Oddly, since we were in the land of the café au lait, morning coffee was hard to come by in Nantes, at least in the quantities required for alertness at a scientific conference. So Kathy Whaler and I took a detour one morning and stopped off at the bustling train station’s take-out coffee stand.
Like most at the conference, Whaler, a professor of geophysics at the University of Edinburgh, was a luminary. You couldn’t move through the crowd at the conference center without bumping into people who have done the defining modern-day work on the inner Earth. The level of expertise was so high that when I would ask someone a question, I almost invariably got redirected. “Check with so-and-so, who wrote a big paper on that and is standing right over there.” And then, when I would find that person—the one who knew the answer better than anyone else on Earth—there was always demurring and more redirecting for another fragment of evidence from someone else.
It was the culture of a meeting like this not to be definitive. Because the discipline itself is still evolving, the meeting was less about certainties than about tracking the slow progress toward certainties. That meant it was the place for presenting careful summaries of what was agreed on, exploring new findings, detailing methodologies to figure out more and more precise ways of reading what the data were telling you, comparing contradictory interpretations. It was where you wore a sackcloth to admit errors, accepted criticism, basked in praise, defended your approach, and, maybe most important, found out what others were looking at. The focus was on the challenges that remain, from the arcane to the urgent. So, for example, are there radioactive elements in the core? What is the chemical composition of the deep mantle? How can you look into the heart of the Earth’s magnetic field without interference from the thick mantle and the crust? How is the field changing over time?
Reading what’s going on within the core itself was where Whaler came in. She was at Cambridge working on her doctorate under the supervision of David Gubbins when an astounding batch of new satellite data came in. These were figures from MAGSAT, the first satellite that could read the whole magnetic vector—both direction and strength—across the whole planet. Operated by the National Aeronautics and Space Administration (NASA) and the US Geological Survey, it collected data for about half a year, ending in the late spring of 1980. (An earlier set of satellites, POGO, launched from 1965 to 1969, was the first to do a general map of the field, but only looked at its strength, not its direction.)
The MAGSAT data were superb. For the first time, researchers could look at the global structure of the field. Whaler was hooked, she told me as we walked across a bridge over the Loire with our coffees to rejoin the others. All of a sudden, you could compare these precise figures from the satellite with data from modern observatories on the ground and try to reconcile the two sets of numbers. And not only the modern numbers but the whole archive. It meant putting together data stretching back to sixteenth-century sailors’ measurements of declination and inclination with the nineteenth-century measurements captured by Gauss’s magnetic union and Sabine’s magnetic crusade. It meant combining those with the record in the rocks that Brunhes and others were beginning to read in the twentieth century and with the new findings from the seabed floor. Finally, a big picture of the field over time was emerging. It was intoxicating.
At an initial pass, Whaler, Gubbins, and others put together a 380-year record of the field and its variation over time, Gubbins explained in an article in Scientific American in 1989. Some of the findings were expected. At the surface of the Earth, the field looked like that of a bar magnet lying along the same axis as the one on which the planet spins. These are the endless looping lines that flow from the south magnetic pole out into space and then back into the Earth at the magnetic north pole. The denser the lines are, the stronger the field is.
Their reconstructed 380-year record also catalogued what’s known as the “westward drift” in the field over the past few centuries. The idea emerged in the late seventeenth century, when Halley suspected that the field was listing to the west, for which he found evidence in London’s shifting measurements of declination. Gubbins’s team tested that idea by tracking the field line on the planet where declination is 0, or where the compass points to both magnetic and geographic north. Its official name is the agonic line, after the Greek phrase for “without angle.” In 1700, for example, the line ran midway through the Atlantic Ocean, curved over the Gulf of Mexico, and ran straight through the Great Plains. By 2017, it had drifted so far west that it was on the Pacific side of South America, careening up through the middle of Minnesota. This was the longitudinal prime meridian that Halley and so many others had sought hundreds of years ago, believing it would bisect the Earth into two neat halves and solve the problem of navigation at sea. Gubbins’s model showed that the agonic line is wildly unpredictable. Back in the early seventeenth century, for example, it ran up through Africa, looped over Norway, slid down past Greenland across the top of South America and out into the Pacific before scooting up to the North Pole via Southern California.
And then there was the strength of the field. Scientists have kept a continuous record of the field’s strength since 1840, after Gauss worked out how to measure it. And while it’s possible to figure out the field’s strength from proxies going back further in time—like ancient terracottas and lavas and mariners’ measurements—that’s not considered to be as precise as direct measurements. So to geophysicists, 1840 is a key line between observed, indisputable measurements and those that are derived from other evidence. When Gubbins looked at his maps going back over time, it was absolutely clear that the dipole had declined since those first measurements in 1840. Looking at terracottas magnetized two thousand years ago during the Roman era, the team could work out a sustained and remarkable weakening from that time.
But why was it weakening? Every single reading taken until then had measured the magnetic field as it manifests itself at the surface of the Earth. But the field, as Gauss showed mathematically in 1838, is generated within the Earth. Between the outer edge of the Earth’s core and the surface was nearly 3,000 kilometers of mantle and crust, potentially interfering with the magnetic signal from the core. What if the field looked different closer to its source?
The trick there was to work out how to strip away any magnetic interference from the crust and the thick mantle and to see what was happening to the field as close as possible to where it originated—at the outer edge of the dynamo. Gubbins and his team wanted to know more precisely what drove the Earth’s magnetic power, how it had evolved, and where it was headed. They were convinced that looking at the field at the core–mantle boundary would give them some clues.
By 1985, Whaler, Gubbins, his graduate student Jeremy Bloxham (now at Harvard), and others had figured it out, at the same time as a separate group working independently at the Scripps Research Institute in California. They used the mathematical methods devised by James Clerk Maxwell in the nineteenth century to project their readings from the Earth’s surface down to the bottom of the mantle, to where the mantle hugged the outer core. The Gubbins group started with the 1980 data and then reached back in time to 1777, making maps that captured both the direction of the field and its intensity at the core. The maps showed the number of field lines and where they exited and entered the core’s surface. That’s known as the magnetic flux over an area. Then they colored the outgoing flux shades of red depending on how intense it was, and the incoming flux, blue.
This picture of Earth’s core was a revelation, a bewildering hodgepodge of swirls and colors that betrayed a far more complicated field than anyone had imagined. As Gubbins explained, if the maps had been describing a simple two-pole system, the northern portion of the image ought to have been blue and the southern, red. It would have been deepest blue at the magnetic north pole, aligned with the Earth’s axis of spin, and deepest red at the magnetic south. That would reflect the fact that the lines converge at the poles, increasing intensity. In addition, the core’s magnetic equator, close to the geographic equator, would have shown up as a boundary between red and blue where no flux penetrated the surface.
Instead, the map of the core showed elements of a two-pole system but also other anatomy deep within the outer core. It was like being able for the first time to see inside the human body with a magnetic resonance imaging machine, discerning the shape of the liver and heart and lungs. For one thing, while the north was mainly blue, and the south mainly red, that was not absolute. There was red where blue was expected and blue where red was expected. Also, several blobs showed up where the flux was either greater or less than expected. Two of the low-flux patches were around the poles, the opposite of what researchers would expect. And then there were the two curious patches below the southern Atlantic Ocean and Africa. They were strong, running in the opposite direction from the way the dipole would demand: in instead of out. Not only that, but the one under Africa was moving westward at the astonishing pace of about one-third of a degree of longitude every year. As for the dipole itself, the map showed what Gubbins and his team believed to be evidence of two columns of liquid spinning in the outer core, separated from the rest of the molten metal making up the outer core. Those columns of movement seemed to be supporting the dipole. And the patch moving under Africa appeared to be what was undermining the dipole.
The elements were now in place to begin to see the kinetic workings of the molten inner Earth, to glimpse its very heart pumping. The trail of discoveries to get there had hopscotched from the development of the simplest compass by the Chinese in the centuries before the common era to Gauss’s proof in the nineteenth century that the Earth’s magnetic force was within the planet itself, from the realization in the seventeenth-century garden near London that the field was changeable to the twentieth-century seismic-wave data proving that the core was both liquid and solid. All these pieces of the magnetic puzzle so arduously assembled over so many centuries had fallen into place to produce a series of maps that showed not just what was in the core, and how that core gyrated, but also how it gyrated over time.
Since then, new data have poured in. By 1999, the Danes had launched the Ørsted satellite, named after Hans Christian Ørsted. It remains in orbit. Originally, it captured the whole field vector, but since 2006 has been recording only intensity. The Germans launched the CHAMP satellite in 2000, which orbited for ten years until it reentered the atmosphere and burned up. The SAC-C, launched by a robust international coalition including NASA, orbited from 2000 to 2013. In 2013, the European Space Agency began the Swarm mission—a trio of satellites measuring the whole field at the same time. Together, the satellite data represent an unbroken, high-grade record of the Earth’s magnetic field, as seen from above, for the better part of two decades.
By 2000, the geophysicist Andrew Jackson, another of Gubbins’s graduate students, had developed what is now a widely used, more precise computer model allowing researchers to see what was going on at the boundary between the mantle and core stretching back four hundred years. Between his model and others, critical changes in the field at that boundary have become clear. The blue reversed-flux patch Gubbins and his team discovered has kept growing and has kept moving westward. In 1984, it joined forces with a similar, smaller patch underneath Antarctica. By 1997, this formation had attached itself to the field in the northern hemisphere, meaning that a massive blob of blue now runs through the red of the southern hemisphere almost from the magnetic equator to the south pole. The non-dipole part of the magnetic field is getting stronger, a dramatic shift for a single human generation.
As for the dipole’s overall intensity, it is waning too. Since the critical year 1840, it has decayed by about 10 percent, measured from the surface of the Earth. And it’s the part of the field that is changing most slowly—because it is the biggest—while other wily structures in the field, led by a sinuous gyre, strain ever more wildly to break free.