pEarth’s magnetic field is like a big bar magnet, with a north pole and a south pole. But Earth’s magnetic field does something no bar magnet ever does: it changes polarity — the north and south poles switch places./p

pThe most recent flip began about 42,000 years ago, and lasted for a thousand years. Geologists have had a tough time nailing down the consequences of the change, though. In part, that’s because it’s hard to sync up the observations they’ve made in different parts of the world./p

pBut the new study reports a way to calibrate all of that work. Researchers looked at rings in kauri trees from New Zealand. The trees were alive at the time of the field reversal, and lived for up to 1700 years. When they died, they were preserved in peat bogs. The trees allow scientists to compare the results of the tree-ring analysis with other studies./p

pScientists examined each year’s rings for carbon-14 — a radioactive element that’s more abundant when the magnetic field weakens. They found that carbon-14 went way up as the field reversed — suggesting the field was much weaker./p

pThe weaker field let in more cosmic rays — heavy particles from the galaxy. That could have led to an ice age, massive lightning storms, and even extinctions./p

pEarth’s magnetic field is overdue for another flip. That doesn’t mean we’ll see a reversal anytime soon. But when the next one happens, it could have some unhappy consequences.br / /p

pemScript by Damond Benningfield/em/p

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