Update on the Oblate Spheroid

A correspondent sent me a link to THIS MAGAZINE ARTICLE. This, in turn, is based on THIS ACADEMIC PAPER. The articles are concerned with the shape of the Earth and the changes which are taking place to it.

When I went to University I was much interested in this and was amused to find that the north to south flowing Mississippi had its source closer to the centre of the Earth than its mouth, so it was flowing uphill!

But the author of the scientific paper has gone rather further into the problem than I did and has found that global warming is the cause - and it is accelerating. Ice has been removed from the poles so the Earths surface has risen. Meanwhile the melt water has moved to lower latitudes and depressed the Earth's surface. The rocky spheroid is becoming less oblate.

But the affect on the Earth's gravity is the opposite. Moving mass (water) to the equator makes the gravitational geoid more oblate!

I found this difficult to comprehend so used Google Gemini to summarise the academic paper - (this is long and very dense) - and it came up with this:-

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Summary of Mapping Global GNSS Vertical Velocities to Solid Earth Figure Change (Kotsakis, 2026)

A 2026 study published in the Journal of Geophysical Research: Solid Earth reveals that the physical shape of our planet is changing at an accelerating pace.

Earth is not a perfect sphere; it bulges slightly at the equator due to its rotation. However, using precise Global Navigation Satellite System (GNSS) tracking stations, geologist Christopher Kotsakis analyzed movement across Earth’s crust between 1997 and 2015. The findings show a clear global pattern: the polar landmasses are rising (uplift), while regions near the equator are sinking (subsidence). This ongoing movement means the solid surface of the Earth is gradually becoming slightly rounder and less flattened at the poles.

Between 1997 and 2000, polar regions were rising at a rate of roughly 0.5 millimeters per year. By 2015, that speed had doubled to about 1 millimeter per year, demonstrating an overall acceleration in surface deformation.

The primary driver behind this shift is the redistribution of water mass across the planet, largely fueled by climate change. As ice sheets and glaciers in places like Greenland and Antarctica melt, the immense weight pressing down on the polar crust lightens. Consequently, the rocky ground underneath springs back up (elastic rebound).

At the same time, all that melted ice flows into the global ocean network, adding heavy water weight to lower latitudes and causing the seafloor and equatorial regions to sink under the new load.

Interestingly, this creates a subtle paradox between Earth's physical ground and its gravitational field. While the rocky crust itself is becoming slightly rounder, the mass of water collecting around lower latitudes causes the planet's gravitational field to remain flattened. Overall, the study highlights how dynamic Earth's solid surface is in response to shifting climate and ocean dynamics.

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