Around 42,000 years ago, a reversal of the Earth’s magnetic poles caused major environmental changes, species extinction events, and long-term alterations in human behavior. These are the main conclusions of a study recently published in ‘Science’ and carried out by an international team of more than thirty researchers from institutions and universities on three continents under the direction of Alan Cooper, from the South Australian Museum in Adelaide.

The discovery was made possible thanks to the discovery of a huge 60-ton trunk of kauri (the largest tree species in New Zealand), 42,000 years old and preserved almost intact in a swamp. The tree’s rings indicate that it lived for 1,700 years and just at a time when the world, magnetically at least, was turned upside down. The information obtained from this “exceptional witness” raises important questions about the impact that pole reversals and “magnetic excursions” can have on the climate and the life forms that suffer them. “Before this work,” explains Chris Turney, one of the authors of the research, “we knew that 42,000 years ago a lot happened around the world, but we didn’t know exactly why. Now, for the first time, we have been able to accurately date what happened when the Earth’s magnetic fields were last reversed. ‘

The magnetic field exists thanks to the flow of molten iron in the Earth’s outer core, which is prone to chaotic changes that not only weaken the field but also cause the poles to drift and sometimes completely reverse. The magnetic orientations of minerals in rocks record long-lasting reversals, but they cannot capture the details of a change that lasted only a few centuries, such as 42,000 years ago. In the geological record of our planet, there are numerous cases of magnetic inversion, episodes during which the magnetic poles ‘migrate’ from their usual positions and end up interchanging. If something like this happened today, it is quite possible that the event would wreak havoc on telecommunications networks and electronic equipment. But the environmental impact of such events was virtually unknown.

The most recent magnetic reversal, known as the “Laschamps excursion,” was a relatively short-lived event (less than a thousand years in total) that occurred around 41,000 years ago and offers the best opportunity we have to study the consequences of a shock. the extreme change in the earth’s magnetic field. However, and even though several palaeoenvironmental records indicate that this event coincided in time with significant changes in climate and ecology, there was not enough data so far to establish a cause-effect relationship between the two. In this study, Cooper and his colleagues present a new atmospheric radiocarbon record, but this time precisely dated thanks to the kauri rings, miraculously preserved for millennia in New Zealand wetlands. The new record allowed the researchers to align other global radiocarbon records, ordering them in time and giving them, for the first time, meaning.

In their work, the researchers discovered a significant increase in atmospheric radiocarbon just during the period of weakening of the magnetic field strength that preceded the polarity reversal. And they showed that this weakness had important consequences. Cooper and his colleagues, indeed, found that during its less intense phase, when the Earth’s magnetic field was barely 6% of its current value, there were substantial alterations in the concentration and circulation of atmospheric ozone, which led to its turn to climate and environmental changes across the globe, including species extinction episodes, which scientists have observed in climate records from 42,000 years ago. In other words, the study shows that fluctuations in the Earth’s magnetic field can affect both temperature and air circulation on a global scale, with major changes in climate and ecology around the world.