Rising Sea Levels Shown To Slow Down Earth's Rotation

The Earth's rotation has slowed over the last few thousands years . Rob Byron/Shutterstock
Robin Andrews 11 Dec 2015, 21:35

With global temperatures increasing at a truly unprecedented rate, glaciers and ice sheets are melting, causing sea levels to begin to swallow up cities and islands across the world. A new study published today in Science Advances confirms that changing sea levels also affect the rotation of the Earth. By quantifying how potent this effect is, these researchers have possibly solved a longstanding conundrum called “Munk’s enigma.”

Walter Munk, an American physical oceanographer based at the Scripps Institution of Oceanography in La Jolla, California, wrote a paper in 2002 that highlighted a supposed discrepancy between historic sea level rises, the amount of glacial melting, and the rotation of the planet. The Earth’s axial spin will change slightly if the distribution of the planet’s mass begins to change, which includes the change caused by moving frozen water mass on land into the sea.

Munk pointed out that, although 20th century sea level rise should have made a change to the Earth’s rotation, there was no available observational data of this effect. A team of researchers – led by Jerry Mitrovica, a professor of geophysics at Harvard University – decided to investigate this mystery. Using a combination of calculations and computer modeling, they believe that they’ve found an answer to this riddle hidden in the last ice age.

During the beginning of that period, global sea levels dropped dramatically as the world’s water was locked up in glaciers and ice sheets across the world. At the end, the massive melting caused the sea levels to rise. This would have had a significant effect on the rotation of the Earth, and Munk took this into account in his 2002 paper.

Image credit: How have sea level changes affected the Earth’s rotation? digidreamgrafix/Shutterstock

This new study claims that his estimates of the effect of the last ice age were erroneous. In addition, the authors suggest that he used a flawed model of the internal structure of the planet, and that the 20th century glacial melting was 30 percent less severe than Munk assumed. Taking all these into account, the team then recalculated the effect that the rising sea levels would have had on the rotation of the Earth.

It is accepted knowledge that the Earth’s rotation has been slowing for at least the last 2,500 years. Although the authors show that this is partly explained by the interactions of the Earth’s molten outer core and its more mushy, rocky mantle, part of this slowdown may be due to the newly estimated rise in sea levels following the last ice age.

One way to measure how Earth’s rotation changes is to compare “universal time” (that measured by an atomic clock) and the appearance of eclipses. The difference between these two measures of time indicates how the Earth’s rotation may have changed – a difference known as “clock error.”

Since 500 B.C.E. the Earth’s rotation has had a clock error, or has “slowed,” by 16,000 seconds, about 4.5 hours. Of this, a loss of 6,000 seconds has been due to the changing sea levels. This is equivalent to a loss of 2.4 seconds per year. This study represents the first time that this loss matches with the rate of sea level rise since the last ice age – and Munk’s enigma has potentially been solved.

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