Polar Ice Loss Makes Solid Earth Rounder While Oceans Flatten It

Earth’s rocky surface is becoming rounder, but the planet measured together with its oceans is becoming flatter. That apparent contradiction is the central finding of a study published Sept. 9 in JGR Solid Earth, which used satellite-navigation measurements to detect changes in the planet’s shape.

From 2011 through 2015, the solid poles rose and Earth’s rocky equatorial bulge shrank by about 1 millimeter per year, according to the analysis. The reported rate was roughly twice the 0.5 millimeter per year measured for the solid surface from 1997 through 2000. The shift is small on a human scale, but it represents a measurable deformation of the planet driven largely by polar ice loss and the redistribution of water.

Ice loss changes the load on Earth’s crust

Earth is naturally wider around the equator and flatter at the poles because of its rotation. Its equatorial diameter is about 7,926 miles, or 12,756 kilometers. The new result concerns changes layered onto that established shape, rather than Earth approaching a perfect sphere.

The mechanism is a planetary-scale loading change. When ice disappears from polar regions, it removes weight from the crust. The unloaded rock responds by rising slowly. Water and other redistributed mass moving toward lower latitudes exert additional pressure there, reducing the equatorial bulge in the solid planet.

Christoforos Kotsakis, a professor of geodesy at Aristotle University of Thessaloniki in Greece, developed a geometric method for extracting that global shape change from measurements made by global navigation satellite systems, or GNSS. These are the satellite constellations commonly associated with positioning and navigation, but fixed ground receivers also use their signals to measure tiny movements of Earth’s surface over time.

That measurement network presents a difficult geometry problem. GNSS stations are sparse near the North and South poles, while remote oceans offer few stable places to install receivers. Covering those areas directly would require stations on buoys. Kotsakis’ method was intended to recover the larger change in Earth’s figure despite those gaps.

The oceans reverse the result

The rounding applies only to solid Earth. Once the oceans are added to the calculation, water moving toward the equator builds a larger ocean bulge than the underlying rock loses. The combined solid and ocean planet is therefore becoming flatter at a reported rate of about 0.8 millimeter per year.

Both descriptions can be correct because they measure different physical boundaries. GNSS stations follow motion in the crust, revealing how the solid surface responds to changing loads. A planet-wide shape that includes the ocean surface also captures where the transferred water accumulates. In this case, the water’s contribution overwhelms the rounding of the rock below it.

The study also compared the geometric signal from GNSS with measurements of Earth’s gravity field. Moving ice and water alter both surface loading and gravity, but the two measurement approaches did not align perfectly. That mismatch reflects the fact that they observe mass redistribution through different physical signals and with different spatial coverage.

Used together, however, the methods can provide complementary views of changes such as Greenland and Antarctic ice loss and the movement of ocean water. GNSS records the crust’s geometric response, while gravity observations track changes associated with the shifting mass itself.

The measured motion is only around a millimeter annually, yet it is large enough for a global satellite-navigation network to resolve. Polar ice loss is no longer visible only as a change in glaciers or sea level: it is also lifting Earth’s solid poles, pressing down its rocky equatorial bulge and changing which answer is correct when scientists ask what shape the planet is becoming.

By David Whitaker — Associate editor for AMI’s aerospace and drone systems desk, translating flight systems, aircraft programs, spaceflight, and UAV developments into accessible technical stories.