NASA | JPL-Caltech | 2018 Sep 19
A typical desk globe is designed to be a geometric sphere and to rotate smoothly when you spin it. Our actual planet is far less perfect -- in both shape and in rotation.
Earth is not a perfect sphere. When it rotates on its spin axis -- an imaginary line that passes through the North and South Poles -- it drifts and wobbles. These spin-axis movements are scientifically referred to as "polar motion." Measurements for the 20th century show that the spin axis drifted about 4 inches (10 centimeters) per year. Over the course of a century, that becomes more than 11 yards (10 meters).
Using observational and model-based data spanning the entire 20th century, NASA scientists have for the first time identified three broadly-categorized processes responsible for this drift -- contemporary mass loss primarily in Greenland, glacial rebound, and mantle convection. ...
With these three broad contributors identified, scientists can distinguish mass changes and polar motion caused by long-term Earth processes over which we have little control from those caused by climate change. ...
What drives 20th century polar motion? ~ Surendra Adhikari et al
- Earth and Planetary Science Letters 502:126 (15 Nov 2018) DOI: 10.1016/j.epsl.2018.08.059