Buried in mud for millennia, some of New Zealand’s ancient kauri trees are revealing surprising clues about Earth’s climate—past, present, and future.
In February 2019, Mark Magee was scraping the bucket of his 45-ton excavator through a hillside when it hit something 30 feet down that wouldn’t budge.
It was high summer in the Southern Hemisphere, and Magee, a construction foreman, was clearing a platform for a new geothermal power plant near Ngāwhā, a tiny community in New Zealand’s Northland Region, the long peninsula that stretches from the city of Auckland to the country’s northern tip.
He called in additional digger drivers to help. Gradually, as the machines peeled away the mudstone encasing the obstinate object, they realized it was a tree—and no ordinary tree. More and more of it appeared, a seemingly endless log. When the thing lay uncovered, complete with a medusa-like rootball, it measured 65 feet long and 8 feet across, and weighed 65 tons.
It was a kauri tree, a copper-skinned conifer endemic to New Zealand. The indigenous Māori hold the species sacred, and use its honey-colored softwood for traditional carvings and ocean-going canoes. Though this kauri tree had clearly been buried for thousands of years, Magee was astonished to see leaves and cones stuck to its underside that were still green.
The kauri tree, or Agathis australis, is one of the largest and longest-lived tree species in the world. An individual kauri can live for more than two millennia, reaching 200 feet tall and more than 16 feet in diameter. Today, the living trees grow only in remnant pockets in northern New Zealand, where the national Department of Conservation lists them as threatened, due to a century of heavy logging, forest clearing for agriculture, and, more recently, the onslaught of a deadly fungus-like pathogen.
Yet for tens of thousands of years, kauri forests dominated a vast swath of the upper North Island. As the trees grew, they recorded information in their annual rings about the climate and makeup of the atmosphere. When they fell, some of the heaviest plunged deep into nearby peat bogs, where they stayed mostly unchanged for millennia.
Beyond their use in dating, the radiocarbon patterns in kauri rings may also have something to teach us about changes in Earth’s geomagnetic field. In 1859, the sun erupted in a massive solar flare and coronal mass ejection, sending a barrage of radiation toward Earth. The resulting day-long geomagnetic storm—called the Carrington Event—took out the nascent telegraph system. The aurora was visible in Mexico, Cuba, Hawaii, and Queensland. The flares were so bright in the northeastern United States that people could read the newspaper by their light, and gold miners in the Rocky Mountains began preparing breakfast in the middle of the night because they mistook the glow for dawn.
Radiocarbon research has pinpointed even larger solar storms further back in history. In 2012, Japanese researcher Fusa Miyake used Japanese tree rings to show that a much more powerful flare—now called a Miyake Event—occurred around 774 AD, leaving a dramatic radiocarbon spike in the wood. The same year, the medieval Anglo-Saxon Chronicle—a history written in the 9th century—recorded an eerie “red crucifix” in the heavens. An identical radiocarbon spike showed up in swamp kauri rings at the same time, according to a Swiss study that included contributions from Hogg and Boswijk. Scientists identified similar Miyake Events in 994 AD and 5480 BC. High-resolution swamp kauri records could reveal yet more from tens of thousands of years ago.
When Hogg adjusted the radiocarbon dates of the tree based on the calibration curve, he realized they spanned the onset of the Laschamps Event. The kauri had sprouted into a sapling around 42,715 years ago; 1,600 years later, it crashed into the mud. Hogg and Lorrey worked with Alan Cooper of the South Australian Museum and Chris Turney of the University of New South Wales to match the information in the tree’s rings with other natural archives to build a precise sequence of events. The story they tell, relayed in a paper published in Science this February, is one of global environmental catastrophe.
When the Ngāwhā kauri was a few centuries old, Earth’s magnetic field weakened dramatically. Coincidentally, the sun went into one of its periodic, century-long dormant periods, called a “Grand Solar Minima,” in which it produces many fewer sunspots than usual and gives off less energy.
This, in turn, decreased the flow of solar wind around the planet, which normally provides another protective magnetic field. With little defense against cosmic and solar radiation, Earth’s atmosphere was bombarded for a few hundred years with ionizing particles from space. Lightning likely raged, auroras shimmered even in temperate skies, and weather patterns abruptly altered. “It would have been freaky as…,” says Cooper. The Ngāwhā kauri lived through it all.
Over the following centuries, ice sheets rapidly expanded across North America, while Australia shifted to a more arid climate, its inland lakes drying up. Cooper suggests those changes are what drove the extinction of numerous species of Australian megafauna—like the giant, wombat-like diprotodon—more than 10,000 years after modern humans arrived on the continent. The implication is that humans alone weren’t responsible for the extinctions: They had only to stake out diminishing waterholes to finish off the ill-fated animals.
42,000 years ago is also when figurative art first appeared in caves all over the world, from Europe to Indonesia. Cooper thinks the onset of the Laschamps Event might have something to do with that, too. Perhaps the adverse weather conditions and very high UV levels forced our ancestors into caves, where the images they would normally have painted on more ephemeral surfaces like cliffs or trees were instead preserved for millennia on cave walls. Intriguingly, Neanderthals went extinct just afterwards – around 40,900 years ago. The study’s authors suggest the combination of the solar minima and the weakened magnetic field triggered the climate changes and the flow-on effects on humans and animals. “It explains a bunch of patterns around 42,000 years ago that had previously been mysteries,” says Cooper.
The study also suggests that, contrary to the established wisdom, severe geomagnetic and solar events can affect global climate, though the scientists are quick to point out that they don’t explain current warming. The prospect opens up a whole new multi-disciplinary field of research, Cooper says. “It gives you a view of a much more changeable planet, and a much more changeable climate system.”
The kauri dug up in Ngāwhā was the key to it all—a kind of Rosetta Stone. “It’s very unusual to get the Earth’s geomagnetic field this low,” says Hogg, “and to get such a long-lived tree that grew right through it—we should be doing more with it. This is just the start.”
https://www.biographic.com/swamp-sentinels/