And given that much of this is related to the loss of polar ice, a changing climate would appear to be at least partly —although perhaps not wholly — responsible. “If we lose mass from the Greenland ice sheet, we are essentially putting mass elsewhere. And as we redistribute the mass, the spin axis tends to find a new direction. And that’s what we mean by polar motion,” said Surendra Adhikari, a researcher with Caltech and NASA’s Jet Propulsion Laboratory who conducted the work with his colleague Erik Ivins. The new research appeared Friday in Science Advances.
Ivins emphasizes that the study doesn’t explicitly attribute the motion of the pole to human-caused climate change — noting that “the word human is not in this paper.” The study simply wasn’t aimed at identifying any causes of mass loss — it merely observed these losses using NASA’s twin GRACE satellites, which measure mass and gravitational changes at the Earth’s surface, and tied them to the resulting polar motion.
At the same time, however, much research has suggested that the warming of the Earth is behind considerable loss of polar ice mass, not only in Greenland and Antarctica, but also smaller glaciers around the world. NASA research itself, for instance, finds that Greenland is losing 287 billion tons of ice per year, while Antarctica is losing 134 billion tons.
The Earth’s spin axis or axis of rotation — which is simultaneously imaginary, and yet nonetheless key to understanding and visualizing the nature of the planet’s movement — has never stayed precisely in place. Movement of material within the planet’s insides, for instance, causes it to shift subtly. So does mass change at the Earth’s surface, which can come from shifts in ice sheets, or even possibly in major atmospheric wind currents. Scientists have tracked such movements for 115 years, and up until the year 2000, the North Pole was moving slowly towards Canada (with corresponding motion of the South Pole).
Since then, however, the new study finds that the motion shifted sharply and now the North Pole is moving towards the U.K. and Europe. The motion has also sped up, though it still isn’t very large. The movement towards Canada was at around 7 to 8 centimeters per year, Adhikari said, and the movement towards the U.K. is now about 16 to 18 centimeters per year.
The study authors provided this graphic to illustrate their findings on the North Pole’s motion:
Why does the pole move? The answer is that if you redistribute mass on a rotating body like the Earth, you also subtly change how it spins. This is called the conservation of angular momentum, and is often illustrated with the analogy of a twirling figure skater whose rate of spin changes when he or she extends or retracts arms or a leg.
“Imagine we have a perfect sphere that is rotating about some spin axis,” says Adhikari. “And if you remove some material from any location … the pole must be heading toward the direction where you lose the mass. For example, just imagine that Greenland is the only region where we lose mass. Then in principle, the pole must be moving toward Greenland. That’s the kind of law of nature in a way.”
Indeed, previously scientists thought that the loss of ice mass from Greenland and Antarctica alone was driving a polar shift. The new study says they’re still playing a huge role in it, but there’s a third factor as well, one that this study is new in identifying — the continents, and how much water they contain. “In some locations we are storing more water, maybe through extra precipitation, and in some regions, we are losing more water,” Adhikari said. “And this pattern is contributing substantially to this shift in the general direction of the polar motion.”
Other recent research has also tied the loss of polar ice to subtle changes in the Earth’s rotation, suggesting that these losses can slow the planet’s spin, in addition to shifting the location of the pole itself.
Ivins believes that with future research, it may be possible to actually use this same polar motion approach to determine whether from 1899 to the present, there was ever any past mass deviation from Greenland that’s at all comparable to what’s currently happening to the ice sheet.
“We think we now have identified a new tool, and that’s the polar motion data, to be able to put bounds on how large the natural variability may be in terms of ice mass….and therefore to be able to make a stronger statement on how unique the current and apparently global warming related ice mass loss is” for Greenland, he says.
While there may be a good case for attributing Greenland’s or Antarctica’s ice loss to human induced climate change — even though the paper itself does not do so — the changes in continental water storage, which also play an important role in the result, are tougher to attribute.
These land water storage changes could be related to changes in the frequency or intensity of precipitation or droughts — which, itself, could be tied to human-induced climate change — but they could also indicate places where humans are simply pulling much more water out of the land for their use, through withdrawals from rivers, lakes or groundwater.
Adhikari estimated that about 40 percent of the Earth’s polar movement is due to Greenland, 25 percent to Antarctica, and 25 percent to changes in water storage on continents.
The study also contained a second major finding — in addition to the overall polar motion trend in the direction of the U.K., there is a smaller polar wobble back and forth, over a period of around a decade, that the paper suggests is also due to changes in how much water is stored on the continents and where it is stored. Here’s an image capturing these shorter and smaller scale swings, alongside images of where mass was being gained, and lost, around the world:
The research is, above all, a reminder of the enormously powerful geophysical forces in the background of our lives. And if you agree that human greenhouse gas emissions are behind the melting of Greenland and Antarctica, then, well, there’s another conclusion: We humans have become a geophysical force.
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