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Dendrochronology: How Climate Catastrophes Show Up in Tree Rings
Dendrochronology is the art and science of using tree rings to count the age of a tree. Tree rings change in width from year to year because growing conditions are variable from year to year. Thus, a tree-ring chronology is a year-by-year record of what trees “thought” of their growth conditions in the past, for a particular species in a particular geographic region. (1) Tree rings yield knowledge of ancient droughts, fires, cold spells, and heat waves. Knowledge of past environmental downturns helps to understand what is happening in our environment now and what may happen in the future.
One of the best chronologies in the world is the continuous record of Irish oak growth for each of the last 7,400 years developed by Irish dendrochronologists from the large number of ancient Irish oaks preserved in Irish bogs. They have discovered, for example, that a “tree-ring event”, occurred in 3195 B.C., at the time that prehistoric people built the henge at Stonehenge in Europe and the Egyptians started their civilization in the Near East! (2) Another fine tree-ring chronology is the continuous record developed over the past century by researchers with the American Laboratory of Tree-Ring Research at the University of Arizona. (3,4) What is the history of dendrochronology, how does the system work, and what are examples of climate catastrophes it has identified? History of DendrochronologyThe recognition that trees produce an annual growth ring goes back at least to Theophrastus (370 -285 B.C.), a student of Aristotle (384 B.C. – 322 B.C.), who figured out the fresh growth formed on the outer circumference of a tree. Leonardo da Vinci (1452 – 1519 A.D.) deduced the relationship between ring width and moisture availability, which he used to reconstruct past weather from earlier ring widths. In the 1700s, people were using the annual character of the rings to count back from known felling dates to look at specific growth years. In the 1800s, Charles Babbage (1791 – 1871 A.D.) presciently predicted (in 1837) that cross-dating trees--that is, overlapping the patterns of wide and narrow rings from tree to tree, would eventually allow the construction of long chronologies using the ancient trees in bogs and lakes. (5-7) American Astronomer-Dendrochronologist Andrew Ellicott DouglassIn the 1900s, the American astronomer Andrew Douglass (1867-1962), who ran northern Arizona’s Lowell Observatory at Flagstaff, became interested in how earth’s weather responded to variations in sunspots, the powerful magnetic fields on the surface of the sun. He hit on the brilliant idea that tree growth observed in tree rings might correlate with sunspot activity over time—that is, when sunspot activity was high, tree growth would be high. He also noticed that year-to-year variation (short-term growth) in tree ring growth depended on the changing amounts of rainfall from year to year. Longer-term growth trends, he postulated, were due to changes in solar activity over time. He determined that the Arizona tree-ring record supported an 11-year solar cycle back to about AD 1700, but a ten-year cycle in the 1700s. (8-9) Hooked on tree-ring counting as a means to better understanding the sun, he decided to extend the tree-ring record backwards in time as far as he could by overlapping the wide and narrow growth-ring patterns from living Arizona pines to the patterns from ancient archaeological timbers in various ruins in the Southwest. “If the trees were growing over some common period he could match their ring patterns over their common years,” explains Irish dendrochronologist Dr. Mike Baillie. (10) Douglass acquired timbers from ancient Pueblo ruins and elsewhere, extending his chronology back to 1284 A.D., but became frustrated with a “floating series” of trees, which he could not link to any part of his existing chronology. Finally, on June 22, 1929, a charred roof timber turned up from the Whipple ruin at Show Low, Arizona, which enabled Douglass to link the modern and floating chronologies, thereby extending his uninterrupted tree-ring chronology back to 701 A.D. Suddenly the American Southwest had the best archaeological chronology in the world! European DendrochronologyThe main researchers interested in dendrochronology in Europe were Germans Bruno Huber (1930-1999) and Veronica Giertz, who by the 1960s, had produced a 1000-year central German oak chronology, while another investigator Ernst Hollstein, working at Trier to the northwest, built another, which is almost identical. (11) Later, a group of oaks from one field at Croston Moss in Lancashire, England, provided a continuous chronology from 3200 B.C. to 970 B.C, with just a single gap between 1680 and 1580 B.C. Baillie points out the need to pay careful attention to stubborn “gaps” because they may suggest climatic catastrophes that expunged trees from the record. Two different groups of European workers found all the pieces of timber needed to complete the Belfast and Gottingen tree-ring chronologies, which were in precise synchronization back to 5000 B.C. By the early 1980s, dendrochronologists had constructed chronologies dating back 7000 years. Since then German workers have pushed back the tree-ring chronologies to 9000 years ago! (12)
Dendrochronologies Useful in Delineating Causes of Historic Famines and PlaguesTree-ring chronologies have proved exceptionally useful in dating archeological sites and providing invaluable environmental information about the past. This information is independent of the potential bias present in human history. The human written record frequently speaks of terrible “famines” and “plagues”, but seldom are the physical causes delineated. Plagues and famines, however, do not “just happen”. Something causes them to happen—a physical cause or causes. Why have historians over the millennia failed to record what they saw as the causes of the famines and plagues they observed? “If something really significant had happened, surely it would have been recorded?” argues Baillie. (13) The reasons are fourfold. First, the physical causes in many cases were physically remote from the affected population(s). For example, a volcano that loaded the atmosphere with dust could dim the sun, precipitating famines and population dislocation far from the volcano, even on the other side of the earth. Second, people assume that the historical record is more detailed than it really is. Much of it has been lost. Third, early writers varied in their critical faculties. Fourth, conditions may have been adverse to record keeping. “Witnesses to truly catastrophic events may have been in no position to make a record, having had other priorities such as survival.” (14) Fifth, people in the position to selectively edit or destroy parts of the historical record may have done so for a variety of reasons. Sixth, evolutionist Charles Darwin and geologist Charles Lyell during the 19th century rejected the idea of catastrophism (“environmental determinism”) suggested by tree ring abnormalities, and instead strongly advocated uniformitarianism, the assumption that the natural processes of the past are the same as those that can be observed operating in the present. Even today, there is a strong lobby against catastrophism. (15) Baillie argues, “So deep is the conditioning that the environment is not a significant factor that no attempt is made to understand the causes of famines and plagues which are perceived as ‘acts of God’”. (13) Example 1: The Narrow Ring Event in the Mid-17th Century B.C.During the 1980s, tree-ring workers in Arizona noted that bristlecone pines, which live at around 9,000 feet, are exquisitely sensitive to temperature, especially in years affected by so-called volcanic dust-veils—where volcanoes blast dust and water vapor high into the stratosphere and cause cooling on the earth’s surface. (16-17) Bristlecones, because of their dense, highly resinous wood, are not susceptible to invasion by bacteria, fungus, or insects that prey upon most plants. Investigators identified an extreme frost event in 1627 B.C., evidenced by an irregular tree ring signature, which prompted Irish dendrochronologists to examine their Irish oak specimens dating around 1627 B.C. Dr. Baillie and his colleagues discovered among their 22 bog oak samples from that period one Irish oak with a band of seven narrow rings starting in 1628 B.C., and another series of trees showing the narrowest rings of their lifetime during the decade of the 1620s. (16) Arizona bristlecone and Irish bog oak researchers call their combined observations “historic synchronicity”, which suggests that whatever happened in 1627 B.C. affected the entire globe. One source concludes: “1628 BC, then, and the years immediately following, were bad years in areas as widely separated as California, Northern Ireland, the Aegean (presuming [volcano Santorini/]Thera was the culprit), and even, perhaps, China. (18) Santorini was a volcanic island in Greece (Thera), known to have erupted in the mid-second millennium B.C. (19) The absolute dating of the Santorini eruption by dendrochronology confounds archeologists who believe it erupted in the 16th, not the 17th, century, B.C., i.e., between 1550 B.C. and 1500 B.C. The dendrologists’ reckoning of the timing of the Santorini eruption, which carbon dating and Greenland ice core findings support, suggests the need for a rescaling of the well-established “conventional Egyptian chronology” (i.e., the rulers of ancient Egypt), which archeologists are debating, according to one source. (20-21) Example 2: The Narrow Ring Event in the Sixth Century A.D.In 1983, Stothers and Rampino drew attention to a major “dry fog” in the year 536 A.D. recorded in Mediterranean histories. They attributed this fog to a volcanic dust-veil, the most serious in 2,000 years. (22) Unfortunately, the European oak chronologies, including the Irish oak chronology, do not validate their assessment. Though the growth ring for 536 A.D. is somewhat narrow, a much more severe growth reduction took place across 540 A.D., an event recorded in tree rings around the world. Moreover, the Greenland ice core data available up to 1997 showed “no good evidence” for a volcanic acid signal in the time window 536-545 A.D. (23) What does this mean? Something awful happened around 540 A.D., as described in detail elsewhere. (24) The Plague of Justinian (540-451 A.D.) broke out in the Eastern Roman Empire. Here we find no attempt in the historical record to determine the cause of the plague, as noted above. However, several Roman observers wrote about bad weather at the time. For example, one eyewitness, a Syrian bishop named John of Ephesus, described the extraordinary events during the years 535 and 536 A.D. as follows: “There was a sign from the sun, the like of which had never been seen or reported before. The sun became dark and its darkness lasted for 18 months. Each day it shone for about four hours, and still this light was only a feeble shadow. Everyone declared that the sun would never recover its full light again.” Cassiodorus writing in Italy also referred to a dimming of the sun: “We have had a spring without mildness and a summer without heat ... The months which should have been maturing the crops have been chilled by north winds. Rain is denied and the reaper fears new frosts.”(24, 25) If a volcanic dust-veil did not cause the narrow tree-ring event between 536 A.D. and 545 A.D., what did? The next most likely cause of atmospheric loading after volcanism is either comet dust or airbursts associated with comets, e.g., multiple Tunguska-class impacts, as described elsewhere. (26) British astronomers Victor Clube and William Napier suggest that earth was at increased risk of bombardment in the time window 400-600 A.D. (27) Dendrochronology: The Past as FutureTree-ring dating methods have identified intermittent severe climate catastrophes in the past 7,000 years, raising the question, what caused the catastrophes and their associated plagues and famines? The human historical record is rich in allusions to catastrophes (e.g., the parting of the Red Sea during the Hebrew Exodus from Egypt, and manna dropping from heaven, among dozens of others). Until recently, however, most people have attributed these bizarre events to metaphorical thinking, i.e., they didn’t really happen. Or did they? Dendrochronology, supported by carbon dating and Greenland ice core chemical analyses, compels reassessment of the historical record. Moreover, the narrow tree-ring findings that are not associated with volcanic acid signatures in Greenland ice cores suggest we should be thinking about alternative sources of atmospheric dust, i.e., comet dust. Dendrochronology makes us smarter about what the future may hold. Notes:
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