The geomagnetic field, generated in the liquid outer core, provides a shield from cosmic radiation that can cause damage to man-made satellites, electrical power grids, and the ozone layer (1). In the absence of this shield, the solar wind might gradually erode the atmosphere, eventually robbing the planet of its water (2). But the dipole magnetic field has decreased in strength over the last 160 y at an alarming rate (3), motivating continued spirited debate (4⇓–6) over whether we are in the early stages of a geomagnetic pole reversal. Some even believe we are overdue. But 160 y is just a blink of the eye in geological terms. The record is too short to have great confidence about what these trends might mean about causal processes and the future. In PNAS, Trindade et al. (7) provide insightful context for these modern changes in the form of a magnetic record from South America that extends ∼1,500 y into the past.

We know much about the recent waning geomagnetic field; since the time of Gauss in 1840, we have had a rich data source in the form of geomagnetic observatory records. In the satellite era, we have a deluge of data from magnetometers in orbit. From analyses of these data, we know that the field-intensity decrease is concentrated in an area known as the South Atlantic Anomaly (SAA), a vast swatch stretching from South Africa to Chile. Amazingly, when one estimates the field structure beneath this region at the core–mantle boundary, one finds patches of reversed polarity that most likely reflect flux being expulsed from the core (8). In numerical simulations, reversed flux patches are a harbinger of field reversal (9). Hulot et al. (10) have suggested that the patch beneath South Africa is largely responsible for the modern dipole field decrease.

Paleomagnetism, and especially its archeological brand known as archeomagnetism, could potentially provide longer-term context to understand the recent dipole decay. But until recently, archeomagnetic data from the SAA area have been lacking. The first data from the Limpopo River Valley of South Africa, Zimbabwe, and Botswana (Fig. 1) suggest that the SAA is a recurrent feature (11, 12). Subsequent analyses of Pleistocene basalts support this recurrence (13). Moreover, a hypothesis (11) based on the Limpopo data posits that the recurrence reflects a kind of top-down influence on the geodynamo. Specifically, the magnetic field may be driven to small scales as the core’s liquid iron flows beneath the African large low-shear-velocity province, an immense region of unusually hot and/or dense mantle defined by anomalously slow seismic shear waves (14) (Fig. 1). This could stimulate flux expulsion and the formation of reversed flux patches. One prediction of this hypothesis is that reversed flux patches should advect westward with the core flow. However, no comparable magnetic data have been available from South America to test these ideas.

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Fig. 1.

Geomagnetic field intensity and the prominent low-strength region known as the SAA are highlighted. The magnetic field estimated at the core–mantle boundary shows reversed flux patches (22), areas where the field is opposite to its expected polarity. The core–mantle boundary above the African reversed flux patch is unusually hot and/or dense and part of the African large low-shear-velocity province (14). Trindade et al. (7) report geomagnetic direction changes similar to those seen in the Limpopo Valley area of South Africa (11), with an ∼200-y time lag.

The situation has now changed with the work of Trindade et al. (7) who have provided a spectacular archive of geomagnetic field behavior from the very center of the modern SAA. The data come from a seemingly unusual source: stalagmites in the Pau d’Alho cave of Brazil (Fig. 1). Most archeomagnetic data are derived from clay materials that were fired to high temperature. During cooling, their constituent magnetic minerals record the ambient magnetic field. In contrast, stalagmites acquire their magnetization one drop at a time. Water containing magnetic minerals drops onto the stalagmite and, once deposited, these minerals can align with Earth’s magnetic field. These speleothem data do not record absolute field strength like a fired archeomagnetic sample might, but they can provide relative intensity information and, importantly, a log of magnetic directions. In addition, they provide a huge advantage relative to the spot readings of most archeomagnetic datasets in that they can be virtually continuous.

This decade has seen a renaissance in speleothem magnetization studies (15, 16). U-Th radiometric age dating (17) provides the all-important timescale, whereas continued investigations in the discipline of rock magnetism provide insight into the magnetic mineral carriers and their magnetic recording fidelity. Using this arsenal of tools, Trindade et al. (7) show how the stalagmites they sampled hold minute magnetite particles expected to faithfully preserve the field.

The records are not perfect. Magnetic inclinations from one stalagmite are systematically offset by about 10°, possibly due to magnetic grain rolling. The relative magnetic intensity records show short-term scatter that is probably experimental noise rather than geomagnetic signal. However, these minor imperfections do not detract from the exciting results. In particular, Trindade et al. (7) focus on the robust directional data and note rapid variations exceeding 0.10°/y between approximately 860 to 960 CE and approximately 1450 to 1750 CE. These lag the record of rapid field variations recorded from the Limpopo region (11, 12) by ∼200 y. The new Pau d’Alho speleothem data thus support the recurrence of fast field variations linked to the SAA and westward advection of anomalous flux patches (11).

The results of Trindade et al. (7) highlight the potential for further tracing of field behavior associated with the SAA back in time and space to discriminate between far-reaching but differing viewpoints on the nature of the geodynamo. In one interpretation, reversed flux patches are purely intrinsic to the flow of iron in the core, without any influence of the overlying mantle. Hence, the occurrence of flux expulsion, reversed flux patches, and other anomalous features such as field-strength spikes (18) or high secular variation (19) would not have any geographic preference. Reversals should also not nucleate in any preferred location beneath the mantle. In contrast, in the top-down hypothesis, the core–mantle boundary stimulates flux expulsion and formation of reversed flux patches, and this could occasionally lead to a field reversal (11). In yet another interpretation, a bottom-up control on the geodynamo driven by the interaction of the inner core with the fluid outer core could also lead to geographic preferences in geomagnetic field behavior departing from that of a simple dipole (20).

The tracking of anomalies denoted by Trindade et al. (7) gives a nod to top-down control of the geodynamo, but more data will be needed before the community can fully evaluate these viewpoints and better parse the related processes. Irrespective of the details of that outcome, the recurrence of changes documented by Trindade et al. (7) provides motivation to use the past as a guide to the future. The nadir of the last occurrence of low intensity recorded in the Limpopo (11) and efforts to forecast the field (21) give us reason to believe that the global field-intensity decay will continue in the coming century. This should be a call to arms to further improve the resiliency of satellites and infrastructure as our planetary magnetic shield becomes ever more imperfect.