About 42,000 years ago, the earth was ravaged by oddities. The magnetic field collapsed. There were ice sheets over North America, Australasia and the Andes. Wind belts shifted across the Pacific and Southern Oceans. Australia was hit by prolonged drought; The largest mammals on that continent became extinct. People went to caves to make ocher-colored art. Neanderthals died forever.
Through it all, one giant kauri tree stood tall – until it died after nearly two millennia and fell into a swamp, where the chemical data embedded in its flesh was immaculately preserved. That tree, excavated a few years ago near Ngawha Springs in northern New Zealand, finally allowed researchers to adjust a tight timeline to what previously seemed like an intriguing but only vaguely correlated series of events.
What if, the researchers argue, the magnetic field crash spawned the climate changes of the time? And to think that the Ngawha kauri tree had witnessed the whole thing.
"It must have seemed like the end of days," said Chris S.M. Turney, a geoscientist at the University of New South Wales in Sydney, Australia, and part of a large team that detailed the findings in a study published Thursday in Science& # 39; And this tree has been through all that. That is unbelievable. "
By comparing the age data of the tree ring and the radiocarbon concentrations of that kauri tree and three others of similar age with recent dating information from two stalagmites in China's Hulu Caves, Dr. Turney and his 32 co-authors were able to determine when the tree lived and died. That gave them what they called a & # 39; calibration curve & # 39; which allowed them to convert radiocarbon dates from that period into calendar years.
Scientists from various disciplines said the kauri data was a dazzling addition to the radiocarbon gun and that it was long awaited.
"For a radiocarbon person, the kauri records are just great," said Luke C. Skinner, a paleoclimatologist at the University of Cambridge who was not involved in the study. He said the fossil kauri trees were the primary way for scientists to obtain radiocarbon information from so long ago.
The tree underwent a prolonged magnetic field disintegration, a period known as the Laschamp excursion, when the magnetic poles tried in vain to switch places. As a result, Dr. Turney and his co-authors were able to use the new data to describe more precisely when that excursion took place and to find out what else was going on, including the bizarre climate and the extinctions.
“It was suddenly, god, these things are actually happening all over the world at the same time, all at the same time,” said Dr. Turney. "It was just an extraordinary revelation."
That discovery unleashed a multiple thought experiment. Earth's magnetic field, constantly generated deep within the planet's molten outer core, protects against dangerous galactic and solar rays. Were all those special climatic, biological and archaeological phenomena 42,000 years ago related to the wasted magnetic field? Had the collapse changed the course of life on Earth? And what about other magnetic field perturbations, including that time 780,000 years ago when the magnetic poles actually switched places?
Scientists have been trying to find answers to these questions since the fact of polar polar reversals was established several decades ago. As a result, this latest effort has received a huge amount of attention.
"It's pretty brave," said Catherine G. Constable, a geophysicist with the Scripps Institution of Oceanography in San Diego who was not involved in the study.
Using advanced global climate model simulations that enabled chemical interactions, Dr. Turney and his colleagues used the timeline generated by the kauri tree to find out what the climate was like during the excursion.
The data revealed "modest but significant changes in atmospheric chemistry and climate," the paper said. Among them: a somewhat depleted ozone layer; slightly increased ultraviolet radiation, especially near the equator; a jump in tissue-damaging ionizing radiation; and auroras as close to the equator as the 40th parallel, which would run through the center of the continental United States in the Northern Hemisphere and through the bottom tip of Australia to the south.
Adding a period of low sun activity, known as grand solar minima, produced more dramatic effects. A peculiar, centuries-long series of deposits of beryllium-10 isotopes has been identified in Greenland ice cores, dating back to the Laschamp excursion 42,000 years ago. Such isotopes are created when cosmic rays occupy the upper atmosphere; in the geologic data, they indicate times when the earth experienced a diminished magnetic field and sometimes solar changes.
In the more extreme computer scenario, which takes into account solar effects, ultraviolet radiation increased by 10 to 15 percent from the norm and ozone decreased by about the same amount. Those effects flowed through the climate system, Dr. Turney said:
“It was actually like a perfect storm,” he said.
The simulations suggest that the weakened magnetic field caused some of the climate changes 42,000 years ago, and that those changes may have had greater consequences: wiping out many of Australia's large mammals, hastening the end of the Neanderthals, and housing art. while people hid for a long time to avoid skin-damaging ultraviolet rays, the authors suggested.
In fact, the effects were so striking that the researchers renamed the years leading up to the middle of the Laschamp excursion. They call it the Adams Transitional Geomagnetic Event.
“The Adams event appears to represent an important climatic, environmental and archaeological frontier that was previously unrecognized,” the team writes, concluding, “Overall, these findings raise important questions about the evolutionary consequences of geomagnetic reversals and excursions by the deeper geological record. "
The new name is a tribute to British humorist Douglas Adams, author of & # 39; The Hitchhiker's Guide to the Galaxy & # 39; and the book and radio series & # 39; Last Chance to See & # 39; about extinction. It's also a nod to Mr. Adams that "the answer to life, the universe, and everything" is 42 – of which Dr. Turney said it reminded him of the timing of the magnetic episode 42,000 years ago.
& # 39; It just seems creepy & # 39; he said with a laugh. "How did he know that?"
The interpretation is destined to create controversy. Some of the scientists who read the paper admired the breathtaking links between disciplines.
“One of the strengths of the paper, just from the perspective of its scientific work, and not necessarily the analytical science that it does, is simply the extent to which it brings together all of these disparate sources of information to make its case,” said Jason E Smerdon, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory in New York, who was not involved in the study. He called it a & # 39; tour de force & # 39 ;.
Likewise, James E.T. Channell, an emeritus professor of geophysics at the University of Florida who was not involved in the study but was a peer reviewer, said scientists have been hampered for half a century by whether a diminishing magnetic field is affecting life. The paper opens new avenues for research.
"If we knew enough about the timing of excursions, maybe we could look at the problem again," he said.
But other scientists said the extensive analysis made them question whether there were other explanations for some of the phenomena during the Laschamp excursion.
"It opens a can of worms instead of solving a series of questions," said Dr. Skinner.
Like several others who were interviewed, he was concerned whether the Adams Event nomenclature would create confusion in the scientific literature, and whether it was necessary. But he praised the newspaper for stimulating discussion.
"I am certainly more excited about this topic today than I was yesterday," he said.