Magnetic reversal caused massive climate shifts

About 42,000 years ago, a reversal of the Earth’s magnetic poles triggered massive climate shifts and caused environmental changes to sweep across the globe, according to new Australian-led research.

Scientists have long known that the planet’s magnetic field periodically flips, with the north and south poles switching places. The last known reversal – which was temporary and technically known as the “Laschamps excursion” – occurred 41,000–42,000 years ago. If such an event happened today, it would wreak havoc on satellites and electrical grids, but its environmental impact is less well understood.

This new study, published in Science, suggests that the Laschamps excursion coincides with significant environmental and ecological changes, including growing ice sheets, mass extinctions, and even the rise of cave art.

To learn this, the researchers built a precisely dated atmospheric radiocarbon record using the tree rings of massive subfossil New Zealand kauri trees (Agathis australis) that were alive during this period. Not only did the trees record changes in radiocarbon levels during the pole reversal, but the growth rings also acted as a natural timestamp.

1. An ancient kauri tree log from ngawha nz. Credit nelson parker
An ancient kauri tree log from Ngāwhā, New Zealand. Credit: Nelson Parker

“For the first time ever, we have been able to precisely date the timing and environmental impacts of the last magnetic pole switch,” says Chris Turney, co-lead author of the study from the University of New South Wales (UNSW).

“Using the ancient trees we could measure, and date, the spike in atmospheric radiocarbon levels caused by the collapse of Earth’s magnetic field.”

This allowed the researchers to build a detailed timeline of how Earth’s atmosphere changed, showing there was a significant increase of radiocarbon during the Laschamps excursion. They compared this with other records of environmental changes from caves, ice cores and peat bogs, and then incorporated it into global climate models to look at the environmental impacts.

Most previous research had focused on what happened during the reversal, when the magnetic field was reduced to 28% of its current strength. But this study reveals the most dramatic impacts occurred into the lead-up to the reversal, when the field dropped to 0–6% of its current strength.

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Red ochre –which may have been used as an ancient form of sunscreen –is a common cave art motif. The centre of this cave art from Spain is believed to be almost 42,000 years old. Credit: Paul Pettitt,courtesy Gobierno de Cantabria.

“We essentially had no magnetic field at all – our cosmic radiation shield was totally gone,” Turney explains.

This left the planet vulnerable to solar flares and cosmic rays.

“Unfiltered radiation from space ripped apart air particles in Earth’s atmosphere, separating electrons and emitting light – a process called ionisation,” Turney explains. “The ionised air ‘fried’ the ozone layer, triggering a ripple of climate change across the globe.”

The team posit that the magnetic reversal – and subsequent radiation exposure – may be linked to the growth of ice sheets and glaciers across North America at the time, as well as shifts in major wind belts and tropical storms.

Several other major events also occurred around 42,000 years ago, including the disappearance of Australian megafauna and the extinction of the Neanderthals. Both could be linked to these widespread environmental changes, perhaps partially due to an inability to adapt.

The timing also coincides with the appearance of figurative cave art. The researchers suggest that increased UV radiation from a weak magnetic field may have driven humans to seek more shelter – and may even explain the use of red ochre as early sunscreen.

The lead-up to the Laschamps excursion, the authors write in their paper, “appears to represent a major climatic, environmental, and archaeological boundary that has previously gone largely unrecognized”.

According to Agathe Lisé-Pronovost, a paleomagnetic geologist at the University of Melbourne who was not involved the study, this new research is fascinating because a hypothetical link between the magnetic field and climate is a long-standing question.

“Much of the discussion in the literature has been speculating what processes may possibly link events that happened at about the same time,” she explains. “It remains largely unclear if and how the magnetic field of our planet, which is generated in the outer core, may impact what is happening at the surface.”

This study, she says, is the first of its kind to bring together “new quality data and an original modelling approach”.

UNSW’s Professor Chris Turney at the Chronos 14Carbon-Cycle facility. Credit: UNSW Sydney

It may help provide a framework to study the potential environmental and evolutionary shifts during the last full magnetic reversal, 780,000 years ago – and could help us understand the implications of a future reversal.

Over the past 170 years, the Earth’s magnetic field has weakened by around 9%, leading scientists to speculate that a reversal might be imminent. Increased exposure to solar storms and other cosmic radiation could be devastating to our satellites and electrical infrastructure – and Turney warns it could be devastating to the climate, too.

“Our atmosphere is already filled with carbon at levels never seen by humanity before,” he says. “A magnetic pole reversal or extreme change in Sun activity would be unprecedented climate change accelerants.”

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