(bottom) and
Holocene (top) The African humid period extended over most of Africa: The Sahara and eastern, southeastern and equatorial Africa. In general, forests and woodlands expanded through the continent. A similar wet episode took place in the tropical Americas and Asia, including the
Makran region, the
Middle East and the
Arabian Peninsula; the episode appears to relate to the same
orbital forcing as the AHP. An early Holocene monsoonal episode extended as far as the
Mojave Desert in North America. In contrast, a drier episode is recorded from much of
South America where
Lake Titicaca,
Lake Junin, the discharge of the
Amazon River and water availability in the
Atacama were lower. The discharge of the
Congo,
Niger,
Nile,
Ntem,
Rufiji, and
Sanaga rivers increased. Runoff from
Algeria, equatorial Africa, northeastern Africa and the western Sahara was also larger. Increased discharge led to changes in the morphology of the river systems and their
alluvial plains, and the Senegal River expanded its riverbed, breached dunes and re-entered the Atlantic Ocean.
Flora and fauna of the Sahara During the African humid period, lakes, rivers,
wetlands and vegetation including grass and trees covered the Sahara and
Sahel, creating a "Green Sahara" with a
land cover that has no modern analogues. Evidence includes pollen data, archaeological sites, evidence of faunal activity such as
diatoms,
mammals,
ostracods,
reptiles and
snails, buried
river valleys, organic-rich
mats,
mudstones,
evaporites as well as
travertines and
tufas deposited in subaqueous environments. The vegetation cover then extended over almost all of the Sahara and consisted of an open
grass savannah with
shrubs and trees, with a moist savanna vegetation getting established in the mountains. In general, the vegetation expanded northward to
27–
30° northern latitude in West Africa with a Sahel boundary at about
23° north, as the Sahara was populated by plants that today often occur about - farther south. The northward movement of vegetation took some time and some plant species moved faster than others. Plants that perform
C3 carbon fixation became more common. The
fire regime has important effects on the vegetation and fauna; during the AHP, with some northern areas becoming wet enough that vegetation could sustain burns, and more southern areas becoming too wet. Forests and plants from the humid tropics were concentrated around lakes, rivers and the Atlantic Ocean coast of
Senegal; waterbodies were also settled by aquatic and partially aquatic plants and the Senegalese coast by
mangroves. The landscape during the AHP has been described as a mosaic between various vegetation types of semi-desert, humid or tropical origin rather than a simple northward displacement of plant species, There was no southward displacement of Mediterranean plants during the Holocene and on the Tibesti Mountains cold temperatures may have restricted the expansion of tropical plants. Pollen data often show a dominance of grasses over humid tropics trees. The tree
Lophira alata and others may have spread out of the African forests during the AHP, and the
Lactuca plants may have split into two species under the effects of the AHP and other climate changes in Africa during the Holocene. The Sahara climate did not become entirely homogeneous; its central-eastern parts were probably drier than the western and central sectors and the
Libyan sand sea was still a
desert although pure desert areas retreated to small core areas or became
arid/
semiarid. An arid belt may have existed north of 22° latitude and towards the Nile Delta, or the vegetation and the African monsoon might have reached 28–31° northern latitude; in general conditions between 21° and 28° northern latitude are poorly known. Dry areas may have persisted in the
rain shadows of mountains and could have supported arid climate vegetation, explaining the presence of its pollen in
sediment cores. In addition, north–south gradations in vegetation patterns have been reconstructed from charcoal and pollen data.
Fossils record changes in the animal fauna of the Sahara. This fauna included
antelopes,
baboons,
birds,
cane rats,
catfish,
clams,
cormorants, crocodiles, elephants, frogs,
gazelles,
giraffes,
hartebeest,
hares,
hippopotamuses,
molluscs,
Nile perches,
pelicans,
rhinoceroses,
snake-eagles, snakes,
tilapia,
toads,
turtles and many more animals, and in Egypt there were
African buffaloes,
spotted hyenas,
warthogs,
wildebeest and
zebra. Additional birds include
brown-necked raven,
coot,
common moorhen,
crested grebe,
glossy ibis,
long-legged buzzard,
rock dove,
spur-winged goose and
tufted duck. Some
migratory birds may have changed their flying route in response to the AHP. Large herds of animals lived in the Sahara. Some animals expanded over the whole desert, while others were limited to places with deep water. Humid periods in the Sahara may have allowed species to cross the now-desert. A reduction in open grasslands at the beginning of the AHP may explain the decline of the populations of some mammals during and a
population bottleneck in
cheetahs at the start of the humid period, while leading to the expansion of the population of other animals such as
Hubert's multimammate mouse and
Natal multimammate mouse.
Lakes and rivers of the Sahara , with present-day
Lake Chad highlighted in green A number of lakes formed or expanded in the Sahara and the
Hoggar and
Tibesti Mountains. The largest of them was
Lake Chad which increased to at least ten times its present-day size to form Lake Megachad or Megalake Chad, then the largest lake on Earth. This enlarged Lake Chad reached dimensions of in north–south and east–west direction respectively, covering the
Bodélé Depression and perhaps as much as 8% of the present-day Sahara desert. It influenced the climate itself; for example rainfall would have been reduced at the centre of the lake and increased at its margins. Lake Chad was possibly fed from the north by rivers draining the
Hoggar (Taffassasset drainage) and Tibesti Mountains, from the
Ennedi Mountains in the east through the "Eastern palaeorivers" and from the south by the
Chari-
Logone and
Komadugu Rivers. The Chari River was the main tributary while the rivers draining the Tibesti formed
alluvial fans/the Angamma
river delta at their entry into northern Lake Chad. Skeletons of elephants, hippos and hominins have been found in the Angamma delta, which is the dominant shoreline feature of northern Lake Chad. The lake overflowed into the Niger River during highstand through the
Mayo Kebbi and the
Benue River, eventually reaching the
Gulf of Guinea. Older dune systems were submerged by Lake Chad. Among the large lakes which may have formed in the Sahara are
Lake Megafezzan in Libya and
Lake Ptolemy in Sudan. Quade
et al. 2018 raised some doubts about the size and existence of some of these lakes such as Lake Ptolemy, Lake Megafezzan,
Lake Ahnet-Mouydir; it is possible that giant lakes only formed in the southern part of the Sahara. Other lakes are known from
Adrar Bous in
Niger,
Era Kohor and
Trou au Natron in the
Tibesti Mountains, I-n-Atei in the
Hoggar, at Ine Sakane and in
Taoudenni in
Mali, the Garat Ouda and Takarkori Lakes in the
Acacus Mountains, Chemchane in
Mauretania, at Guern El Louläilet in the
Great Western Erg, and Hassi el Mejnah and Sebkha Mellala both in
Algeria, at Wadi Shati and elsewhere in the Fezzan in
Libya, at Bilma, Dibella, Fachi and Gobero in the
Ténéré, in Niger and at "Eight Ridges", El Atrun, Lake Gureinat, Merga, "Ridge", Sidigh, Wadi Mansurab,
Selima and
Oyo in Sudan. The
lakes of Ounianga merged into two large lakes and overflowed, either above surface or underground. Mosaics of small lakes developed in some regions, such as the
Grand Erg Occidental.
Wetlands also expanded during the AHP, but both their expansion and subsequent retreat were slower than that of lakes. The Saharan topography prevents rapid drainage of accumulated water, thus favouring the development of waterbodies. The
Niger River, which had been dammed by dunes during the LGM, formed a lake in the
Timbuktu region that eventually overflowed and drained at some point during the AHP. In some parts of the Sahara
ephemeral lakes formed such as at
Abu Ballas,
Bir Kiseiba,
Bir Sahara,
Bir Tarfawi and
Nabta Playa in Egypt, which may relate to later Egyptian religions, or
swamp-lakes such as at Adrar Bous close to the
Air Mountains. Ephemeral lakes developed between dunes, and a "freshwater archipelago" appears to have existed in the Murzuq basin. All these lake systems left fossils such as fish,
limnic sediments and fertile soils that were later used for
agriculture (El Deir,
Kharga Oasis). Finally,
crater lakes formed in
volcanic fields such as
Trou au Natron and
Era Kohor in the Tibesti, and sometimes survive to this day as smaller remnant lakes such as Malha crater in the
Meidob volcanic field. Potentially, the increased availability of water during the AHP may have facilitated the onset of
phreatomagmatic eruptions such as
maar formation in the
Bayuda volcanic field, although the chronology of volcanic eruptions there is not well known enough to substantiate a link to the AHP. Increased precipitation resulted in the formation or reactivation of river systems in the Sahara. The large
Tamanrasset River flowed from the
Atlas Mountains and Hoggar westward towards the Atlantic and entered it in the
Bay of Arguin in
Mauritania. It once formed the 12th largest watershed in the world and left a
submarine canyon and riverine sediments. Together with other rivers it formed
estuaries and
mangroves in the Bay of Arguin. Other rivers in the same area also formed submarine canyons, and sediment patterns in marine
sediment cores and the occurrence of
submarine landslides in the area have been related to the activity of these rivers. Rivers such as the
Irharhar in
Algeria, Libya and
Tunisia and the
Sahabi and
Kufra rivers in Libya were active during this time although there is some doubt that they had perennial flow or that they reached the sea directly; they appear to have been more important in earlier humid periods. Small watersheds,
wadis and rivers discharging into
endorheic basins such as Wadi Tanezzuft also carried water during the AHP, leading to increased erosion. In Egypt, some rivers active during the AHP are now
gravel ridges. In the
Air,
Hoggar and Tibesti Mountains, the so-called "Middle
Terrace" was emplaced at this time. The rivers and lakes of the Sahara may have acted as pathways for the spread of humans and animals; animals that might have propagated through the Sahara in these waterbodies are
Nile crocodile and the fish
Clarias gariepinus and
Tilapia zillii. The name
Tassili n'Ajjer, which means "plateau of the rivers" in
Berber, may be a reference to past river flows. On the other hand, intense flows of these rivers may have made their shores dangerous to humans and thus created additional impetus for human movement. Now-dry river valleys from the AHP in the eastern Sahara have been used as analogues for former river systems on
Mars.
Humans of the Sahara Conditions and resources were ripe for first
hunter-gatherers,
fishermen and, later,
pastoralists; the exact chronology – when humans returned in the Sahara after the onset of the AHP – is disputed. They may have come either from the north (
Maghreb or
Cyrenaica) where the
Capsian culture was located, the south (
Sub-Saharan Africa), or the east (
Nile Valley). The human population in the Sahara rapidly increased during the AHP, interrupted by a brief decline between 7,600 and 6,700 years ago. Traces of human activity have been found in the
Acacus Mountains where caves and
rock shelters were used as basecamps for humans, such as the Uan Afuda
cave and the Uan Tabu and
Takarkori rock shelters. The first occupation in Takarkori took place between 10,000 and 9,000 years ago; about five millennia of human cultural evolution are recorded there. At
Gobero in the
Ténéré desert a
cemetery has been found, which has been used to reconstruct the lifestyle of these former inhabitants of the Sahara, and at Lake Ptolemy in
Nubia humans settled close to the lake shore, using its resources and perhaps even engaging in
leisure activities. At that time, many humans appear to have depended on water-bound resources, seeing as many of the tools left by the early humans are associated with
fishery; hence this culture is also known as "
aqualithic" although substantial differences between the cultures of various places have been found. The greening of the Sahara led to a
demographic expansion and especially in the Eastern Sahara human occupancy coincides with the AHP. Conversely occupation decreased along the Nile valley, as it became inhospitable to human settlement due to flooding extending to the Nile delta. Humans moved into the Sudanese segment of the Nile valley only by about 11,000 years ago. Humans were hunting large animals with weapons that have been found in archaeological sites and wild
cereals occurring in the Sahara during the AHP such as
brachiaria,
sorghum and
urochloa were an additional source of food. Humans also domesticated
cattle,
goats and
sheep. Cattle domestication may have occurred especially in the more environmentally variable Eastern Sahara, where the lack of lakes (cattle having high requirements of
drinking water) may however have limited the occurrence of cattle. Animal husbandry picked up in earnest around 7,000 years ago when domestic animals came to the Sahara, and a population boom may be linked to this change in cultural practice; cattle and goats spread southwestwards from northeasternmost Africa from 8,000 years before present.
Dairying has been demonstrated in some locations and cattle-husbandry is supported by the frequent depiction of cattle in
rock paintings. The relative importance of
hunter-gatherer practices and pastoralism, and whether people were sedentary or migratory, is unclear. The
Dufuna canoe, one of the oldest known ships in the world, appears to date to the Holocene humid period and implies that the waterbodies of that time were navigated by humans. The cultural units "Masara" and "Bashendi" existed in
Dakhleh Oasis during the AHP. In the Acacus Mountains, several
cultural horizons known as Early and Late Acacus and Early, Middle, Late and Final Pastoral have been identified while in
Niger the
Kiffian culture has been related to the beginning of the AHP. Ancient civilizations thrived, with farming and animal husbandry taking place in
Neolithic settlements. Possibly, the domestication of plants in Africa was delayed by the increased food availability during the AHP, it only took place around 2,500
BC. Humans created
rock art such as
petroglyphs and
rock paintings in the Sahara, perhaps the largest density of such creations in the world. Scenes include animals and everyday life such as
swimming which supports the presence of past wetter climates. One well-known such petroglyph location is the
Cave of Swimmers in the
Gilf Kebir mountains of Egypt; other well known sites are the
Gabal El Uweinat mountains also of Egypt,
Arabia and the
Tassili n'Ajjer in
Algeria where rock paintings from this time have been discovered. Humans also left
artifacts such as
Fesselsteine and
ceramics in what today are inhospitable deserts. North Africa together with East Asia is one of the first places where
pottery was developed probably under the influence of increased availability of resources during the AHP. The humid period also favoured its development and spread in West Africa during the 10th millennium
BC; the so-called
"wavy line" or "dotted wavy-line" motif was widespread across Northern Africa and as far as
Lake Turkana. A similar but more circumscribed spread occurred with the Ounan
arrow points. These populations have been described as
Epipaleolithic,
Mesolithic and
Neolithic and produced a variety of
lithic tools and other assemblages. In West Africa, the cultural change from the African
Middle Stone Age to the
Late Stone Age accompanied the beginning of the AHP. In Sudan, the beginning of the early Khartoum culture coincides with the initiation of the AHP.
Genetic and archaeological data indicate that these populations which exploited the resources of the AHP Sahara probably originated in
Sub-Saharan Africa and moved north after some time, after the desert got wetter; in return, the AHP facilitated the movement of some
Eurasian populations into Africa, and bidirectional travel across the Sahara more generally. The northward spread of
Macrohaplogroup L and
Haplogroup U6 genomic lineages may reflect this tendency but did not result in widespread genetic exchanges between Northern and sub-Saharan Africa. Elsewhere, newly formed or expanded water courses may have restricted human mobility and isolated populations. These favourable conditions for human populations may be reflected in
paradise myths such as the
Garden of Eden in
The Bible and
Elysium and the
Golden Age in
Classical Antiquity, while a possible role in the development of the
Afroasiatic and the spread of the
Niger-Congo and
Nilo-Saharan languages is debatable. References in Egyptian chronicles about moist lands along the
Red Sea may record the wet conditions of the late AHP.
Additional manifestations in the Sahara The expanded vegetation and
soil formation stabilized previously active
dunes, eventually giving rise to the present-day
draa dunes in the
Great Sand Sea of Egypt for example, although there is uncertainty about whether this stabilization was widespread.
Soil development and biological activity in soils are attested in the
Acacus Mountains and the
Mesak Settafet area of Libya, but evidence of soil formation/
pedogenesis such as
bog iron or
kaolinite formation are described from other parts of the Sahara and Sahel as well. In the Selima Sand Sheet, the landscape underwent erosional truncation and
bioturbation. Erosion by increased runoff delayed soil development until after a few millennia after the onset of the AHP. The Central and Southern Sahara saw the development of
alluvial deposits while
sebkha deposits are known from the Western Sahara. Lightning strikes into soil left
lightning-altered rocks in parts of the Central Sahara. The increased precipitation recharged
aquifers such as the
Nubian Sandstone Aquifer; presently, water from this aquifer maintains several lakes in the Sahara, such as the
Lakes of Ounianga. Other
groundwater systems were active at that time in the
Acacus Mountains,
Air Mountains, in the
Fezzan and elsewhere in Libya and the
Sahel. Raised groundwater tables provided water to plants and was discharged in depressions, lakes,
springs and valleys, forming widespread
carbonate deposits and feeding lakes and wetlands. The formation of lakes and vegetation reduced the export of dust from the Sahara. This has been recorded in
marine cores, including one core where dust export decreased by almost half, and in
Italian lakes. In coastal places, such as in
Oman,
sea level rise also reduced the production of dust. In the Mediterranean, a decreased dust supply was accompanied by increased sediment input from the Nile, leading to changes in
marine sediment composition. Conversely, the increased vegetation may have yielded more
volatile organic compounds in the air. Whether the strengthening of the
monsoon enhanced or reduced
upwelling off Northwestern Africa is debatable, with some research suggesting that the strengthening in upwelling decreased
sea surface temperatures and increased the biological productivity of the sea, while other research suggests that the opposite occurred; less upwelling with more moisture. However, regardless of whether upwelling increased or decreased, it is possible that the strengthening of the monsoon boosted productivity off the coasts of Northern and Western Africa because the increased river discharge delivered more nutrients to the sea. The decline of dust input may have caused the cessation of
deep-water coral growth in the eastern Atlantic during the AHP by starving them of nutrients.
Arabia Precipitation in
Dhofar and southwestern Arabia is brought by the African monsoon, and a change to a wetter climate resembling Africa has been noted in southern Arabia and
Socotra from
cave and river deposits. It possibly reached as far as
Qatar.
Holocene paleolakes are recorded at
Tayma,
Jubbah, in the
Wahiba Sands of
Oman and at
Mundafan. In the
Rub al-Khali lakes formed between 9,000 and 7,000 years ago and dunes were stabilized by vegetation, although the formation of lakes there was less pronounced than in the Pleistocene. One of these lakes eventually overflowed to produce an
outburst flood north of the Rub al-Khali. The
Wadi ad-Dawasir river system in central
Saudi Arabia became active again with increased river runoff into the
Persian Gulf. Wadis in Oman eroded across LGM dunes and formed
accumulation terraces. Episodes of increased river discharge occurred in
Yemen and increased precipitation is recorded in the caves of Hoti, Qunf in
Oman, Mukalla in Yemen and
Hoq Cave in
Socotra. Increased precipitation resulted in increased
groundwater flow, generating groundwater-fed lakes and
carbonate deposits. Forests and
wildfire activity expanded across parts of Arabia. Freshwater sources in Arabia during the AHP became focus points of human activity and herding activity between mountains and lowlands occurred. In addition,
karstic activity took place on exposed
coral reefs in the Red Sea and traces of it are still recognizable today. Increased precipitation has been also invoked to explain decreased salinities in the Red Sea, increased sedimentation and increased river inflow, while dust input decreased.
Rock art depicts wildlife that existed in Arabia during the humid period. The Arabian
neolithic coincides with the humid period, with archaeological sites such as
cairns appearing with its beginning, and settlement of southern
Mesopotamia during the
Ubaid period may coincide with the wet epoch. The humid period in Arabia did not last as long as in Africa, deserts did not retreat as much and precipitation may not have reached the central and northern part of the peninsula past
Oman and the
Yemen Highlands; northern Arabia remained somewhat drier than southern Arabia,
droughts were still common and the land and still produced dust. One study has estimated that the amount of rainfall in the Red Sea did increase to no more than . Whether some former lakes in Arabia were actually
marshes is contentious.
East Africa Nile discharge was higher than today and during the early African humid period, the Nile in Egypt flooded up to higher than it did recently before
flood control. The increased flooding may have turned the
Nile Delta and Nile Valley
marshy and inhospitable and could explain why many archaeological sites along the Nile were abandoned during the AHP, with violent conflicts taking place at
Jebel Sahaba archaeological site. Early after the Younger Dryas, the Blue Nile would have been the major source of waters for the Nile. Waters from the Nile filled depressions like the
Fayum Depression to form a deep lake with
anoxic bottom waters and reaching above sea level, probably once a geomorphic barrier was breached.
Wetlands and
anastomosing channels developed in the
Nile Delta as sediment supply increased. In addition, Nile tributaries in northwestern Sudan such as
Wadi Al-Malik,
Wadi Howar and
Valley of the Queens became active during the AHP and contributed sediments to the Nile.
Wadi Howar was active until 4,500 years ago, and at the time often contained dune-dammed lakes,
swamps and
wetlands; it was the largest Saharan tributary of the Nile and constituted an important pathway into sub-Saharian Africa. Conversely it appears that
Lake Victoria and
Lake Albert were not overflowing into the White Nile for all of the AHP, and the White Nile would have been sustained by overflow from
Lake Turkana. There appears to be a tendency over the course of the AHP for the discharge of the Blue Nile to decrease relative to that of the White Nile. The
Blue Nile built an
alluvial fan at its confluence with the White Nile, and
incision by the Nile reduced flooding risk in some areas which thus became available for human use.
Closed lakes in East Africa rose, sometimes by hundreds of metres.
Lake Suguta developed in the
Suguta Valley, accompanied by the formation of
river deltas where rivers such as the
Baragoi River entered the lake. In turn, Lake Suguta overflowed into the
Kerio River, this adding water to
Lake Turkana where increased discharge by the
Turkwel River led to the formation of a large
river delta. The
Omo River remained its principal inflow but the relative role of other water sources increased compared to present-day conditions. A deep lake filled the
Chew Bahir basin and together with
Lakes Chamo and
Abaya formed a river system flowing into Lake Turkana, which itself overflowed on its northwestern side through the Lotikipi Swamp into the
White Nile. Deposits from this lake highstand form the Galana Boi
Formation. The increased water depth reduced water mixing in Lake Turkana, allowing organic material to build up. This overflowing large lake was filled with
freshwater and was populated by humans, typically in bays, along capes and protected shorelines; the societies there engaged in
fishery but could probably also fall back on other resources in the region. The Ethiopian
Lake Abhe expanded to cover an area of , much larger than the present-day lake, in the "Abhe IV"–"Abhe V" lake cycle. The enlarged lake covered a large area west of the present-day lake, present-day lakes
Afambo,
Gamari and
Tendaho, reducing
Borawli,
Dama Ale and
Kurub to islands, and possibly overflowed to the Red Sea. The maximum water level was reached during the early Holocene as river discharge increased, but was later limited by partial overflow and did not rise above again. Deep thermal
groundwater recharge occurred in the region. About 9,000 years of human occupation are documented at the lake.
Archaeological sites indicate that people obtained resources from the lake and followed its rise and decline. The cultural traditions at Lake Abhe appear to be unusual by AHP/African standards.
Lake Zway and
Lake Shala in Ethiopia joined with
Lake Abiyata and
Lake Langano to form a large waterbody which began overflowing into the Awash River. Nearly all lakes in the
East African Rift were affected by the AHP:
Lake Ashenge and
Lake Hayq also in Ethiopia,
Lake Bogoria,
Lake Naivasha and
Lake Nakuru-
Lake Elmenteita which merged, all in
Kenya, and
Lake Masoko in
Tanzania. Lakes formed in the
caldera of the
Menengai volcano and in the Chalbi region east of Lake Turkana; the lake covered an area of about . A large and deep
Lake Magadi formed in the early Holocene, generating the "High Magadi Beds" sediments. This lake was fed by now-dry
waterfalls and possibly from the neighbouring lake Koora. In the
Danakil Depression of Ethiopia freshwater conditions became established. Lakes formed in depressions on the mountains around
Lake Kivu. Some of these lakes became connected through overflow: Lake Nakuru-Elmenteita drained northward through the Menengai caldera, Baringo-Bogoria Suguta into Lake Turkana and from there into the Nile, carving
gorges along the way. Lake Naivasha drained south through Lake Siriata into Lake Magadi-Natron. Overflow of several of these lakes allowed animals including
Nile crocodiles and fish to propagate to the individual lake basins, but at the same time hindered the propagation of many land-based mammals. River systems in the southern Kenya Rift region became active.
Glaciers stopped retreating or briefly expanded in East Africa at the beginning of the AHP before continuing retreat. On
Mount Kilimanjaro they may have expanded during the AHP after a phase during the
Younger Dryas where the mountain was ice free, but the
tree line also rose at that time, accompanied by
soil formation. The wetter climate may have destabilized the neighbouring
Mount Meru volcano, causing a
giant landslide that removed its summit. In the
Rwenzori Mountains, increased precipitation during the AHP has been linked to the occurrence of
rockfalls.
Erosion in catchments of East Africa increased with the beginning of the humid period but then decreased even before its end, as the increased
weathering led to the formation of
soils, these in turn to the establishment of a vegetation cover that subsequently reduced additional erosion. Increased
weathering resulted in the increased consumption of
atmospheric during the AHP. Surprisingly, and contrary to the patterns expected from precessional changes, the
East African Rift also experienced a wetter climates during the AHP, reaching as far south as
Lake Rukwa and
Lake Cheshi into the Southern Hemisphere. In the region of the
African Rift Valley and
African Great Lakes,
pollen evidence points to the occurrence of forests including
rainforest vegetation due to the increased precipitation, while today they occur only in limited areas there. Denser vegetation also occurred at
Lake Turkana, with wooden vegetation covering almost half of the dry land although grasslands remained dominant. Development of forest vegetation around the African Great Lakes created an interconnected environment where species spread, increasing
biodiversity with effects on the future when the environment became fragmented. Vegetation cover also increased in the
Afar region and
Ericaceae plants spread at high elevations. Forests and moisture-requiring vegetation expanded in the
Bale Mountains. Different types of vegetation, including dryland vegetation, existed at
Lake Malawi and
Lake Tanganyika however, and vegetation did not change much. The wetter climate led to the formation of the Halalee
paleosoil in the Afar region and decreased wildfire activity in the Rwenzori. In East Africa, the AHP led to improved environmental conditions in terms of food and water supply, allowing early human populations to survive, grow in size and settle new regions without requiring major changes in food gathering strategies.
Pottery techniques such as the "dotted wavy line" and "Kanysore" are associated with fishing and foraging communities. In Somalia, the "Bardaale"
lithic industry is linked to the AHP. Earlier wet and dry periods in East Africa may have influenced the
evolution of humans and allowed their spread across the Sahara and into
Europe. Animal species took advantage too, with
midge populations increasing in Lake Victoria.
Other parts of Africa and the rainforest realm Lake Bosumtwi in
Ghana rose during the AHP. Evidence there also suggests a decrease in
wildfire activity took place. Tropical forests expanded in
Cameroon Highlands and the
Adamawa Plateau of
Cameroon and moved upward at
Lake Bambili also in
Cameroon, causing an upward shift of
afromontane vegetation. The core of the
rainforest was probably unaltered by the African humid period, perhaps with some changes in species and an expansion of their area. There is some evidence that an "Equatorial humid period", mechanistically linked to equatorial insolation and extending into the
Amazon, may have taken place in the eastern
Congo region at the same time as the AHP or around its beginning and end. The
peatlands of
Central Congo started developing during the African humid period and peat continues to accumulate there to this day, albeit with a slowdown in the
Cuvette Centrale after the end of the African humid period. In the Gulf of Guinea, increased sedimentation and changed sedimentation patterns from increased river runoff decreased the activity of submarine
cold seeps offshore present-day Nigeria. On
São Nicolau and
Brava in the
Cape Verde Islands, precipitation and erosion increased. In the
Canary Islands, there is evidence of a moister climate on
Fuerteventura,
La Gomera and
Tenerife, the
laurel forests changed perhaps as a consequence of the AHP. Recharge of
groundwater levels have been inferred from
Gran Canaria also in the Canary Islands, followed by a decrease after the end of the AHP.
Choughs may have reached the Canary Islands from North Africa when the latter was wetter.
Levant and Mediterranean High latitude Africa has not undergone large scale changes in the past 11,700 years; the
Atlas Mountains may have blocked the monsoon from expanding further north. However, soil and
tufa, river valley and
cave deposits showing a moister climate in southern
Morocco, increased precipitation in the Algerian highlands, vegetation changes in the
Middle Atlas, several floods in
Tunisian rivers and ecosystem changes which impacted
steppe-dependent
rodents of Northern Africa have been linked to the AHP. In the
Pleistocene and
Holocene humidity in the Mediterranean is often correlated to humidity in the Sahara, and the early-mid
Holocene climate of
Iberia,
Italy,
Negev and
Northern Africa was wetter than today; in
Sicily wettening correlates with ITCZ changes in Northern Africa. Mediterranean precipitation is brought by Mediterranean
cyclones and the
westerlies; either increased precipitation from the westerlies, northward moisture transport from Africa or monsoonal precipitation extending into the Mediterranean may have rendered it wetter. While it is established, the nature of the connection between the African Monsoon and Mediterranean precipitation is unclear and it was winter rainfall that increased predominantly, although separating monsoonal and non-monsoonal precipitation can be difficult. The
Mediterranean Sea became less saline during the AHP, in part due to increased precipitation from the
westerlies but also from increased river discharge in Africa, leading to the formation of
sapropel layers when the increased runoff led to the Mediterranean becoming more stratified and
eutrophied, with changes in the main water masses of the sea. The S1 sapropel layer is specifically associated with the AHP and with increased discharge of the Nile and other African rivers. These processes together with decreased dust transport by wind led to changes in the sediment patterns of the Mediterranean, and increased marine nutrient availability and
food web productivity in the Mediterranean, which impacted the development of deep-sea
corals. In the
Levant, wetter conditions during the AHP are recorded from
Jeita Cave in
Lebanon,
Soreq Cave in
Israel and desert varnish in the
Negev, while the
Dead Sea has variously been reported to have grown or shrunk during the AHP. Such a decline, if it took place, and a decline of other southern European lakes were low during this period. This is unlike some earlier wet periods in the Sahara, and whether the monsoon reached the southern Levant region during the AHP or there was increased winter precipitation is contentious. The northern Mediterranean may have been drier, with more
wildfire activity, during the AHP, but increased summer precipitation in the
European Alps has been associated with the AHP.
Southern Africa The effects, if any, of the African humid period on Southern Africa have been unclear. Originally it was proposed that the orbitally driven changes would imply a dry period in Southern Africa which would have given way to moister conditions as the northern AHP ended, as the ITCZ should shift its average position between the two hemispheres. However, the lack of paleoclimatology data with sufficient time resolution from Southern Africa has made it difficult to assess the climate there during the AHP. More recently obtained paleoclimate data have suggested however that southern Africa was actually wetter during the AHP rather than drier, reaching as far as
Rodrigues Island in the Indian Ocean and as far as the catchment of the
Orange River. The area between
Lake Tanganyika and
Lake Malawi has been interpreted as the limit of the AHP's influence. Conversely, and consistent with the opposite reaction pattern of the Southern Hemisphere, the
Zambezi River reached its lowest discharge during the AHP, and precipitation in the Central African Plateau and
Zambia decreases in computer simulations of a Green Sahara. Thus, the AHP may not have reach southern or southeastern Africa. There may have been opposite changes in precipitation between southeast Africa and tropical East Africa, separated by a "hinge zone". Particular changes occurred in central southern Africa, where a dry period co-occurred with an expansion of
Lake Makgadikgadi; presumably the lake during this dry interval was nourished by increased wetness over the
Okavango River catchment in the
Angolan Highlands due to the AHP;
peatlands formed in Angola during the AHP. In general there is little consistency between Northern and Southern Africa in terms of hydrological changes during the
Holocene, and nowhere are both the start and end of the AHP apparent. Orbitally-mediated changes in Northern Hemisphere climate affected the Southern Hemisphere through oceanic pathways involving
sea surface temperatures. Additionally, wetter periods unrelated to the AHP may have occurred after deglaciation in Southern Africa.
Numerical estimates Estimates of the exact amount of increased precipitation vary widely. During the African humid period, Saharan rainfall increased to , and values exceeding may have spread to 19–21° northern latitude. In the eastern Sahara, a gradient from increment in the north to in the south has been identified. An area with less than may have remained in the Eastern Sahara however, although its driest parts may have received 20-fold more precipitation than today. Precipitation in the Sahara probably reached no more than , with large uncertainty. Other reconstructed values of the precipitation increase indicate an annual increase of about in Africa, with strong regional variation. From lake levels and other proxies, precipitation increases of 20–33%, 25–40%/23-45% or 50–100%/40–150% have been inferred for East Africa, with an increase of 40% reconstructed for Northern Africa. In the early Holocene, there appears to have been an eastward- and northward-decreasing trend of humidity. Additionally, at
Tayma in Arabia a threefold increase appears to have occurred and precipitation in the
Wahiba Sands of
Oman may have reached .
Effect on other climate modes The
El Niño–Southern Oscillation is a major climate variability mode. Paleoclimatology records from
Ecuador and the
Pacific Ocean indicate that during the early and middle Holocene
ENSO variability was suppressed by about 30–60%, which can be only partially explained through
orbital forcing. The Green Sahara may have suppressed
ENSO activity, forcing a
La Niña–like climate state, in one
climate model this is accompanied by decreased
upwelling and deepening of the
thermocline in the Eastern Pacific as the
Walker circulation expands westward. Easterly winds in the western Pacific Ocean increase, while they decrease in the eastern. In addition,
Atlantic Niño sea surface temperature patterns develop in the Atlantic Ocean and the
South Atlantic High weakens while the circulation of the
South Atlantic Ocean changes. However, the existence of serious
model bias in the depictions of the equatorial Atlantic sea surface temperatures is a problem for simulations of its past climate and the Atlantic Niño state may only occur during marginal AHP conditions, being suppressed during its maximum. Remote effects of the AHP on climate have also been studied, although many changes are model-dependent and may also be inaccurate due to incorrect depictions of atmospheric dust distribution. Whether the reduced albedo of the Sahara during the AHP contributed to, or increased cloud cover counteracted, the warming of the
Holocene thermal maximum is model-dependent; dust changes did not have a major effect. The AHP would also influence SSTs in the Indian Ocean, although there is not much evidence about the mid-Holocene sea temperatures there. The AMOC transports heat from the Southern into the Northern Hemisphere and is implicated in starting the Holocene AHP an earlier AHPs after the end of an ice age. Various studies have been conducted to determine which effects reduced dust supply and the greening of the Sahara would have had on its intensity, with conflicting results on which effects would predominate. Increased heat transport either through the atmosphere or the ocean would result in warming in the
Arctic. Gaetani
et al. 2024 found that Green Sahara climate simulations feature warming throughout the Northern Hemisphere and a strengthening of the
westerlies and their precipitation in the Atlantic but a decline along the North American
West Coast. There are also changes in the
North Atlantic Oscillation during winter. The simulated temperature patterns, however, only poorly match temperature reconstructions.
Remote precipitation and the AHP The Sahara greening intensified the Indian and Asian monsoons, warming and increased precipitation across most of the
Tibetan Plateau especially late in the monsoon season, and climate simulations including a green Sahara reproduce the reconstructed palaeoclimates there better than these without. In a climate model, there is a shift in precipitation from snow to rain. The strengthened and expanding monsoons of Africa and Asia alter the atmospheric circulation of the planet, inducing a northward-shifted and wetter
East Asian Monsoon and drying across tropical South America and central-eastern North America. In East Asia, a strengthened
anticyclone over the West Pacific delivers more moisture to northeastern China and
Indochina, and less to central and south-eastern China. The reduced dust emission warms the North Atlantic and increases westerly flow into the
North American Monsoon, strengthening it. The far-field precipitation changes reach as far as Europe and Australia. Discrepancies between modelled and reconstructed northward extension and precipitation in the Asian monsoon regions and the
North American Monsoon area may be explained through these remote effects. Sun
et al. 2020 proposed that the greening of the Sahara during the AHP can increase precipitation over the
Middle East even if neither the African nor the Indian monsoons reach it. During spring, the increased vegetation forces anomalous atmospheric circulations that direct moisture transport from the Mediterranean, the Red Sea and eastern tropical Africa into the Middle East, increasing precipitation and agricultural productivity there. This could explain increased precipitation in the Middle East during the AHP: A wet climate occurred in the Middle East during the early Holocene, leading to the
Ubaid period of settlement in
Mesopotamia, followed by dry phases around 5,500 years ago and a concomitant reduction in simulated
wheat yield.
Hurricanes and the AHP One climate model has indicated that a greener Sahara and reduced dust output would have increased
tropical cyclone activity, especially over the Atlantic but also in most other
tropical cyclone basins. Changes in the intensity of the storms, decreases in
wind shear, changes in atmospheric circulation and less dust in the atmosphere, which results in warmer oceans, are responsible for this phenomenon, while
tropical wave activity may have increased or decreased. The net effect could be a global increase in tropical cyclone activity, a westward shift within the ocean basins and in the
Atlantic Ocean a shift towards later dates. While there are no good
paleotempestology data for the time of the African humid period that could confirm or refute this theory and many of these records are specific for particular locations,
hurricane activity including past strikes in
Puerto Rico and in
Vieques appear to correlate with the strength of the
West African Monsoon and increased precipitation on the northern
Yucatan Peninsula during the middle Holocene could be explained by increased hurricane activity during the AHP. On the other hand, at
Grand Bahama Bank and the
Dry Tortugas of
South Florida a decrease of hurricane activity took place during the AHP and dust emission is not always anti-correlated to hurricane activity. Finally, the northward movement of the ITCZ during the AHP may have caused a corresponding northward movement of
tropical cyclogenesis areas and storm tracks in the Atlantic Ocean, which could also explain decreased hurricane activity in the Bahamas and Dry Tortugas. == Fluctuations ==