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GEOSCIENCES

Polarized opinion

Declining strength of Earth's magnetic field fuels debate on the chances of the poles reversing for the first time in 780,000 years

Aurora Boreal registrada em 18 de janeiro de 2005 na região do lago Bear, no Alasca

The current status of the Earth’s magnetic field is dividing opinion among experts who study the origin and behavior of this invisible shield that protects the planet against high-energy particles from space. Various scientific methods have shown that the strength of the field is gradually declining, leading some researchers, such as Italian geophysicist Angelo De Santis, from the National Institute of Geophysics and Volcanology in Rome, to suggest that the Earth’s magnetic poles may be starting a slow reversal process that will eventually result in the North and South Poles switching places, as previously occurred 780,000 years ago. Others argue that the field’s strength regularly fluctuates without causing the poles to flip. “A reversal of the poles does not seem likely any time soon,” says geophysicist Ricardo Trindade, from the Institute of Geophysics, Atmospheric Sciences, and Astronomy at the University of São Paulo (IAG-USP).

Generated by the movement of an incandescent ocean of liquid iron deep inside the Earth, the donut-shaped magnetic field surrounds the entire planet (see infographic on page 49). It reaches 63,000 kilometers away from the Earth on the side facing the Sun, and up to 10 times farther on the opposite side. The strength of the planet’s magnetic field, however, is not stable, and has been getting weaker since at least 1832, when German physicist and mathematician Carl Friedrich Gauss first measured its intensity. Since then, more frequent and accurate measurements from ground and satellite observatories have confirmed that the field’s strength is decreasing at a rate of 17 nanoteslas (nT) per year—it measures 66,000 nT at the poles and 22,000 nT in an area over the southern hemisphere that stretches from Africa to South America. At this rate, some areas of the field could, within the next few centuries, become weak enough to trigger a reversal of the magnetic poles, a process that can last anywhere from hundreds to thousands of years and leave the Earth more exposed to particles from the Sun and other regions of space.

One reason geophysicists are interested in finding out when the poles will next flip is that these particles can cause damage to satellites, energy supply networks, and the planet’s atmosphere, as well as living creatures. Over the last 30 years, numerous groups have suggested that this reversal could begin within decades, while others claim that it is unlikely to happen for thousands of years. In an attempt to estimate when such an event might next occur, researchers input millions of years of past and present data into mathematical models to predict the magnetic field’s future behavior.

Angelo De Santis is one of those who believes a major change could start soon. The Italian researcher and his colleagues analyzed the rate at which the weakest area of the magnetic field—known as the South Atlantic Anomaly (SAA)—has grown over the last 400 years. The SAA is a particularly weak region of the Earth’s magnetic field, where particles from solar winds are able to reach the upper layers of the atmosphere. It has remained stable for a long time, but has been increasing in size at an accelerated pace since the 1800s. Today it spans 80 million square kilometers, an area 10 times larger than 200 years ago. In an article published in the journal Natural Hazards and Earth Science Systems in 2013, the group stated that if this rate of expansion continues, the SAA would cover almost the entire southern hemisphere as early as the 2030s.

According to the researchers, such an occurrence would signify a point of no return. With the field so weak over such a vast area, the poles would begin to reverse. In the last 70 million years, the poles have flipped every 250,000 years or so on average (the last time it happened was 780,000 years ago). “There is still not any convincing evidence that the magnetic field is close to being reversed,” De Santis said in an interview by email. “I would rather say we have some clues that something special is taking place.”

De Santis is not alone. In the late 1980s, British geophysicist David Gubbins, then a researcher at the University of Cambridge, UK, proposed that the decreasing strength of the magnetic field over the last century could be explained by a shift towards the south pole of a warmer structure located deep within the planet, whose surface location would correspond to an area in southern Africa. In an article published in Nature in 1987, Gubbins suggested that if the two facts were related, the weakening of the magnetic field “could occasionally lead to a reversal of the poles.” In a 2006 article in the journal Science, having analyzed the strength of the magnetic field over time by studying archaeological records, he reaffirmed his belief that the poles could soon flip, concluding that the field has been weakening at an increasing pace, from 2 nT per year 400 years ago to roughly seven times that rate from 1840 onwards.

Deep origin
In 2002, French geophysicist Gauthier Hulot and his team at the Paris Institute of Earth Physics backed up Gubbins’s argument. By feeding satellite measurements of the field’s strength into mathematical models, they were able to infer what was happening inside the Earth. In a paper published in Nature, the researchers concluded that regions of the outer core that transport heat in an opposing direction to the main flow of the molten iron ocean weakened the magnetic field at the surface, proposing that abnormal flow patterns would precede the beginning of a pole reversal.

Ricardo Trindade, an IAG-USP researcher who specializes in analyzing the magnetic field based on archaeological materials, recently discovered that the field over South America began to weaken 200 years earlier than previously thought (see Pesquisa FAPESP issue no. 244). Even so, he believes the poles will not begin to reverse for at least a thousand years. “Those who say a reversal is imminent are only looking at the data from the last few decades or centuries, which despite being more accurate, does not allow us to identify the natural fluctuations of a phenomenon that occurs on a scale of hundreds of thousands of years,” he says.

In a recent study, geophysicist Wilbor Poletti, a PhD student supervised by Trindade, analyzed the strength of the magnetic field by examining ceramic artifacts and volcanic rocks from the last two millennia. In an article published in Physics of the Earth and Planetary Interiors in January, the group suggests that the field has been weakening at its current pace for 1,300 years. “There has been no acceleration in the rate of decline,” says Poletti. “If the current pace does not change, it will take 2,400 years for the strength to reach close to zero, causing the poles to flip.”

Another study published this year estimates that a reversal of the magnetic field is a distant prospect. Based on data from the past 2 million years, American and British geophysicists Bruce Buffett and William Davis, from the University of California, Berkeley, created a model that predicts how the field will evolve in the future. Presented in the journal Geophysical Research Letters in February, the results indicate that there is no significant risk of a new reversal in the next 50,000 years.

What might happen to the planet when the poles flip? According to astronomer Douglas Galante, a researcher at the Brazilian Synchrotron Light Laboratory in Campinas, São Paulo State, one problem would be the increased number of electrically charged particles reaching the Earth from the Sun. The Earth’s magnetic field usually deflects these high-energy particles towards the poles, which is what creates the aurora borealis and the aurora australis phenomena.

“With no magnetic field, or with a very weakened field, particles carried by solar winds would interact with the gases in our atmosphere, changing their composition and causing various effects,” says Galante. If the reversal process lasts for hundreds of years, it could reduce the ozone layer and destroy greenhouse gases, leading to a global cooling. If it continues for too long, part of the atmosphere could even be swept into space. In the short term, as well as the effects on the atmosphere and energy and telecommunications systems, the increased number of these particles and higher levels of ultraviolet radiation entering the atmosphere could affect living creatures and increase cancer rates, including humans.

Project
Historical analysis of South America’s geomagnetic field (no. 13/16382-0); Grant Mechanism PhD Grant; Principal Investigator Ricardo Ivan Ferreira da Trindade (IAG-USP); Grant Beneficiary Wilbor Poletti Silva; Investment R$133,627.11.

Scientific articles
POLETTI, W. et alContinuous millennial decrease of the Earth’s magnetic axial dipolePhysics of the Earth and Planetary Interiors. Vol. 274, pp. 72–86. Jan. 2018.
DE SANTIS, A. et alToward a possible next geomagnetic transition? Natural Hazards and Earth Systems Science. Vol. 13, pp. 3395–403. 23 Dec. 2013.
GUBBINS, D. Mechanism for geomagnetic polarity reversalsNature. Vol. 326, pp. 167–69. Mar. 12, 1987.
GUBBINS, D. et al. Fall in Earth’s magnetic field is erraticScience. Vol. 312, no. 5775, pp. 900–2. May 12, 2006.
HULOT, G. et al. Small-scale structure of the geodynamo inferred from Oersted and Magsat satellite dataNature. Vol. 416, pp. 620–3. Apr. 11, 2002.
BUFFETT, B. and DAVIS, W. A probabilistic assessment of the Next Geomagnetical ReversalGeophysical Research Letters. pp. 1845–50. Feb. 13, 2018.

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