GNSS vertical velocities show solid Earth becoming slightly less flattened

Christopher Kotsakis, of the Department of Geodesy and Surveying at Aristotle University of Thessaloniki, has published a geometric framework in the Journal of Geophysical Research: Solid Earth for converting global GNSS vertical velocity fields into changes in the solid Earth's figure.

Using ITRF2014 velocities, supplemented by selected polar stations from ITRF2020, the study covers 1997–2015 and a time-varying network of 886 stations. Polar uplift rises from about 0.5 mm/yr to nearly 1 mm/yr, while equatorial regions subside by a comparable amount. This implies the solid Earth's flattening is decreasing, with an acceleration of roughly 0.4 mm/yr per decade.

The geometric change is not fully mirrored in the gravity field, which has shown increasing oblateness. The paper links the difference to additional contemporary processes such as accelerating ice loss and ocean and hydrological mass redistribution. It also notes that uneven station distribution could bias the result.

Why it matters: The work shows GNSS velocity solutions can act as an independent check on gravity-based monitoring of mass change. It also depends on denser global station coverage and better reference frames, which matters for under-represented regions such as Africa.