Author: Евген&# (---.d.dial.de.ignite.net)
Date: 02-08-04 18:06
Clark Whelton zum Thema:
> CCNet 17/2004 - 4 February 2004
> DID A COMET IMPACT TRIGGER A "NUCLEAR WINTER" in 536 AD?
> -------------------------------------------------------
>
>
> Scientists at Cardiff University, UK, believe they have discovered
> the cause of crop failures and summer frosts some 1,500 years ago -
> a comet colliding with Earth. The team has been studying evidence
> from tree rings, which suggests that the Earth underwent a series of
> very cold summers around 536-540 AD, indicating an effect rather
> like a nuclear winter. The scientists in the School of Physics and
> Astronomy believe this was caused by a comet hitting the earth and
> exploding in the upper atmosphere. The debris from this giant explosion
> was such that it enveloped the earth in soot and ash, blocking out
> the sunlight and causing the very cold weather.
> --Cardiff University, 3 February 2004
>
>
> The beginning of the Dark Ages may have been literal, as well as
> figurative, as the result of a massive volcanic eruption in the 6th
> century, according to a volcanologist at the Department of Energy's
> Los Alamos National Laboratory. Ken Wohletz said an eruption in the
> Indonesian archipelago could have produced a 150-meter-thick cloud
> layer over the entire Earth, triggering a chain of climatic, agricultural,
> political and social changes that ushered in the Dark Ages. Evidence
> supporting the catastrophe includes tree-ring and ice-core measurements,
> indications of a huge underwater caldera, and ash and pumice in the
> same area.
> --Los Alamos National Laboratory, 17 December 2000
>
>
>
> (1) ASTRONOMERS SUGGEST COMET TO BLAME FOR 6TH CENTURY "NUCLEAR WINTER"
> Ron Baalke
>
> (2) SCIENTISTS: COMET CAUSED DARK AGE FROSTS
> The Washington Time, 3 February 2004
>
> (3) COLLISION WITH COMET MAY HAVE HASTENED FIRST PLAGUE EPIDEMIC
> The Independent, 4 February 2004
>
> (4) SUPER-VOLCANO MAY HAVE TRIGGERED GLOBAL COOLING IN 536 AD
> Los Alamos National Laboratory, 17 December 2000
>
> (5) SUPER-DROUGHT MAY HAVE SET OFF EUROPEAN DARK AGES
> University of California, Berkeley, 9 December 1998
>
> (6) ICE CORE EVIDENCE FOR VOLCANIC ERUPTION ~530 AD
> Environmental Catastrophes and Recoveries in the Holocene
>
> (7) JUMBLING OLD EVENTS WITH MODERN MYTHS
> British Archaeology, November 1999
>
> (8) A COMET IMPACT IN AD 536?
> Astronomy & Geophysics, Volume 45 Issue 1, February 2004
>
>
> ===========
> (1) ASTRONOMERS SUGGEST COMET TO BLAME FOR 6TH CENTURY "NUCLEAR WINTER"
>
> Ron Baalke
>
> Contact: Dr Derek Ward-Thompson
> derek.ward-thompson@astro.cf.ac.uk
> 029-2087-5314
> Cardiff University
> February 3, 2004
>
> Astronomers unravel a mystery of the Dark Ages
>
> Undergraduates' work blames comet for 6th-century "nuclear winter"
>
> Scientists at Cardiff University, UK, believe they have discovered the
cause
> of crop failures and summer frosts some 1,500 years ago - a comet
colliding
> with Earth.
>
> The team has been studying evidence from tree rings, which suggests that
the
> Earth underwent a series of very cold summers around 536-540 AD,
indicating
> an effect rather like a nuclear winter.
>
> The scientists in the School of Physics and Astronomy believe this was
> caused by a comet hitting the earth and exploding in the upper atmosphere.
> The debris from this giant explosion was such that it enveloped the earth
in
> soot and ash, blocking out the sunlight and causing the very cold weather.
>
> This effect is known as a plume and is similar to that which was seen when
> comet Shoemaker-Levy-9 hit Jupiter in 1995.
>
> Historical references from this period - known as the Dark Ages - are
> sparse, but what records there are, tell of crop failures and summer
frosts.
>
> The work was carried out by two Cardiff undergraduate students, Emma Rigby
> and Mel Symonds, as part of their student project work under the
supervision
> of Dr Derek Ward-Thompson.
>
> Their findings are reported in the February issue of Astronomy and
> Geophysics, the in-house magazine of the Royal Astronomical Society.
>
> The surprising result of the new work is just how small a comet is needed
to
> cause such dramatic effects. The scientists calculate that a comet not
much
> more than half a kilometre across could cause a global nuclear winter
> effect. This is significantly smaller than was previously thought.
>
> Dr. Ward-Thompson said: "One of the exciting aspects of this work is that
we
> have re-classified the size of comet that represents a global threat. This
> work shows that even a comet of only half a kilometre in size could have
> global consequences. Previously nothing less than a kilometre across was
> counted as a global threat. If such an event happened again today, then
once
> again a large fraction of the earth's population could face starvation."
>
> The comet impact caused crop failures and wide-spread starvation among the
> sixth century population. The timing coincides with the Justinian Plague,
> widely believed to be the first appearance of the Black Death in Europe.
It
> is possible that the plague was so rampant and took hold so quickly
because
> the population was already weakened by starvation.
>
> ================
> (2) SCIENTISTS: COMET CAUSED DARK AGE FROSTS
>
> The Washington Time, 3 February 2004
> http://www.washtimes.com/upi-breaking/20040203-033138-9377r.htm
>
> CARDIFF, Wales, Feb. 3 (UPI) -- Scientists at Cardiff University in Wales
say a "nuclear winter" that caused summer frosts in the sixth century was
caused by a comet.
>
> Using evidence from tree rings, the scientists at the School of Physics
and Astronomy determined that Earth underwent a series of very cold summers
around 536-540 A.D. because a comet hit the planet and exploded in the upper
atmosphere.
>
> Comet debris from the explosion enveloped Earth in soot and ash, blocking
out the sunlight and causing the cold weather.
>
> This period, known as the Dark Ages, has yielded sparse records but what
records do exist tell of crop failures and summer frosts.
>
> An article published in the February issue of Astronomy and Geophysics,
the in-house magazine of the Royal Astronomical Society, said that only a
relatively small comet was needed to cause such a dramatic effect.
>
> The scientists calculated the comet was not more than a third of mile
across -- significantly smaller than previously thought.
>
> Copyright 2004, The Washington Time
>
> =============
> (3) COLLISION WITH COMET MAY HAVE HASTENED FIRST PLAGUE EPIDEMIC
>
> The Independent, 4 February 2004
> http://news.independent.co.uk/world/science_medical/story.jsp?story=487550
>
> By Steve Connor, Science Editor
>
> A collision between Earth and a passing comet in the 6th century AD may
have caused the collapse of agriculture, mass famine and indirectly led to
the bubonic plague in Europe, a study has suggested.
>
> Scientists have calculated that a relatively small comet, or fragment of a
comet, could have caused huge amounts of dust and debris to be ejected into
the atmosphere, blocking the sun for months at a time.
>
> The resulting crop failures and famine would have allowed bubonic plague
to spread easily among a physically weakened population.
>
> Studies of tree rings - from preserved oaks retrieved from Irish bogs to
ancient American pine trees - have shown that plant growth around the world
almost stopped between about 536AD to 545AD. Chinese records from this time
refer to a "dust veil" obscuring the skies. Mediterranean historians record
a "dry fog" that blocked out much of the sun's heat for more than a year.
>
> Scientists have suggested two causes, both involving the ejection of dust
or debris into the atmosphere to block the sun and so prevent
photosynthesis.
>
> One idea is that a super-volcano erupted, but neither the volcano nor its
acidic deposits have been identified, Derek Ward-Thompson, who carried out
the latest study at Cardiff University, said. The other proposal involved a
collision with a big asteroid or comet, but there was no direct evidence
such as a crater.
>
> However, Dr Ward-Thompson and his colleagues Mel Symonds and Emma Rigby
believe a much smaller comet which exploded in the atmosphere could easily
have generated the dust and debris in the 6th century catastrophe. "The
surprising result of these calculations is just how small a comet fragment
we have estimated was needed to cause the observed effects," Dr
Ward-Thompson said.
>
> "A comet less than 1km in diameter has not been previously considered to
represent a global hazard - as opposed to a local hazard - let alone one
0.5km across," he said.
>
> Using information gathered from the impact of Comet Shoemaker-Levy with
Jupiter in 1994, the scientists have produced a model of how comet fragments
would behave if they collided with Earth. "The comet plunges into the upper
atmosphere leaving an effectively hollow tube behind it, where it has been,
and into which the surrounding air has not yet had time to diffuse," the
scientists write in the journal Astronomy and Physics.
>
> "This tube then acts rather like a gun barrel, focusing much of the energy
of the airburst explosion along the tube and carrying with it much of the
comet debris," they write.
>
> As a result, the plume would have spread around the world in a massive
fountain of debris. "This period coincides with a mass population decrease
in Europe. This is commonly known as the Justinian plague, and is believed
to be the first appearance of the Black Death in Europe," the scientists
say.
>
> They said that if such an event happened today, a large percentage of the
population could face starvation.
>
> Copyright 2004, The Independent
>
> =============
> (4) SUPER-VOLCANO MAY HAVE TRIGGERED GLOBAL COOLING IN 536 AD
>
> Los Alamos National Laboratory, 17 December 2000
> http://www.lanl.gov/worldview/news/releases/archive/00-165.shtml
>
> Contact: John Webster, webster@lanl.gov, (505) 667-5543
>
> LOS ALAMOS, N.M., Dec. 17, 2000 -- The beginning of the Dark Ages may have
been literal, as well as figurative, as the result of a massive volcanic
eruption in the 6th century, according to a volcanologist at the Department
of Energy's Los Alamos National Laboratory.
>
> Ken Wohletz said an eruption in the Indonesian archipelago could have
produced a 150-meter-thick cloud layer over the entire Earth, triggering a
chain of climatic, agricultural, political and social changes that ushered
in the Dark Ages.
>
> Evidence supporting the catastrophe includes tree-ring and ice-core
measurements, indications of a huge underwater caldera, and ash and pumice
in the same area, said Wohletz, who discusses his work modeling such an
eruption today (Dec. 17) at the fall meeting of the American Geophysical
Union.
>
> The 6th century was a turbulent, unsettling period in human history. The
Roman Empire began to fall; nomads of central Asia migrated to Europe and
the Near East; civilizations in Persia, Indonesia and South America
collapsed; major religions experienced considerable change as natural events
were viewed as omens.
>
> Many of these social transformations resulted from widespread crop
failures and the explosion of plague around the globe, which in turn were
caused by major climatic changes, Wohletz said. Beginning in about the year
535, according to historical and archeological records, the weather was
colder and drier, sunlight diminished, snow fell in summer and regions of
persistent drought suffered floods.
>
> Wohletz was a resource for a book postulating that the climate changes
resulted from a huge volcanic eruption. The book, "Catastrophe: A Quest for
the Origins of the Modern World" by David Keys, was published earlier this
year.
>
> Wohletz said he worked with Keys to try to identify a volcano that could
produce such dramatic climate change. "We came up with an eruption that
would certainly be the largest in recorded history, some four or five times
bigger than the (1815) eruption of Tambora, which is usually considered the
biggest eruption in the past few millennia," he said.
>
> Such an explosion, he said, would eject some 200 cubic kilometers of
material, and one-third to one-half of it would be lofted into the
stratosphere, where it would remain suspended for months to years while
being carried around the globe.
>
> "It would have produced enough dust and water vapor (in the form of ice
crystals) to form a cloud layer 150 meters thick over the entire globe, and
that's a conservative estimate," he said, adding that a cloud of particles
that thick may have diminished the transmission of sunlight by as much as 50
percent.
>
> Wohletz said tree-ring data collected around the world and ice-core
measurements in Greenland and Antarctica support the possibility of a huge
eruption in the 6th century. Ocean depth measurements between Sumatra and
Java where Krakatoa exploded in a well known 1883 eruption indicate the
presence of a caldera up to 50 kilometers in diameter, and a recent survey
uncovered evidence of ash and pumice layers formed in the area during the
appropriate time frame.
> Under a likely scenario, a large volcano, which Wohletz calls
proto-Krakatoa, connected the islands of Sumatra and Java. When it erupted
and then subsided, it created the Sundra Strait and left a ring of smaller
volcanoes, including the present day Krakatoa. The ash, dust and water vapor
blown into the stratosphere would disperse across both the Northern and
Southern Hemispheres.
>
> "This volcano would have had the potential to be a major player in
destabilizing the climate around the world," he said. "An eruption that
could produce a caldera 50 kilometers across would have been big enough."
>
> Although definitive evidence for such a catastrophic eruption has not been
discovered, the possibility deserves a full-scale field study, Wohletz said,
in part because of the potential impact on the world if another such
catastrophe happens.
>
> "(Key's book) is the first detailed account of how closely humanity is
linked to the natural world," he said. "If the natural world goes through
some large upheaval, we'll all be affected."
>
> =============
> (5) SUPER-DROUGHT MAY HAVE SET OFF EUROPEAN DARK AGES
>
> University of California, Berkeley, 9 December 1998
> http://www.berkeley.edu/news/media/releases/98legacy/12-9-1998a.html
>
> BERKELEY -- A tiny Christian village in ancient Greece and a long-dead
river have provided the first clues that a terrible drought may have set off
the European Dark Ages, at least in the eastern Mediterranean.
>
> The clues come from the archaeological site of Nemea, Greece, where
ancient athletic games were held. Stephen G. Miller, professor of classics
at the University of California, Berkeley, announced the results of recent
digs near the site in an evening lecture on Tuesday, Dec. 8.
> While excavating deep into the bed of the small Nemea River which runs
through the site, Miller made an unexpected discovery. He found a much
larger, older river bed that had completely run dry during the first half of
the 6th century A.D.
>
> At about the same time, said Miller, an agricultural community of early
Christians at Nemea became impoverished and short on water. They built new
wells but used them for only a short period of time.
>
> The Christian community at Nemea was totally destroyed by invading Slavs
in A.D. 585, but Miller said the archaeological evidence clearly indicates
the community had already fallen on hard times 50 years earlier.
>
> Miller believes these clues, taken together, provide the first evidence of
an extraordinary drought in the eastern Mediterranean that may have laid the
groundwork for that region's later devastation by plague and marauding
Slavic tribes.
>
> His evidence lends weight to a recent theory that the Dark Ages were
brought on by a cataclysmic event, a violent volcanic eruption in 536 A.D.
that cloaked the skies in volcanic dust and cast the world into cold
darkness for more than a year.
>
> According to popular belief, based on Gibbon's classic "Fall of the Roman
Empire," barbarians brought about the fall of classical civilization by
sacking Rome in 476 A.D. Byzantine civilization in the eastern
Mediterranean, however, collapsed about 100 years later after a bubonic
plague swept the region and after invasions by Slavic tribes and Persians.
>
> The evidence of drought at Nemea and its implications for classical
civilization represent a new direction for Miller, whose excavations at the
Greek site since 1974 have led to the reconstruction of much of the original
setting for the Panhellenic Games, including the stadium and track, a
bathhouse and a temple to Zeus.
>
> In 1996, Miller led a revival of the ancient games by holding a modern day
foot race on the ancient track. Hundreds of participants from around the
world, including many celebrities, journeyed to Nemea to take part in the
race. The Nemean games will be held again in the year 2000, on June 3 and 4.
>
> In 1997, Miller and his team set out to find the course of the ancient
Nemea river in order to divert water there and save artifacts scattered
throughout the area.
>
> What he discovered, however, were three older rivers and an excellent
stratigraphy with which to decipher changes that had occurred since the
early Mycenean era in the 12th century B.C.
> "We could see cycles of aridity and dampness for more than 3,000 years,"
said Miller. "There had been a large river during the early Christian era;
then, suddenly, there was no river."
> The team confirmed through excavation that the river had not been diverted
somewhere else, but had actually stopped flowing in the first half of the
6th century A.D.
>
> That period corresponds with worldwide accounts of a massive climatic
change, caused perhaps by a volcanic eruption, perhaps in New Guinea in 536
A.D., according to a theory proposed recently by the astrophysicist R. B.
Stothers and others, said Miller.
>
> The strange effects were recorded by observers from Rome to China who
noted that the sun went dark for more than a year and all the crops failed.
>
> "The Sun gave forth its light without brightness, like the Moon, during
this whole year, and it seemed very much like the Sun in eclipse, for the
beams it shed were not clear," wrote Procopius of Greece in 536 A.D.
>
> Said another source, the Roman writer Cassiodorus: "We have had a winter
without storms, spring without mildness, summer without heat. Whence can we
hope for mild weather, when the months that once ripened the crops have been
deadly sick under the northern blasts? ... Out of all the elements, we find
these two opposed to us: perpetual frost and unnatural drought."
>
> Miller said that further confirmation of this unnatural climatic period
comes from tree-ring data in several parts of the world. Ancient trees such
as the 4,000-year-old bristle cone pines in California show that the years
around 540 A.D. were those of the least growth in four millennia.
>
> The possibility that massive climatic change caused the fall of the
Byzantine Empire was discussed by climatologist William James Burroughs in
his 1997 book, "Does the Weather Really Matter?" But he noted that the one
piece of evidence missing was a sign of drought in the eastern
Mediterranean.
>
> "We now have that evidence," said Miller.
>
> ===========
> (6) ICE CORE EVIDENCE FOR VOLCANIC ERUPTION ~530 AD
>
> Environmental Catastrophes and Recoveries in the Holocene
> http://atlas-conferences.com/c/a/i/q/21.htm
>
> Lars Berg Larsen, Department of Geophysics, University of Copenhagen
> Coauthors: Marie-Louise Siggaard-Andersen (Alfred-Wegener-Institute for
Polar and Marine Research,Bremerhaven) and Henrik B. Clausen (Department of
Geophysics, NBIfAFG, University of Copenhagen)
>
> In the beginning of the sixth century, early medieval historical data
indicate some events,
> which is believed to be the aftermath of some sort of global climatic
catastrophe. The three-
> ring and ice core data obtained around the world show a temperature
anomaly around this period.
> In the ice cores drilled in Greenland at the sites NGRIP, GRIP, DYE-3 and
the Antarctica sites
> BYRD and DOME C we investigated special events and other anomalies in the
chemistry, isotope
> end ECM data. In the ECM record we have the third largest signal found in
the ECM record the
> last to thousand years. A detailed chemical analysis of the ice covering
the period was made.
> >From the chemical record we identify a volcanic eruption and we estimate
the magnitude of
> the eruption and a possible location. With the chemistry and the isotope
data it is possible
> to do a very precise dating for the eruption. The volcanic eruption is
dated to AD. 527 +/- 1 year. The AD 527 volcanic eruption is the only
eruption in the period and the only likely
> candidate to cause some of the calamities described in the historical data
The dating of
> this volcanic eruption suggest that the event is not the same one
described by other sources.
>
> =============
> (7) JUMBLING OLD EVENTS WITH MODERN MYTHS
>
> British Archaeology, November 1999
> http://www.britarch.ac.uk/ba/ba49/ba49book.html
>
> by Ken Dark
>
> CATASTROPHE
> David Keys
> Century, Ј16.99
> ISBN 1-7126-8069-1 hb
>
> Did a huge volcanic eruption in the early 6th century AD cause profound
global political, economic, cultural and religious changes? David Keys, a
well-known journalist specializing in archaeological news, argues that it
did. Drawing extensively on the work of archaeologists, historians, and
scholars in other fields, as he makes clear, Keys suggests that an eruption
led to a global environmental `catastrophe'. This, he proposes, brought ruin
to the Roman Empire, the Avar kingdom in Central Asia and to Teotihuacan in
Mesoamerica, and led to the formation of later England, France, China and
Japan.
>
> It is a bold thesis, and one which touches on current fears about
environmental problems as a global threat. Moreover, while aspects of the
argument are not entirely new (for example, the 6th century environmental
crisis is credited to Mike Baillie's work), at first sight Keys offers a lot
of supporting evidence for his broader interpretation. However, much of the
apparent evidence presented in the book is highly debatable, based on poor
sources or simply incorrect.
>
> The chapters on Britain illustrate the limitations of the book as a whole.
Sites (such as Mothecombe) are mislocated and archaeological evidence (as at
Dinas Emrys) misquoted in detail. Unfounded assertions about population (as
at Killibury) and desertion (as at Chun) abound.
>
> Important sites which might cause problems for the argument (for example,
Dinas Powys) are absent altogether. As for textual evidence,
pseudo-historical and historical material is intermingled, and few
specialists will accept that late medieval `Arthurian' literature contains
any reliable information about the 6th century, the topic of a whole chapter
of this book.
>
> Nonetheless, both the global scope and the emphasis on the 6th century AD
as a time of wide-ranging change are commendable, and the book contains some
fascinating and obscure information which will be new to many. However, it
fails to demonstrate its central thesis and does not offer a convincing
explanation for the many changes discussed.
>
> Dr Ken Dark is an early medieval specialist at the University of Reading
>
> ==============
> (8) A COMET IMPACT IN AD 536?
>
> Astronomy & Geophysics, Volume 45 Issue 1, February 2004
>
> Emma Rigby1, Melissa Symonds2 and Derek Ward-Thompson2
> 1Cardiff University, UK (now at Edinburgh University, UK) 2Cardiff
University
>
> Emma Rigby, Melissa Symonds and Derek Ward-Thompson review the evidence
for the possibility that a comet may have impacted the Earth in historical
times, and discuss the size of the putative comet.
>
> Abstract
>
> A global climatic downturn has previously been observed in tree-ring data
associated with the years AD 536-545. We review the evidence for the
explanation of this event which involves a comet fragment impacting the
Earth and exploding in the upper atmosphere. The explosion would create a
plume, such as was seen during the impact of comet Shoemaker-Levy 9 with
Jupiter. The resulting debris deposited by the plume on to the top of the
atmosphere would increase the opacity and lower the temperature. We
calculate the size of the comet required, and find that a relatively small
fragment of only about half a kilometre in diameter could be consistent with
the data. We conclude that plume formation is a by-product of small comet
impacts that must be added to the list of significant global hazards posed
by near-Earth objects.
>
> The Earth is bombarded every day by debris from space. The majority of
this debris takes the form of very small particles of dust. These objects
are known as meteoroids which, as they run into the Earth's atmosphere,
produce meteors - also known as shooting stars. Such objects are rarely
hazardous. However, there are also much less frequent collisions with larger
objects ranging in size from tens of metres to kilometres across, which may
be asteroids or comets. Asteroids are primarily rocky or metallic in
composition, whereas comets are composed mainly of a variety of ices with
some rock. Objects of this size are generally more of a hazard. In fact, the
UK government even set up a Near Earth Object Task Force to evaluate the
risks of impacts from such objects (Atkinson et al. 2000). Depending on the
size and strength of the material of the meteoroid, it may explode in the
atmosphere before reaching the ground. Such an event is known as an
airburst. An airbursting object releases energy in the form of a shockwave,
which can devastate large areas and trigger forest fires. Airbursts can also
produce a high-altitude haze of particles, such as was seen in the 1908
Tunguska event.
>
> In AD 536 an event occurred which caused significant climatic change.
Dendrochronological (tree-ring) evidence in oak trees salvaged from Irish
peat bogs indicates a sequence of colder than average summers at this time
(Baillie 1991, 1999). A similar effect is seen in Fennoscandian pine trees
(Briffa et al. 1990), and a study of European oak tree data as a whole shows
that the event starts in 536 and lasts until 545 (Baillie 1994). Figure 1
shows the typical variation in tree-ring widths.
>
> North American bristle-cone and foxtail pines (La Marche 1974, Scuderi
1990), Mongolian tree rings (D'Arrigo et al. 2001) and Argentinean tree-ring
data (Baillie 1999) all show the same effect. The decrease in rate of growth
in these years corresponds to a global temperature decrease of up to 3 °C
(Briffa et al. 1990, Scuderi 1990). In fact, 536 is noted as one of the
coldest two or three years globally in the last 2000 years (Briffa et al.
1990, Scuderi 1993, Baillie 1999). Figure 2 shows the results for the Irish
oak trees and the US foxtail pines.
>
> There are also some limited historical records from the same period, which
record the presence of a "dust veil". Chinese records refer to obscured
skies and summer frosts (Baillie 1999). Mediterranean historians also record
a "dry fog" event. Procopius (Dewing 1916) records that:
>
> "the Sun gave forth its light without brightness, like the Moon, during
this whole year, and it seemed exceedingly like the Sun in eclipse",
>
> and Michael the Syrian (Chabot 1901) states:
>
> "the Sun was dark and its darkness lasted for eighteen months; each day it
shone for about four hours, and still this light was only a feeble shadow".
>
> The sixth-century British monastic writer Gildas talks of large-scale
fires and widespread destruction of the landscape at around the same time
(Winterbotham 1978). Previously Gildas has been thought to have been talking
allegorically, with his many biblical references. However, he may have been
describing the actual events taking place at this time.
>
> There have been two theories put forward to account for this cataclysmic
climatic event. One idea is that a giant "super-volcano" erupted in 536,
causing the effects described above (Stothers 1984, Keys 1999). The
alternative scenario invokes an impact by an asteroid or comet (Clube and
Napier 1984).
>
> The super-volcano theory has several problems. Firstly, no terrestrial
volcano can be satisfactorily identified with this event. Secondly, a
super-volcano would be expected to produce significant acidity in the
atmosphere. This acidity would be recorded in the polar ice caps. Numerous
ice-core studies have been carried out in both Greenland and Antarctica
(see, for example, Clausen et al. 1997, Hammer et al. 1997). None of these
has found evidence for a significant acid layer around 536 of the sort that
would be caused by the eruption of a super-volcano.
>
> There are small acid layers associated with 528 and 533, but they are not
sufficiently strong that they can be related to a super-volcano (Clausen et
al. 1997). In addition there is an Antarctic ice acid layer dated as 504±40
(Hammer et al. 1997), which has been argued could be related to the 536
event (Keys 1999). However, once again this is not the depth of layer that
one would expect of a super-volcano (Baillie 1999). Other measurements have
proved inconclusive. For example, in one case an ice core broke up across
the crucial period (Zielinski et al. 1994) and in another the exact dating
proved controversial (Hammer 1984).
>
> The scenario involving an impact by an asteroid or comet may have seemed
far-fetched at one time, but recent scientific evidence indicates that such
impacts may be more common than was once believed. The impact of comet
Shoemaker-Levy 9 (SL9) with Jupiter in 1994 provided dramatic and direct
evidence of the damage that even quite small impacts can generate. Figure 3
shows an example of some images of the SL9 impact event. In addition, the
impact of a comet or asteroid is now believed to be responsible for the
considerable climatic changes at the end of the Cretaceous period (Alvarez
et al. 1980).
>
> No known impact crater can be linked to the 536 event so, if it were due
to a comet collision, then the crater may lie unrecognized in a region such
as the seabed (unlikely, given that the seabed has now been very well
surveyed), or else the comet must have been destroyed in an airburst. If the
airburst had sufficient energy to ignite one or more large-scale forest
fires (Hills and Goda 1993, Adushkin and Nemchinov 1994), we can calculate
whether the soot emissions from these fires were sufficient to cause the
observed climatic effects.
>
> During the Cold War much effort went into modelling the secondary effects
of a conflict involving nuclear weapons, including large-scale fires and
related atmospheric effects, referred to collectively as a nuclear winter
(see, for example, Turco et al. 1983, 1990, Crutzen and Birks 1992). The
basic hypothesis is that the extensive fires started by nuclear explosions
would generate a sufficiently large quantity of soot in the upper atmosphere
to affect its radiation balance and cause a change in the Earth's albedo.
The net result is reduced solar heating at the Earth's surface and hence a
lower temperature. This would produce significantly colder summers and hence
reduced tree-ring growth. We can use such models to study fires started by a
comet impact, since they provide estimates of the soot yields from
appropriate masses of relevant flammable materials. We choose the model of
Turco et al. (1990) to make our comparison. This model finds that 1011 kg of
soot injected into the troposphere is sufficient to produce the effect known
as a nuclear winter.
>
> It has been found that airbursting comets can start fires more easily than
asteroids (Hills and Goda 1993). This occurs because comets tend to
dissipate their energy higher in the atmosphere, which has two main effects.
The high-altitude energy dissipation allows them to illuminate a much larger
area of the Earth. Simultaneously the high altitude of the airburst makes it
more difficult for the associated atmospheric shockwaves to reach the
Earth's surface and blow out the fire. We note that calculations on the size
of an impacting comet required to ignite pine forests show that an object
radius of more than 100 m is needed for such an ignition, setting an
effective lower limit.
>
> Dendrochronological data and historical records indicate that Northern
Europe is the most likely geographical location for the impact (Baillie
1999). However, we note that this suggestion is not critical to the model of
large-scale fires, and any equivalent land area would suffice, provided that
it was appropriately forested. In the sixth century Europe's most flammable
species of tree was the pine tree and forests in Northern Europe were
largely composed of pine trees (Rackham 1993), so a forest fire there is
possible. A study of fire behaviour from large-scale biomass burning
(Einfeld et al. 1991) provides the following equation for the amount of
smoke injected into the troposphere by large-scale forest fires:
>
> M>X = M>FA>BF>CE>X (1)
>
> where MX is the mass yield of a particular gaseous or particulate species
X, MF is the fuel load in terms of mass per unit area, AB is the area
burned, FC is the fractional consumption of the total fuel load and EX is
the emission factor for the species of interest. We can use this equation to
estimate what size of forest fire would have been required to produce the
cooling recorded in the dendrochronological data.
>
> To calculate MF we assume one tree per hundred square metres of forest
(Rackham 1993), which when combined with a typical tree mass gives a fuel
load of M>F ~20 kg m^2.
>
> For EX we take a value of 1% (Turco et al. 1990) and for the fractional
consumption FC we assume 10%. Inserting these values into equation 1, we
find that we would need a forest fire to cover the whole of Northern Europe
(5 1012 m2) to inject into the troposphere the 1011 kg of soot required by
the nuclear winter model described above. Such a large conflagration seems
somewhat unlikely as there would almost certainly have been some form of
historical or archaeological record of such a fire. Furthermore, the
climatic effects may not last for the time required by the tree-ring data,
since soot in the troposphere has a tendency to precipitate out.
Consequently, forest fires may have added to the events of 536, but they
cannot explain all of the data.
>
> Atmospheric plumes Go to: Choose Top of page The AD 536 event Forest
fires Atmospheric plumes
> When comet SL9 broke up into many pieces and collided with Jupiter in
1994, astronomers had their first opportunity in modern times to study in
detail a large comet impact with a planet. Many previously unpredicted
effects were seen, including giant plumes above the impact sites and huge
scars where each fragment had hit. This led several people to model in
detail the effects of such collisions. The similarity was noted between the
impact of SL9 with a largely gaseous body such as Jupiter and the airburst
of a comet impacting the Earth.
>
> Figure 4 shows some results from a model of a 15 megaton equivalent yield
comet airburst over the Earth (Boslough and Crawford 1997). The comet
plunges into the upper atmosphere, leaving an effectively hollow tube behind
it, where it has been, and into which the surrounding air has not yet had
time to diffuse. This tube then acts rather like a gun barrel, focusing much
of the energy of the airburst explosion along the tube and carrying with it
much of the comet debris. This is what causes the observed plume, as
material is fired along the tube by the force of the explosion.
>
> The plume expands once it has exited the top of the atmosphere and the
material within it then falls back on to the top of the atmosphere. This is
the explanation for the observed scars on Jupiter. However, figure 4 has
been used to model the Tunguska comet impact of 1908. This was a much
smaller comet than SL9, but the chief effects appear to change only in
scale. The sequence of events remains the same: impact; airburst; plume.
>
> An eye-witness drawing of the Tunguska event actually resembles the model
plume of figure 4, and many witnesses across northern Russia and Europe
noted that the sky was light at night for a few days afterwards. This effect
can be explained as noctilucent clouds caused by material from the plume.
>
> There was no significant global cooling in 1908 so any comet required to
explain the 536 event must be larger than the comet which exploded in the
air over Tunguska. The best estimate for the size of the Tunguska comet is a
few tens of metres in diameter. We can use the model illustrated in figure
4, together with an estimate of the optical depth of the 536 dust veil, to
calculate the size of comet fragment required to cause the 536 event.
>
> The dry-fog event noted in the Mediterranean area has been estimated from
the description of the Sun to correspond to an optical depth of 2.5
(Stothers 1984). However, we would question this estimate. It was based
purely on assumptions of what was meant in historical records by "the Sun
appearing like the Moon" and hence by how much this implied the Sun had
dimmed. We can make alternative estimates based on the tree-ring data, which
require a decrease in temperature of only up to 3 °C (Briffa et al. 1990,
Scuderi 1990).
>
> The temperature of the Earth's surface,
> TE
> is proportional to L^ј I
> where LI
> is the incident luminosity at the surface of the Earth. If the dust veil
had an optical depth of then LI
> would be decreased by e
> and the temperature at the Earth's surface under the dust veil,
> TD is given by: T>E+TEe^t4 (2)
>
> Taking TE 300 K and using the maximum temperature decrease of 3 K, we
calculate the maximum value required for is 0.04, which is considerably
less than the previous estimate.
>
> We make the assumption that the entire mass of the comet is destroyed in
the airburst and ejected via the plume. Thereafter the dust condenses out
and is deposited on to the top of the atmosphere as small particles of dusty
debris. This debris is then spread around the globe roughly uniformly. The
dust-to-gas mass ratio in a comet can range from 0.1 to 10 (see, for
instance, Weiler et al. 2003 and references therein), so we take a value of
order unity (i.e. Mdust 0.5 Mcomet).
>
> The optical depth is then related to the number of dusty particles Nd by
the equation: (3)
> where a is the typical radius of a particle and RE is the radius of the
Earth. Nd is simply given by the ratio of the dust mass of the comet to the
mass of a dusty particle. Hence Nd is given by the ratio of the comet
density to the dust particle density (a factor 0.5 - see, for example,
Fitzsimmons et al. 1996, and references therein), multiplied by the fraction
of the comet's mass that is dust (0.5), multiplied by the ratio of the cube
of the comet radius RC to the particle radius a. Thus: (4)
>
> and the comet radius is given by: (5)
>
>
> We know the radius of the Earth ( 6.4x10^6 m) and we assume a typical
particle radius to be similar to that of particles in the interstellar
medium of 10^6 m (e.g. Fitzsimmons et al. 1996). Hence we find that a of
0.04 requires a comet radius of only 300 m.
>
> Clearly such calculations contain many approximations and assumptions.
Some of the numbers that go into this estimate are rather uncertain. We
particularly note that we are relying on atmospheric effects to spread the
dust evenly over the top of the atmosphere. Nonetheless, the result is
interesting. It puts the comet size as intermediate between the Tunguska
comet and SL9 and makes it significantly smaller than the comet thought
responsible for the dinosaur extinction.
>
> A comparison between the 536 event and the SL9 impact with Jupiter can
provide a useful cross-check to our size calculations. The fragments of SL9
that collided with Jupiter ranged in diameter from 300 m to 2 km. The scars
created on Jupiter were typically about the size of the Earth or larger.
This appears to corroborate our basic premise that a relatively small
fragment could have been responsible for the 536 event. We note that a
collision with a comet or asteroid of this size has been predicted to occur
on the Earth once in several thousand years (Morrison et al. 1994).
Consequently we should not be surprised to find evidence for such an impact
within the 7000-year dendrochronological record.
>
> The surprising result of these calculations is just how small a comet
fragment we have estimated was needed to cause the observed effects. A comet
less than a kilometre in diameter has not been previously considered to
represent a global hazard (as opposed to a local hazard), let alone one half
a kilometre across. For example, the UK government's Near Earth Object Task
Force classified impacts from objects in the size range 300-1500 m to be a
"large sub-global hazard" (Atkinson et al. 2000) and only recommended
monitoring programmes of near-Earth objects greater than a kilometre in
size. Yet our estimates show that if plume formation is a common by-product
of cometary airbursts (as is believed to be the case), then a relatively
small comet fragment can have a global effect.
>
> A possible origin for this object could be the Taurid meteor stream. This
stream is the result of the fragmentation of a much larger comet some 20 000
years ago (Bailey et al. 1990). The Earth passes through the Taurid stream
in November and June each year, resulting in regular meteor showers normally
consisting of microscopic dust particles. A fragmentation of the stream is
thought to have occurred around 500 (Clube and Napier 1984). It is possible
that a fragment could then have collided with the Earth as it passed through
the beta portion of the stream early in 536, causing the atmospheric
dust-veil event.
>
> One other historical aspect of the period around 536 may also be
significant. This period coincides with a mass population decrease in
Europe. This is commonly known as the Justinian Plague, and is believed to
be the first appearance of the Black Death in Europe (Russell 1968). The
relevance to our hypothesis is that crop failures for several years in a row
while the dust veil was present may have caused widespread starvation,
leading to an increased susceptibility to disease among the remaining
population. Small comets had previously been thought to be less hazardous
than small asteroids, but our estimates show that a small comet fragment can
have a global effect.
>
> If such an event happened today, and crops failed over a significant part
of the globe for several consecutive years, then once again a large
percentage of the world's population would face starvation.
>
>
> References
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>
> The authors thank Mike Baillie, Mark Bailey, Martin Johnson, Ted
Johnson-South and David Williams for interesting and helpful discussions.
>
> Copyright 2004, Astronomy & Geophysics
>
>
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