In the 1990s, the Earth’s axis underwent a major shift. It is normal for the Earth’s axis to move by a few centimeters each year. But, in the 1990s, the direction of polar drift shifted suddenly and the rate of the drift accelerated. The reason for this sudden change was previously unclear, but a team of scientists in Beijing recently published a paper that shows that the main driver of the change in direction of the axial shift was glacier melt caused by global warming.
The Earth’s spin axis is the figurative line about which the Earth rotates. The poles, north and south, are situated at either end of the spin axis. By contrast, the magnetic poles — the ones you can find using a compass — are usually offset from the geographic poles, and their location shifts with the magnetic field.
“The Earth rotates around its axis somewhat like a spinning top,” explained Suxia Liu in an interview with GlacierHub. “If the weight from one area is moved to another area, the spinning top will start to lean, causing the rotation axis to change.” Liu co-led the paper with colleague Shanshan Deng at the Chinese Academy of Sciences.
Bernhard Steinberger, a researcher at the GFZ German Research Centers for Geosciences who did not work on this paper, explained how glaciers influence mass distribution. “The Earth always orients itself relative to the pole in a way to move masses as far as possible away from the pole,” he wrote in an interview with GlacierHub. “As an example, if there is a glacier growing on Greenland, the Earth’s orientation will change in such a way that Greenland is further away from the pole. If a glacier melts on Greenland, it would change in the opposite direction.”
Prior to human-caused forces, the primary drivers of polar drift were ocean currents and the movements of the molten rock deep below the Earth’s surface. The research team reanalyzed existing data to determine what role terrestrial water storage — how water is dispersed above Earth’s surface and in oceans and groundwater — played in the shift. They determined that the key driver of the directional change was glacier melt, and the change in mass distribution that arose from it.
Most of the world’s glaciers are above ground, and when these glaciers melt, the water that they contain moves into bodies of water. “Shifting water storage away from above-ground glaciers in one area on the Earth’s surface to another results in the polar shift due to the weight change,” said Liu.
In 1995, the direction of the planet’s polar drift abruptly shifted from southward to eastward. Today, scientists can connect polar drift to glacier loss using gravitational data from NASA’s twin Gravity Recovery and Climate Experiment (GRACE) satellites, which launched in 2002. However, since the satellites had not yet been launched in the 1990s, researchers had to piece together the reasons for the sudden directional shift without these detailed gravitational records.
Besides the greenhouse gasses emissions that cause glaciers to melt, other human activity is responsible for changes in Earth’s hydrosphere. The team also found that groundwater pumping was, and continues to be, a factor in polar drift. Groundwater pumping for drinking water, irrigation, and manufacturing has been common practice since the 1960s. The United States alone uses 82.3 billion gallons of groundwater each day. The water that is removed from below ground ends up in the atmosphere as it evaporates from irrigated crops or in the ocean as runoff from irrigation systems into rivers, redistributing mass around the world and altering the planet’s rotation.
A groundwater pumping station in California’s Central Valley. Source: Chris Austin/Flickr
As the Earth’s glaciers recede at unprecedented rates, the planet’s mass is being constantly redistributed. The team’s findings suggest that we can expect to see the hydrosphere continuing to cause the Earth’s axis to shift in coming years.
Though imperceptible to humans without the use of specialized instruments, the tremendous mass of the Earth has been shifting more than ever recorded. Since 2005, the rate of polar drift has increased by about four centimeters per year. “The shift is nothing that a regular person might notice in their day-to-day life,” said Steinberger. “One would really have to wait for millions of years in order to notice something.” It is, however, a stark reminder of the magnitude of humans’ effects on the planet.