Ancient comets, plagues and the collapse of civilisations can be traced in the natural world, writes Claire O'Connell
Counting tree rings is an easy way to work out the age of a felled tree. But a closer look at the woody circles could also help shed light on some of the darkest periods in human history, according to palaeo-ecologist and Queen's University Belfast emeritus professor Mike Baillie.
He believes that ancient tree-ring records, backed up with ice core data and mythological stories, suggest that brushes with comets set the scene for two devastating pandemics - the plague of Justinian and the Black Death.
In the 1980s Prof Baillie helped to compile a 7,000-year chronology of tree rings from modern trees, historic and archaeological timbers and bog oaks. The aim was to calibrate radiocarbon dating, but when he went back to the data for a second look, he noticed some unusual patterns in the record. Of particular interest was the width of individual rings: the wider ones chart favourable conditions for growth while narrow rings indicate the tree didn't like what was going on, explains Prof Baillie.
"I became interested in extreme events where the trees were very badly affected and a number of trees put on their narrowest growth rings in their entire lifetime," he says. "Some of the events were at interesting dates. One was around AD 540, which is the start of the Justinian plague, a really traumatic event for humans."
Narrow tree rings crop up for this time in chronologies from many parts of the world, says Prof Baillie. "When trees all over the place show extremely bad conditions, you are left with a global environmental event. And from a scientific point of view you want to know what caused the environmental event."
Historical records don't point specifically to a cause for the misery, but detail plenty of symptoms of bad times around the world such as famine and the collapse of civilisations.
"The assumption was that we were dealing with the effects of big volcanoes," says Prof Baillie. But data from ice cores in Greenland show no signs of a volcano around AD 540: "That raises questions then of what on earth happened."
He believes something else loaded the atmosphere with dust, which screened out sunlight and caused cooling, leading to famine and other catastrophes. "The jump was to suggest if it wasn't a volcano, the next most likely cause was something extra-terrestrial, a brush with a comet," he says, adding that it would not have been a single, "dinosaur-killer" comet, rather that debris from a passing comet might have rained on the earth.
For further pieces of the jigsaw he turned to mythology and found that many stories for this period involve sky deities. In one Irish version, the sky god Lugh is seen rising brightly in the west with a long arm, which Prof Baillie believes describes a cometary event. "The notion that these stories are just made up fairy tales is fundamentally wrong. The fact that these stories were retold and conserved really accurately shows they were important," he says.
While the comet link is not proven, spikes of ammonium in ice cores could back it up. "Ammonium occurs in comets but it also occurs on earth, especially as the result of forest fires," explains Prof Baillie. "But when you find that there's a spike at AD 539, given the story that I'm putting together you think maybe this isn't a forest fire, maybe this is an impact signature."
Prof Baillie notes that ice cores also show a pronounced ammonium spike in AD 1348, around the time of the Black Death. Again there is no evidence of volcanoes, and you see another downturn in the tree rings from that time.
"So you are now linking the two great plagues with tree-ring effects and ammonium, and you are left thinking is this a signature of extra-terrestrial involvement?" asks Prof Baillie, who has recently published two books on his findings. "In my view, it requires only one person to find the right material from one or other of these events and show that there really was some sort of cosmic dimension to it. Then the story would flip into the mainstream."
Prof Mike Baillie will give two free public lectures about his work at 1pm and 6.30pm on Wed, February 14, at the National Museum of Ireland in Kildare Street, Dublin 2. See www.museum.ie