Aride und humide Phasen in der Sahara Afrikas während
des Holozäns in einer stark vereinfachten schematischen
Darstellung. Verändert nach Kevin White & David
J. Mattingly (2006) Versunkene Seen in der Sahara.- SdW,
September 2006, S. 51 (Original-Abb. von Barbara Aulicino).
folgende Text wurde der Website "Fezzan
Project - Palaeoclimate and environment"
entnommen und ist leider nicht mehr online. Die umfangreichen
Literaturangaben können im Originaltext eingesehen
werden. Texthervorhebungen und zusätzliche Absätze
durch den Verfasser. (ehemaliger URL: http://www.cru.uea.ac.uk/~e118/Fezzan/fezzan_palaeoclim.html
- evtl. über
beachten Sie, dass die jeweiligen humiden und ariden Phasen
nicht unbedingt den gesamten Raum der Sahara betrafen,
z.B. in der zweiten holozänen Feuchtphase die nördlichen
Zonen der Sahara nur sehr geringe zyklonale Niederschläge
erhielten (vgl. Sie bitte den Text unten!) [date
of access: 28.08.06]
Holocence wet phase:
most important wet periods in terms of archaeology and the
development of human society occured during the Holocene
(the last 10,000 years). By about 10 ka
[cf. humide Phase A], rainfall was plentiful
and most of the Sahara was vegetated; in the south, vegetation
zones were displaced some 400 km north of their present-day
positions, and fauna from the equatorial regions had
migrated north into the Sahara (...).
about 10 ka and 8 ka, it is believed that rainfall in
the Sahara was generated by the interaction between mid-latitude
weather systems and the inter-tropical convergence zone,
where warm dry Saharan air meets cooler moist air originating
over the eastern tropical Atlantic (...). It is the
northwards migration of this moist oceanic air in the form
of the West African Monsoon that today brings rainfall to
the Sahel (the semi-arid transition zone between the hyper-arid
Sahara and the humid equatorial regions) in summer.
it is not thought that the West African Monsoon penetrated
any further north than today between 10 ka and 8 ka.
The rainfall-generating systems during this period were
the result of semi-permanent low pressure regions sustained
by remnant ice-sheets over North America and northern Europe,
and resulted in precipitation throughout the Sahara from
south to north (...). The monsoon would have remained active
over the Sahel.
Holocence arid interval:
wet episode described above was interrupted by century-scale
arid episode sometime around 8 ka, which was most probably
due to the collapse of the remnants of the Laurentide Ice
Sheet in North America (...). This would have caused a massive
injection of cold fresh water into the North Atlantic, altering
oceanic and atmospheric circulation and lowering sea surface
temperatures, which would have reduced the moisture content
of the atmosphere by suppressing evaporation from the ocean
surface temperatures would have reduced the intensity of
atmospheric convection and hence its capacity to sustain
rainfall-generating weather systems. These changes represented
the transition to full interglacial conditions in the northern
Holocene wet phase:
after 8 ka, wetter conditions returned to much of the Sahara
as the northern hemisphere warmed, and were certainly fully
established by 6.5 ka [cf. humide
Phase B]. However, the northernmost parts
of the Sahara remained dry.
most likely explanation for this situation is that the transition
to full interglacial conditions was associated with increased
solar heating of the northern hemisphere due to changes
in the tilt of the Earth (...). This would have intensified
the West African Monsoon, which may have penetrated to some
30 degrees north, some 10 degrees further north than at
the disappearance of the ice sheets vastly reduced the interaction
between mid-latitude and tropical weather systems that previously
had generated rainfall in the northern Sahara (as well as
much of the central and southern Sahara).
two Holocene wet phases thus represented very different
climatic regimes; it is probably that the perennial vegetation
of the first phase gave way to semi-arid seasonal savannah
in the second, and that in the latter the survival of human
populations required greater ingenuity.
Holocene Saharan Desiccation:
is widespread evidence that the onset of the hyper-arid
conditions that characterise the Sahara today occurred at
around 5 ka (...). It is believed that the desiccation occurred
in two phases, and was the result of changes in the Earth's
orbital parameters which resulted in reduced solar heating
of the northern African landmass that caused a weakening
of the West African Monsoon.
levels remained high in some areas after the onset of hyper-aridity,
and lakes are likely to have persisted in some regions.
Open water bodies and near-surface groundwater would have
sustained a reduced human population in many regions; (...)
has found evidence of human activity and open water bodies
in the Fezzan outside of the Wadi al-Ajal as late as about
Garamantian civilisation in the Wadi el-Agial appears to
have developed soon after this time (...), and it is likely
that people settled in the wadi in increasing numbers as
access to water became more and more difficult in other
parts of the Fezzan. Where such oasis refuges did not exist,
people would have migrated to the the Saharan margins and
the Nile Valley; it is plausible that a large influx of
Saharan refugees was one of the factors that led to the
development of Egyptian Dynastic civilisation; certainly
a knowledge of astronomy and various religious themes appear
to have been common to Pharaonic Egypt and pre-Dynastic
Saharan cultures (...)."
Weitere Forschungen zur Klima- und Besiedlungsgeschichte
der Sahara, des grössten Trockenraums der Erde, vorgestellt
Grüne Vergangenheit der Sahara.- 12. Internationale
EIKE-Klima- und Energiekonferenz (IKEK-12) am 23. und 24.
November 2018 in Aschheim/München.
R. & St. Kröpelin (2006)
Holocene Occupation in the Sahara: Motor of Africa's Evolution.-
Science 11 Aug 2006: Vol. 313, Issue 5788, pp. 803-807.
date of access: 29.09.2019]
Radiocarbon data from 150 archaeological excavations in
the now hyper-arid Eastern Sahara of Egypt, Sudan, Libya,
and Chad reveal close links between climatic variations
and prehistoric occupation during the past 12,000 years.
Synoptic multiple-indicator views for major time slices
demonstrate the transition from initial settlement after
the sudden onset of humid conditions at 8500 B.C.E. to
the exodus resulting from gradual desiccation since 5300
B.C.E. Southward shifting of the desert margin helped
trigger the emergence of pharaonic civilization along
the Nile, influenced the spread of pastoralism throughout
the continent, and affects sub-Saharan Africa to the present
H.-J. & St. Kröpelin (1987)
Howar: Paleoclimatic Evidence from an Extinct River System
in the Southeastern Sahara.- Science 17 Jul 1987:
Vol. 237, Issue 4812, pp. 298-300. [last
date of access: 29.09.2019]
Field research into the climatic history and shifting
of the East Saharan desert has furnished evidence that
during Quaternary time the present extremely arid western
part of Upper Nubia (northern Sudan) was temporarily linked
to the Nile by way of a hitherto unknown 400 kilometer
long tributary. From about 9500 to 4500 years ago, lower
Wadi Howar flowed through an environment characterized
by numerous ground water outlets and freshwater lakes.
Savanna fauna and cattle-herders occupied this region,
which today receives at most 25 millimeters of rainfall
per year. At that period the southern edge of the eastern
Sahara was some 500 kilometers further north than today
and ground water resources were recharged for the last
F. & H. Kehl (SFB 69) Eine eigene Website zum Thema
der 'Western Desert of Egypt" mit zahlreichen Literaturangaben.
dazu auch die interne Seite:
glaziale und postglaziale Vegetationsgeschichte Afrikas"
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