Future — Magnetic Pole Reversal

Background

The evidence of major variations in the geomagnetic field derives from sedimentary and volcanic lava records. These records provide the timing and location of the magnetic pole from the angle of dip and the horizontal orientation of magnetite material in the sediments and lava flows. Unfortunately, sedimentary records are limited in that they tell us more about the pre- and post-geomagnetic records than about the actual reversal process itself. Lava records, on the other hand, being laid down in a matter of days (instead of thousands of years in the case of sediments) provide much more detail in the description of events in a geomagnetic pole reversal.

Evidence of reversals in the geomagnetic field indicates 200 reversals over the last 178 million years. This includes a 35-million year hiatus during the Cretaceous period, and 33 reversals in the last 25 million years. Over the last 70 million years, "polarity reversals have been taking place at an increasing rate". In fact, there have been 9 major reversals in the past 3.6 million years (i.e. on average one every 400,000 years), with the most recent one, 730,000 years ago. Even more noteworthy is evidence of a looping excursion by the geomagnetic field some 28,000 years ago.


Geomagnetic polarity during the late Cenozoic Era. Dark areas denote periods where the polarity matches today's polarity, light areas denote periods where that polarity is reversed. (view large)

How long does the process take?

It has been assumed by paleomagnetists that geomagnetic pole reversals do not occur instantaneously from one polarity state to the other, but occur over a period that typically spans a few thousand years. However, evidence from lava flows indicate rapid shifts, perhaps taking only a few years, or "fast enough that it would virtually be possible to watch a compass needle move". Data from the Steens Mountain lava beds in southeastern Oregon, for example, indicate that the geomagnetic poles were moving between 3 degrees and 8 degrees per day!

Earth's Magnetic Field

Earth's magnetic field — the force that protects us from deadly radiation bursts from outer space — is weakening dramatically.

Scientists have discovered that its strength has dropped precipitously over the past two centuries and could disappear over the next 1,000 years.

The effects could be catastrophic. Powerful radiation bursts, which normally never touch the atmosphere, would heat up its upper layers, triggering climatic disruption. Navigation and communication satellites, Earth's eyes and ears, would be destroyed and migrating animals left unable to navigate.

'Earth's magnetic field has disappeared many times before — as a prelude to our magnetic poles flipping over, when north becomes south and vice versa,' said Dr Alan Thomson of the British Geological Survey in Edinburgh.

'Reversals happen every 250,000 years or so, and as there has not been one for almost a million years, we are due one soon.'

For more than 100 years, scientists have noted the strength of Earth's magnetic field has been declining, but have disagreed about interpretations. Some said its drop was a precursor to reversal, others argued it merely indicated some temporary variation in field strength has been occurring.

But now Gauthier Hulot of the Paris Geophysical Institute has discovered Earth's magnetic field seems to be disappearing most alarmingly near the poles, a clear sign that a flip may soon take place.

Using satellite measurements of field variations over the past 20 years, Hulot plotted the currents of molten iron that generate Earth's magnetism deep underground and spotted huge whorls near the poles.

Hulot believes these vortices rotate in a direction that reinforces a reverse magnetic field, and as they grow and proliferate these eddies will weaken the dominant field: the first steps toward a new polarity, he says.

And as Scientific American reports this week, this interpretation has now been backed up by computer simulation studies.

How long a reversal might last is a matter of scientific controversy, however. Records of past events, embedded in iron minerals in ancient lava beds, show some can last for thousands of years — during which time the planet will have been exposed to battering from solar radiation. On the other hand, other researchers say some flips may have lasted only a few weeks.

Exactly what will happen when Earth's magnetic field disappears prior to its re-emergence in a reversed orientation is also difficult to assess. Compasses would point to the wrong pole — a minor inconvenience. More importantly, low-orbiting satellites would be exposed to electromagnetic battering, wrecking them.

In addition, many species of migrating animals and birds — from swallows to wildebeests — rely on innate abilities to track Earth's magnetic field. Their fates are impossible to gauge.

As to humans, our greatest risk would come from intense solar radiation bursts. Normally these are contained by the planet's magnetic field in space. However, if it disappears, particle storms will start to batter the atmosphere.

'These solar particles can have profound effects,' said Dr Paul Murdin, of the Institute of Astronomy, Cambridge. 'On Mars, when its magnetic field failed permanently billions of years ago, it led to its atmosphere being boiled off. On Earth, it will heat up the upper atmosphere and send ripples round the world with enormous, unpredictable effects on the climate.'

Causes

Scientific opinion is divided on what causes geomagnetic reversals. One theory holds that they are due to events internal to the system that generates the Earth's magnetic field. The other holds that they are due to external events.

Internal events

Many scientists believe that reversals are an inherent aspect of the dynamo theory of how the geomagnetic field is generated. In computer simulations, it is observed that magnetic field lines can sometimes become tangled and disorganized through the chaotic motions of liquid metal in the Earth's core.

In some simulations, this leads to an instability in which the magnetic field spontaneously flips over into the opposite orientation. This scenario is supported by observations of the solar magnetic field, which undergoes spontaneous reversals every 9-12 years. However, with the sun it is observed that the solar magnetic intensity greatly increases during a reversal, whereas all reversals on Earth seem to occur during periods of low field strength.

Present computational methods have used very strong simplifications in order to produce models that run to acceptable time scales for research programs.

External events

Others believe that geomagnetic reversals are not spontaneous processes but rather are triggered by external events which directly disrupt the flow in the Earth's core. Such processes may include the arrival of continental slabs carried down into the mantle by the action of plate tectonics at subduction zones, the initiation of new mantle plumes from the core-mantle boundary, and possibly mantle-core shear forces resulting from very large impact events. Supporters of this theory hold that any of these events could lead to a large scale disruption of the dynamo, effectively turning off the geomagnetic field. Because the magnetic field is stable in either the present North-South orientation or a reversed orientation, they propose that when the field recovers from such a disruption it spontaneously chooses one state or the other, such that a recovery is seen as a reversal in about half of all cases.

Brief disruptions which do not result in reversal are also known and are called geomagnetic excursions.

Is Anything Starting?

In 1998, NASA noticed a large change in the Earth's magnetic field. This may be the beginning of the Magnetic Pole Reversal. But what lead time do we have before it flips?

There is also magnetic anomalies in the south Atlantic that suggest a big change in the Earth's magnetic field.

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