Kauri trees reveal ancient geomagnetic chaos

Australian scientists have dug up evidence connecting multiple massive shake ups to the prehistoric world - the extinction of the Neanderthals, the appearance of cave art, massive swings in global temperature and climate change - to a weakening of the planet’s magnetic field nearly 42,000 years ago. Adam Murphy reports...

Adam - Earth has a magnetic field. And it does more than just point your compass one way or the other; it keeps the planet safe from the sun. Without it, particles beaming in from the sun would just strip off the ozone and leave the planet vulnerable to massive doses of UV radiation. But it's not static. It changes, it weakens, and occasionally it flips all together. One such weakening happened about 42,000 years ago, and when it happened, a lot of other big things were happening as well.

Chris - Massive growth of the ice sheets over North America; shifting tropical rain belts; shifting winds over the Southern ocean; the extinction of megafauna in Australia and more arid conditions; the demise of the Neanderthals; all happening effectively at the same time. And precisely coincident just when the poles were switching.

Adam - That's Chris Turney from the University of New South Wales, who's been studying this changing in the magnetic field, which is called Laschamp event. One issue when you're looking back that far, though, is pinning down exactly when something happened. Did the magnetic field change before or after these other things? And this new research has been pinning a much more accurate date using New Zealand's plant life.

克里斯,有这些美丽的大树;塞弗ral metres across, but can live up to two millennia; they're called kauri. They've been there for millions of years. And effectively, these trees have died and fallen into peat bogs and wetlands, and then been beautifully preserved. As a result of which, these trees provide a year by year record of a climate, through the patterns of the growth rings, but they also photosynthesise the carbon from the atmosphere, and lay that down as wood. And that gives us a measure of the radioactive levels of carbon from the upper atmosphere. And during the Laschamp - during the switch from North to South and South to North over several hundred years - the magnetic field effectively collapsed almost to nothing, less than 6% of what it is today. And the practical upshot of which was the shield - protecting the air for all these high energy cosmic rays formed from supernovae - basically just was flung wide open. And as a result, what you see in the trees year by year is this huge spike in radioactive carbon which is laid down in the trees. You find this big spike, so it's really distinctive.

Adam - Because this happened 42,000 years ago, and with a love for the Hitchhiker's Guide to the Galaxy, the team have named all this, the magnetic field changes and the events that followed, the "Adams Event" after author Douglas Adams. But what would it have been like to live through?

Chris - Aurorae through the skies; lightning bolts; people were hiding out in caves... in fact we see an explosion of rock art at that time, which implies people were actually hiding away, because there would been increased UV and terrible climate changes. It must've seemed like the end of days, it must've been an extraordinary time to live from. And it would have gone on for decades. During this Laschamp period the sun's activity dropped a lot as well. And it's almost like the perfect storm, you've got a weaker magnetic field... everything's like the worst case scenario possible.