Earth’s magnetic field is our great protector, shielding us from dangerous incoming solar radiation that would otherwise make life on Earth almost impossible. However, the strength of the field has changed through geological time, with the poles of the planet’s magnet switching dramatically at somewhat random intervals – roughly between 200,000 and 5 million years. Although the field strength has been dropping for the past two centuries, a new study, published in the Proceedings of the National Academy of Sciences, suggests it is not in danger of flipping any time soon.
Our planet’s magnetic field appears to be quite chaotic: It undergoes not only reversals, but also excursions wherein the poles “wander,” changing their coordinates on the surface of the planet rapidly with respect to geological time, before suddenly switching back to “normal.” Although the most recent reversal occurred 780,000 years ago, the poles temporarily flipped during an excursion in the middle of the last ice age 41,000 years ago.
Today, we can accurately trace the changes in the orientation and the strength of the Earth’s magnetic field in real time, but predicting when it will suddenly flip again is no easy task. The current rate of its degradation has led some scientists to think that it will bottom out in 2,000 years or so, leaving the planet hazardously unprotected against our Sun’s radiation. Whenever the magnetic field has reached a nadir in the geological past, it tends to flip soon afterwards.
This new study, led by the Massachusetts Institute of Technology (MIT), looked at the strength of the Earth’s magnetic field over the last 5 million years by examining volcanic rocks. These ancient lava flows provide an ancient record of the planet’s field strength, locked in as they cooled below a particular temperature called the Curie point.
Image credit: When is the next big reversal due? Triff/Shutterstock
They found that the average field strength, as recorded in Galápagos Archipelago lava flows, was roughly 21.6 microteslas, representing the equatorial average. At the Antarctic, the average field strength was previously found to be 33.4 microteslas, representing the polar average. The modern-day field strength of the planet, despite its recent diminishing, is roughly twice as powerful in both locations. So although the field strength is still decreasing, it isn't decreasing to a "dangerous low" that will cause the poles to flip.
By this measure, it doesn’t appear that we are due for a reversal any time soon. However, as lead author Huapei Wang indicated in a statement, the behavior of the Earth’s magnetic field is incredibly unpredictable: “Sometimes you won't have a flip for about 40 million years; other times there will be 10 flips in 1 million years. On average, the duration between two flips is a few hundred thousand years. The last flip was around 780,000 years ago, so we are actually overdue for a flip.”
As it turns out, this study is the first to measure the Earth’s ancient magnetic field strength from the equator: This provides a magnetic “snapshot” that previous studies have missed, according to Wang. In addition to this, Wang and his colleagues suggest that it was easy to incorrectly measure magnetic field strengths from volcanic rocks. So it could be that the assumption that the Earth’s magnetic field is dropping to a dangerous low, as many studies purport, is wrong.
As for the when the next reversal is due? The random nature of the Earth’s internal dynamo means that, according to the study, no one can be sure.