Carlo Laj1 and Catherine Kissel report that the rapid decrease of the geomagnetic field intensity in the last centuries has led to speculations that an attempt to a reversal or an excursion might be under way. Here we investigate this hypothesis by examining past records of geomagnetic field intensity obtained from sedimentary cores and from the study of cosmogenic nuclides. The selected records describe geomagnetic changes with an unprecedented temporal resolution between 20 and 75 kyr B.P. We find that some aspects of the present-day geomagnetic field have some similarities with those documented for the Laschamp excursion 41 kyr ago. Under the assumption that the dynamo processes for an eventual future reversal or excursion would be similar to those of the Laschamp excursion, we tentatively suggest that, whilst irreversible processes that will drive the geodynamo into a polarity change may have already started, a reversal or an excursion should not be expected before 500–1000 years.
Since the first measurements made by the observatories, the Earth's magnetic field intensity has been diminishing at an average rate of 5% per century, about an order of magnitude faster than the rate expected for simple decay by diffusion. This has led to speculations that the geomagnetic field might be in the early stages of a polarity reversal or an excursion.
Gubbins (1987) noted that a patch of flux occurring below the southern tip of Africa had opposite sign to that expected for a dipole field in the Southern Hemisphere, when extrapolated at the core mantle boundary (the South Atlantic Anomaly or SAA). He suggested that “the present fall of the dipole field is directly related to the intensification and southward migration of this and other similar patches and that the fall may occasionally leads to polarity reversal.”
More recently, Hulot et al. (2002) have compared the 2002 data from the Danish Oersted Satellite with those obtained from the US satellite Magsat that operated in 1979/1980 and interpreted variations of the magnetic field over this 20 years period down to previously inaccessible length scales. Hulot et al. (2002) showed that growth and poleward migration of these reversed flux patches account almost entirely for the decrease in the dipole field in the last two centuries. They postulate “that the present magnetic state of the Earth may be one in which the geodynamo operates before reversing,” consistent with, but with a much better spatial resolution than Gubbins' (1987) suggestion.
This last point was also addressed by Olson (2002) in a News and Views comment associated with the Hulot et al. (2002) article. Although stating that it is premature to conclude that the present day downward directed field in the Southern Hemisphere will continue to grow until the present field is entirely canceled, this author also concluded that “the rapidly evolving reverse flux pattern suggests that an attempt to reversal may be under way.”
Very recently, De Santis and Qamili (2015 and references therein) have considered the evolution of the South Atlantic Anomaly (SAA) and its progressive extension over the last 400 years. They find that there has been an acceleration in the rate at which the SAA expands. Also, fitting the surface extension of the SAA with power laws or logarithmic functions, in analogy with critical phenomena, they define a critical time tc when the SAA area will reach almost an hemisphere in the very near future (tc = 2034 ± 3 yr AD). They interpret tc not as the exact moment of a geomagnetic reversal or excursion, but rather as the time when irreversible processes that will drive the geodynamo into a polarity change will start. Even more recently, it has been suggested that low field values of the SAA might be a recurrent phenomena related to the unusual CMB of the region (Tarduno et al., 2015).
Different factors, that certainly must be considered when trying to assess whether or not the field is entering a reversal, have been carefully scrutinized by different authors. Olson and Amit (2006)and Constable and Korte (2006) have independently examined past records of geomagnetic field intensity and concluded that the variations of the field strength over the past few hundred years are not anomalous compared to values observed over a 7500 years interval of the CALS7.K2 model of Korte and Constable (2005). These authors also note that, although decreasing fast, the intensity of the present dipole is higher than its average over the entire Brunhes chron, while it is known that polarity reversals and excursions coincide with significant lows in intensity that last longer than the directional changes.
The present reversal rate is 3.7/Myr and the last reversal (M-B) occurred at 0.78 Ma, so that it may appear that the next reversal is overdue. However, Constable and Korte (2006) have shown that the probability of observing a chron as long as the present Brunhes one is not improbably low [using the Cande and Kent (1995) time scale for reversal and assuming that the irregular times for reversals are described by a Poisson model].
The question of the rate of decrease of the geomagnetic field strength has also been considered by Gubbins et al. (2006). Intensity data are available since 1840 and while in the GUFM1 model, Jackson et al. (2000) linearly extrapolated the post-1840 axial dipole component back in time to 1590, Gubbins et al. (2006)have suggested that the rate of decrease of g01g10 has not been regular in the last centuries: it has been of the order of 2 (±1) nT/yr prior to 1840 when it suddenly changed to 15 nT/yr. Extrapolating to the core surface, they have proposed that this rapid decrease originates from patches and poleward motion of reverse magnetic flux in the Southern Hemisphere, in agreement with previous results. They found little or no evidence of field strength changes prior to 1840 supporting the idea of a stable dipole up to 1840 (Gubbins et al., 2006). Finlay (2008) gave a similar interpretation, but Suttie et al. (2011) did a more rigorous assessment of data errors than Gubbins et al. (2006) and found that the recent dipole decay most likely extends back to ~1590 AD. Their work suggest a decay rate of ~11 to 15 nT/yr in the period since 1590.
Is this very high rate of decrease of the field strength (one order of magnitude larger than expected from simple decay by diffusion) a sign for an impending geomagnetic instability (reversal or excursion)? Here we examine this hypothesis by comparing the present rate to what can be observed from the past, similarly to Olson and Amit (2006) and Constable and Korte (2006). Different from these authors, however, instead of using a model, we base our observation on a paleomagnetic stacked record of geomagnetic field intensity that extends over the past 75 kyr (GLOPIS-75; Laj et al., 2004) combined with records of the 10Be (Muscheler et al., 2004) and 36Cl (Wagner et al., 2000a) concentrations in the Greenland ice cores. The GLOPIS-75 record is characterized by a very precise evaluation of past intensity changes with sedimentary and volcanic data combined, coupled with an age model of unprecedented resolution. The time interval of 75 kyr comprises two episodes of low intensity coinciding with the Laschamp and the Mono Lake excursions, and a period of low intensity that did not lead to a directional reversal. It is thus possible to investigate whether the rate of change could be a specific characteristic in these two different states of the geodynamo.
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