Earth's magnetic field is known to have flipped many times throughout our planet's history. Its dipole magnetic field, like that of a bar magnet, remains about the same intensity for thousands to millions of years, and then – for incompletely known reasons – it occasionally weakens and reverses direction. That weakening and reversal was supposed to take on the order of several thousand years. A new study by an international team of scientists, however, demonstrates that the last magnetic reversal – 786,000 years ago – actually happened very quickly, in less than 100 years, or roughly the span of a modern human lifetime. The study will be published in the November 2014 issue of Geophysical Journal International.

The discovery comes as new evidence indicates that the intensity of Earth's magnetic field is decreasing 10 times faster than normal, leading some geophysicists to predict a reversal within a few thousand years.

Though a magnetic reversal is a major planet-wide event driven by convection in Earth's iron core, there are no documented catastrophes associated with past reversals, despite much searching in the geologic and biologic record. Today, however, such a reversal could potentially wreak havoc with our electrical grid, generating currents that might take it down.

And since Earth's magnetic field protects life from energetic particles from the sun and cosmic rays, both of which can cause genetic mutations, a weakening or temporary loss of the field before a permanent reversal could increase cancer rates. The danger to life would be even greater if flips were preceded by long periods of unstable magnetic behavior.

Paul Renne, director of the Berkeley Geochronology Center and a University of California Berkeley professor-in-residence of Earth and planetary science is a coauthor of the study. He said:

We should be thinking more about what the biologic effects would be.

The new finding is based on measurements of the magnetic field alignment in layers of ancient lake sediments now exposed in the Sulmona basin of the Apennine Mountains east of Rome, Italy. The lake sediments are interbedded with ash layers erupted from the Roman volcanic province, a large area of volcanoes upwind of the former lake that includes periodically erupting volcanoes near Sabatini, Vesuvius and the Alban Hills.

Italian researchers measured the magnetic field directions frozen into the sediments as they accumulated at the bottom of the ancient lake.

Scientists at University of California, Berkeley then used argon-argon dating, a method widely used to determine the ages of rocks, whether they're thousands or billions of years old, to determine the age of ash layers above and below the sediment layer recording the last reversal.

Because the lake sediments were deposited at a high and steady rate over a 10,000-year period, the team was able to interpolate the date of the layer showing the magnetic reversal, called the Matuyama-Brunhes transition, at approximately 786,000 years ago. This date is far more precise than that from previous studies, which placed the reversal between 770,000 and 795,000 years ago. Renne said:

What's incredible is that you go from reverse polarity to a field that is normal with essentially nothing in between, which means it had to have happened very quickly, probably in less than 100 years. We don't know whether the next reversal will occur as suddenly as this one did, but we also don't know that it won't.

Bottom line: Imagine that compasses now pointed south instead of north. It's not as bizarre as it sounds. It will happen when Earth's magnetic field reverses. A new study indicates the last reversal – 786,000 years ago – happened in roughly the span of a human lifetime.

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