A simulation from NASA’s Virtual Earth System Laboratory shows just how much Earth’s spin axis is drifting and wobbling every year, in part because of climate change driven by humans.
The simulation shows the average speed of Earth’s polar motion, which is described as an imaginary line that runs vertically through the North and South poles. When our planet spins, it “drifts and wobbles” – similar to how a spinning top teeters. This movement wanders through the Earth’s crust, causing the polar motion to drift. NASA’s virtual simulation visualizes data from two separate studies conducted over the last two centuries. During the 20th century, the spin axis drifted 10 centimeters (4 inches) each year, which is more than 10 meters (33 feet) from 1900 to 2000. Between 2003 and 2015, scientists attributed this shift to a trifecta of geophysical sources, including the melting of the world’s ice sheets, glacial rebound, and mantle convection.
Before we get into the three causes, let us set the stage. Our planet isn’t a perfect sphere. Unlike the smooth globes we learned about in school, Earth has high mountains and deep ocean trenches that, when combined, distribute weight unevenly across the planet’s surface. As ice sheets melt and the Earth continues to build on itself, mass on Earth is redistributed in ways that change its rotation.
"We assembled models for a suite of processes that are thought to be important for driving the motion of the spin axis. We identified not one but three sets of processes that are crucial – and melting of the global cryosphere (especially Greenland) over the course of the 20th century is one of them," said first author Surendra Adhikari in a statement
As temperatures through the 20th century increased, Greenland’s ice sheet has decreased in mass by about 7,500 gigatons – or more than 20 million Empire State buildings in weight. While ice sheets globally have contributed to rising sea levels, the study notes that the Arctic nation is the greatest contributor over the last century. As ice sheets melt, sea levels do not rise evenly around the world because of a combination of our world’s characteristics, including tectonic shifts and the ever-changing seas above it.
But that’s just one part. A 23-year record of satellite data shows that glacial rebounds, which also contribute to a varying distribution of sea level rise, affects polar motion. As glaciers melt or move, the earth that was once depressed by them slowly begins to rise back to its original form – in much the same that after you press your hand into a mattress, it slowly rises back to its original form once pressure is relieved.
Lastly, the authors note that a movement of tectonic plates, known as mantle convection, also helps throw our planet off-center. This happens below Earth’s crust – as the mantle is heated from below, areas that are hotter rise upward while cooler areas sink, resulting in a horizontal movement of plates.
The authors say if climate change continues to accelerate Greenland’s melting ice sheet, we can expect to continue seeing polar motion picking up.