PLEASE NOTE:
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CCNet-ESSAY: CLIMATIC CHANGES IN PREHISTORY AND HISTORY
From Ken Hsu <ken@erdw.ethz.ch>
This is the English translation of an article which I am writing
(in
German) for the Neue Zürcher Zeitung. I thought you might be
interested
in this subject matter.
Ken Hsu
--------------------------
CLIMATIC CHANGES IN PREHISTORY AND HISTORY
The press has been reporting an impending catastrophe of global
warming
as an inevitable consequence of the greenhouse effect of the
industrially produced carbon dioxide. The scientific community
has,
however, not reached a consensus. The orthodox opinion, based
mainly on
computer-modelling, relates the current global warming to the
atmospheric
greenhouse. Their opponents suggest, on the contrary, that the
greenhouse-effect leads to increased cloud cover, more
snowfall in
polar regions, and global cooling. That there has been a
global-warming
trend since the last century is, however, a well-established
fact. What
I would like to explore is not so much the reason why, but to
present
the record of climatic changes during the prehistorical and
historical
times.
The earth underwent episodes of continental glaciation during the
last
two million years. North America, Scandinavia, and Europe were
covered
by ice sheets during the last Ice Age. The ice caps started
their
retreat after they reached their greatest extent, when Swizerland
was
entirely covered under ice. The post-glacial warming began some
16,000
or 15,000 years ago, but there was one last glacial advance some
11,000
years ago, The climate of Zurich then was as cool as that of
the
Engadine now, and Lake Zurich was frozen for several months every
year.
The finest suspension from the "glacial milk" could
then settle to the
lake bottom, to form a clay lamina above the melt-water deposit
of the
spring and summer. Accumulating annually, the laminated sediments
are
called varves.
A wholly new epoch, called the Holocene, had its beginning after
the
last glacial retreat, some 10,000 years ago; it has been
warm since.
The uninformed have the impression that the global climate has
not
changed until the recent global warming. In fact, the average
temperatures on earth have varied within the range of 10 to 20 C.
The
average masks, however, the significance of regional variations.
Besides, not so much the temperature but the precipitation
changes are
more important to the well being of the society. A cold spring in
northern Europe, for example, does not make a big difference in
the
tempeture record, but late planting could result in crop
failures.
Colder years in China may not seem significant in the terms of
global
average, but the cooling could bring forth a long drought with
dire
consequences. Scientists studying sediments and ice cores have
found
only faint signals of alternating of cold and warm millenia.
Historians
have recorded, however, all too obvious impact of climatic
changes on
the history of civilization.
The first half of the Holocene was warmer, and the Climatic
Optimum
ended several thousand years ago. The global average may have
changed a
degree or two, but lakes freeze when subzero temperatures prevail
in
the winter. Zurichsee has not been frozen for more than 30 years,
but
Alpine lakes above 1600 or 1800 m freeze every year. Studying the
varves of Silvaplana, my student Andreas Lehmann found no
Holocene
varves older than 4000 years, when there was no
"glacial-milk"
sediment.. The conclusion is inescapable: There were no varves
because
the Engadine lakes were not frozen every year. There were no
"glacial
milk" deposits when there were no Alpine glaciers!
I was excited by Lehmann's discovery and called my former student
Dr.
Kerry Kelts at Minnesota. He headed our Limnology Laborary at
ETH-Z
before accepting a professorship at University of Minnesota.
Kelts was
not surprised. He told me drily: "I have been telling you
all those
years of the 4000 BP event, and you did not listen. There was a
global
cooling when the Climatic Optimum came to an end."
Yes, I was told of the strange things and I did not quite
appreciate
the significance of those scientific findings. Prof. Nicola
Petit-Maire, at University of Marseilles, described the vast
lacustrine
deposits in the Sahara desert: the sediments were laid down
during a
humid phase between 9,500 to 4,000 BP. Rainfall was so abundant
then
that Mali was not a desert but land of great lakes. The
Cro-Magnon
people came across the Strait of Gibraltor from Spain to
the savannas
of Sahara. They hunted elephants, rhinoceros, buffaloes,
hippopotamus,
antelopes, and giraffes, as depicted in their wonderful rock
paintings.
The deserts of North Africa expanded, however, and an early
clustering
of cold centuries around 5200 BP caused the deteriozation of
environments. Hunters and grazers left Sahara and settled on as
farmers
the alluvial plains of Egypt. The cooling and aridity continued
and the
last of the Saharan lakes dried up 4,000 BP, ending the Saharan
civilization, at about the same time when the glaciers advanced
in the
Alps.
Mild and wet climate prevailed during the Climatic Optimum in the
Near
East. I visited the Canannite City Arad on the edge of the Negev
Desert: it was a populous settlement of several thousand
inhabitants
during the Early Bronze Age. Suddenly Arad was abandoned. The
deserted
city showed no signs of destruction by war, the exodus was
necessitated
by a shortage of water supply. Indeed, the centuries-long drought
in
the Middle East was the cause of the collapse of the Early Brone
Age
civilization in Mesopotamia, as Prof. H. Weiss of Yale and his
colleagues concluded. A marked increase in aridity caused the
abandonment of setttlements in the north and the collapse of the
Akkadian Empire in the south. The impact of was extensive: there
were
synchronous collapse of the civilizations in Hindus Valley and in
Egypt. The climatic catastrophe started around 2200 BC and came
to an
end 300 years later. This was the expression of the 4000 BP Event
in
Middle East.
In central Europe, the 4,000 BP Event brought not aridity, but
increased precipitation. The cold and wet climate caused the
advance of
the Alpine glaciers. In the region of Prealpine lakes, the Lake
Dwellers had enjoyed warm and dry climate, and they had built
villages
on the shores of lowland lakes. When the cold and wet climate
came, the
settlements were flooded, the Lake Dwellers had to leave their
homes,
when the lake-level rose. The Zurich archaeologists discovered,
for
example, that the villages on the shores of the lake were
abandoned
about 2,400 BC, and they remained uninhabitated for about 800
years.
In northern Europe, cattle farming had brought prosperity
to the
megalithic kingdoms. The 4000 BP Event brought forth late springs
and
cold and wet summers. Crops were not harvested because of late
planting, and cattle were famished when it became impossible to
make
hays. The Indo-Europeans of northern Europe had to move. Carrying
battle axes and corded-ware pottery, they went to southern
Russia, from
there to southeastern Europe, to Anatolia, to Persia and India.
and to
northwest China.
The 4000 BP event hit China also. When the legendary King Huangti
ruled
in China, at about 3,000 BCE, mulberry trees grew in north China
where
elephants and rhinoceros roamed. The climate turned cold and arid
then.
Yu, the first king of the Xia Dynasty, received credit for having
tamed
devastating floods. He may in fact not have done more than his
predessors, except flooding eased when rain storms ceased their
visitation.
There was a return to warmer and more humid climate during the
second
millenium. The agricultural economy of the Near East flourished
during
Late Bronze Age. The cluster of the warm centuries was followed,
however, by the cold and more arid Mediterranean climate of the
Iron
Age. The Hittite Empire perished, and Mycenae was sacked by the
Dorian
invaders. The People of Sea came from the north. They invaded
Libya and
Egypt, from where they were turned back; the Philistines had to
settle
in Palestine. The cluster of the centuries of darkness started
shortly
before 1,000 BCE and continued until the 7th century when Greek
city-states rose and when Rome was founded.
In central Europe, a severe cooling ended the Late Bronze Age.
The
temperature drop in the higher latitudes was accompanied
not by
aridity but by an increase of precipitation. The alpine glaciers
advanced again, and the water level of the midland lakes rose.
The Lake
Dwellers of Zurich again had to abandon their settlements around
the
time of 800 BC. Meanwhile, the tree-line was depressed, when
broad-leafed trees were replaced by conifers.
Most remarkable is the dominance of the Urnfield People. The
indigenous
population were the Neolithic farmers, and they had been in
central
Europe since the 6th millennium BCE. The immigrants from western
Europe
brought with them their bell-shaped beakers. The immigrants from
northern Europe came during the second half of the Third
Millennium BC:
they were Indo-Europeans, identifiable by their corded
ware/battle ax
culture. The Bronze-Age civilizations flourished, before the
Urnfielders came around 1250 BC. They were warriors, they
built
castles on hill-tops, they hunted, raised cattle, cultivated
fields.
They traded, and made wars. The Urnfielders were Indo-Europeans,
and
archaeologists could trace their origin as far north as to the
Lausitzer region of east Germany. Coud they have come from far
north?
The Urnfielders cremated their dead, whereas the indigenous
peoples had
buried their dead by inhumation. The custom of cremation
evolved in
Scandinavia during the Bronze Age. The northerners came
south when
there was a dramatic climatic ("Subatlantic")
deteriozation during the
late Bronze Age. The same colder and wetter summers had driven
the
Indo-Europeans from their homeland in northern Europe during the
second
half of the Third Millennium. The Urnfielders seem to represent
the
second wave of the Indo-European invaders. They too had to
abandoned
their home when they could no longer harvest their fields or feed
their
cattle.
The climate was warm and wet in central China when the Bronze Age
came
to China. The global cooling hit also at the end of the second
millenium BCE: there were many cold and dry years. Drought and
famine
were commonplace during the reigns of the four last West Zhou
kings Li,
Hsuan, You, and Ping. Weakened by forign invasion and internal
rebellions, King Ping was forced to abandon Xian: the capital was
moved
to Loyang at the beginning of the Eastern Zhou Dynasty, 771 BCE.
Arid
northwest was the source of eolian dusts. Silt-size particles
settle in
North China as loess, while clay-sized dust was transported
across the
Pacific and deposited in the ice of Greenland.
Warmer climate prevailed during the Greek, Roman, and Hellenistic
eras.
The first cluster of cold centuries came shortly before the
birth of
Christ. The mass migration of the Helvetians in Caesar's time may
have
been related to bad harvests. The climate continued to
deteriorate
during the first Christian centuries: one German tribe after
another
left their farms and ranches in northern Germany. The coldest
years
were to come during the age of the Migration of German Nations,
from
the late 4th century to the early 6th century.
Historical records in China also indicate a little climatic
optimum
during the Spring and Autumn Era and the Age of Warring States
(722-221
BCE). Rice, the staple crop in south China, grew in Shandong,
Henan, and
Hebei, - the provinces which are now noted for their winter cold
and
aridity. The climate continued to be warm and wet during the Qin
and
Western Han years (221-29 BC), and settlements on the Silk Roads
on the
edge of the Taklimakan Desert were built. The first cold years
came
also at the time of Christ's birth. The good government of an
able
ruler Wang Mong could not prevent the Hunger Years. There was
little
peace nor prosperity during the next two centuries of the Eastern
Han
Dynasty, and the climate got even worsewhen the Yellow Turban
Rebels
rose in rebellion: 800,000 people died during the age of chaos
when the
the Han Empire collapsed. Extremely cold and dry climate finally
arrived in central Chin. The worst drought was recorded during
the
reign of the Jin emperors, continuing during the decade 281-290
CE.,
reaching an catastrophic climax in the year 309 CE: Chinese
historians
stated that the rivers "Jiang, Han, He, Le were all
desiccated, so
that people could wade across." Jiang is Yangzi, and
He is Yellow
river!
The peoples of Europe enjoyed the warmest climate during the
Medieval
Optimum, a stable warm period from 900 to 1350 AD. It was the Age
of
the Vikings. Fields for planting and meadows for grazing in
Scandinavia
expanded to areas north of the Arctic Circle, and the tree-line
advanced significantly to higher altitude. The record of the
Greenland
ice cores indicates that the warming started in the 7th century
and
reached its zenith during the 10th, 11th, and 12th century.
High-pressure and sunny weather prevailed over Britain, Germany,
and
Scandinavia, such warmer and dry summers are a
"heaven-sent" gift to
cattle-farmers.
In China the empire was reunited toward the end of the 6th
century; the
Sui, Tang and early Sung years were again a period of prosperity.
This
age of Chinese glory (600-965 AD) was a warm period. The desert
was
greening at the time of the Medival Optimum. The cities on the
Silk
Road were rebuilt. The greening of desert provided the economic
basis
for the strength of northern Asia tribes. Xia, Liao, and Jin all
scored
military victories over the Sung emperors. The Seljuk Turks went
west
in the 11th century and founded a colony in eastern Anatolia.
Finally
the blooming of population in the Mongolian steppes provided the
foundation of the Ginghis Khan's conquest.
The Mongol Empire declined at the start of the Little Ice Age.
The
centuries from 1400 to the mid-1800s were an era of many cold and
stormy winters in northern Europe. The glaciers came again down
to
overhwhelm the Alpine meadows. The cold and wet climate rendered
crop-
and cattle-farming equally difficult. The Thirty-Year War raged
during
the coldest of the Little Ice Age, while the people of the
maritime
nations went to America.
The last years of the Ming Dynasty were cold and dry in central
China.
The worst came during the forty-odd years, 1601 to 1644 CE:
historians
recorded two episodes of "ba-lien-da-han," eight years
of extremely
severe drought. Hungry peasants rose in rebellion in Henan where,
it
was reported, not a drop of rain fell during three whole years!
The Little Ice Age came to an end and the most recent warming
trend
started during the 19th century, in Europe and in China. The 20th
has
been a century of global warming, but the trend was interrupted
by cold
spells during the 1960s and 1970s. The continuing global warming
of the
1980s and 1990s brought alarm to a sensitized public. Alpine
glaciers
have been retreating, and will rapidly disappear if the warming
trend
should persist.
Separating "noises" from "signals,"there
seems to have been cycles of
the climatic changes in historical time,with an approximate
periodicity
of about 1200 years: six hundred cold years alternating with six
hundred
years of warmth. There is little evidence of variation of
greenhouse gases
during the last several millenian, and the climatic changes are
correlated
to cyclic variations of solar activities.