly known as polar motion. A 120-year-long record of polar motion features interannual and multidecadal fluctuations of 20 to 40 milliarcseconds (mas) superimposed on a secular trend of about 3 mas/year pointing toward Hudson Bay. Polar motion is driven by a collection of Earth’s surface and interior processes. However, how these processes operate and interact to yield the observed signal remains an enigma. Here we show that predictions made by an ensemble of physics-informed neural networks fully explain the main features of the observed polar motion. Glacial isostatic adjustment and mantle convection primarily account for the secular trend. Mass redistribution on the Earth’s surface results in a relatively weak trend, but explains about 90% of the interannual and multidecadal variations. Torques at the core-mantle boundary contribute to a small but non-negligible part of both the secular trend18 and multidecadal fluctuations. These findings have broad implications for constraining various geophysical and climatological processes and exploring their possible interactions.