Friday, December 8

The Earth’s magnetic poles (probably) aren’t about to flip, scientists say

The Earth’s geomagnetic field, which scientists have been warning about for hundreds of years, isn’t about to suddenly flip over after all, according to a new study.

It now looks like the magnetic North Pole will remain in the north and the magnetic South Pole will stay in the south — at least for a few thousand years or so.

“In the geological time perspective we are currently in a period of very strong geomagnetic field,” geoscientist Andreas Nilsson of Sweden’s Lund University said in an email. “So there is a long way to go before a polarity reversal.”

Nilsson is the lead author of research published this month by the National Academy of Sciences that studied a large weakness in the geomagnetic field known as the South Atlantic Anomaly, or SAA.

The study notes that the Earth’s magnetic field has been getting steadily weaker since the first geomagnetic observatories were established in the 1840s, while the SAA weakness has grown larger over that time.

That’s led some scientists to theorize that the geomagnetic field is decreasing in strength just before it completely reverses direction — something it has done several times in the past, according to layers of rock laid down over millions of years that show previous reversals.

But the new research has found that large geomagnetic anomalies have happened before, and relatively recently in geological time, without causing a field reversal.

These anomalies typically fade away a few hundred years later — and there’s no sign that the SAA will be any different, Nilsson said.

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Nilsson and his colleagues studied how the Earth’s magnetic field has changed over the last 9,000 years by looking at the iron in volcanic rocks, ocean sediments and in some cases burned archaeological artifacts.

Those include clay pots fired in ancient kilns thousands of years ago, which sometimes contain small amounts of an iron ore called magnetite. The magnetite lost its alignment when it was heated in the firing process, and the grains became magnetized again by the geomagnetic field when they cooled down, resulting in a record of the field’s strength, Nilsson said.

The study shows that the current state of the Earth’s magnetic field is similar to that of about 600 BC, when it was dominated by two large weaknesses over the Pacific Ocean.

The anomalies over the Pacific, however, faded away over the subsequent 1,000 years, and it’s likely that the SAA will as well, Nilsson said — probably in about 300 years, leaving a stronger and more even geomagnetic field.

A geomagnetic field reversal probably wouldn’t be catastrophic, but it would definitely be inconvenient.

Scientists think the field is generated by the flow of molten iron at the Earth’s core, about 1,800 miles below the surface. It acts as a shield against deadly solar radiation, and it also makes magnetic compasses work.

Geological studies have shown that the geomagnetic field has reversed 10 times in the last 2.6 million years alone. The last time was about 780,000 years ago — an event known as the Brunhes-Matuyama reversal.

But although the process is linked to movements in the molten core, it isn’t well understood — and scientists aren’t sure when the next reversal will occur.

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“The Earth’s magnetic field reverses on average every 300 to 400 thousand years,” explained Adrian Muxworthy, a professor of Earth and planetary magnetism at Imperial College London who wasn’t involved in the study. “But it’s chaotic. It isn’t regular. There have been periods where it hasn’t reversed for up to 30 million years, but we’re kind of due one.”

The geological records of previous reversals show it can take 500 to 2,000 years for the Earth’s magnetic field to completely reverse by growing gradually weaker in the prevailing direction and gradually stronger in the opposite direction, he said.

Muxworthy notes that while modern navigation systems, such as the Global Positioning System (GPS), now rely on orbiting satellites, the navigational satellites themselves still rely on the geomagnetic field for their alignments.

It’s also likely that satellites in low orbits that are currently projected by the Earth’s magnetic field could be damaged by greater amounts of solar radiation during a field reversal, although they could be protected by making them heavier, he said.

At its weakest, the geomagnetic field would be about 20 percent of what it is now, which would result for a time in increased solar radiation at the surface, although probably not enough to affect life there, he said.

One curious side-effect of a complete field reversal, however, would be that the spectacular auroras that now occur mainly above the poles would happen all over the globe.

“It would actually be quite exciting,” Muxworthy said. “Just as we now get the Northern and Southern Lights, we’d see them at all latitudes, including over the equator.”

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Nilsson cautions that while his study of the South Atlantic Anomaly suggests it will fade away without trouble in a few hundred years, there’s still a possibility that the Earth’s magnetic field will start to reverse anyway, although scientists see no sign that it will.

But “we can definitely be wrong,” he said.

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