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Earth's Magnetic Field Is Fading

John Roach
for National Geographic News
September 9, 2004
 
Earth's magnetic field is fading. Today it is about 10 percent weaker than it was when German mathematician Carl Friedrich Gauss started keeping tabs on it in 1845, scientists say.

If the trend continues, the field may collapse altogether and then reverse. Compasses would point south instead of north.

Not surprisingly, Hollywood has already seized on this new twist in the natural-disaster genre. Last year Tinseltown released The Core, a film in which the collapse of Earth's magnetic field leads to massive electrical storms, blasts of solar radiation, and birds incapable of navigation.



Entertainment value aside, the portrayal wasn't accurate, according to scientists who say the phenomenon of Earth's fading magnetic field is no cause to worry.

"The field has reversed many times in the past, and life didn't stop," said Gary Glatzmaier, an earth scientist and magnetic field expert at the University of California, Santa Cruz.

Glatzmaier is keeping an eye on our planet's weakening magnetic field as he tries to learn more about how Earth's geodynamo works. The geodynamo is the mechanism that creates our planet's magnetic field, maintains it, and causes it to reverse.

Magnetic Shield

Earth's geodynamo creates a magnetic field that shields most of the habited parts of our planet from charged particles that come mostly from the sun. The field deflects the speeding particles toward Earth's Poles.

Without our planet's magnetic field, Earth would be subjected to more cosmic radiation. The increase could knock out power grids, scramble the communications systems on spacecraft, temporarily widen atmospheric ozone holes, and generate more aurora activity.

A number of Earth's creatures, including some birds, turtles, and bees, rely on Earth's magnetic field to navigate. The field is in constant flux, scientists say. But even without it, life on Earth will continue, researchers say.

"There are small fluctuations, which lead to nothing, and large ones, which we know from the geologic record are associated with reversals," said Peter Olson, a geophysicist at Johns Hopkins University in Baltimore, Maryland.

When molten lava erupts onto the Earth's crust and hardens, it preserves a snapshot of Earth's polarity, much in the way that iron filings on a piece of cardboard align themselves to the field of a magnet held beneath it.

According to Earth's geologic record, our planet's magnetic field flips, on average, about once every 200,000 years. The time between reversals varies widely, however. The last time Earth's magnetic field flipped was about 780,000 years ago.

"We hear the magnetic field today looks like it is decreasing and might reverse. What we don't hear is it is on a time scale of thousands of years," Glatzmaier said. "It's nothing we'll experience in our lifetime."

But several generations from now, humans just may witness a reversal. By then, Glatzmaier said, scientists will better understand the process and be prepared to cope with the effects.

Geodynamo

Scientists believe the magnetic field is generated deep inside the Earth where the heat of the planet's solid inner core churns a liquid outer core of iron and nickel.

The solid inner core is thought to be a mass of iron about the size of the moon that is heated to several thousand degrees Fahrenheit. Heat radiated by this inner core builds up at its boundary with Earth's liquid outer core, causing the fluid there to expand.

"When it expands it becomes a little less dense [and more] buoyant. So it starts to rise. That's convection," Glatzmaier said. "Hot fluid rises, then cools off and sinks again."

The convection generates an electric current and, as a result, a magnetic field.

Additional currents are created as Earth cools. Some of the molten iron solidifies onto the inner core, releasing lighter material in the process. The rotation of the Earth also generates forces that curve the flow of fluid as it rises, twisting the magnetic field.

All of these currents constantly replenish the magnetic field, a maintenance process that prevents it from decaying.

Typically each newly generated field lines up in the direction of the existing magnetic field. But every now and again, some force will cause the new field to line up in the opposite direction. This process can lead to a net weakening of Earth's magnetic field.

Over time a new field can continue to grow. This further weakens the original magnetic field. If the process continues, the two fields would eventually cancel each other out. Earth's magnetic field would collapse and then, maybe, flip.

"But more likely than not what will happen is the original [field] will get stronger again and overwhelm the instability," Glatzmaier said.

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