-Sea Level, Ice, and Climatic Change (Proceedings of the Canberra
Symposium, December 1979). IAHS Publ. no. 131.
African cSimatic changes in late Pleistocene and
Holocene and the general atmospheric circulation
S, E, NICHOLSON
Clark University,
Graduate School of
Worcester,
Massachusetts
01610, USA
H, FLOHN
Meteorologisches
Institut,
Universitat
FR Germany
Geography,
Bonn,
Bonn,
ABSTRACT
This paper summarizes the environmental and
climatic changes which took place in Africa from the late
Pleistocene through the Holocene and the general atmospheric circulation patterns which likely corresponded to
them. Three major periods are considered: (a) a period
of aridity and dune building c. 20 000-12 000 BP in which
the Sahara advanced considerably southward; (b) a moist,
lacustrine period c. lO 000-8000 BP; and (c) a second
moist, lacustrine period toward c. 6500-4500 BP in which
the entire Sahara desert contracted considerably. The
prevailing atmospheric circulation patterns are theorized
on the basis of corresponding changes of surface boundary
conditions - primarily changing thermal character - and
known dynamic behaviour of the atmosphere. This
summarizes an article which will soon appear in
Climatic
Change (Nicholson & Flohn, 1980). Information on the
environmental conditions prevailing in Africa during
these periods is found also in Street & Grove (1976,
1979), Rognon (1976) and Rognon & Williams (1977).
CLIMATIC AND ENVIRONMENTAL CONDITIONS IN AFRICA
The period c. 20 000-12 000 roughly corresponds to the last
maximum (c. 18 OOO BP) and end of the Wisconsin glacial. At that
time the prevailing tropical and subtropical aridity resulted in
a 200-500 km southward expansion of the Sahara (Fig. 1 ) , and
semiarid conditions probably prevailed throughout most of the
now-humid equatorial regions, except perhaps those receiving the
most extreme rainfall amounts today. These conditions are traced
through dunal horizons along the east-west expanse of the present
Sahel and Soudan, evaporites and sand evidencing desiccation of
lakes and blockage of rivers, stream and river deposits indicative
of reduced discharge, and pollen indicative of more arid environments. Lake Chad was totally desiccated and in many places dunes
blocked the course of the Niger, Senegal and Nile rivers. In
northern Africa, however, the environment was likely more humid
than at present, with both reduced evaporation and increased
rainfall producing stream terraces, lakes and marshes, and dune
stabilization.
Between c. 12 000 and 10 OOO BP tremendous changes took place.
Vast lakes covered East Africa/ the now semiarid southern margins
295
296
S.E. Nicholson & H. Flohn
18 0 0 0 B P
Fig. 1 Proposed atmospheric circulation scheme c. 20 000-12000 BP (dark shading,
areas more humid than today; light shading, areas drier than today; inset, present
position of the ITCZ in winter and summer; from Nicholson & Flohn, 1980).
1 0 - 8 000 B P
Fig. 2 Proposed atmospheric circulation scheme c. 10 000-8000 BP (shading key,
see Fig. 1 ; ITCZyviN refers only to the position over southern Africa; from Nicholson
& Flohn, 1980).
African climate changes
297
of the Sahara, and even parts of the present desert. During the
maximum of this episode, c. lO 000-8000 BP (Fig. 2 ) , rivers such
as the Senegal, Niger and Nile were considerably stronger than
today, discharging tremendous volumes of water; Lake Chad, now
about 4 m deep, then stood about 38 m above its present level
and was about 10 times its present size; and the depth of Lake
Naivasha increased some 200 m. The vegetation cover devastated
by the Pleistocene aridity returned to achieve a state more
luxuriant than at present in all of these areas. Vegetation and
stream deposits also indicate in the Saharan regions a lack of
pronounced seasonality, i.e. rainfall of both tropical and
temperate origin distributed rather evenly throughout the year.
Wetter conditions prevailed also in the Saharan highlands and
probably in the northeast, but dunes and evaporites, stream
downcutting, and vegetation indicate that a more arid environment
prevailed in the northwest.
A second lacustrine episode lasted from c. 6500 to 4500 BP
(Fig. 3 ) . It is considered distinctly different from the first
not only because of the transitory arid episode c. 7000 BP which
separated them, but also because of qualitative differences in
the climatic conditions which likely prevailed during the two.
This is suggested both by the spatial patterns of the dominant
climatic anomalies, and by empirical evidence of a change in the
rainfall seasonality in the two periods. Again lakes covered
large areas of the Sahara and its semiarid margins, rivers discharged significantly more flow than at present, and vegetation
iTCZ
SUM
/ /TV-
ITCZ
WIN
-15°S
6 500-4 500 BP
Fig. 3 Proposed atmospheric circulation scheme c. 6500-4500 BP (dark shading,
areas more humid than today; hatching, areas drier than during previous period, but
more humid than at present; from Nicholson & Flohn, 1980).
298
S.E. Nicholson & H. Flohn
types corresponding to more humid conditions spread into the present semiarid regions. Civilizations of pastoral nomads flourished throughout all but the most arid core of the Sahara, and the
desert contracted considerably as a consequence of increased
rainfall along both its tropical and temperate margins. The
subtropical areas were most affected by this episode, and the
lakes there often reached higher stands than during the previous
one. However, during this period rainfall south of the Sahara
was strongly seasonal and clearly tropical. The equatorial
tropics were less markedly affected: lake levels in East Africa
were still high compared to the present but significantly lower
than in early Holocene, and areas such as the Guinea Coast may
have been as dry as now.
FACTORS CHANGING THE ATMOSPHERIC CIRCULATION
Thermal factors play a dominant role in determining the character
of, the general atmospheric circulation and hence the thermal
variations provoked by the presence of ice sheets in the
Pleistocene and early Holocene decisively influenced the atmospheric circulation patterns prevailing then. The degree of global
ice cover and hemispheric warming was different in each of the
three climatic episodes discussed earlier, creating different
sets of atmospheric boundary conditions. On the basis of these
conditions and the environmental and climatic characteristics
evidenced in Africa, the atmospheric circulation patterns creating
these climates and environments can be hypothesized, using basic
meteorological principles. The main changes would have taken the
form of displacement and weakening of intensification of present
circulation features, and changes between primarily zonal (eastwest) flow or meridional flow (strong northerly and southerly
perturbations superimposed upon the east-west flow). In
particular, four factors must be considered in hypothesizing
these changes.
Effect
of hemispheric
temperature
temperature
difference)
gradient
(i.e.
equator-to-pole
Theoretically, an increased temperature gradient, which would
result from the presence of northern continental ice sheets,
should result in stronger westerlies, an equatorward displacement
of circulation features, and intensification and shrinking of the
Hadley cell and associated subtropical high. Temperature
gradients also determine the location of the transition between
tropical Hadley and extra-tropical Rossby circulation (i.e.
location of the subtropical high) and influence the wave
character of the Rossby circulation (i.e. the number and position
of waves characterizing the circumpolar westerly currents)
(Flohn, 1964).
Thermal contrast
between
the two
hemispheres
At present the Southern Hemisphere, in comparison, to the
Northern, is much cooler and its temperature gradient much
greater. This results from the varying amounts and distribution
African climate changes
299
of land and ocean in the two hemispheres and, especially, from
the contrast between an extremely cold Antarctic continent and a
relatively "warm" Arctic ocean. The stronger temperature
gradient produces a more intense atmospheric circulation in the
Southern Hemisphere. If this asymmetry is responsible for the
present Northern Hemispheric location of the meteorological
equator or ITCZ (Kraus, 1977; Flohn, 1978), decreased contrast
between the hemispheres, as produced by intensive continental
glaciation in the Northern Hemisphere, should displace the
meteorological equator to a position more coincident with the
geographical equator, i.e. southwards.
Baroclinic
zone: steep temperature
gradients
in subpolar
regions
For dynamic reasons the zone of steepest temperature gradients
must coincide with a jet, or wind maximum, in the circumpolar
westerlies, according to the thermal wind equation. Within the
Northern Hemisphere westerlies, the polar-front jet fluctuates
strongly in time and space and can hardly be detected separately
in long term averages. A baroclinic zone (sometimes described as
the Arctic Front) tends to develop in subpolar latitudes along
the ice margins; each increase of the horizontal temperature
gradient strengthens the westerly flow. Such a situation
prevailed also during the glacial peaks when this baroclinic zone
was displaced just south of the ice margins around lat. 38°N in
North America, around 45°N in Europe and sometimes merged with
the subtropical jet.
Surface
temperatures
Very roughly generalizing, higher (lower) surface temperatures
should increase (reduce) global evaporation, with consequential
changes of rainfall. Similarly, warmer or cooler surface conditions may affect the stability of the atmospheric column, hence
influencing rainfall by suppressing or enhancing the vertical
motion associated with cloud development. Certainly the
generally cooler temperatures prevailing during glacials should
have had a negative influence on precipitation; the thereby
affected rainfall decrease was most marked in areas influenced
by the subtropical high and in the tropical oceans, where it was
enhanced by strong equatorial upwelling of cool water, as
indicated by a belt of low temperatures along the equator at the
Pacific and Atlantic Ocean and caused by an intensification of
the trade winds (Parkin & Shackleton, 1973). Albedo changes
imposed by the landscape changes (presence of glaciers, modification of lakes and vegetation) during glacials should have also
affected the Earth's heat budget and must have significantly
modified atmospheric circulation and climate.
ATMOSPHERIC CIRCULATION OVER AFRICA DURING LATE PLEISTOCENE AND
HOLOCENE
The late Pleistocene tropical and subtropical aridity c. 20 00012 OOO BP (Fig. 1) was probably a consequence of both the presence
of ice sheets in the Northern Hemisphere and the comparatively
300
S.E. Nicholson & H. Flohn
small changes in the Southern Hemisphere, which would have
reduced thermal contrast between the hemisphere. The continental
ice would have increased the hemispheric temperature gradients,
caused circulation features to be displaced toward the equator
and intensified the subtropical high. The result would have been
westerly flow and frequent mid-latitude cyclones over northern
Africa. These features, and their year-round persistence, could
explain the wetter conditions in North Africa. The corresponding
displacement of the subtropical high would have displaced the
aridity maximum from northern Mauritania and southern Algeria to
West Africa (e.g. Senegal, Mali, Mauritania). Increased upwelling
resulting from the stronger subtropical high and lower sea-surface
temperatures both would have reduced evaporation and energy for
convection and storm formation. The decreased thermal contrast
between the Northern and Southern Hemispheres would have displaced
the ITCZ to equatorial latitudes, thereby disrupting the subSaharan"monsoon"system. These were probably more important
factors in the aridity.
The transition in the millenia before 10 OOO BP to the
lacustrine episode coincided with the accelerated melting of
higher-latitude continental ice. The wetter tropical and subtropical environment c. lO 000-8000 BP (Fig. 2) resulted from the
atmospheric circulation changes induced by the disappearance of
the ice: decreased hemispheric temperature gradient, increased
inter-hemispheric thermal contrast, and higher sea-surface
temperatures. In response to these changes, the subtropical high
would have retreated poleward, removing its aridifying influence
from the southwestern Sahara and displacing it into the northwest; the "monsoon" system of West Africa would have been
re-established as the ITCZ again advanced into more northern
latitudes; and increased evaporation and more frequent and
energetic storms would also have acted to increase rainfall.
However, the remaining North American ice would have still been
an imposing force on the general circulation, so that a quasistationary cyclone and westerly flow over northeastern Africa
would have enhanced rainfall there. The wetter conditions over
the Sahara may have resulted in part from that: the reduced rainfall seasonality evidenced there could be explained by the
frequent occurrence of Soudano-Saharan depressions formed by the
interaction of the cyclonic trough in the westerlies (Fig. 2)
with disturbances in the easterlies near the ITCZ.
During the latter lacustrine episode c. 6500-4500 BP (Fig. 3)
conditions were somewhat different. The Northern Hemisphere
reached its thermal optimum then but the Southern Hemisphere
appears to have been cooling since c. 9000 BP (Hays, 1978); this
would have minimized the Northern Hemisphere temperature gradient
and maximized inter-hemispheric thermal contrast. These changes
would have weakened the North Atlantic subtropical high and
possibly displaced it somewhat northward (as under present summer
conditions), thereby diminishing its aridifying influence in the
northwest. The northeast was probably still experiencing
increased cyclonic activity, in part a downstream effect imposed
by the still present core of the Canadian ice. The increased
temperature contrast between the hemispheres would have provoked
African climate changes
301
a more northward advance of the ITCZ, which would explain the
markedly wetter conditions in the present central Sahara and
Sahel-Soudan region south of it as well as the lack of such a
pronounced lake phase in the more equatorial regions. Unlike
during the previous wetter phase south of the Sahara, the
position of the ITCZ (rather than the year-round Soudano-Saharan
disturbances) was probably the main factor increasing rainfall.
This would account for the increased rainfall seasonality, as
compared with the previous period.
REFERENCES
Flohn, H. (1964) Grundfragen der Palaoklimatologie im Lichte
einer theoretischen Klimatologie. Geol. Rdsch.
54, 504-515.
Flohn, H. (1978) Comparison of Antarctic and Arctic climate and
its relevance to climatic evolution. In: Antarctic
Glacial
History
and World Palaeoenvironments
(ed. by E. M. van
Zinderen Bakker). Balkema, Rotterdam.
Hays, J. D. (1978) A review of the Late Quaternary climatic
history of Antarctic Seas. In: Antarctic
Glacial
History
and
World Palaeoenvironments
(ed. by E. M. van Zinderen Bakker).
Balkema, Rotterdam.
Kraus, E. G. (1977) Subtropical droughts and cross-equatorial
energy transports. Mon. Weath. Rev. 105, 1009-1018.
Nicholson, S. E. & Flohn, H. (1980) African environmental and
climatic changes and the general atmospheric circulation in
late Pleistocene. Climatic
Change 2, 313-348.
Parkin, D. W. & Shackleton, N. (1973) Trade-wind and temperature
correlations down a deep-sea core off the Saharan coast.
Nature
245, 455-457.
Rognon, P. (1976) Essai d'interprétation des variations climatiques
au Sahara depuis 40 000 ans. Rev. Geogr. Phys.
Geol. Dyn. 18,
251-282.
Rognon, P. & Williams, M. A. (1977) Late Quaternary climatic
changes in Australia and North Africa: a preliminary interpretation.
Palaeogeogr.,
Palaeoclimatol.,
Palaeoecol.
21,
285-327.
Street, F. A. & Grove, A. T. (1976) Environmental and climatic
implications of Late Quaternary lake-level fluctuations in
Africa. Nature
261, 385-390.
Street, F. A. & Grove, A. T. (1979) Global maps of lake-level
fluctuations since 30 000 BP.
Quatern.
Res. 12, 83-118.