Photo: NASA
Earth is not a perfect sphere. Its rotation produces an equatorial bulge and makes the planet slightly flattened at the poles. New research suggests that the solid Earth is gradually becoming less flattened, with polar regions rising faster while areas near the equator are sinking, ScienceAlert reports.
The study, led by geodesist Christopher Kotsakis of Aristotle University of Thessaloniki, found that the rate of uplift at the poles roughly doubled during the period examined. The findings were published in the Journal of Geophysical Research: Solid Earth on September 9, 2026.
“Our results indicate an acceleration of polar uplift accompanied by a corresponding increase in the rate of equatorial subsidence. This means that the overall figure of the solid Earth is becoming slightly less flattened,” Kotsakis wrote.
Why is Earth changing shape?
The ground beneath us may seem completely stable, but Earth’s crust responds continuously to changes in the distribution of mass on its surface.
When a large amount of mass is added to a region, the crust can sink under the additional load. When that mass is removed, the crust gradually rebounds upward, much like a compressed spring.
One example is glacial isostatic adjustment. During the last Ice Age, which ended roughly 11,000 years ago, enormous ice sheets covered parts of the Northern Hemisphere. Although the ice has long disappeared, the crust in those regions continues to rise in response to the loss of that weight.
Modern ice loss is now contributing to the process as well. Greenland and Antarctica are losing large amounts of ice, reducing the load on the crust beneath them and allowing the underlying rock to rise. At the same time, the meltwater flows into the oceans, redistributing mass toward lower latitudes and adding weight to other parts of the planet.
What the scientists measured
Scientists have been monitoring changes in Earth’s large-scale shape since the 1970s through measurements of the geoid — a gravity-defined reference surface that reflects how mass is distributed throughout the planet.
The geoid and the physical surface of the solid Earth are not exactly the same thing.
Kotsakis wanted to determine whether the rocky part of Earth was changing in the same way as the gravity-defined geoid. To do this, his team analyzed data from a global network of Global Navigation Satellite System (GNSS) stations, which can detect extremely small vertical movements of Earth’s surface. The researchers examined observations collected between 1997 and 2015.
The results showed a clear change in the vertical motion of the planet’s surface.
Between 1997 and 2000, the poles were rising at an average rate of about 0.5 millimeters per year. By the 2011–2015 period, the rate had increased to nearly 1 millimeter per year. At the same time, the equatorial regions showed increasing subsidence of a similar order of magnitude.
In other words, the solid Earth was becoming progressively less flattened.
But the geoid is becoming more flattened
The researchers found an apparent contradiction: while the solid Earth is becoming less oblate, the geoid is becoming more flattened.
There is a physical explanation for this difference.
As polar ice melts, the solid crust underneath the former ice sheets rebounds upward. However, the water released by melting does not remain at the poles. It spreads through the oceans and shifts mass toward lower latitudes.
The solid Earth responds primarily to the changing surface load, while the geoid responds directly to the redistribution of mass and its gravitational effect.
As a result, the two measurements can move in opposite directions at the same time: the rocky surface becomes slightly less flattened, while the gravity-defined geoid becomes more flattened. The study finds that this interpretation is consistent with observed changes in the Earth's gravitational field.
The change is tiny but measurable
The changes are extremely small — on the order of millimeters per year — and cannot be noticed by people or seen in ordinary photographs of Earth from space.
Nevertheless, modern geodetic instruments are sensitive enough to detect such movements over long periods.
The study estimated an acceleration of roughly 0.25 millimeters per year per decade in both polar uplift and equatorial subsidence. The authors say the size of the signal is greater than would be expected from glacial isostatic adjustment alone, suggesting that ongoing loss of ice mass and its redistribution are contributing to the observed changes.
The findings illustrate how the melting of polar ice can affect not only sea levels and ocean circulation but also the physical shape and gravitational field of the entire planet.
