Secular Polar Motion from Non-Surface-Loading Processes Estimated Using Satellite Gravimetry

Summary Polar motion (PM) provides an independent, observation-based constraint on global mass redistribution, but interpretation of its long-term variability is primarily limited by uncertainty in the glacial isostatic adjustment (GIA) contribution, for which conventional model-based estimates exhibit substantial differences. Here we develop a refined approach that uses primarily GRACE and GRACE Follow-On (GRACE-FO) observations to estimate the secular PM associated with non-surface-loading processes, predominantly GIA. We restore the pole tide contribution removed during standard GRACE processing to reconstruct the total degree-2, order-1 spherical harmonic coefficients and the corresponding PM excitation, then isolate the non-surface-loading component as the difference between this excitation and the GRACE/GRACE-FO-derived excitation from contemporary surface loads and pole tides. This strategy avoids uncertainties associated with atmospheric winds, ocean currents, and core–mantle processes present in geodetic PM records. The resulting secular trends are 1.77 mas/yr in χ1 and − 2.49 mas/yr in χ2, differing markedly from commonly used GIA model predictions (e.g. 1.01 and − 4.67 mas/yr from Peltier et al., 2018). We validate our estimate by reconstructing the PM excitation for 1992–2016 using a comprehensive set of cryospheric, hydrologic, atmospheric, and oceanic contributions together with our inferred secular term; the reconstruction agrees closely with geodetic observations. Compared with the previous PM-based approach of Seo et al. (2021), our method yields a substantially smaller 95 per cent confidence region, providing an improved characterization of the long-term secular contribution to PM and a stronger foundation for using multi-decadal PM observations to diagnose twentieth-century ice mass and terrestrial water storage changes.