What Will Happen When the Earth’s Magnetic Field Begins to Reverse?

On the University of Maryland campus, a giant whirligig tries to predict the planet’s next big flip

When a sodium-filled model of the Earth’s outer core spins at full speed, it could generate a dynamo. Courtesy Daniel P. Lathrop

In a large, warehouse-like laboratory on the University of Maryland campus, a stainless steel sphere ten feet in diameter whirls rapidly. It is the largest spinning model of the Earth’s interior ever built and resembles the Star Wars Death Star, only shinier. Geophysicist Daniel Lathrop wants, among other things, to use it to predict when the Earth’s magnetic field will next reverse.

Over the course of our planet’s history, the field has flipped hundreds of times: Magnetic north has slid toward the bottom of the planet while magnetic south has traveled north. Signatures in volcanic rocks reveal that the switch last happened 780,000 years ago, when human ancestors were just learning to make fire.

We’re still here, so we’ll probably survive the next reversal—but we don’t know what to expect. During the reversal, a gradual event that takes about a thousand years, the field will weaken. Without the protection it offers, will our sun’s radiation bombard us? Will migrating birds relying on the field become hopelessly confused? And when will it happen? Some estimates say “soon,” which, for a geophysicist, could be in the next 10,000 years. It could even start tomorrow.

That’s where Lathrop’s sphere comes in. Within is nested another sphere—the space between the two filled with 12 tons of liquid sodium, heated to 250 degrees Fahrenheit. When set spinning, the setup mimics the roiling liquid iron in the Earth’s outer core, which forms electrical currents that generate the magnetic field in a process called a dynamo. His team hopes to find out how Earth’s field forms and evolves. “Any theory they’re able to even rule out will be front-page news to many of us,” says Peter Olson, an earth and planetary scientist at Johns Hopkins University.

Just getting the sphere going was a major feat: eight years on design and construction, two years of experiments with water, and another six months to drain the water and pour in the sodium, an element prone to explosions. “Hazards have to be respected,” Lathrop says.

With the sphere spinning at 45 miles per hour, and a little help from electro­magnets, the team saw short-lived magnetic bursts within the sodium. When the spinning ramps up to nearly 90 miles per hour later this year, the sodium might generate a field without the extra nudge. If so, and with one second of the experiment equaling 5,000 years of Earth time, the researchers could see a reversal before everyone else on the planet.