According to a study of carbon isotopes preserved in ancient trees, several centuries’ worth of climate change, mass extinctions, and even changes in human behavior can be directly linked to the last time Earth’s magnetic field changed its intensity.
“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 radiocarbon levels caused by the collapse of Earth’s magnetic field.” explains geologist and author Chris Turney of the University of New South Wales in Australia.
Earth’s magnetic field shields the surface from harmful radiation and high-energy particles. When this radiation enters the atmosphere, it will interact with the local nitrogen atoms to trigger a nuclear reaction that produces the slightly radioactive carbon -14 isotope, eventually incorporated in the living kauri trees. A high concentration of carbon-14 in fossilized wood samples suggests a weak magnetic field, as more radiation penetrates through to the atmosphere to produce more carbon-14. By analyzing the annual tree rings, the researchers reconstructed the carbon-14 curve for over 40,000 years.
The last complete geomagnetic reversal occurred about 780,000 years ago, with partial reversals happening about 105,000 to 103,000 and 98,000 to 92,000 years ago. The most recent change in Earth’s magnetic field is known as the Laschamp event, named after the lava flows in France where it was first recognised, and it is what scientists call a geomagnetic excursion, when the magnetic fields weakens, but don’t reverses polarity. It took place around 41,000 years ago, and lasted for around 800 years. Based on their measurements, scientists had previously been able to ascertain that Earth’s magnetic field had weakened to about 28 percent of its normal strength during the Laschamp event.
The new study shows that the Laschamp event was preceded by another, even more significant event, when Earth’s magnetic field dropped to only 0-6 percent strength, about 42,000 years ago. The researchers named it “Adams Transitional Geomagnetic Event”, or “Adams Event” — a tribute to science-fiction writer Douglas Adams.
The effects on Earth’s environment of the Adams Event are yet not clear; however, the authors suggest that the disappearing field could have triggered substantial changes in Earth’s atmospheric ozone layer, leading to potentially harmful ultraviolet radiation reaching Earth’s surface and disrupting weather patterns around the world.
The researchers compared their results with records from sites across the Pacific and used it in global climate modelling, finding that the growth of ice sheets and glaciers over North America and large shifts in major wind belts and tropical storm systems coincide with the proposed failure of Earth’s protective shield around 41-42,000 years ago.
The researchers also suggest that the Adams Event changed human behavior as recorded in some of the oldest cave paintings known to date, created almost 42,000 years ago.
“This sudden behavioural shift in very different parts of the world is consistent with an increasing or changed use of caves during the Adams Event, potentially as shelter from the increase of ultraviolet B, potentially to harmful levels, during grandsolar minima or solar energetic particles, which might also explain an increased use of red ochre sunscreen,” they wrote in their paper.
Some authors also link mass extinctions to fluctuations in Earth’s magnetic field. About 40,000 years ago, mammalian fossils in Australia and Eurasia record an important die‐off of large mammals that included Neanderthals in Europe.
Other authors diagree, concluding that there seems to be no correlation between magnetic reversals and extinction rates. Even during a complete reversal, the magnetic field doesn’t just vanish completely. Residual magnetism still offers some protection and Earth’s atmosphere alone is sufficient to protect life from the most harmful effects of radiation.