Geoscience at the BA: What gave the Earth a nasty turn in 540?  September 8, 2000

 An unexplained global environmental event at AD 540, deduced from tree-rings, may have been caused by cometary debris from space. A counterview (espoused by journalist and historian David Keys in a recent book and television programme, which has recently come under attack) holds that it may have been the result of a supervolcano eruption in February 535.

“This debate that needs to be resolved” Mike Baillie of Queen’s University, Belfast told the British Association: “because whatever it was, it may have killed one third of the population of Europe just for starters.”

“The most interesting aspect of this global event is that it is not there in conventional historical sources. Why not? Hints of it appear in mythology, a subject normally taboo for scientists and historians. This issue raises the different approaches of scientists and historians wherein scientists can accumulate items of low significance - weak evidence - whereas historians can dismiss each weak item singly, thus missing the event altogether.

"I am calling for a debate by scientists and historians on how to approach the evidence for catastrophic events of this kind that were previously not known to have taken place. What the scientist is seeking is not proof of cause but merely hints of likely cause to aid in the search for hard evidence.”

Back in the 1970s, when people in Europe started to build tree-ring chronologies in a serious manner, they tended to think in 'local' terms. A north German oak chronology would be 'different' from a south German chronology; an Irish oak chronology would be different from one for England, and so on. This assumption about regional differences was largely based on everyone's experience with local weather - if the weather is different the tree-ring patterns will be different, researchers assumed. We know now that the situation is  not like that at all. There are many years where the vast majority of oak trees exhibit similar responses all the way from Ireland to Poland.

Various workers have built tree-ring chronologies - year-by year records of what trees 'thought' of their growth conditions - back for hundreds, and in some cases thousands, of years. The chronologies were constructed for different reasons. For example, several were constructed just to calibrate the radiocarbon timescale - dendrochronologists could supply wood samples of precisely known age that could be dated by radiocarbon methods.  Others were constructed to date archaeological timbers, while others were intended to reconstruct of temperature or rainfall. The chronologies differ in length; there are hundreds of regional chronologies for the last 500 years. The number of chronologies which cover the whole of the last 2000 years is less than 30, while the number which are continuous back to 5000 BC drops to about eight.

The result of all this chronology building was that by the 1990s some dendrochronologists could make a start on identifying global events. Keith Briffa and colleagues have looked in detail at the mean response of high latitude conifers to temperature - basically this can be deduced from reduced wood density; low temperature in the growing season leads to low density wood for that year. They observe that the years of low wood density were associated with the effects of volcanic eruptions (e.g., 1601, 1783, 1816). We can compare the Northern Hemisphere density record for pine with our own European oak record for ring width. When we do this we see that not all the events in the pine show up as reduced growth in the oaks. So, some events are more severe than others, in terms of the widespread nature of their effects; and they show up in both the pine and the oak records.

Baillie explains: “Trees are supplying information which has, up until now, not been available to historians. This type of information may well have profound implications for our understanding of the types of environmental event that can affect human populations. Put bluntly, there seems to be an environmental component which is largely missing from the history which human scribes have left to us. The trees seem to be able to supply aspects of this missing component.  If this really is the case, some history may have to be rewritten.

Tree-ring events in the last half millennium or so suggest that explosive volcanic eruptions cause cooling which lasts for a year or two. Sometimes there are real effects on the ground such as the 'year without a summer' recorded in 1816 in most of the North Atlantic region, following the eruption of Tambora in 1815. So, in recent times human documentation of failed harvests and outbreaks of disease coincide with the growth downturns observed in the trees.

However as dendrochronologists go further back in time they discover that there is a different class of event. The first event of this kind occurs in the immediate vicinity of AD 540. Around that time several tree-ring chronologies from places as dispersed as Siberia, Fennoscandia, Northern Europe, Western North America and Southern South America all show notable growth downturns at the same time.  “This is highly unusual” says Baillie. Just how unusual it is can be conveyed by the fact that these chronologies quantify the 540 event as 'the worst' or the 'second worst' or 'one of the four worst' events in the last 1500 years. There is no other equivalent event in the last 1500 years.

Baillie says: “One dramatic point about the events around AD 540 is that they do not occur in mainstream history. You cannot lift a conventional history book published before 1995 and read about the cometary bombardment of 536-545; nor can you read about the alleged supervolcano of February 535. You can read about the Justinian plague of 542, but not about the associated environmental events or their causes. So how can this have happened? The trees single out an episode which can best be described as catastrophic and which may, for all we know, mark the start of the Dark Ages, and it isn't there in written history.

“The surprising thing is that if you look closely you can find a reference to a “comet in Gaul, so vast that the whole sky seemed on fire” in 540/541. But to historians such references, preserved in thirteenth century texts, carry no weight. Similarly, the Arthurian stories, with their Celtic antecedents of bright sky gods, Dolorous Blows and “wasteland” come with traditional dates for Arthur's death in 537, 539 and 542. These myths hint strongly at a bombardment vector for the environmental downturn, but are almost universally dismissed as fiction or fantasy by academics. So, the question I wish to pose is this; how does a scientist looking for the physical cause of an abrupt, and catastrophic, environmental event handle inherently weak information? Remember, no-one is trying to use early historical references or myths to prove what happened at 540, all we want are some clues to guide us towards what we should be looking for - what is it most likely to have been?”

“Now, if handling myth is difficult for a scientist, handling odd, and apparently worthless, snippets of information - throwaway lines - is equally difficult for historians. But 540, and other events like it in the more distant past, are potentially so important that they need to be understood. This may require a new approach to handling weak information and more interaction between scientists and historians. My current thinking is that scientists can accumulate the low significance of even individually weak pieces of evidence to arrive at a relatively robust conclusion, whereas historians use the low significance of the individually weak pieces of evidence to dismiss each in turn ending up with a negative conclusion - nothing happened. Unfortunately, the events recorded in the tree rings around 540, and at other times, demand a positive answer and preclude a negative one.”

Baillie is planning to conduct chemical analysis of the growth rings of trees, which grew across the 540 event, looking for the smoking gun that will point to the likely cause.  He hopes to persuade the ice-core workers to analyse dated ice layers relevant to the period to look for similar clues. He is also interested in detailed contouring of trees most affected by the event in the hope of identifying at least one epicentre fo the event – whatever it was.