Researchers demonstrated that the last magnetic reversal 786,000 years ago actually happened in less than 100 years roughly a human lifetime.
    
"It's amazing how rapidly we see that reversal," said University of California, Berkeley graduate student Courtney Sprain.
    
"The paleomagnetic data are very well done. This is one of the best records we have so far of what happens during a reversal and how quickly these reversals can happen," said Sprain.
    
The discovery comes as new evidence indicating 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, researchers said.
    
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, they said.
    
The danger to life would be even greater if flips were preceded by long periods of unstable magnetic behaviour.     

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.
    
Italian researchers led by Leonardo Sagnotti of Rome's National Institute of Geophysics and Volcanology measured the magnetic field directions frozen into the sediments as they accumulated at the bottom of the ancient lake.
    
Sprain and Paul Renne, a UC Berkeley professor-in- residence of Earth and planetary science, 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 at approximately 786,000 years ago.
    
The study appears in the Geophysical Journal International.