african environmental and climatic changes and the … · african environmental and climatic...

AFRICAN ENVIRONMENTAL AND CLIMATIC CHANGES AND THE GENERAL ATMOSPHERIC CIRCULATION IN LATE PLEISTOCENE AND HOLOCENE

SHARON E NICHOLSON

Department of Environmental Sciences University of Virginia Charlottesville Virginia 22903 USA

and

HERMANN FLOHN

Meteorologisches lnstitut Universitiit Bonn 53 BONN 1 FRG

Abstract The paper describes 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 correspond to them Three major periods are considered (l) a period of aridity and du ne building c 20000-12000 BP in which the Sahara advanced considerably southward (2) a moist lacustrine period c 10000-8000 BP and (3) 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 behavior of the atmosphere

I Introduction

Since the beginning of the century numerous atmospheric circulation schemes have been proposed to explain the observed late-Pleistocene and Holocene environmental and climatic changes With the discovery of very high ancient strand lines in the Nakuru basin (East Africa) it became generally accepted that a tropical pluvial (wet period) accompanied the high latitude Ice Ages and the proposed circulation schemes were modified to fit this glacialpluvial correlation However recent research has decisively proved this correlation false showing that marked tropical aridity was synchronous with the peak of the last glacial c 18000 BP and several authors (Rognon and Williams 1977 Rognon 1976 Maley 1977a Street and Grove 1976 Flohn 1977) have modified these hypotheses on the basis of this revised picture of Pleistocene environmental changes in Africa This article summarizes the evidence for Pleistocene and Holocene climatic changes in Africa then briefly reviews basic concepts of atmospheric general circulation and the evolution of hypotheses concerning circulation in glacial and non-glacial periods Attention is focused on three periods c 20000-120001BP c to000-8000 BP and c 6500shy4500 BP corresponding circulation patterns are theorized on the basis of known atmospheric dynamics These episodes represent two extreme scenarios of climate extreme subtropical aridity and (the latter two episodes) subtropical pluvials The possibility of their recurrence is considered

middotPresent address Graduate School of Geography Clark University Worcester MA 01610 USA

Qimatic Change 2 (1980) 313-348 0165-0009800024-0313 $0360 Copyright copy 1980 by D Reidel Publishing Company

314 Sharon E Nicholson and Hermann Flohn

2 Late Pleistocene and Holocene Environment and Climate in Africa

Within the past 20000 years three major episodes occurred in which the climates of Africa differed significantly from those now prevailing over the continent As most of the

continent lies in the tropics the effects of Pleistocene glaciation were minimal climatic

fluctuations expressed themselves in terms of changes in the hydrological balance among precipitation evaporation and runoff Geological evidence for this balance appears in the

form of tremendous variations oflakes and of the extent of the arid zone While evaporation fIIIIII and runoff certainly varied throughout the Pleistocene and Holocene in most cases

rainfall was clearly the decisive factor in these environmental changes which various types of geological and palynological evidence help to trace Tropical and SUbtropical aridity marked the period c 20000-12000 BP which coincided with the last glacial maximum (Wisconsin or Weichsel) A rapid rise of lake levels throughout tropical Africa

followed during the early Holocene period of rapid glacial retreat and global warming before c 10000 BP and maximum levels prevailed to 8000 BP A subsequent

lacustrine period lasted from c 6500 to 4500 BP (Nicholson 1976) At least moderately high lake levels characterized most of the Holocene from about 12000 BP until three or

four millenia ago interrupted only briefly by drier conditions Nevertheless two discrete Holocene lake episodes are considered here because of the widespread arid episode toward 7000 BP and because the nature of the two episodes appears to have been quite different

as explained below Figure 1 shows mean annual rainfall and the location of most sites discussed Figure 2 summarizes local chronologies while Figures 3 4 and 5 summarize

Fig1(a) Mean annual rainfall (em) over Africa (from Nicholson 1980 based on all available years to 1973 at 420 stations)

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Fig l(b) Location of sites discussed w Vgt


conditions for the three episodes considered The reader should refer to these throughout

the discussion Those less interested in the details offered in Section 2 can follow the

discussion of Sections 3 4 and 5 on the basis of the climatic summaries in the figures in the above paragraph and in Section 4

21 Tropical Aridity Synchronous with the Wisconsin Glacial Maximum

Following a latemiddotPleistocene lake phase (Rognon and Williams 1977) the onset of more

arid conditions was variable across the continent but began about 20000 in most areas maximum aridity was probably achieved between 18000 and 14000 BP The formation

and marked expansion of aeolian sand dunes along the southern margin of the present

Sahara was a predominant feature of this episode (Figures 2 and 3) Traces of the dunes

are observed in Mauritania in Senegal (Hebrard 1972 Michel 1973) and Mali where they

blocked the course of the Senegal (Michel 1973) and Niger (Tricart 1965) rivers in

northern Upper Volta across most of Niger (Michel 1973 Chamard and Courel 1975) and Chad (Pias 1957) and as far east as the Sudan (Grove and Warren 1968)

where they encroached into the White Nile Valley Extending down to about 14deg N in

the west and to about 10deg in northern Nigeria Cameroun and the Sudan these formations

represent a 200-500 km southward expansion of the Sahara along its entire eastmiddotwest

extent At the same time the discharge of the Senegal was considerably reduced and

Lake Chad was probably totally dry as were numerous basins and depressions in Niger

and Mauritania which subsequently contained lakes (Michel 1973 Servant 1973)

Numerous radiocarbon dates firmly establish the existence of the dune system in

Senegal Mauritania and Chad from about 20000 to 12000 BP In Chad the dunal horizons are bracketed by lacustrine deposits radiometrically dated at 22000 and

12000 BP On the basis of lithologie similarities and continuity of the outcrops it can

be concluded that nearly all submiddotSaharan dunal deposits are contemporaneous (Michel

1973 Servant 1973 Burke et al 1971) but the systems commencement and termination

may be one or two millenia later in other regions As lower t~mperatures and hence decreased evaporation likely prevailed then a decrease in rainfall along the tropical Saharan

margins can be assumed Similar changes probably affected the entire tropical zone of

Africa although for many regions the evidence is not well fixed in time and in some cases may relate to conditions just prior to 18000 BP Traces of an arid episode coincident

with the last glacial maximum extend as far south as the Guinea Coast region of West Africa throughout East Africa and in parts of the central African equatorial tropics Fluvial deposits indicate rare and torrential rains in parts of Portuguese Guinea presently covered by tropical rain forest (see Michel 1973) pollen indicates a more arid environment

in the Ivory Coast littoral (Assemien et al 1970 cited in Michel) and extensive gully

erosion and other morphological features and the desiccation of Lake Chad suggest drier conditions in Cameroun western Zaire (de Ploey 1965) and southern Nigeria (Hurault

1971 1972 Hervieu 1970 cited in Michel 1973) Even in equatorial regions where tropical rain forests now prevail relatively dry conditions probably characterized the period about 18000 years ago In much of Zaire subsurface deposits of aeolian sand

bull bull

African Environmental and Climatic Changes in Late Pleistocene and Holocene 317

SMgal ---------- ----- --- --shyMauritanian

_________ bull _________---4 ------ shy_-~Littoral Sbkha N Drahmacha ----_ --_-shySbkha Chmchan -- - ~ - --- shyMauritanian _ _ _______---- ------------------- 2Adrar

Niger Laks ------__--- --- -- -----------__---__--- - Adrar Bous ---- ------ ----Ahaggar (Hoggar) ---------------_ Tibesti ---------- Lake Chad __ __ --_- _- _-

-

Lak Rudolf MOO _~_ _ __ ____ _____ -- bullbull e

Lake Nakuru --_- --------_ -bull---------_ -----_ Lake ---_ -- __ _Naivasha Lake Magadi

_-- - - - --- ----_ _ --_ -- gt

Lak ----_ - _Victoria Sudan - _----____

While Nile __--_ -

Blue Nite __ shy----_ Egypt-Wadis - ----------__ ___deg2 Lowr Awash

Lak AMi ------------_ _------ shy__deg2Dobi-Hante ___ 2 Lake Asal -- -- ------ shy ~-------------

Lake Afrera

30 25 20 15 10 5 o Fig 2 Climatic variations in various parts of Africa from 30000 BP to present -high levels - intermediate levelsmiddot low levels or desiccation bull = radiometric dates (Summary of figures in Nicholson 197 6 and present text)

__ __


w

W

Key to Numbered Locations

1 Great Western Erg 2 Saouara terrace 3 Touat 4 Erg Chech 5 Soltanian terrace 6 Hoggar 7 Tibesti 8 Nubia 9 Southern Tunisia

10 Mauritanian ergs dunes blocking Senegal

11 Dunes Dakar Mbour Cayor Thies

12 Dunes Niger Bend 13 Dunes near Mopti 14 Erg of Ouagadou dunes

ih northern Upper Volta

N

30deg

il 1

20deg 16 t31~ 15 14 18

19 10deg

0deg

10deg

20deg

pollen rVidence or

51-_-shy_____---_---shy --shy --_----_5

15 Dunes Bandiagara plateau 16 Dunes near Dhar Nema 17 Kano erg 18 Dunes near Fort Lamy

Lake Chad 19 Dunes northern

Cameroun 20 Dunes EI Obeid

Kordofan 21 Qoz dunes 22 Dunes blocking White

Nile 23 Portuguese Guinea 24 Ivory Coast littoral 25 Western and southwestern

Nigeria 26 Cameroun

50deg E N

30deg

20deg wi

10deg

0deg

10deg

20deg

50deg E

27 Ruwenzori 28 Mount Kenya 29 Cherangani Hills 30 Muchoya Swamp 31 Lake Victoria 32 Lakes Magadi Naivasha

Nakuru 33 Lake Rudolf 34 Four Galla Lakes 35 Afar Lakes 36 Awash and Bulbula Rivers 37 Blue Nile 38 White Nile 39 Western Desert 40 Lake Albert

Fig 3 Summary Map of Oimatic Conditions c 20000-15000 BP (modified from Nicholson 1976)


underlie forests and woodland (Livingstone 1975) and prior to 12500 BP the evergreen

forest of Uganda virtually disappeared (Kendall 1969) In the Stanley Pool area of the

Congo soils sediments and geomorphology suggest that relatively arid conditions prevailed until at least 10000 years ago Increased aridity in coastal areas of southwestern Africa is also likely as upwelling was stronger during glacials and the cold Benguela current was more extensive running not only along all of the Angolan coast but extending as far north as the Congo-Gabon area and possibly into the Gulf of Guinea (Giresse et al 1976 Giresse 1975 1978) In general the tropical rain forest had probably retreated from all areas except those presently extremely rich in precipitation parts of Gabon the extreme south of Cameroun and possibly the northern Congo and coastal regions of

Liberia and southern Sierra leone and some highland areas

In much of Eastern Africa there is decisive evidence that arid conditions occurred

simultaneously with those along the southern margins of the Sahara in late Pleistocene

In this case several factors complicate interpretation the varying climatic regimes within eastern Africa differentiation between local conditions and conditions in the remote source regions of water bodies and the sensitivity of local climate and the hydrologic regime of individual basins to climatic change Therefore the apparent times and degree of

the observed hydrologic changes are not quite uniform at the sites considered Nevertheless it is clear that in most areas considered increased aridity prevailed from c 20000 or

18000 BP to 12000 BP The Blue Nile was then actively aggrading its bed and floodshy

plain with sands indicative of a semi-arid environment (Williams and Adamson 1974) While this flow regime may have resulted from either local aridity or drier conditions in

the rivers source region the Ethiopian highlands there are independent indications of

greater aridity in parts of Ethiopia After about 20000 BP the discharge of the Awash

and possibly Bulbula (Gasse 1975) diminished and lake levels fell in Ethiopia and the

- Afars Territory (Gasse ]975 Rognon 1974) during the aridity maximum c ]7000shy

12000 BP Lake Abhe dried up completely within a few millenia after attaining a

maximum of 6000 km2 and 150 m depth At the same time the White Nile was lower than today and probably a strongly seasonal river as it was receiving no outflow from the

East African lakes (Williams and Adamson 1974) The levels of Lakes Mobutu Sese Seko

(Albert) Manyara (Uvingstone 19751979) Magadi Nakuru Turkana (Rudolf) Victoria

and probably Navivasha (Butzer et al 1972) were low in the late Pleistocene prior to

12000 BP For Lakes Victoria and Mobutu Sese Seko direct evidence of extreme aridity

covers only two millenia the former completely lacked an outlet for at least 2000 years

prior to 12500 BP (Kendall 1969) as did the latter before 18000 BP and from c14000 to 12500 BP (Harvey 1976 cited in Uvingstone 1979) Both lakes were probably below their Nile outlets throughout the late Pleistocene depriving the river of the presently substantial contribution of equatorial waters However a brief interruption of this late Pleistocene aridity in East Africa about 17000 BP is evidenced by diatoms in Lakes Mobutu Sese Seko and Manyara (Harvey and Holdship unpublished dissertations cited in Livingstone 1979 and Street and Grove 1979) Further research is needed to establish whether or not this brief humid oscillation occurred elsewhere in East Africa

Evidence for the period c 20000-12000 BP in northern Africa is less conclusive

320

II

30deg 30deg

20deg

10deg

0deg

10deg

20deg


IOuarghla 2 Piedmont north of Aures 3 Great Western Erg 4 Laghouat and Biskra 5 Sou them Tunisia and

Schotts 6 Saouara 7 Tarfaya 8 Erg Chech 9 Touat

10 Mauritanian wadis 11 Sebkha de Chemchane

lakes in the Adrar and Tagant

12 Sebkha de NOrahmacha and coastal streams

13 Senegal River 14 Baoule River 15 Gambia River 16 Araouanc 17 Lake Faguibine

18 Hoggar 19 Adrar Bous 20 Lake Bilma 21 Lake Fachi 22 Lake Termit 23 Lake Agadem 24 Lake Manga 25 Tibesti 26 Jebel Aulia 27 Kosti 28 Oanakil depression 29 Afar Lakes 30 Afar Lakes 31 Awash River 32 Galla Lakes 33 Blue Nile 34 White Nile 35 Lake Stefanic 36 Lakc Rudolf 37 Chcrangani Hills

Muehoya Swamp

20deg

10deg

0deg

10deg

20deg

38 Lake Albert 39 Lake Victoria Mount

Kenya 40 Ruwcnzori Lake Kivu 41 Lakes Nakuru and

Naivasha 42 Lake Magadi 43 Lake Manyara 44 Lake Rukwa 45 Kom Ombo 46 Majabat al-Koubra 47 Hodh Ohar Tichitt-Nema-

Oualata 48 Upper Volta 49 Ivory Coast 50 Nigeria 51 Cameroun 52 Lake Botsllmtwi 53 Lake Tanganyika

Fig 4 Summary Map of Climatic Conditions c 10000-8000 BP (modified from Nicholson 1976)

321 African Environmental and Climatic Changes in Late Pleistocene and Holocene

and somewhat contradictory Here it is also difficult to distinguish between the lower

temperatures of the late Pleistocene and increased precipitation in creating a more humid environment However some trends become clear from the summaries by Street and Grove (1976 1979) Rognon (1976) Rognon and Williams (1977) Jakel (I978) and

Sarnthein (I978) The Saharan highlands (Tibesti Hoggar Mr) where today the rare rains

fall in summer then were semi-arid (considerably wetter than today) and received frequent winter rain The Saouara (SW Algeria) formed a terrace during a period of increased flow

from the Atlas Mountains c 24000-14500 BP or later (Conrad 1969) the Soltanian terrace in Morocco was probably contemporaneous (Rognon and Williams 1977) Water

covered the lowlands of Ahnet c 34000-18000 BP or later (Conrad 1969) and a

lacustrine period occurred after about 22000 BP in the regions of Touat and the erg

Chech (Algeria) (Conrad 1969) the Great Western Erg and the erg Chech were inactive

at that time until c 10000 BP or later From 21000 to 15000 BP aquifers significantly

recharged in Algeria and Tunisia where no signs of aridity appeared until 16000 BP or

later (see Street and Gorve 1976 Rognon 1976 Rognon and Williams 1977 Coque

1962) Intense wadi activity (Butzer and Hansen 1968) in southeastern Egypt c 17000shy13000 BP resulted from wetter conditions and as in the highlands penetration ofwinter

extra-tropical rains into more southern latitudes Coincident with this was a major arid interval in the Western Desert ofEgypt (Wendorf et al 1977) and an important Nile terrace

(Butzer and Hansen 1968) which probably resulted from more powerful and more sporshy

adic floods rather than wetter conditions (Rognon 1976) in the source regions of the Nile The above suggests that during most of the late Pleistocene c 20000-14000 BP

relatively wet conditions prevailed over most of North Africa with the exception of

northern Egypt This conclusion is further supported by evidence in the next section of increasing aridity in these regions commencing c 12000 or 10000 BP However at the

- peak of the higher latitude glacial toward 18000 BP a brief arid interval occurred in

some of these regions (Sarnthein 1978) dunes formed near Touat the Great Western Erg the erg Chech in southern Egypt and the Nile Valley in northern Cyrenaica near

Algiers and along the southern piedmont of the Atlas Dune formation in the Great Western Erg was relatively long from c 19800 BP to 17510 BP in other regions the arid interval may have been limited to one or two millenia Thus summarizing conditions

in North Africa it appears that the late Pleistocene up to c 12000 BP was predominantly wet in North Africa except for an arid interval c 18000 BP While in some regions arid conditions prevailed also before that date in most regions one or two arid millenia centered

around that date merely interrupted a generally wet late Pleistocene

22 Early Holocene Lacustrine Jpisode ill the Tropics and Subtropics

At some time between 12000 and 10000 BP rapid changes commenced in most of the regions discussed A lacustrine episode beginning then and attaining a maximum from 10000 to 8000 years ago left traces throughout the southern Sahara the Sahel and the

Soudan from Senegal and Mauritania eastwards to Ethiopia and eastern Africa (Figures 2 and 4) In Mauritania lakes some radiometrically dated formed in the Adrar the Tagant


the sebkha de Chemchane and the sebkha de NDrahmacha and in the dhar TichittmiddotNemashy

Oualata (Hebrard 1972 Chamard 1973 Michel 1973 Gasse 1975) Wadis of western

Mauritania reached the ocean coastal streams cut the Pleistocene dunes between

Nouakchott and Cape Verde hydrological systems developed in the ergs of the Hodh

and ponds and lakes occupied interdunal depressions in the EI Djouf desert and parts of

Mali The Gambia Senegal and Baoule Rivers deepened the Niger discharged an increased

flow volume and the Upper Volta Noire formed a vast lake in the plain of Sourou In

presently desertic regions of northern Mauritania a Soudanian vegetation (indicative of

over 400 mm of rainfall annually) prevailed Soudano-Guinean and Soudan ian vegetation

requiring in the mean at least 800 mm rainfall annually probably covered northern Senegal an area presently dominated by thorny Sahelian steppe

Traces of this wetter episode are observed as far south as the [vory Coast Nigeria

(Michel 1973) and Ghana where Lake Bosumtwi rose some 40 m above its present level

(Talbot and Delibrias 1977) Lakes up to 40 m deep formed in presently desertic areas

of Niger near Agadem Bilma and Fachi (Faure et al 1963 Servant 1973) Lake Chad

with a present mean depth of 38 m then stood 38 m above its present level at that time

it covered an area the size of the United Kingdom and reached a maximum depth of about

340 m in the Bodele depression in the northeast Numerous radiometric dates establish

the peak of the NigerChad lake phase as 9000 to 8000 BP Because Lake Chad depends

to a large extent on rainfall from the humid Cameroun tropics it is not an unambiguous

indicator of sub-Saharan rainfall However such a tremendous increase of the lake without

a simultaneous increase in Sahelian rainfall is highly unlikely and furthermore other lakes

discussed clearly indicate that the presently semi-arid regions south of the Sahara were much wetter than today (Maley 1977)

Numerous radiometric dates for lacustrine deposits in the Sudan indicate that lakes

filled the pans near Jebel Aulia south of Khartoum from 8400 to 6990 BP and existed

near Kosti between 11300 and 8370 BP (Williams etal 1974 Grove 1972) These

lakes were not fed by runoff from the White Nile and imply annual rainfall possibly

three times the present value of 164 mm (Williams et al 1975) About 12000 years ago

the White Nile became a less seasonal river as the levels of Lakes Victoria and Albert rose

and the lakes overflowed into the river from then until about 8000 BP the White Nile

was higher and broader than today (Williams and Adamson 1974 Williams 1966 Williams et al 1975) Also toward 12000 BP the Blue Nile ceased aggrading its bed

changed to a meandering course and began depositing instead of sands and gravels finer alluvial material indicative of wetter conditions Increased rainfall in the Ethiopian highlands may have produced these changes

During the peak of the lacustrine episode between 10000 and 8000 years ago the lakes in Ethiopia and the Afars Territory were also deeper and more expansive From

9700 to 7000 BP Lake Turkana (Rudolf) stood 80 m above its present level about the same time the Galla lakes were much deeper all united and overflowing into the Awash

River which discharged a highly increased volume of water and Lake Stefanie now a

marsh was probably a deep and extensive lake (Butzer 1971 b Grove et al 1975 Grove

and Goudie 1971) A lake filled the Danakil depression from 8940 to 5600 BP and the


levels of the Afars lakes (Abhe Assala Afrere and tllOse in the Dabimiddotllanle basin) rose

sharply (Gasse 1975 Grove 1972) A large number of radiocarbon dates confirm the

abrupt rise about 10000 years ago and the persistence of these high level stages over

several millenia This lacustrine episode also affected East Africa where a great number

of radiocarbon dates are available to confirm the chronology The levels of LIkes Rukwa

Nakuru Naivasha Magadi Victoria (Butzer el at 1972) Kivu (Degens and Becky

1974) and probably Mobuto Sese Seko Tanganyika and Manyara (Livingstone 1975

w 1979) rose tremendously toward 10000 BP or earlier these maximum stands as much as 180 m above the present lakes were maintained until c 8000 BP Vegetational

changes support the lake evidence of a change to wetter conditions in East Africa

c 10000 BP (Livingstone 1975) A change to wetter conditions probably also occurred in more western equatorial regions about this time In the Congo and parts of Zaire the

transition to moister conditions was probably about 10000 BP (de Ploey 19631965)

but this may have been even earlier in coastal areas as the cold Benguela current parshy

ticularly strong and extensive c J8000 BP disappeared from the Angolan coast between 12860 and 11730 BP (Kouyoumontzakis and Giresse 1975)

The Saharan highlands of Tibesti Mr Tassili and the Hoggar experienced lacustrine

phases contemporary with the formation and expansion of lakes along the southern fringes of the Sahara and in East Africa within the period 12000 to 8000 BP Lake and

marsh deposits near Tamanrasset in the Hoggar date to I 1580 and 8380 BP (Rognon

1967 1976) and lakes existed from about 9000 to 7000 BP in the Adrar Bous of northeastern Arr (Clark et al 1973) The rivers on these massifs which had previously

carried coarse sand and gravel began depositing finer-grained and well sorted alluvium

indicative of a denser vegetation cover increased and less seasonal rainfall and less erosive

flow perennial marshes formed in the rivers floodplains (Rognon and Williams 1977

Rognon 1976 Servant 1973) Pollen from these highlands indicates the presence of

both Mediterranean and tropical flora including large tropical herbivores (Rognon 1976

Rognon and Williams 1977 Servant 1974) Rognon interprets these stream deposits as

evidence that these regions regularly received both winter rains of northern origin represhysented in Tibesti by the Middle Terrace dating from 13700 to 7380 BP (Hagedorn

and Jakel 1969 Pachur 1975) and summer tropical rains the presence of both

Mediterranean and tropical pollen supports this conclusion In Tibesti runoff of the

tropical rains activated wadis on the southern flanks formed the Angama terrace and

contributed to the waters of Pa1eolake Chad until 7000 years ago (Rognon 1976)

Today these highland areas lie south of the limit of winter rainfall Increased wadi activity in Upper Egypt from 11500 to 9500 BP (Butzer 1971a) also related to winter rains

may provide additional evidence of a further southward penetration of the winter rains in North Africa in early Holocene times

Evidence of corresponding conditions in other parts of North Africa is summarized by Rognon (I 976) Once again the patterns are less decisive than for regions south of the

Sahara but it seems that increased aridity prevailed in the Northwest during this tropical lacustrine episode while parts of eastern Algeria Tunisia and possibly areas further east were relatively humid It is not clear whether climate in the more western areas was arid


relative to the present or simply drier than during the previous humid period in late Pleistocene The Saoura began downcutting its floodplain after 14000 BP during a dry phase lasting until c 6000 BP the Great Western Erg and the Erg Chech were inactive

after c 10000 BP (Ortlieb 1975 Conrad 1969 cited in Rognon) At Laghouat aeolian sands were deposited from about 12000 to 8000 BP and near Ouarghla evaporites developed and dunes were active from before 9500 BP to some time after 7900 BP (radiometrically dated) (Estorges et al 1969 Aumassip et al 1972 cited in Rognon) A semi-arid episode near Biskra was interrupted by two brief humid periods c 10000 and filii 5400 BP and other indications of a drier episode contemporary with the Moroccan

dune deposits are found to the south of the Atlas in Algeria and on the piedmont north of Aures vegetation changes evidence a dry episode in the Algerian Maghreb from 14000 to 9000 years ago and the Chotts and Atlas region of Tunisia were relatively arid between

16000 and 8000 years ago (G E Williams 1970 Couvert 1972 Coque 1972 cited in Rognon) However within this arid period there occurred brief humid phases in the Maghreb at the peak of the tropical lake episode 9280-7500 BP in the Tunisian piedmont

north of Aures and 8500-7500 BP in the Algerian Maghreb (Ballais 1974 Coque 1972 cited in Rognon) The subsistence patterns including land snails and density of occupation sites of the Capsian civilization which inhabited eastern Algeria and southern

Tunisia from about 10000 to 7000 BP hint at a longer humid phase during these three millenia (Lubell 1977) In the southern Libyan desert an arid interval ended c 9500 BP from 8500 to about 6000 BP numerous playa lakes existed there (Haynes 1977 Pachur 1975) partly sustained by runoff from Tibesti After 10000 BP several wetter phases occurred also in the Western Desert of Egypt (31degE 22degN) and in Upper Egypt (Nubia)(Wendorf et al 1976) Butzer 1971 a Butzer and Hansen 1968)

23 A Second Humid Phase during the Neolithic

Following a brief arid episode toward 7000 BP in many of the tropical and subtropical regions discussed above (see Rognon and Williams 1977 Nicholson 1976 Street and Grove 1976 Livingstone 1979) a second lacustrine phase occurred from about 6500 to about 4500 BP when a change began toward the present aridity (Figures 2 and 5) While the entire early and mid-Holocene is characterized by some authors (Rognon 1976 Rognon and Williams 1977 Street and Grove 1976) as primarily one lacustrine or humid episode two distinct lacustrine periods are distinguished here for several reasons The tropical arid episode toward 7000 BP the evidence for which is summarized in Nicholson (1976) and Rognon (1976) seems to signal a significant climatic discontinuity marked also by the termination of the long arid period in numerous parts of extra-tropical North Africa toward 6000 BP and by falls of numerous East African lakes although they still maintained levels much above the present stands Another sign of this climatic discontinuity is the abrupt change of stream flow regime of the Blue Nile (Williams et al 1975) and in the Saharan highlands (Rognon 1976 Rognon and Williams 1977) After about 7000 BP stream deposits suggest a semimiddotarid more seasonal and more torrential rainfall regime At this time the Blue Nile ceased aggrading and began down cutting (Williams et al 1975)

African Environmental and aimatic Changes in Late Pleistocene and Holocene 325

W 10deg 0deg 10deg 20deg 30deg 40deg 50deg E N N

2 middot2middot

30deg 30deg

6

r 20deg 20deg

12

10deg 10deg

0deg 0deg

10deg 10deg

20deg 20deg


1 Maghreb 16 Hoggar 31 White Nile 2 Biskra 17 Adrar Bous 32 Afar Lakes 3 Tarfaya 18 Lake Bilma 33 Awash River 4 Ougarta 19 Lake Fachi 34 Four Galla Lakes 5 Saouara 20 Lake Agadem 35 Lake Rudolf 6 Touat and erg Chech 21 Lake Termit 36 Ruwenzori Lake Kivu 7 Sebkha NDrahmacha 22 Lake Manga 37 Lake Victoria 8 Senegal River 23 Lake Chad 38 Lakes Nakuru and 9 Gambia River 24 Northern Tibesti Naivasha

10 Tichitt 25 Southern Tibesti 39 Lake Manyara 11 Yelimane 26 Jebel Marra 40 Main Nile 12 Lake near Mopti 27 Khartoum 41 Majabat al-Koubra 13 Ghat 28 Lake Tjeri 42 Libyan playa lakes 14 Tihodaine 29 Ethiopian Highlands

15 Djanet 30 Blue Nile



and the level of the White Nile fell by 2 to 3 m (WUliams 1966) Further evidence of a

more torrential rainfall regime derives from the destruction of dams in valley lakes in Tibesti after about 7000 BP (Gabriel 1977) The runoff from the Tibesti massif ceased to reach Paleolake Chad for a brief period c 7000 BP terminating construction of the

Angamma terrace and both the pollen profile and sedimentation rate in the lake changed

markedly at this time (Maley 197 7b ) Other characteristics distinguish the second lacustrine episode from the one in early

Holocene DUring the latter period increased rainfall prevailed primarily in the semi-arid

sub tropics south of the Sahara and in the tropics further south rather arid conditions characterized northwestern Africa the northern fringes of the Sahara However during

the period c 6500 to 4500 BP wetter than present conditions affected both the

temperate and tropical margins of the Sahara considerably shrinking the desert belt Significantly the tropical area along the Guinea Coast of West Africa may have been relatively dry if the midmiddotHolocene regression of Lake Bosumtwi (Ghana) evidenced by

Talbot and Delibrias (1977) occurred during this period During early Holocene that area was clearly wetter than today Meteorologically two different explanations are needed

to explain the two sets of environmental conditions explanations based on atmospheric

boundary conditions prevailing during the two periods

During the period 6500-4500 BP frequently termed the Neolithic (archaeologically)

or Atlantic (climatically) numerous gullies were active in Senegal and Mauritania Boghe

in southern Mauritania was surrounded by Soudanian type tropical vegetation steppe

with small trees or shrubs and isolated forest stands in place of the present thorny Sahel

steppe the discharge of the Senegal and Gambia rivers and their affluents was greater

than now nearby depressions were inundated indicating higher floods of the rivers In

Mauritania ponds and lakes occupied interdunal depressions of the Majabat almiddotKoubni

and the trend toward the present aridity did not begin until at least 4800 BP even as

late as 4450-3700 BP the climate there was still much wetter than today with many

lakes existing in the Tichitt area (l8degN) (Michel 1973 Munson 1971 Hebrard 1972

Elouard 1973) Numerous radiometric dates also confirm a lacustrine episode in the

Spanish Sahara and northern Mauritania from c 7000 to c 4000 BP (Delibrias et at 1976)

Further east in Niger and Chad there was a similar return to wetter conditions about

6500 BP The Niger formed a lake near Mopti and lakes in the Tenere and eastern Niger again reached high stands between then and about 4000 BP (Servant 1973 Michel

1973) Between 7000 BP and 6000 BP the surface of Lake Chad rose rapidly mainshy

taining its high stand until c 4000 BP but the lake was smaller than during the previous humid period before 7000 BP Concurrently with this second humid period Neolithic

peoples spread into the driest regions of the Slhara (Gabriel 1977)

In the Sudan the Nile valley and parts of eastern Africa similar trends are evidenced a second humid period c 6500~4500 BP followed a brief arid period toward 7000 BP

Neolithic sites and fossil flora and pollen evidence this phase in much of the Sudan (Williams ct al 1975 Wickens 1975 Melugh 1971) After 7000 BP the height of

the Nile flood again increased five thousand years ago floods were at least 5 m higher

African Environmental and Qimatic Changes in Late Pleistocene and Holocene 327

than today and a low flood stage was not reached until about 4000 BP when wadi

activity (indicative of winter rains) was also reduced in Upper Egypt to a minimum after a period of active flow during the Neolithic (Butzer 1971abGrove 1972) As mentioned earlier after 7000 BP the flow of the Blue Nile changed to a regime indicative of more arid conditions and more torrential rains but floods were still 5 m or more above the present level until some four thousand years ago and until then the river was still a sinuous and suspended-load channel indicating a less arid rainfall regime (Williams 1975

-Williams et al 1975) While Lakes Victoria Naivasha and Manyara appear to have continuously maintained

high stands from at least 10000 BP until 5000 BP or later the levels of the former two lakes fell somewhat about 7000 years ago at which time Lake Magadi began to

rapidly dry up to its present size Lake Nakuru evidenced the brief arid episode toward 7000 BP described for other areas and afterward regained only moderately high stands (Butzer et al i 972 Livingstone 1975) Significantly the sedimentary record of Lake

Naivasha suggests that until about 2500 BP a single annual cycle of wet and dry seasons

prevailed rather than the present two wet periods and two dry periods over the year

(Livingstone 1975) Lakes in the Afar and in Ethiopia as well as Lake Rudolf generally

evidenced a brief arid interval about 7000 years ago or a change to lower levels then and

about this time the discharge of the Awash considerably diminished (Gasse 1975) In general in these regions as in the sub-Saharan lands a second lacustrine phase reigned

until about 4000 years ago or later

In the Saharan highlands and other parts of the central Sahara and in much of North

Africa this humid lacustrine phase c 65004500 BP is evidenced As elsewhere there

are signs of an arid millenium about 7000 BP in Tibesti and Air when rivers began

depositing large and badly sorted material characteristic of a drier climate with a shorter more pronounced rainy season and stream flow from Tibesti into Lake Chad ceased about

the same time terminating construction of the Angama terrace In Chad fresh-water diatoms disappeared and vertisol soils developed indicating a tropical climate with conshy

trasting seasons (Servant 1974) Nevertheless wetter than present conditions prevailed in these areas throughout the

Neolithic c 6500 AOOO BP After the arid interval runoff from the northern part of Tibesti continued to feed playa lakes in the Libyan desert (Pachur 1975) such lakes were numerous in the desert of southern Libya until about 6000 BP (Haynes 1977) and from c 6500 to 4500 BP at 28deg to 30

0 N in Libya (Petit-Maire and Delibrias personal

communication) At the time the Sahelian summer rains may have reached to c 300 N in

Libya and the Atlas Mountains (Sarnthein 1978) Lowland marshes developed in the northwest Sahara as did lakes in the highland areas while pastoralists occupied now dry sites throughout the Sahara including the Tenere desert the area to the north of Tibesti and the south of Libya (Rognon 1976) Neolithic peoples populated the Hoggar until at

least 5450 BP and the Adrar Bous where lakes date to 5700 and 5000 BP until at

least 5180 BP To the southeast of Tibesti Lake Ounianga Kebir was 40 m higher than today toward 6160 BP while lakes in the northern part of the massif toward 5300 to

5100 BP were surrounded by a mixture of Mediterranean and Sahelian vegetation


After a humid period 7600 to 5800 BP in which Mediterranean vegetation prevailed in the Tassili near Ghat a semi-arid vegetation developed between 5500 and 4500 BP to be succeeded by deposits of aeolian sand which mark the present desert there The Tassili near Djanet was also less arid from 8000 to 4000 BP and Mediterranean vegetation existed in the Hoggar until 5450 BP (Rognon 1976)

As for areas outside the highlands (Rognon 1976) the dry episodes which had premiddot vailed in southern Tunisia and near Laghouat and Ouarghla (Algeria) had ended 6000

appears to be the end of a long dry episode in the Saouara region of southwestern Algeria and in southern Morocco Wetter conditions prevailed in the region of the Saouara and its terrace was covered with vegetation from about 6500 to 4500 BP and lakes existed in the Erg Chech from 6000 to 3000 BP coincident with the entire Saharan Neolithic The Touat region also became wetter and the dunes of southern Morocco became inactive toward 6000 BP There is vegetational evidence of increased precipitation in the Maghreb from about 8500 to 4000 BP during the core of this humid period c6500 to 4000 BP rainfall may have been 300 mm to 600 mm greater than today according to

Couvert (1972) Silts containing freshwater mollusca near Biskra indicate a humid episode c 5400 BP and calcareous deposits and paleosols suggest similar conditions in the Monts dOugarta (Algeria) between 5000 and 4000 BP Marsh deposits rediometrically

dated to 4900 BP evidence this episode at Tihodafne Butzers (1957) reconstruction of Ubyan and Egyptian climate on the basis of fauna flora and archaeology also represents a humid period north of the Sahara even in now hyperarid areas of Kufra and Tibesti rainfall then amounted to 200--400 mm annually (Gabriel 1977 lakel 1978)

3 General Atmospheric Circulation

31 Present Circulation Features

A simple model (Figure 6) of the present general atmospheric circulation provides an appropriate starting point for developing a scheme of late Pleistocene and Holocene

circulation Near the surface it is represented by a series of wind systems and cellular pressure belts in continuous motion about the hemisphere

(a) subpolat low pressure belt with travelling cyclonic centers (b) zone of surface westerlies with travelling midmiddotlatitude cyclones (c) subtropical high pressure belt with quasimiddotstationary anticyclones most pronounced

over the oceans (d) zone of surface easterlies (trade winds) most regular over the oceans (e) intertropical convergence zone (ITCZ) with variable winds and frequent rains

In the upper troposphere (9-14 km) zone (b) broadens and extends above zones (c) and (d) towards the equator here large meanders with a sequence of troughs and ridges travel eastwards and interact with lowmiddotlevel disturbances frequently observed in zone (e) These features are all displaced seasonally towards the summer hemisphere the center of


NORTH POLE

SOUTH POLE

Fig 6 Features of the general atmospheric circulation winds (arrows) high pressure cells (H) low pressure cells (L) a subpolar lows b = westerlies and mid-latitude cyclones c = subtropical highs d surface easterlies (trades) e = intertropical convergence lone f = vertical Hadley cells with rising motion near equator subsidence in subtropics

zone (c) for example varies in the northern hemisphere seasonally between 32degN in

January and 42degN in July (latitudinal averages) and in the southern hemisphere from 35degS to 27degS A more extreme example is the large-scale tropical monsoon system

which represents the seasonal displacement of zone (e) Extending in the tropics between

West Africa and the Philippines and situated near the equator in winter zone (e) migrates as far as 20deg to 30

0 N during the boreal summer and a belt of moisture-laden westerlies

develops on its equatorial flanks Two features are prime factors determining the rainfall regimes to the north and south

of the Sahara The described West African monsoon controls the onset of the summer

rainy season south of the Sahara and along its southern fringes annual rainfall is more or

less proportional to the number of months in which an area receives the moist monsoon flow so that lands become progressively drier northward toward the Sahara North of

the Sahara a very different situation prevails The dominant winter rainfall regime is controlled by the migration of cyclones associated with zone (b) into North Africa a

phenomenon generally restricted to the cooler months when all circulation features are

displaced equatorward Two interacting branches or modes of the atmospheric circulation can be distinguished

In the tropics the dominant mode is the Hadley type described as a screw-like (helical)

cell in a meridional-vertical plane Ascending motion prevails in the ITCZ (zone (e)) while subsidence occurs in the anticyclonic cells (zone (e)) which are separated in the upper troposphere by troughs in the westerlies Rainfall is associated with ascending motions concentrated in meso-scale disturbances (with a diameter of 100-300 km) in


the tropics or travelling cyclones with a diameter on the order of 500-3000 km in midshyand higher latitudes subsidence leads to aridity even jf the large-scale vertical components

are only on the order of cms or even mms The second circulation mode the Rossby type is represented by the extratropical

westerlies (zone (braquo which dominate the layers between the upper troposphere near 200 mb (and 850 mb) as a large irregular vortex around one or two centers in polar regions with a continuous chain of wave-like meandering distortions The subtropical anticyclonic belt (zone (craquo coinciding with the most intense westerly flow aloft (the

subtropical jetstream) is common to both modes A lesser known and infrequently occurring system the Saharan disturbances or

Soudano-Saharan depressions described by Dubief and Queney (1935) develops from the

interaction of these two modes In the sub tropics this interaction (Hohn 1975) is characshyterized by a large-scale meandering motion of the upper westerlies at 150-300 mb

extending above the tropical easterlies into low latitudes (Figure 7) Over northern Africa this happens most frequently but by no means exclusively over the western sector with troughs extending as far south as 10deg N At their equatorial fringes these meanders or

tilted troughs frequently trigger the development of tropical easterly waves (zone (draquo into cyclonic vortices which travel ahead of the upper air troughs northeastward across the Sahara In low latitudes they bring heavy showers and thunderstorm clusters after raining out their convective activity may be reduced to heavy duststorms but they can again produce heavy rain after entraining moist Mediterranean air

Another noteworthy atmospheric characteristic relates to the thermal contrast between the glaciated Antarctic continent and the Arctic Ocean where only thin drift ice separates the unfrozen ocean from the atmosphere the markedly different heat budgets thereby produced result in Antarctic-Arctic temperature differences on the order of 20-30degC

=gt Tracks 01 Saharan Depressions

l at

SUBTROPICAL JET 300

Fig 7 Interaction of upper-air troughs with low-level tropical vortices

331 African Emironmentai and Climatic Changes in Late Pleistocene and Holocene

in the annual average near the surface and II-12degC in the troposphere (Flohn 1967 1978) Consequently the southern hemispheric circulation is distinctly stronger than that of the northern hemisphere the described circulation belts lie more equatorward in the southern hemisphere and the average annual position of the meteorological equator or ITCZ is near 6degN reaching as far as ISoN in the northern summer The distribution of land and ocean further differentiates the thermal character of the two hemispheres Land comprises 40 of the northern hemisphere but only 19 of the southern in subpolar

latitudes the contrast rises to 80 versus 0 The result is strong zonal flow patterns in the southern hemisphere with prevailing stormy westerlies and more meridional flow patterns in the northern hemisphere

32 Factors Changing the Atmospheric Circulation

Thermal factors playa dominant role in determining the character of the general atmosshypheric 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 main changes would have taken the form of displacement and weakening or intensification of present circulation features and changes between primarily zonal (eastmiddotwest) flow or meridional flow (strong northerly and southerly

oscillations superimposed upon the east-west flow) In particular four factors must be

considered in hypothesizing these changes

(1) Effect of hemispheric temperature gradient (ie equator-to-pole temperature difference) 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 (cf Fiohn 1964 Hess 1959 Pahnen and Newton 1969) Temperature gradient determines also the location of the transition between tropical Hadley and extra-tropical Rossby circulation (Le location of the Subtropical High) and influences the wave-character of the Rossby circulation (Le the number and position of waves characterizing the circumpolar westerly currents) (Fiohn 1964 Lamb

and Woodroffe 1970)

(2) 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 disshytribution of land and ocean in the two hemispheres and expecially 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 sou them hemisphere If this asymmetry is responsible for the present northernhemispheric location of the meteorological equator (Kraus 1977 Flohn 1978) decreased contrast

332 Sharon E Nicholson and Hermann Fiohn

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 Le southwards

(3) 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 (Hess 1959) Within the northern-hemisphere westerlies the polar-front jet III 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 38degN in North America around 45degN in Europe and some times merged with the

subtropical jet

(4) 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 or precipitation the thereby affected rainfall decrease was most

marked in areas influenced by the subtropical 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 and Shackleton 1973) Albedo changes imposed

by the landscape changes (presence of glaciers modification of lakes and vegetation) during glacials should have also affected the earths heat budget and must have Significantly modified atmospheric circulation and climate

33 Development ofPleistocene General Circulation Theories

Long ago Penck (1914) theorized that a general equatorward displacement of climatic zones characterized glacial episodes Later authors (eg Klute 1930 Budel 1949) retained this idea and further assumed that a narrowing of the tropical rain belt and possibly a general weakening of the atmospheric circulation accompanied this displacement Klute (1949) and Poser (1948) added another element a shift of midmiddotlatitude cyclone tracks the massive ice sheets must have acted as barriers to the development and proshypagation of these cyclones displacing them southward of the continen tal margins

Viete (1950) Willett (1950) and later Kutzbach et al (1968) further developed theories by considering thermal contrasts imposed by the presence of large glaciated areas Both the meridional temperature gradient between tropical and temperate latitudes


and the zonal contrast between land and water would have been greater during glacials

and would have exerted marked influence on the atmospheric circulation In view of these contrasts Uiboutry (1965) likened the resulting northern hemisphere circulation to the present circulation of the southern hemisphere which must have experienced less drastic changes during the Pleistocene and suggested that a near coincidence of climatic and geographic equators would have also resulted during glacials

Fiohn (1952) introduced another important concept differentiation between zonal and meridional general circulation modes He suggested that the former mode basically east-west flow with strongly developed westerlies and weaker troughs and ridges prevailed

during interglacials Accordingly meridional circulation types dominated during glacials weaker westerlies and strong north-south components (ridges troughs and cellular elements-cyclones and high pressure centers)

For some time the tropical pluvials and other changes which occurred in parts of Africa were considered to be simple consequences of the presence of ice sheets in higher latitudes and hence more or less synchronous with glacial episodes (see Simpson 1957 for example) This meant for the Sahara according to common theory a southward

displacement of the entire desert during glacials The Dubief (1953)-Balout (1955) hypothesis a notable exception suggested that the shift of the desert margin was greater along the tropical southern margin hence a general expansion of this desert occurred during glacials a hypothesis more or less in accord with data derived from recent research summarized in this paper Butzer (1957) questioned the concept of glacialpluvial synshychroneity and suggested that atmospheric changes which provoked the African and Near East pluvials be considered independently of glacial circulation changes Later his (Butzer et ai 1972) was the first of a series of papers which unequivocally show that increased subtropical and tropical aridity accompanied the last glacial maximum the last so-called African pluvial occurred during the period of rapid warming in early Holocene

Numerous plaUSible explanations have since been put forth to explain this subtropical aridity and subsequent lacustrine episode (Street and Grove 1976 Rognon and Williams 1977 Rognon 1976 Maley 1977a Flohn 1977) In addition to the previously discussed displacement andor intensification of the subtropical high pressure cells and other atmosshypheric circulation features these explanations also include changes in coastal upwelling the slope and nature of the ITCZ and the increased role of a system with only secondary importance today the Soudano-Saharan depressions In reappraising the evidence of climatic change and combining elements of several of these theories this article attempts to reconcile the known changes with atmospheric dynamics as altered by the varying ice cover of both hemispheres The interaction of the two hemispheres and the interaction of tropical and extra-tropical systems are emphasized

34 Atmusph eric Circulation during the Late Pleistocelle

At the peak of the last glacial about 18000 years ago ice sheets covered large areas of the northern-hemispheric continents summers were much cooler than now and the seasonal variation of ice cover of temperatures and of atmospheric circulation would


TABLE I Probable atmospheric boundary conditions and related changes c 20000-12000 BP

Northern Hemisphere

ex tensive area covered by ice minimum seasonal variation baroclinic zone displaced equatorward temperature gradient increased wrt today Hadley cell much stronger than today

Southern Hemisphere

increase of sea ice (not greatly differing from today) baroc1inic zone displaced slightly equatorward temperature gradient increased somewhat wrt today Hadley cell somewhat stronger than today

IntermiddotHemispheric Asymmetry

thermal contrast and dirferences in circulation intensity much less than at present

have been minimal (Table I) The northern hemisphere was in general cooler and because

temperature changes were greater at higher latitudes than in the tropics the hemispheric

temperature gradient from equator to pole would have been considerably greater than

today In contrast the southern hemisphere differed little from its present state having

experienced mainly an increase of sea ice surrounding the Antarctic continent extreme

winter temperatures but summer temperatures similar to todays (Burckle and Clarke

1977) The main effect there was probably an equatorward displacement of the Antarctic

Polar Front (Hays et al 1974) the baroclinic zone of the westerlies but equator-pole

temperature gradient may have been somewhat greater than at present since the greatest cooling was probably at high latitudes and the hemispheric temperature somewhat lower IIffII These differential changes in the two hemispheres would have minimized the thermal contrast between the two hemispheres

At this time and essentially throughout most of the interval 20000-12000 BP

increased aridity prevailed throughout the now semi-arid sub tropics and humid tropics

south of the Sahara and in East Africa The prevailing situation at that time in North

Africa is less clear Sarnthein (1978) presents evidence of an arid interval of 2000 to

3000 years duration about 18000 years ago He places the global aridity maximum at

18000 BP Le synchronous with the glacial maximum and estimates that sand dunes tnen covered 50 of the land between 30

0 N and 30

oS However after this time and up

to about 12000 years ago wetter conditions prevailed in much of North Africa including the entire Northwest the Saharan highlands and southern Egypt which was then receiving

winter rains Thus it seems that during most of the period of tropical desiccation

c20000 12000 BP more humid conditions prevailed in North Africa despite the transitory arid interval c 18000 BP

It is possible to theorize on the basis of the boundary conditions described earlier

what circulation patterns produced the environmental conditions in Africa c 20000shy

12000 BP as summarized above However it must be recalled that within this period

335 African Environmental and aimatic Changes in Iate Pleistocene and Holocene

a change occurred between conditions causing Pleistocene ice sheets to expand (preshy

maximum) and conditions causing the disintegration of the ice sheets (post-maximum)

This suggests a change c 18000 BP from an ke-build-up circulation pattern to an

ice-melt circulation It is likely that this change was predominantly in the higher latitudes

while the prevailing circulation over Africa and the tropics was imposed primarily by the

presence of the ice mass Hence the major environmental changes in tropical Africa coincided with the near disappearance of the ice masses after c 12000 BP and its accelerated growth pre-22OOO BP North Africa on the other hand would have been

influenced to a greater extent by the transition in the higher latitudes c 18000 BP from icemiddotbuild-up circulation to ice-melt circulation This may account for some of the

controversy surrounding North Africa the lack of synchroneity between changes there

and in tropical Africa and the shortmiddotterm fluctuations in North Africa about 18000 BP

(transitory dune building) Here however the theorized circulation patterns are assumed

to be generally valid for the overall period c 20000-12000 BP and relate to conditions prevailing during the major part of that period aridity in tropical Africa but relatively

humid conditions in North Africa

The ice sheets covering the northern continents evoked a strongly baroclinic zone along

and to the south of their margins displacing the westerly flow-coincident with zone (b)

(Figure 6)-south of their present position and into North Africa (Figure 8) This would

account for winter rains in the Hoggar Tibesti and southern Egypt Because seasonal

variation was minimal parts of North Africa presently in a winter rainfall regime probably

experienced a year-round rainy season thus they were wetter during the late glacial The

presence of ice sheets should also have increased the hemispheric temperature gradient

strengthening westerly flow intensifying the Hadley cell including the easterly trades

and displacing the subtropical high southward over the Atlantic This effect should have increased subsidence in the subtropical high pressure cell and intensifield upwelling

(associated with the pressure cell) the aridifying influence of the subtropical high and

upwelling would be thereby enhanced The equatorward displacement of this system

would shift its influence from the far Northwest of Africa to the western sub-Sahara

further enhancing rainfall in the former region and supressing it in the latter

The minimized thermal contrast between the hemispheres would theoretically result

in an equatorward displacement of the meteorological equator (ITel) from its present

pOSition (Figure 8) This should certainly have suppressed its northward summer excursion preventing the moist rain-bearing monsoon flow from reaching the presently semishy

arid regions south of the Sahara Intensification and equatorward concentration of the

Hadley cells of both hemispheres may have totally suppressed especially during northern

summer the development of this flow but such a dramatic circulation change has not yet been evidenced

A more important factor in the subtropical and tropical aridity was probably increased

upwelling and lower ocean temperatures Equatorial surface temperatures were 4_7 0 e lower than today in winter (Hays et al 1973) hence equatorial upwelling must have been

stronger and even more frequent than now at least in the Pacific and Atlantic Presently

in regions of upwelling the flux of sensible heat is regularly directed downwards whilp


SII

L quasI

permanen

-30( -15middotN

- omiddot

-15deg5

-30middot5

18000 BP Fig 8 Proposed atmospheric circulation scheme c 20000-12000 BP (dark shading areas more humid than today light shading areas drier than today inset present position of lTeZ in winter and summer)

evaporation is reduced over tropical oceans to 20-30 of normal (TrempeL 1978) Extrapolating this the sea surface temperatures prevailing during the last glacial (16deg 18degC in large areas of the tropical ocean in winter and probably very close to the equator)

likely reduced evaporation to zero or even negative values with mist and condensation at the cold water surface

Several points of evidence support the accuracy of certain features of this late Pleistocene atmospheric circulation scenario Intensification of the subtropical high (CLIMAP Project 1976) is suggested by sediments in deep-sea cores west of Dakar (Parkin and Shackleton 1973) which show evidence of stronger NE trade winds over

Africa McIntyres et al (1975) palaeotemperature reconstructions based primarily on distribution of floral and faunal species in ocean cores evidence increased upwelling along the Northwest African coast and an increased southward extension of this region of cold water Giresse et al (1976) evidence the same phenomenon along the Southwest African coast (Angola Congo Gabon) This can be interpreted as intensification and

337 African Environmental and Qimatic Changes in Late Pleistocenc and Holocene

equatorward displacement of the subtropical highs of both hemispheres In the southern

hemisphere equatorward displacement of circulation features is suggested by palaeoshy

ocean temperatures (Hays ef at 1974 Burckle and Clarke 1977) and snowline depression

in the Southern American Andes (Hastenrath 1971) which indicates predominantly

westerly winds at 29deg S Furthermore the scenario is in accord with most numerical

models of Pleistocene circulation (Gates 1976) and with the marked northward shift

of the Kalahari desert (Southern Africa) decisively evidenced by Heine (1979)

35 Atmospheric Circulation c 10000-8000 BP

About 12000 years ago perhaps coinciding with the Bolling or the Allerod warm phase

a marked change of the circulation pattern led the way to the first subtropical lacustrine

episode ice sheets of the northern hemisphere abruptly commenced retreating while sea

ice and glaciers rapidly diminished in the southern hemisphere parts of which reached a

thermal maximum about 9000 years ago (Hays 1977 Mercer 1977) (Table II) Thus

at least the sub-antarctic and arctic areas of the southern hemisphere were then warmer

than today while the northern hemisphere was cooler than now although warmer than

during the preceding glacial Accordingly thermal contrast between the two hemispheres

was probably increasing Much of the North American ice sheet remained during the period

10000middotmiddot8000 BP but the European ice had considerably diminished the North Atlantic

was only gradually warming

Throughout the African tropics a drastic change occurred toward 12000 BP when

aridity abruptly gave way to a period of increased rainfall and high lake levels in areas

extending from East Africa throughout the southern Sahara westwards to Senegal and

Mauritania At the same time aridity set in northwestern Africa but the northeast was

relatively wet The peak of the lacustrine episode and synchronous aridity in the northwest

was about 10000-8000 BP

TABLE II Probable atmospheric boundary conditions and related changes c 1OOOO~-8OOO BP

Northern Hemisphere

warming ice melting rapidly nearly gone from European sector seasonal variation probably small (but greater than during previous period) temperature gradient somewhat greater than today Hadley cell weaker than during previous period but stronger than today

Southern Hemisphere

polar and sub-polar latitudes reached thermal maximum hemispheric temperature gradient weakened Hadley cell weak wrt today and previous period

InterHemispheric Asymmetry

differences in circulation intensity and thermal contrast about like today but much greater than during previous period


-300 N

_15degN

- Omiddot

- 15deg5

-300 S

10 ~8000 BP

Fig 9 Proposed atmospheric circulation scheme c 10000middot-8000 BP (shading key see Fig 8 ITCZwin refers only to the position over Southern Africa)

While the North Atlantic gradually warmed up (Ruddiman and Mcintyre 1977) the

northern-hemisphere temperature gradient was decreasing and its maximum was displaced

northward This probably provoked a weakening and northward displacement of the

northern Hadley cell (Le the North Atlantic subtropical high) (Figure 9) This would

have displaced the aridity maximum in western Africa from the sub-Sahara to the Northshy

west and somewhat diminished its aridifying influence coastal upwelling would have

also been less intense than during the preceding glacial The northward displacement or

this system would sufficiently explain the advancing sand dunes and other signs of aridity

in Morocco and western Algeria its retreat from the southwestern Saharan margins would

have enhanced the more humid conditions setting in there after 12000 BP but the cause

of the rainfall increase which affected the entire sub-Saharan region equatorial tropics and

East Africa must be found elsewhere An explanation probably involves a re-establishment

of the humid SW monsoon in West Africa a northward displacement of the Tel (provoked

by the increasing interhemispheric thermal contrast) possibly beyond its present position

and increased oceanic evaporation accompanying the warming of the equatorial ocean

with prevailing downwelling In the area affected by the upwellingdownwelling change (at least gOLat x (40--110)oLong or about 15 x 106 km2

) the maritime evaporation may

have increased by 50 percent or more and this was probably the most signil1cant factor

African Environmental and Qimatic Changes in Latc Pleistocene and Holocene 339

A special study of this change in ocean cores with a high sedimentation rate should be

suggested An additional factor the Saharan depressions produced by tilted upper air troughs

was probably decisive in establishing the rainfall regime which created extensive lakes

along the entire southern Sahara and its fringes As indicated this system develops from

the interaction of troughs in the uppermiddotlevel westerlies with the humid air south of the

ITCZ (Figure 9) The remaining presence of considerable ice mass over North America

the lesser ice sheets of northern Europe and the slow northward retreat of the North

Atlantic High (in relation to the warming of the North Atlantic) led to a quasi-stationary

trough over eastern North America and a secondary one over eastmiddotcentral Europe As a

result the frequency and intensity of the travelling upper-level troughs of the Westerlies

should have only slightly diminished with respect to 18000 BP and they must have

reached somewhat more southerly latitidues than at present the northward advance of

the ITCZ and remiddotestablishment of the SW monsoon must have led to more frequent

interaction between the two systems This implies more frequent development of the

Saharan depressions and similar storms and their possible occurrence through most of

the year Thus Saharan depressions would account for both wetter conditions throughout

the southern Sahara and the less seasonal rainfall regime evidenced in the highlands The

frequent presence of the troughs in the upper-level Westerlies over North Africa also

accounts for the increased rainfall evidenced in the Northeast In contrast the high

pressure regime dominant in the Northwest blocked these and related cyclonic systems

in this region which became arid toward 12000 or 10000 BP

36 Atmospheric Circulation c 6500-4500 Bp

A significant change toward 7000 BP was marked by the somewhat earlier disappearance

of the Scandinavian ice sheets and by a brief arid interval or change to generally more

arid conditions in many parts of the African tropics and sUbtropics With the disappearance

of the Scandinavian ice and the disintegration of the Laurentide ice sheet the European

part of the northern hemisphere reached a thermal maximum toward 6000 BP while

the southern hemisphere was cooling (Hays 1978 Burckle 1978) (Table II) The Canadian

Arctic reached a thermal maximum around 4500 BP after the final disappearance of

the Laurentide icemiddotsheet (Barry et al 1977) this coincided with the onset of desiccation

in the Sahara During the interval 6500~4500 BP both the northern and southern

margins of the Sahara were considerably wetter than today This shrinking of the arid

belt occurring in both the east and west of the Sahara was accompanied by increasingly

semi-arid climates in these regions as opposed to the less seasonal rainfall regime of

c J0000-~8000 BP East Africa was drier than during the early Holocene (after

10000 BP) but considerably wetter than at present

Differences between this lacustrine episode in tropical Africa and the one in early

Holocene are described in Section 23 In view of the contrasting hemispheric surface

characteristics prevailing during the two periods this is not surprising While during the

earlier period ice sheets had a considerable influence on circulation in the northern


TABLE III Probable atmospheric boundary conditions and related changes c 6500-4500 BP

Northern Hemisphere

temperature maximum (climate optimum) only remaining ice small part of Laurentide ice sheet temperature gradient probably somewhat smaller than today circulation features displaced poleward Hadley cell weaker than today

Southern Hemisphere

probably cooling (wrt 10000 BP) but warmer than today temperature gradient somewhat less than today circulation features displaced poleward Hadley cell weaker than today

Inter-Hemispheric Asymmetry

maximum asymmetry thermal contrast and differences in circulation intensity probably much greater than today

hemisphere this factor had markedly diminished by 6500 BP During the earlier period the northern hemisphere was thus relatively cool while the southern reached a thermal maximum this situation was probably reversed toward 6000 BP with the southern

hemisphere cooling while the northern had attained a thermal maximum An attempt is

made to explain the coeval African climates on the basis of these atmospheric boundary

conditions As the northern hemisphere gradually warmed its temperature gradient would have

decreased weakening the subtropical high over the Atlantic expanding it poleward and

diminishing the coastal upwelling associated with it (Figure 10) There is empirical

evidence (Petit-Maire 1979) that upwelling off the Northwest coast was considerably weaker than at present The increased rainfall in northwestern Africa can be interpreted as a

result of these changes The remaining presence of the Laurentide ice sheet over North America produced a quasi-stationary trough there which as during the period 10000shy8000 BP resulted in frequent troughs and cyclonic activity over North Africa this as

during the previous period would account for increased rainfall in northeastern Africa The increased rainfall in the southern Sahara probably related to the development of

Saharan disturbances as discussed previously These however were probably less frequent than during the period c 10000-8000 BP and limited only to certain seasons thus

accounting for the semi-arid conditions (nevertheless wetter than present) which set in in many areas after 7000 BP However one other plausible explanation might be formulated (Figure 10)

If the increased thermal contrast between the hemispheres (thermal maximum in the north coeval with a cooling southern hemisphere) resulted in a northward displacement of the meteorological equator or ITeZ the rainy season south of the Sahara would have been longer at all latitudes north of about lOoN and the summer monsoon rains associated with the ITez would have reached further northward into the southern Sahara This

---------

African Environmental and Qimatic Changes in Late Pleistocene and Holocene 34t

L

ITCZ SUM

_--- ITCZ

WIN

- omiddot

-15middotS

-30middotS

6500~4500 BP

Fig 10 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)

would explain the pronounced seasonality of rainfall in the highlands after 7000 BP as well as the increased (with respect to present) rainfall along the entire southern margin of the Sahara The single rainy season at Lake Naivasha before 2500 BP is in accord with

this hypothesis and if the mid-Holocene regression of Lake Bosumtwi dates to this period it provides clear evidence of such a situation with less rainfall south of lOoN Analysis of modern and historical climatic data (Nicholson 1976 and unpublished data) shows that such a situation is realistic

4 Outlook for the Future

The coincidence of the two Saharan moist periods with the warmest phase of the Holocene in the Subantarctic (c 9000 BP first phase) and in the European area (c 6000 BP second phase) suggests a correlation between temperature increase and precipitation

increase Such a correlation at least in a global sense has been suggested also in some circulation models (Wetherald and Manabe 1975) Kellogg (1977 Figure 8) and Sarnthein (1978) have indicated that not only in the Sahara but in many other regions of the earth


the climate had been wetter than now about 6000 years ago In this paper only the possible

return of a moist climate in the Saharan belt shall be discussed in view of the cautious remarks by Kellogg Such a situation would almost certainly benefit the welfare and prosperity of the inhabitants and nations of the area even if some necessary adaptations might prove to be difficult Can we indeed expect such a moist climate as a likely pattern for a future warmer earth (Kellogg 1977 p 29) which may result from an anthroshypogenically-induced increase of atmospheric carbon dioxide Can climatic history repeat itself A more or less exact r~plica of the climatic past depends on the recurrence of the atmospheric boundary conditions which then prevailed Le the recurrence of the former

status of the slowly changing subsystems of the climatic systems These include the extension of the continental ice and of the vegetation together with such variable physical surface properties as albedo and soil moisture

Three essential boundary conditions have changed considerably since the two Holocene

moist periods in which the Sahara could nourish a relatively dense population of game

hunters (c 9500 BP) and more importantly cattle-raising nomads (Neolithic c 6000 BP) (Gabriel 1977) The first is the distribution of snow and ice the second

relates to man-made surface modification alteration and destruction of the natural vegetation cover the third is the atmospheric composition especially dust and CO2 bull Four

aspects of these changes are considered below

(1) During the first moist period the Laurentide ice in Eastern Canada covered still between the two-thirds and half of its original area and even the smaller Scandinavian

ice still existed During the cool season the situation differed little from today the area decked by a permanent snow cover between late October and early May was about the

same then and now This snowcover maintains a high albedo especially in the nonshyforested tundra areas the albedo of snow-covered forests is about 40 and that of an undisturbed snowcover is about 60-30 However the warm-season contrast between

9500 BP and today is substantial the albedo must have approached 60 instead of the

present 10-20 Thus even during the warm season (June-September) production and outflow of cold air must have been tremendous while it is minimal today As the Arctic drift ice also migrated northward during the COUrse of the Holocene this is also true for

the Atlantic Ocean area 10000 years ago the flow patterns of summer should have resembled those of the present transition seasons with many outbreaks of cold air across Central Europe and with elongated troughs aloft across the central Mediterranean into the central and western Sahara triggering substantial rainstorms which nourished under normal conditions a grassland or dry savanna vegetation

(2) The dry interval between 8000 and 7000 BP could be interpreted as the result of the change of two large-scale boundary conditions the disappearance of the Scandinavian Ice and (nearly simultaneously) the catastrophic incursion of the sea into Hudson Bay (Ives et al 1975) DUring the latter event approximately 3 million km 3 of ice were transshyported as huge icebergs into the Atlantic Ocean and melted away especially in the region of the Gulf Stream this produced a sea-level rise of 8 m a substantial cooling of the waters of the Canary Current and a sharp reduction of evaporation (for details see Flohn and Nicholson 1979)


(3) In the second moist period after 6500 BP the Scandinavian Ice had disappeared

and the distribution of sea-ice and temperature in the North Atlantic could not have been

much different from today However in Eastern Canada three separate ice-sheets still

remained (cf Lamb 1977 Figure 164) the Labrador ice the Keewatin ice and the

Baffin Island ice together more than 05 x 106 km 2bull As a result the surface area near

Hudson Bay producing cold air during the warm season must have been increased by a

factor of three (with respect to today) by large snow fields and drift-ice This should

- have maintained a winter-spring circulation type during the summer season about 6000

years ago ie a rather permanent trough penetrating from Labrador deeply towards the

southwest and probably triggering as now during spring and winter a secondary European

trough originating somewhere between 0deg E and 20deg E This secondary trough could not

have been fixed in its position as during the first moist period by the ice and snow fields

of Scandinavia rather its position should have varied (as now) according to the physical

laws controlling the baroclinic Rossby waves This quite plausible pattern should also

have lead to the maintenance of extra-tropical rainfall at the northern flank of the Sahara

and to a triggering of Saharan depressions also during summer

The above-mentioned physical boundary conditons over Eastern Canada do not exist

today in a foreseeable future (say during the next I00~200 years) it is highly unlikely

that they can be re-established since the transition from a cold climatic phase (as the

Little Ice Age between about 1550 and 1850 AD) to a full glacial lasts much more than

these 300 years (Flohn 1979) On the other hand an extension of the tropical summer

rains on the southern flank of the Sahara seems to be possible if (as expected) during a

global warming the subtropical anticyclones and the trades of the northern hemisphere

weaken This could lead to a decreased frequency and intensity of coastal and equatorial

upwelling to higher evaporation and to an extension of the area (and season) of the

SW-Monsoon with its tropical summer rains This positive development should be

restricted to the area south of about 20-21 oN

(4) Recent models (Charney 1975 Berkofsky 1976 Ellsaesser etal 1976Otterman

1974) evidence a bio-feedback mechanism in which decrease or destruction of vegetation

reduces rainfall in marginal areas by increasing albedo It is not unlikely that during and since

the Neolithic revolution (6~8 millenia ago) an anthropogenic reduction of vegetation~

desertification~has been taking place This would have slowly contributed to the gradual

natural desiccation beginning about 5000 DP and would have been particularly intense

during the last century or so However recent representative rainfaII data~~at least in areas

like Rajasthan the Sahel or Tunesia since about 1900-do not reveal the occurrence of a

long-term downward trend It must therefore be concluded that this bio-feedback

mechanism is only a weak and slowly effective long-term process Nevertheless if

indeed rainfall should increase during a global warming along the southern flank of the

Sahara a re-established vegetation cover must be protected ami maintained before a

lasting effect can be reached this is a socio-economic problem In other words the weak

positive feedback mechanism as described above can be interrupted only by a major

concerted effort under increasing popUlation stress Any beneficial climatic change with

increasing rainfall can only become effective and permanent if supported by strong


counter-measures against man-made desertification From arguments (1) and (3) we should conclude that only along the southern flank of the arid belt an extension of the tropical summer rains should be possible In the central and northern Sahara any permanent increase of rainfall is unlikely along the Mediterranean coast even a decrease of rainfall could be expected From argument (4) we must conclude that a reconstruction of a permanent green vegetation lowering the albedo and enhancing rainfall is a serious socio-economic problem under present popUlation stress Only a very powerful natural change with dominant equatorial downwelling together with a powerful combat of man-made desertification could result in a long-lasting improvement of the present climatic

conditions in the southern Sahara-Sahel belt

Acknowledgements

This work has funded in part by the Atmospheric Research Section of the National Science Foundation under Grant ATM 77-21547 to the University of Virginia The

authors are grateful to the articles reviewers among them M Sarnthein for valuable

critiques 0 f the manuscript

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(Received July 13 1979 in revised form March 3 1980)


2 Late Pleistocene and Holocene Environment and Climate in Africa

Within the past 20000 years three major episodes occurred in which the climates of Africa differed significantly from those now prevailing over the continent As most of the

continent lies in the tropics the effects of Pleistocene glaciation were minimal climatic

fluctuations expressed themselves in terms of changes in the hydrological balance among precipitation evaporation and runoff Geological evidence for this balance appears in the

form of tremendous variations oflakes and of the extent of the arid zone While evaporation fIIIIII and runoff certainly varied throughout the Pleistocene and Holocene in most cases

rainfall was clearly the decisive factor in these environmental changes which various types of geological and palynological evidence help to trace Tropical and SUbtropical aridity marked the period c 20000-12000 BP which coincided with the last glacial maximum (Wisconsin or Weichsel) A rapid rise of lake levels throughout tropical Africa

followed during the early Holocene period of rapid glacial retreat and global warming before c 10000 BP and maximum levels prevailed to 8000 BP A subsequent

lacustrine period lasted from c 6500 to 4500 BP (Nicholson 1976) At least moderately high lake levels characterized most of the Holocene from about 12000 BP until three or

four millenia ago interrupted only briefly by drier conditions Nevertheless two discrete Holocene lake episodes are considered here because of the widespread arid episode toward 7000 BP and because the nature of the two episodes appears to have been quite different

as explained below Figure 1 shows mean annual rainfall and the location of most sites discussed Figure 2 summarizes local chronologies while Figures 3 4 and 5 summarize

Fig1(a) Mean annual rainfall (em) over Africa (from Nicholson 1980 based on all available years to 1973 at 420 stations)

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0deg 0deg

10deg 10deg

20deg 20deg













































































NORTH POLE

SOUTH POLE



























l at

SUBTROPICAL JET 300










and Woodroffe 1970)








subtropical jet





















Northern Hemisphere


Southern Hemisphere



















0 N and 30













































SII

L quasI

permanen

-30( -15middotN

- omiddot

-15deg5

-30middot5

































Northern Hemisphere


Southern Hemisphere





-300 N

_15degN

- Omiddot

- 15deg5

-300 S

10 ~8000 BP































































Northern Hemisphere


Southern Hemisphere
















---------


L

ITCZ SUM

_--- ITCZ

WIN

- omiddot

-15middotS

-30middotS

6500~4500 BP






































































Acknowledgements




References






















































































































( r ALGfER


u il-p Is ~QGnaJ1uS


40cMP r 0 bull ~- ~


bull E~G bull o~s ~






(~ L Chad J o



I ~


Mt KENIA

NAIRoeIt7

bull

~
































bull bull





-



_-- - - - --- ----_ _ --_ -- gt


While Nile __--_ -



Lake Afrera


__ __


w

W







N

30deg

il 1

20deg 16 t31~ 15 14 18

19 10deg

0deg

10deg

20deg

pollen rVidence or

51-_-shy_____---_---shy --shy --_----_5






Nigeria 26 Cameroun

50deg E N

30deg

20deg wi

10deg

0deg

10deg

20deg

50deg E




























320

II

30deg 30deg

20deg

10deg

0deg

10deg

20deg









Muehoya Swamp

20deg

10deg

0deg

10deg

20deg

















































































































2 middot2middot

30deg 30deg

6

r 20deg 20deg

12

10deg 10deg

0deg 0deg

10deg 10deg

20deg 20deg













































































NORTH POLE

SOUTH POLE



























l at

SUBTROPICAL JET 300










and Woodroffe 1970)








subtropical jet





















Northern Hemisphere


Southern Hemisphere



















0 N and 30













































SII

L quasI

permanen

-30( -15middotN

- omiddot

-15deg5

-30middot5

































Northern Hemisphere


Southern Hemisphere





-300 N

_15degN

- Omiddot

- 15deg5

-300 S

10 ~8000 BP































































Northern Hemisphere


Southern Hemisphere
















---------


L

ITCZ SUM

_--- ITCZ

WIN

- omiddot

-15middotS

-30middotS

6500~4500 BP






































































Acknowledgements




References




















































































































































bull bull





-



_-- - - - --- ----_ _ --_ -- gt


While Nile __--_ -



Lake Afrera


__ __


w

W







N

30deg

il 1

20deg 16 t31~ 15 14 18

19 10deg

0deg

10deg

20deg

pollen rVidence or

51-_-shy_____---_---shy --shy --_----_5






Nigeria 26 Cameroun

50deg E N

30deg

20deg wi

10deg

0deg

10deg

20deg

50deg E




























320

II

30deg 30deg

20deg

10deg

0deg

10deg

20deg









Muehoya Swamp

20deg

10deg

0deg

10deg

20deg

















































































































2 middot2middot

30deg 30deg

6

r 20deg 20deg

12

10deg 10deg

0deg 0deg

10deg 10deg

20deg 20deg













































































NORTH POLE

SOUTH POLE



























l at

SUBTROPICAL JET 300










and Woodroffe 1970)








subtropical jet





















Northern Hemisphere


Southern Hemisphere



















0 N and 30













































SII

L quasI

permanen

-30( -15middotN

- omiddot

-15deg5

-30middot5

































Northern Hemisphere


Southern Hemisphere





-300 N

_15degN

- Omiddot

- 15deg5

-300 S

10 ~8000 BP































































Northern Hemisphere


Southern Hemisphere
















---------


L

ITCZ SUM

_--- ITCZ

WIN

- omiddot

-15middotS

-30middotS

6500~4500 BP






































































Acknowledgements




References






















































































































__ __


w

W







N

30deg

il 1

20deg 16 t31~ 15 14 18

19 10deg

0deg

10deg

20deg

pollen rVidence or

51-_-shy_____---_---shy --shy --_----_5






Nigeria 26 Cameroun

50deg E N

30deg

20deg wi

10deg

0deg

10deg

20deg

50deg E




























320

II

30deg 30deg

20deg

10deg

0deg

10deg

20deg









Muehoya Swamp

20deg

10deg

0deg

10deg

20deg

















































































































2 middot2middot

30deg 30deg

6

r 20deg 20deg

12

10deg 10deg

0deg 0deg

10deg 10deg

20deg 20deg













































































NORTH POLE

SOUTH POLE



























l at

SUBTROPICAL JET 300










and Woodroffe 1970)








subtropical jet





















Northern Hemisphere


Southern Hemisphere



















0 N and 30













































SII

L quasI

permanen

-30( -15middotN

- omiddot

-15deg5

-30middot5

































Northern Hemisphere


Southern Hemisphere





-300 N

_15degN

- Omiddot

- 15deg5

-300 S

10 ~8000 BP































































Northern Hemisphere


Southern Hemisphere
















---------


L

ITCZ SUM

_--- ITCZ

WIN

- omiddot

-15middotS

-30middotS

6500~4500 BP






































































Acknowledgements




References














































































































































320

II

30deg 30deg

20deg

10deg

0deg

10deg

20deg









Muehoya Swamp

20deg

10deg

0deg

10deg

20deg

















































































































2 middot2middot

30deg 30deg

6

r 20deg 20deg

12

10deg 10deg

0deg 0deg

10deg 10deg

20deg 20deg













































































NORTH POLE

SOUTH POLE



























l at

SUBTROPICAL JET 300










and Woodroffe 1970)








subtropical jet





















Northern Hemisphere


Southern Hemisphere



















0 N and 30













































SII

L quasI

permanen

-30( -15middotN

- omiddot

-15deg5

-30middot5

































Northern Hemisphere


Southern Hemisphere





-300 N

_15degN

- Omiddot

- 15deg5

-300 S

10 ~8000 BP































































Northern Hemisphere


Southern Hemisphere
















---------


L

ITCZ SUM

_--- ITCZ

WIN

- omiddot

-15middotS

-30middotS

6500~4500 BP






































































Acknowledgements




References

































































































































































































































2 middot2middot

30deg 30deg

6

r 20deg 20deg

12

10deg 10deg

0deg 0deg

10deg 10deg

20deg 20deg













































































NORTH POLE

SOUTH POLE



























l at

SUBTROPICAL JET 300










and Woodroffe 1970)








subtropical jet





















Northern Hemisphere


Southern Hemisphere



















0 N and 30













































SII

L quasI

permanen

-30( -15middotN

- omiddot

-15deg5

-30middot5

































Northern Hemisphere


Southern Hemisphere





-300 N

_15degN

- Omiddot

- 15deg5

-300 S

10 ~8000 BP































































Northern Hemisphere


Southern Hemisphere
















---------


L

ITCZ SUM

_--- ITCZ

WIN

- omiddot

-15middotS

-30middotS

6500~4500 BP






































































Acknowledgements




References





























































































































































































NORTH POLE

SOUTH POLE



























l at

SUBTROPICAL JET 300










and Woodroffe 1970)








subtropical jet





















Northern Hemisphere


Southern Hemisphere



















0 N and 30













































SII

L quasI

permanen

-30( -15middotN

- omiddot

-15deg5

-30middot5

































Northern Hemisphere


Southern Hemisphere





-300 N

_15degN

- Omiddot

- 15deg5

-300 S

10 ~8000 BP































































Northern Hemisphere


Southern Hemisphere
















---------


L

ITCZ SUM

_--- ITCZ

WIN

- omiddot

-15middotS

-30middotS

6500~4500 BP






































































Acknowledgements




References

































































































































l at

SUBTROPICAL JET 300










and Woodroffe 1970)








subtropical jet





















Northern Hemisphere


Southern Hemisphere



















0 N and 30













































SII

L quasI

permanen

-30( -15middotN

- omiddot

-15deg5

-30middot5

































Northern Hemisphere


Southern Hemisphere





-300 N

_15degN

- Omiddot

- 15deg5

-300 S

10 ~8000 BP































































Northern Hemisphere


Southern Hemisphere
















---------


L

ITCZ SUM

_--- ITCZ

WIN

- omiddot

-15middotS

-30middotS

6500~4500 BP






































































Acknowledgements




References






























































































































and Woodroffe 1970)








subtropical jet





















Northern Hemisphere


Southern Hemisphere



















0 N and 30













































SII

L quasI

permanen

-30( -15middotN

- omiddot

-15deg5

-30middot5

































Northern Hemisphere


Southern Hemisphere





-300 N

_15degN

- Omiddot

- 15deg5

-300 S

10 ~8000 BP































































Northern Hemisphere


Southern Hemisphere
















---------


L

ITCZ SUM

_--- ITCZ

WIN

- omiddot

-15middotS

-30middotS

6500~4500 BP






































































Acknowledgements




References



























































































































subtropical jet





















Northern Hemisphere


Southern Hemisphere



















0 N and 30













































SII

L quasI

permanen

-30( -15middotN

- omiddot

-15deg5

-30middot5

































Northern Hemisphere


Southern Hemisphere





-300 N

_15degN

- Omiddot

- 15deg5

-300 S

10 ~8000 BP































































Northern Hemisphere


Southern Hemisphere
















---------


L

ITCZ SUM

_--- ITCZ

WIN

- omiddot

-15middotS

-30middotS

6500~4500 BP






































































Acknowledgements




References


































































































































Northern Hemisphere


Southern Hemisphere



















0 N and 30













































SII

L quasI

permanen

-30( -15middotN

- omiddot

-15deg5

-30middot5

































Northern Hemisphere


Southern Hemisphere





-300 N

_15degN

- Omiddot

- 15deg5

-300 S

10 ~8000 BP































































Northern Hemisphere


Southern Hemisphere
















---------


L

ITCZ SUM

_--- ITCZ

WIN

- omiddot

-15middotS

-30middotS

6500~4500 BP






































































Acknowledgements




References



























































































































































SII

L quasI

permanen

-30( -15middotN

- omiddot

-15deg5

-30middot5

































Northern Hemisphere


Southern Hemisphere





-300 N

_15degN

- Omiddot

- 15deg5

-300 S

10 ~8000 BP































































Northern Hemisphere


Southern Hemisphere
















---------


L

ITCZ SUM

_--- ITCZ

WIN

- omiddot

-15middotS

-30middotS

6500~4500 BP






































































Acknowledgements




References

















































































































































Northern Hemisphere


Southern Hemisphere





-300 N

_15degN

- Omiddot

- 15deg5

-300 S

10 ~8000 BP































































Northern Hemisphere


Southern Hemisphere
















---------


L

ITCZ SUM

_--- ITCZ

WIN

- omiddot

-15middotS

-30middotS

6500~4500 BP






































































Acknowledgements




References























































































































-300 N

_15degN

- Omiddot

- 15deg5

-300 S

10 ~8000 BP































































Northern Hemisphere


Southern Hemisphere
















---------


L

ITCZ SUM

_--- ITCZ

WIN

- omiddot

-15middotS

-30middotS

6500~4500 BP






































































Acknowledgements




References

































































































































































Northern Hemisphere


Southern Hemisphere
















---------


L

ITCZ SUM

_--- ITCZ

WIN

- omiddot

-15middotS

-30middotS

6500~4500 BP






































































Acknowledgements




References






















































































































---------


L

ITCZ SUM

_--- ITCZ

WIN

- omiddot

-15middotS

-30middotS

6500~4500 BP






































































Acknowledgements




References





















































































































































































Acknowledgements




References






















































































































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