The shape of our planet is changing slowly, and two ways of measuring it now show apparently opposite trends.
Earth is not a perfect sphere. Its rotation makes it slightly flattened at the poles and wider around the equator. This difference is tiny at our scale, but it can change when the surface deforms or when large quantities of matter move. A new study tracked these changes using measurements made by satellites.
Model of Earth’s gravitational field obtained from data collected by the GRACE mission. The colors and relief represent local variations in gravity.
Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio
Researcher Christos Kotsakis analyzed the vertical movements recorded by GNSS stations distributed across the globe. The principle is simple: these stations make it possible to determine whether the ground is rising or sinking over the years. The data studied cover the period from 1997 to 2015 and come from international networks used to measure Earth precisely.
The result shows a gradual change. In polar regions, the average uplift increases from about 0.5 to 1 mm per year. Near the equator, the movement goes in the opposite direction, with subsidence of a comparable order of magnitude. Taken together, these movements make Earth slightly less flattened over time.
To understand the origin of these movements, we must look in particular at ice. When a large mass of ice disappears, the weight exerted on the ground decreases, allowing it to rise. The water produced by this melting is then redistributed across the oceans. In addition, the ground is still slowly readjusting after the disappearance of the ancient ice sheets.
Yet measurements of the gravitational field do not tell exactly the same story. They depend not only on Earth’s shape, but also on the distribution of masses. Water moved from polar regions toward lower latitudes can therefore alter gravity without producing the same geometric change. The shape of the ground and the shape inferred from gravity can thus evolve differently.
This apparent disagreement is therefore not a contradiction between two instruments. GNSS stations directly measure the movement of the solid surface, the ground on which they are anchored. Gravitational observations, meanwhile, are sensitive to matter, whether rock, ice, or water. The two methods observe two different consequences of the same changing planet.
The measurements nevertheless remain imperfect. GNSS stations are numerous in Europe, North America, and certain regions of Asia, but much rarer at the poles and over the oceans. It is therefore necessary to estimate what happens between them. The study tests the errors associated with this uneven distribution before interpreting the global signal.
Future networks with better coverage should make it possible to track this evolution more precisely. More accurate vertical measurements will also help separate the effects of current melting, continental water, and the slow readjustment of the ground inherited from ancient glaciations.
