evidence: A flip-flop of Earth’s magnetic poles between 42,000 and 41,000 years ago briefly but dramatically shrank the magnetic field’s strength — and may have triggered a cascade of environmental crises on Earth, a new study suggests. With the help of new, precise carbon dating obtained from ancient tree fossils, the researchers correlated shifts in climate patterns, large mammal extinctions and even changes in human behavior just before and during the Laschamps excursion, a brief reversal of the magnetic poles that lasted less than a thousand years. It’s the first study to directly link a magnetic pole reversal to large-scale environmental changes, the team reports in the Feb. 19 Science. [...] Surprisingly, the most intense effects did not occur during the actual pole reversal, the team found, but in the several hundred years leading up to it, spanning about 42,300 to 41,600 years ago. During the actual reversal, the field was only about 28 percent as strong as it is today. But during that transitional period, field strength shrank to about 6 percent of its current strength. The researchers dubbed this the "Adams Transitional Geomagnetic Event" — for Douglas Adams, author of The Hitchhiker’s Guide to the Galaxy series. Adams is often associated with the number 42, said in his books to be the answer to "the ultimate question of life, the universe, and everything."