Melting glaciers, a spoke in Earth’s wheel

So claims a new study. This is why

Updated: December 20, 2015 2:23 am

Glacier, polar wander, Melting glacier, Science Advances journal, Walter Munk, American oceanographer William Richard Peltier,

THE STUDY

Rising global temperatures are causing glaciers to melt at the poles, leading to an influx of large amounts of water into the ocean and resulting in a shift in the “weight” of the planet. This meltdown from higher altitudes is moving towards the Equator, swelling up the water bodies in the region. According to a new study by a group of researchers from Harvard University, the shrinking of glaciers, and the resultant “slight shift” of the North Pole — a phenomenon known as “polar wander” — is “causing the Earth to slightly wobble as it spins on its axis”.
The study, published in the Science Advances journal, also claims that the shift in Earth’s weight has slowed down its rotation by 1.7 milliseconds a century — similar to how figure skaters begin to slow down when they reach their arms out away from their body. This has also increased the duration of a day by about a thousandth of a second over the 20th century.

THE BREAKTHROUGH

The research examined the changes in Earth’s rotation in the context of rising sea-levels in the 20th century. The researchers used a “new model and cross-referenced rotation information with ancient astronomical observations”. The results showed that the increase in sea-level was about 1-1.5 millimeters per year. They also factored in the viscous nature of Earth’s interior and the motion in the planet’s core. This showed a co-relation between rotation changes and estimated sea-level rise and led the scientists to conclude that Earth’s rotation had slowed down, though very little. But if the melting continues, the effect can be more drastic, the researchers said.

MUNK’S ENIGMA

In a study in 2002, Walter Munk, an American oceanographer based at the Scripps Institution of Oceanography in La Jolla, California, found a “discrepancy between historic sea level rises, the amount of glacial melting, and the rotation of the planet”.

In his study, Munk pointed out that “although 20th century sea-level rise should have brought a change in Earth’s rotation, there was no available observational data to this effect”. This inconclusive result was referred to as “Munk’s Enigma” and the exact relation between melting glaciers and Earth’s rotation remained unknown till this latest study put a number to the slow-down.

WHAT MUNK MISSED

Munk’s study was derailed because of a few incorrect assumptions.

# New measurements show that glacier melting is about 30 per cent less severe than what Munk assumed.

# Munk had overestimated the average increase in sea level ( 2mm).

# He ignored the internal structure of Earth – which is viscous in form. This means that the rocks that make up the planet tend to “flow and reshape themselves over long periods of time”. This factor too affects the rotation of Earth.

# Interaction between Earth’s rocky mantle (layer between the crust and the outer core) and the planet’s molten metal outer core was discounted.

THE LOOPHOLES

William Richard Peltier, a professor of physics at the University of Toronto, thinks that while the new paper makes “some interesting and important points about the rotational relationships between the mantle and core”, he says that the viscosity data of the Earth’s interior used by the Harvard University researchers is not accurate and and is “not reflective of the way the Earth’s core actually behaves”.
The new research, he says, is a “nice try, but no cigar”.

(Source: Science Advances)

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