Deep records: Yuhji Yamamoto examines drilling cores on the JOIDES Resolution during the 2012 expedition in the North Atlantic. (Courtesy: Peter Lippert, University of Utah)

The Earth’s magnetic poles have reversed 540 times over the past 170 million years. Usually, these reversals are relatively speedy in geological terms, taking around 10,000 years to complete. Now, however, scientists in the US, France and Japan have found evidence of much slower reversals deep in Earth’s geophysical past. Their findings could have important implications for our understanding of Earth’s climate and evolutionary history.

Scientists think the Earth’s magnetic field arises from a dynamo effect created by molten metal circulating inside the planet’s outer core. Its consequences include the bubble-like magnetosphere, which shields us from the solar wind and cosmic radiation that would otherwise erode our atmosphere.

From time to time, this field weakens, and the Earth’s magnetic north and south poles switch places.…