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Scientists tracking melting ice in the Arctic have solved a geological puzzle that shows how quickly the resulting runoff is adding to sea levels and gradually slowing the Earth's spin.
Scientists have known for about 30 years that the additional water would flow toward the Equator from the poles, adding mass to the planet's middle and a bit of drag to its rotation. But new research published this week appears to resolve what's been known as "Munk's Enigma," a decade-old discrepancy in their calculations.
Answering that riddle has helped confirm scientists' recent estimates of how much the oceans rose during the 20th century — and suggests the process is accelerating faster today, Harvard geophysicist Jerry Mitrovica said.
"Sea level is rising differently from place to place, and people are trying to model those differences," Mitrovica said. "If the Greenland ice sheet melts, the sea level rise we'll see on the East Coast of North America is very different from what we'd see if the Antarctic ice sheet melts. Rotation is part of this much bigger problem."
The effect is to slightly reduce how fast the Earth spins, "like a figure skater spreading his or her arms away from the body," said Mitrovica, the lead author of the study that appeared in the peer-reviewed journal Science Advances.
But don't start making plans for what to do with that extra daylight just yet: It'll amount to about a thousandth of a second per day by the end of the century, he said.
"We're talking about really, really small effects," he said.
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Munk's Enigma is named for oceanographer Walter Munk, who theorized in 2002 that scientists could mathematically confirm the rate of sea-level rise in the 20th century from its effects on the planet's rotation. He tried to prove that by comparing modern observations to those of historical astronomers and even ancient records of events like eclipses, while factoring in the post-Ice Age retreat of the glaciers in the Northern Hemisphere. But he was left with "a really puzzling problem," Mitrovica said.
"Munk is saying it should affect rotation in known ways, but we don't see a record of it in observation," he said. But Mitrovica and his colleagues say they squared that contradiction by factoring in something that Munk didn't: The effect of the Earth's middle layer, or mantle, turning around its outer core of molten iron. That provides a slight kick backward, like a hamster kicking its wheel backwards as runs.
"It turns out that difference is precisely the difference that is produced by this core-mantle or hamster-wheel connection," Mitrovica said. And now the calculations square with what scientists now know about sea-level rise during the 1900s.
"This gives us much more confidence. If you can't get the 20th century right, how are you going to convince someone you can project to the end of the 21st century," he said.
And when he gives talks on rising oceans, Mitrovica said, "People don't care about the global average, they care about what's happening in their backyard. "We use these rotation changes in our calculations of seal-level change. They are one of the reasons sea level changes from place to place."
But as with a lot of climate science, there's a downside. At the time Munk was doing his calculations, scientists estimated the world's oceans rose an average of 2 millimeters a year over the previous century. While the new paper helps confirm that figure was more like 1 to 1.5 mm a year, it's currently about 3 to 3.5 mm.
"In the old way, we thought we were going from two to three. In our new understanding of the 20th century, we're going from one and a half to three. And what that means in the acceleration in sea-level rise is worse than we thought."
And with that cheery thought, enjoy your infinitesimally longer day.
Follow Matt Smith on Twitter: @mattsmithatl
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