Ancient tree shows result of magnetic pole switch

Tree rings allow the present to learn the lessons of the distant past. Image: By Michael Gäbler, via Wikimedia Commons

A preserved ancient tree trunk records the story of a climate catastrophe more than 40 millennia ago. It could happen again.

LONDON, 5 March, 2021 − Here is the news of the world from 42,000 years ago. Imagine a dramatic shift in global climate during the last Ice Age; a co-incident extinction of one human species and a range of giant Australian mammals; a devastated ozone layer and astonishing displays of auroras over the tropics, all triggered by a simple but unimaginable shift.

It happened when the north and south magnetic poles weakened, then swapped places, and then swapped back again, all in the space of about 800 years. The episode, carefully decoded from the story told by the growth rings of a vast and long-lived ancient tree preserved for 42,000 years in a New Zealand swamp, has been given its own name.

It is an Adams Transitional Geomagnetic Event, and is so called − say the authors in a literary jest rare in a respected science journal − “after the science writer Douglas Adams because of the timing … and the associated range of extinctions”. Famously if enigmatically, Douglas Adams’s book The Hitchhiker’s Guide to the Galaxy proposed that the answer to life, the universe and everything was the number 42.

And the 33 authors from Australia, New Zealand, Europe, China and Argentina claim in their study in the journal Science that their Adams Event “appears to represent a major climatic, environmental and archaeological boundary that has previously gone largely unrecognised.”

The phrase “appears to” is important: what the authors describe is what they think would have been probable, or possible. What they have established is something else: the precise sequence of an event so far not observed in human history, a magnetic reversal.

“For the first time ever we have been able to date the timing and precise environmental impacts of the last magnetic pole switch,” said Chris Turney  of the University of New South Wales.

“A magnetic pole reversal or extreme change in Sun activity would be unprecedented climate change accelerants. We urgently need to get carbon emissions down before such a random event happens again”

“The findings were made possible with ancient New Zealand kauri trees, which have been preserved in sediments for over 40,000 years. Using the ancient trees we could measure, and date, the spike in atmospheric carbon caused by the collapse of the Earth’s magnetic field.”

Just as the planet’s orbit periodically changes, with sometimes dramatic consequences, so too does the planet’s magnetic field. Such collapses and reversals are repeatedly recorded in its bedrock.

Scientists also knew there had been a temporary reversal approximately 41,000 to 42,000 years ago: what they did not have was any precise, year-on-year evidence of the progress of such a switch. Not, that is, until the first close examination of the growth rings of a kauri tree, Agathis australis, that had flourished for 1700 years, through the entire episode, before it fell, only to be preserved with its bark intact, in a bog in New Zealand’s warm, rainy Northland.

The researchers could cross-check the story told by this and other preserved trunks with radio-carbon and climate records revealed in cave deposits, marine sediments and ice cores.

The twin poles began to migrate across the planet, and in the run-up to the magnetic flip the Earth’s magnetic field fell to between zero and 6% of its present strength.

Twin  quandaries

“We had no magnetic field at all − our cosmic radiation shield was totally gone,” said Professor Turney. “Unfiltered radiation from space ripped apart air particles in Earth’s atmosphere, separating electrons and emitting light − a process called ionisation. The ionised air ‘fried’ the ozone layer, triggering a ripple of climate change across the globe.”

Australia, at that time, became more arid, and a number of large Australian vertebrates went extinct at around the same time. At around that time, too, human cave art began to flourish in Europe and Asia, and one species of European human, the Neanderthal, disappeared. How connected these events might be with any geomagnetic reversal is not proven. It remains, the authors concede, a possibility.

But in the past 170 years the Earth’s magnetic field has weakened by 9%, and the north magnetic pole has been on the move. What happened 42,000 years ago could happen again. The research confirms two things.

One is that the number 42 is an answer to something. “The more we looked at the data, the more everything pointed to 42. It was uncanny,” said Professor Turney. “Douglas Adams was clearly onto something after all.”

The other confirmation is that magnetic reversal could be accompanied by climate change. And, the scientists say, the human-triggered climate emergency is bad enough today.

“Our atmosphere is already filled with carbon at levels never seen by humanity before. A magnetic pole reversal or extreme change in Sun activity would be unprecedented climate change accelerants,” said Professor Turney. “We urgently need to get carbon emissions down before such a random event happens again.” − Climate News Network