The North Pole suddenly changed direction and started drifting east around the year 2000. Now scientists have uncovered the reason — and found that humans are to blame.
The geographic North Pole, located in the middle of the Arctic Ocean, is the northern end of Earth's rotational axis. Its location, and that of the South Pole with it, affects GPS measurements, along with results from Earth-observing satellites and ground-based observatories, so keeping track of the pole is important to scientists.
They have been tracking its exact location for 115 years and found that it has drifted as much as 12 metres over that time. Generally, it had been moving in the direction of Canada about seven or eight centimetres a year.
That's because when you remove some mass from a rotating object, the axis tends to move in the direction where you lost the mass. And North America has been losing mass since its ice sheets melted at the end of the last ice age.
No longer headed for Canada
But around the year 2000, the North Pole suddenly changed direction.
"It's no longer moving toward Hudson Bay, but instead toward the British Isles," said Surendra Adhikari, a post-doctoral researcher at NASA's Jet Propulsion Laboratory in Pasadena, Calif., in a news release. "That's a massive swing."
It's also now drifting almost 17 centimetres a year — about twice as fast as it was before.
Adhikari and Erik Ivins, a senior research scientist at the laboratory, decided to figure out why.
In order to do that, they used data from the Gravity Recovery and Climate Experiment (GRACE) satellites. The twin satellites, launched by NASA and the German Aerospace Agency in 2002, remap the Earth's gravitational field and the distribution of its mass each month. The Earth's mass and gravity get redistributed with large movements of water such as snow and ice melting or accumulating on different parts of the Earth.
Adhikari and Ivins found that the North Pole was being pulled by melting ice sheets and reduced mass in Greenland, ice mass loss from west Antarctica, and ice mass gain in east Antarctica. That explained about 70 per cent of the movement, but didn't explain why the North Pole was heading east.
Looking east of Greenland, they realized that the additional mass loss in that direction was caused by the depletion of aquifers and drought in the Indian subcontinent and the region around the Caspian Sea. By adding that in, they could explain nearly all of the North Pole's movement and its direction.
While the amount of mass loss was much smaller than that caused by the melting ice sheets, it was happening at around 45 degrees north — an area that has a very strong effect on the spin axis.
Wobbling pole mystery solved
The researchers realized this might also solve a long-standing mystery — why the North Pole's location wobbles east and west about 0.5 to 1.5 metres every six to 14 years. When they looked at the satellite measurements, they found that an eastward wobble corresponded to wet periods in Eurasia and a westward wobble corresponded to dry periods.
The researchers wrote in the journal Science Advances that the findings suggest that the motion of the North Pole may "offer yet unexploited information about the intensities, duration and globality of wet and dry periods" over the past 115 years.
That kind of knowledge could help improve climate models, they added, and "will have important ramifications for climate change during the 21st century, as we now face an increased intensity of the global water cycle."
Jonathan Overpeck, professor of geosciences at the University of Arizona, who wasn't part of the study, told The Associated Press that the discovery humans are making the North Pole move "highlights how real and profoundly large an impact humans are having on the planet."
Jianli Chen, a senior research scientist at the University of Texas Center for Space Research, first attributed the pole shift to climate change in 2013, and he said this new study takes his work a step further.
"There is nothing to worry about," said Chen, who wasn't part of the NASA study. "It is just another interesting effect of climate change."
Scientists have previously found that humans aren't just making the Earth spin on a slightly different axis — they're also making it spin more slowly.