We’re all familiar with the basic properties of Earth’s rotation. It revolves completely about every 24 hours, and we call that a day. You may not be aware that the earth also wobbles slightly, causing the orientation of the axis to shift by a few inches per year. NASA says it has now pinned down three causes for this shift. As expected, the loss of polar ice is a major contributor.
Over the course of the 20th century, scientists watched in fascination as the Earth’s axis swung back and forth by as much as several meters. On average, it drifts about 4 inches (10 centimeters). Sometimes it was less, and sometimes more. In the year 2000, the axis shifted a whopping 7 inches. During the 20th century, the axis migrated approximately 32 feet (10 meters) in total. But why?
NASA used observational and model-based data from past decades to determine once and for all what factors affect the planet’s wobble. It landed on three main causes: loss of ice mass, glacial rebound, and mantle convection.
The loss of ice is easy to visualize. As the ice caps melt, mass is transferred from the polar regions into the ocean. NASA says Greenland is the primary culprit here. It’s somewhat farther south, so it has lost a lot of ice mass over the past century — about 7,500 gigatons. That’s roughly the same as 20 million Empire State Buildings. This causes about one-third of the wobble.
The observed direction of polar motion, shown as a light blue line, compared with the sum (pink line) of the influence of Greenland ice loss (blue), postglacial rebound (yellow) and deep mantle convection (red). The contribution of mantle convection is highly uncertain.
Glacial rebound is related to the loss of ice, but the mechanism by which it affects Earth’s wobble is different. As glaciers recede, they reveal long-compressed ground that’s suddenly unencumbered by ice. That ground slowly rises back up to its natural level, and that changes the distribution of mass and affects the wobble. NASA says this is also responsible for about one-third of the wobble.
The last third comes courtesy of mantle convection. That’s what causes tectonic plates to shift over the course of eons. As molten rock shifts around in the mantle, it changes the planet’s balance slightly, adding to the axial shift. So, this part of the wobble is decidedly not the doing of humanity.
With the causes identified, scientists can track polar motion and understand which changes are due to human activity and which are a consequence of the nature of Earth. We can also project how the wobble will change in the future based on increased warming and loss of polar ice.
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