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Shannon Hall, Scientific American
Shannon Hall, Scientific American
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The north pole is on the run. Although it can drift as much as 10 meters across a century, sometimes returning to near its origin, it has recently taken a sharp turn to the east. Climate change is the likely culprit, yet scientists are debating how much melting ice or changing rain patterns affect the pole’s wanderlust.
The geographical poles—the north and south tips of the axis that the Earth spins around—wobble over time due to small variations in the sun’s and moon’s pulls, and potentially to motion in Earth’s core and mantle. But changes on the planet’s surface can alter the poles, too. They wobble with every season as the distribution of snow and rain change, and over long stretches as well.
Roughly 10,000 years ago, for example, Earth woke up from a deep freeze and the massive ice sheets sitting atop what is now Canada melted. As ice mass fled, and the depressed crust rebounded, the distribution of the planet’s mass changed and the north pole started to drift west. This pattern can be clearly seen in data from 1899 onward. But a recent zigzag in the north pole’s path (and the opposite movement in the south pole) suggests a new change is afoot.
Around 2000 the pole took an eastward turn; it stopped drifting toward Hudson Bay, Canada, and started drifting along the Greenwich meridian in the direction of London. In 2013, Jianli Chen, a geophysicist at The University of Texas at Austin, was the first to attribute the sudden change to accelerated melting of the Greenland Ice Sheet. The result startled his team.
“If you’re losing enough mass to change the orientation of the Earth—that’s a lot of mass,” says John Ries, Chen’s colleague at U.T. Austin. The team found that recent accelerated ice loss and associated sea level rise accounted for more than 90 percent of the latest polar shift. Of course that includes ice lost across the world, but “Greenland is the lion’s share of the mass loss,” Ries says. “That’s what’s causing the pole to change its nature.”
Could such a dramatic shift be so simple? In a new study published today in Science Advances, Surendra Adhikari and Erik Ivins, two geophysicists from NASA Jet Propulsion Laboratory, think another mechanism might be at play: changes in the amount of water held within the continents. Like Chen’s team, Adhikari and Ivins compared data collected by NASA’s Gravity Recovery and Climate Experiment (GRACE) satellite, which measures changes in Earth’s gravitational field, with Global Positioning System (GPS) measurements of the north and south poles. But Adhikari and Ivins have a couple extra years of data. They also incorporated small-scale features within the GRACE data set that are more directly related to terrestrial water storage.
Although the predominant cause of the pole’s shift still turned out to be Greenland, a recent dry spell that has overrun Eurasia is also driving the pole toward the east, Ivans says. With less rainfall on a continent over time, it starts to shed some bulkAdhikari and Ivins think the sudden shift could be the latest in a series of decadal changes in drift that scientists have been unable to explain. Eurasia, which was quite lush 10 years ago, is not the only continent to experience a drought. “We think this flip is happening all the time,” Ivins says. “It’s a natural phenomenon that characterizes the entire Earth rotation time-series going all the way back to 1899.”
The data do not indicate whether the recent climate changes are man-made, but Chen personally believes the drastic shift in the pole has to be the result of human activities. Meanwhile Ivins thinks he will be able to tease man-made climate change from the data in another six months or so.
Given that polar motion and climate variability seem to be inextricably linked, scientists can look at historical records of the pole’s motion (which date back to well before the advent of GPS and the GRACE satellite) and see changes in Earth’s climate. If those changes are less dramatic than the ones we see today, Ivins says, then scientists could say that global warming has a controlling influence on Earth’s poles.
This article is reproduced with permission from Scientific American. It was first published on April 8, 2016. Find the original story here.
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