Earth's magnetic field, which has reversed itself many times in our planet's history, can do so quickly -- perhaps within a human lifespan, researchers say.

The field, like a bar magnet, has poles -- north and south -- which remain stable for hundreds of thousands to millions of years, but which can, on occasion, reverse direction.

Such reversals were previously thought to happen over thousand of years, but a new study says it may happen in the geological blink of an eye.

An international team of scientists, in a study published in the Geophysical Journal International, say they've determined the most recent such reversal, around 786,000 years ago, occurred with surprising speed, taking less than 100 years -- about a human lifetime.

"It's amazing how rapidly we see that reversal," says UC Berkeley graduate student Courtney Sprain. "This is one of the best records we have so far of what happens during a reversal and how quickly these reversals can happen."

There is evidence the planet's magnetic field might be getting ready for another switch, and it may come sooner than anyone predicted, the researchers say.

Satellite measurements of the Earth's magnetic field suggests it's weakening at a rate 10 times faster than had been believed, growing weaker at around 5 percent every decade rather than 5 percent a century as had been thought.

That could be a precursor to an upcoming reversal, which could happen in less than 2,000 years, the researchers say.

Convection currents in the iron core of the planet drive magnetic field reversals, and although there's no evidence of any geological or biological catastrophes associated with past reversals, a future reversal could potentially wreak havoc with the global electrical grid, generating currents that might cause it to go down, they say.

In addition, since the field protects life on Earth from cosmic rays and particles in the solar wind, which can cause genetic damage, cancer rates might increase if there was a sufficient weakening or even temporary loss of the magnetic field before a permanent reversal was completed, they suggested.

"We should be thinking more about what the biologic effects would be," says Berkeley professor of earth and planetary science Paul Renne.

Much could depend on the speed of the next reversal, something that can't be accurately predicted, he says.

"We don't know whether the next reversal will occur as suddenly as this [last] one did, but we also don't know that it won't," he says.

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