Did Asteroids and Comets Turn the Tides of Civilization?
Chaos from Above
Did Asteroids and Comets Turn the Tides of Civilization?
The heart of humanity seems at times to have lost its cadence, the rhythmic
beat of history collapsing into impotent chaos. Wars raged. Pestilence
spread. Famine reigned. Death came early and hard. Dynasties died, and
Such a time came in the sixth century A.D. The Dark Ages settled heavily
over Europe. Rome had been beaten back from its empire. Art and science
stagnated. Even the sun turned its back. "We marvel to see no shadows of our
bodies at noon, to feel the mighty vigor of the sun's heat wasted into
feebleness," Italian historian Flavius Cassiodorus wrote at the time. "We
have summer without heat. The crops have been chilled by north winds, (and)
the rain is denied."
In China, "the stars were lost from view for three months." The sun dimmed,
the rain failed, and snow fell in the summertime. Famine spread, and the
emperor abandoned his capital amid political and economic disasters.
Then came pestilence. The Justinian plague, named for a Byzantine emperor,
apparently began in central Asia, spread into Egypt, and then swept across
Europe. Hundreds of thousands died.
The world had gone to hell in a hurry, if the historical accounts can be
believed. But with neither evidence of global disaster nor a viable cause,
the records were widely doubted by historians.
New evidence, however, supports the tales of ancient scribes and identifies
brief but brutal times of worldwide ecological catastrophe. The evidence is
in tree rings, which clearly show several years of cold weather that stunted
growth beginning in A.D. 536 and especially after A.D. 540-541. The rings
show similar events that began in 1628 B.C. and 1159 B.C., and rare written
documents of those times seem also to describe cataclysmic social collapse.
What weapon does nature wield that is powerful enough to alter the course of
civilizations within a few years? The most likely explanation, the best fit
with the evidence, is that described by both Chinese and Europeans as
dragons in the sky: Pieces of comets (or perhaps of asteroids) crashed into
Earth, spewing a veil of dust that encircled the world and dimmed the sun.
A much larger and rarer bolide (an exploding meteoric fireball) is assumed
to have ended the reign of the dinosaurs some 65 million years ago. A
smaller and more common one exploded over the Tunguska River in the Siberian
wilderness 91 years ago with 2,000 times the power of the bomb that
devastated Hiroshima in 1945. And just five years ago, astronomers watched
the fragmented comet Shoemaker-Levy 9 plow spectacularly into Jupiter.
I believe the association between the tree-ring data and historical
documents and folktales is real: Earth faced catastrophic environmental
dislocation at or around 1628 B.C., 1159 B.C., and A.D. 540 (and probably in
2354 B.C. and 208 B.C., as well) because of near-miss comets, either through
dust-loading of the atmosphere as Earth passed through the comet's dusty
tail or through direct bombardment by cometary fragments. (They must have
been near misses, because if we had been hit by a full-blown comet in the
past 10,000 years or so, we wouldn't be here today.) This hypothesis is not
proven, but the circumstantial evidence is overwhelming.
The strongest evidence comes from tree rings and the science of
dendrochronology. Tree rings record the age of a tree, with a distinct ring
of growth produced each year. The width of each ring depends on growing
conditions, so each year's growth in a particular area leaves a unique
signature (a reflection of fat, moderate, or lean growing conditions) in the
By calibrating the rings through progressively older trees from a specific
region, archaeologists can build millennia-long chronologies that allow them
to date ancient wooden artifacts. (See Discovering Archaeology, May/June,
page 45.) The pattern of tree rings in an artifact can be matched to the
regional chronology to determine the year in which the tree died.
A less-well-known consequence of these chronologies is that we can now
identify periods in which trees grew very little or not at all. This is
indicated by clusters of extremely narrow rings, which suggest extremely
cold growing seasons. A band of these narrow rings occurred after A.D. 540
and lasted about six years in parts of Europe, Asia, and North America.
Similar ring patterns are found around 1159 B.C. and 1628 B.C. These dates
may coincide with the collapse of Bronze Age civilizations across Eurasia.
They may also be recalled in the biblical book of Exodus and contemporary
records from China.
The first inkling that tree rings might record catastrophic events came in
the mid-1980s from dendrochronologist Val LaMarche and volcanologist Kathy
Hirschboeck. In the extremely long-lived bristlecone pines of the western
United States, they noted a frost-damage ring at 1627 B.C. and suggested it
might reflect the massive eruption of the Santorini volcano in the Aegean
Sea. Similar frost rings followed the eruptions of Krakatoa in Indonesia
(1883) and Katmai in Alaska (1912).
After a major volcanic eruption, Earth is veiled by a layer of fine debris
circulating in the stratosphere. This layer reflects sunlight away from
Earth, causing the surface to cool.
As a result of their suggestion, I searched the ring patterns derived from
oak logs that had been preserved in the peat bogs of Ireland. I found that
many trees exhibited the worst growth - the narrowest rings - of their
lifetimes starting in 1628 B.C. Only a few other such events were seen in
the rings, but two others were at 1159 B.C. and A.D. 540. Those years are
close to dates for acid-rich layers (attributed to volcanic eruptions) that
had been identified in ice cores taken in Greenland. We seemed to be onto
Mandate of Heaven
Then astronomer Kevin Pang of the California Institute of Technology
(Caltech) noted that 1628 B.C. and 1159 B.C. roughly mark the beginning and
end of the Shang Dynasty of Bronze Age China. Both ends of the dynasty
featured, according to ancient Chinese texts, environmental disasters -
dimming of the sun and summer frosts that caused crop failures and famine.
Pang notes also the Chinese concept of "mandate of heaven," wherein a
dynasty reigned only as long as it protected the well-being of its people.
This notion might have originated in the coincidence of dynastic change and
The Caltech team also noted similar descriptions from A.D. 536-545 that
describe climatic disruptions that led to catastrophic famines and great
loss of life.
Much was going on in the world around these three dates. The four centuries
of the Greek Dark Ages, which began after the Mycenaean era of mainland
Greece collapsed amid great social upheaval, are thought to have begun in
the twelfth century B.C. This period also saw the end of the once-mighty
Hittite civilization of Anatolia in the Near East and of Bronze Age Israel.
The situation in Egypt is more ambiguous. Egypt's prosperous New Kingdom
grew out of a century or so of warfare and upheaval known as the Second
Intermediate Period, which itself followed the end of the Middle Kingdom.
The New Kingdom has been dated from 1550 B.C. to 1070 B.C. While that is 70
years later than our two dates (1628 B.C. and 1159 B.C.), the time span is
almost exactly the same. Some scholars have questioned traditional Egyptian
dating, and it seems possible the timing of the New Kingdom, some 3,500
years ago, might be a little off.
Then the volcano hypothesis began to dim. Volcanologists noted that
volcanoes normally would not be powerful enough to collapse dynasties - the
dust and acid, even if sufficient to dim sunlight, washes out of the
atmosphere within a few years. And a review of the ice-core evidence from
Greenland failed completely to confirm an exceptional volcanic eruption at
It appears now that something far more damaging than volcanoes may have been
at work here, especially after seeing unassailable proof that comets can hit
planets: the extraordinary spectacle of comet Shoemaker-Levy 9 crashing into
Jupiter in 1994. Comets appear in Chinese records of events at the beginning
and end of the Shang dynasty. Were the catastrophic environmental downturns
at 1628 B.C., 1159 B.C., and A.D. 540 caused by encounters with comets?
Archaeologists and astrophysicists do not necessarily read each other's
work, and it mostly escaped notice that three British cometary
astrophysicists - Mark Bailey, Victor Clube, and Bill Napier - had published
a highly relevant paper in 1990. They wrote that Earth had been at increased
risk of bombardment by cometary debris in the period A.D. 400-600. They
based their conclusion on the increased number of great meteor showers
during that period.
It's hard to overestimate the devastation that could result from a serious
bolide impact on Earth. The impact of fragments measuring between one and
several hundred meters across can cause fiery, multimegaton explosions that
destroy natural and cultural features across huge areas through fire blasts,
earthquakes, and tidal waves (if the debris arrives over the sea).
The danger in A.D. 400-600, concluded Bailey and colleagues, was of Earth
running into a "cosmic swarm" of objects the size of the one that exploded
over Tunguska, Siberia, in 1908. Some astronomers believe we can expect
Tunguska-type impacts every 50 years on average, while an impact with
explosive power in the 1,000- to 10,000-megaton range - a super Tunguska
event - is likely in any 5,000-year period. Such impacts could trigger
enormous global ecological catastrophe.
Impacts between those two extremes might be expected often enough to account
for these calamities. Direct evidence, however, is scanty. Associating
craters to specific events is problematic at best; the Tunguska event left
no significant crater at all, since the bolide exploded a few kilometers
above the surface. Impacts in or over the ocean would not leave physical
We turned, then, to the written record and oral traditions. Comets were
extraordinary objects that seemed rarely to escape written notice. Zachariah
of Mitylene noted about A.D. 540 that - a great and terrible comet appeared
in the sky at evening time for 100 days." Chinese texts about the same time
say: "Dragons fought in the pond of the K'uh o. They went westward. ... In
the places they passed, all the trees were broken." Similar descriptions are
common throughout the Old World.
Sixth-century events generally are well-dated. But with more ancient
documents and traditions, dating usually is ambivalent at best. This is why
similarly spaced events in the second millennium B.C. are so interesting.
What are the chances of similarly spaced events in both Hebrew and Chinese
histories, both with cometary associations, arising by chance?
There is, I feel, a strong case for the contention that we do not inhabit a
benign planet. This planet is bombarded relatively often. If this story is
correct, we have been bombarded at least three times - and probably five
times - since the birth of civilization some 5,000 years ago. And each time,
the world was changed.
Mike Baillie is a leading dendrochronologist and Professor of Palaeoecology
at Queen's University, Belfast, Northern Ireland. His book, Exodus to
Arthur, describes in detail his theory of comet encounters and turning
points of civilization.