LULU GARCIA-NAVARRO, HOST:
We all learned this in the classroom, right? The Earth is a giant magnet. Molten iron churning deep inside our planet generates a magnetic field that wraps around it. But, sometimes, that field becomes unstable. The north and south poles can flip. And as NPR's Nell Greenfieldboyce reports, the last time this happened, life on Earth may have faced a crisis.
NELL GREENFIELDBOYCE, BYLINE: Around 42,000 years ago, the North Pole wandered around, briefly hanging out down by Antarctica before zooming back up. Geologists know this because lava flows and sediments created back then have bits of iron pointing in weird directions. Now, though, a research team has studied this event in a different way by looking at trees, giant kauri trees that grow in New Zealand.
ALAN COOPER: You can get up to 6 meters wide. I mean, these are enormous trees.
GREENFIELDBOYCE: Alan Cooper with the South Australian Museum says when these trees die, they fall into bogs.
COOPER: The low-oxygen conditions in the bogs can quite often preserve them spectacularly.
GREENFIELDBOYCE: In the journal Science, he and some colleagues say they analyzed well-preserved trees that were alive when the magnetic field flipped out. During these changes, the field got weak. That left the Earth unprotected from cosmic rays coming in from outer space. When those rays hit the atmosphere, they created a certain kind of carbon. It got taken up by the trees and put down as wood.
COOPER: The tree recorded exactly what had happened.
GREENFIELDBOYCE: He says the calendar-like rings in the trees let them create a precise timeline of the magnetic field changes. And he says the timeline matches up with a lot of other interesting things going on.
COOPER: We see the ice of North America, the Laurentide Ice Sheet suddenly start growing really fast.
GREENFIELDBOYCE: Large mammals died off in Australia. The Neanderthals disappeared. The research team also did climate modelling that suggests the weakening of the magnetic field would have made the climate go haywire. And if the sun had one of its periodic solar storms or flares, it would have looked like the end of days.
COOPER: That huge burst of ionizing radiation and ultraviolet coming into the earth would have had massive impacts on people. And so this is what we think actually drove them into caves.
GREENFIELDBOYCE: These are provocative ideas. Brad Singer is a geologist at the University of Wisconsin-Madison. He thinks all this will get scientists digging through other prehistoric records of life and the environment.
BRAD SINGER: And trying to test this proposal that there is a strong impact on these systems by the magnetic field of the Earth.
GREENFIELDBOYCE: He says scientists had long assumed pole reversals would be no big deal. But maybe like asteroids or volcanoes, they could have big impacts. Nell Greenfieldboyce, NPR News.
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