A study of the most recent near-reversals of the Earth’s magnetic field by an international team of researchers, including the University of Liverpool, has found it is unlikely that such an event will take place anytime soon.
There has been speculation that the Earth’s geomagnetic fields may be about to reverse, with substantial implications, due to a weakening of the magnetic field over at least the last two hundred years, combined with the expansion of an identified weak area in the Earth’s magnetic field called the South Atlantic Anomaly, which stretches from Chile to Zimbabwe.
In a paper published in the Proceedings of the National Academy of Sciences, a team of international researchers model observations of the geomagnetic field of the two most recent geomagnetic excursion events, the Laschamp, approximately 41,000 years ago, and Mono Lake, around 34,000 years ago, where the field came close to reversing but recovered its original structure.
The model reveals a field structures comparable to the current geomagnetic field at both approximately 49,000 and 46,000 years ago, with an intensity structure similar to, but much stronger than, today’s South Atlantic Anomaly (SAA); their timing and severity is confirmed by records of cosmogenic nuclides. However, neither of these SAA-like fields developed into an excursion or reversal.
Richard Holme, Professor of Geomagnetism at the University of Liverpool, said: “There has been speculation that we are about to experience a magnetic polar reversal or excursion. However, by studying the two most recent excursion events, we show that neither bear resemblance to current changes in the geomagnetic field and therefore it is probably unlikely that such an event is about to happen.
“Our research suggests instead that the current weakened field will recover without such an extreme event, and therefore is unlikely to reverse.”
The last time a geomagnetic reversal happened was 780,000 years ago. However, geomagnetic excursions, where the field comes close to reversing but recovers its original structure, have occurred more recently.
Earth’s magnetic field is generated in Earth’s convecting liquid iron outer core and protects Earth’s surface from harmful solar radiation. The field has varied on different timescales throughout geological history, and these variations reflect changes deep within the Earth. Two of the field’s most extreme variations are reversals and excursions. During such events, the strength of the field decreases and the magnetic poles rapidly flip polarity, with reversals characterized by the pole retaining an opposite polarity, while excursions are marked by a return to the original polarity. Field strength over the past centuries has also been decreasing strongly; however, through analyzing previous excursions, we infer that Earth’s magnetic field is not in an early stage of a reversal or excursion.
The geomagnetic field has been decaying at a rate of ∼5% per century from at least 1840, with indirect observations suggesting a decay since 1600 or even earlier. This has led to the assertion that the geomagnetic field may be undergoing a reversal or an excursion. We have derived a model of the geomagnetic field spanning 30–50 ka, constructed to study the behavior of the two most recent excursions: the Laschamp and Mono Lake, centered at 41 and 34 ka, respectively. Here, we show that neither excursion demonstrates field evolution similar to current changes in the geomagnetic field. At earlier times, centered at 49 and 46 ka, the field is comparable to today’s field, with an intensity structure similar to today’s South Atlantic Anomaly (SAA); however, neither of these SAA-like fields develop into an excursion or reversal. This suggests that the current weakened field will also recover without an extreme event such as an excursion or reversal. The SAA-like field structure at 46 ka appears to be coeval with published increases in geomagnetically modulated beryllium and chlorine nuclide production, despite the global dipole field not weakening significantly in our model during this time. This agreement suggests a greater complexity in the relationship between cosmogenic nuclide production and the geomagnetic field than is commonly assumed.
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