African humid period

The Sahara was not a desert during the African humid period. Instead, most of northern Africa was covered by grass, trees, and lakes.

The African humid period (AHP) is a climate period in Africa during the late Pleistocene and Holocene geologic epochs, during which northern Africa was wetter than today. This replacement of much of the Sahara desert by grasses, trees and lakes was caused by changes in Earth's orbit around the Sun; changes in vegetation and dust in the Sahara which strengthened the African monsoon; and increased greenhouse gases, which may imply that anthropogenic global warming will also result in a shrinkage of the Sahara desert.

Before the AHP, during the last glacial maximum, the Sahara was much larger than today and had extensive dune fields; many lakes and rivers such as Lake Victoria and the White Nile were either dry or at low levels and the Sahara mostly uninhabited. The African humid period commenced about 14,600–14,500 years ago at the end of Heinrich event 1, simultaneously to the Bølling-Allerød warming; rivers and lakes such as Lake Chad formed or expanded, glaciers grew on Mount Kilimanjaro and the Sahara retreated. Two major fluctuations occurred, one during the Younger Dryas and the other during the 8.2 kiloyear event during both of which temporarily drier conditions returned across Africa. The end of the African humid period came about 6,000–5,000 years ago during the Piora Oscillation cold period when the Sahara occupied its present position. While some evidence points to an end 5,500 years ago in the Sahara, in the Sahel, Arabia and East Africa the period appears to have taken place in several steps such as the 4.2 kiloyear event.

The AHP led to a widespread settlement of the Sahara and the Arabian Deserts, and had a profound effect on African cultures, such as the birth of the Pharaonic civilization and the pyramids after its end, and potentially also the development of widespread Golden Age myths. They lived as hunter-gatherers until the agricultural revolution, domesticating animals such as cattle, goats and sheep. They have left archeological sites and artifacts such as one of the oldest ships in the world; and rock paintings such as those in the Cave of Swimmers and in the Acacus Mountains.

Earlier humid periods in Africa were postulated after the discovery of these rock paintings in now-inhospitable parts of the Sahara. These earlier periods likely influenced the evolution of modern humans. When the African humid period ended, humans gradually abandoned the desert in favour of regions with more secure water supplies, such as the Nile Valley and Mesopotamia, where they gave rise to early complex societies.

Research historyEdit

Herodotus in 440 BC and Strabon in 23 AD discussed the existence of a greener Sahara, although their reports were at first questioned owing to their anecdotal nature. In 1850 the researcher Heinrich Barth discussed the possibility of past climate change leading to increased wetness in the Sahara after discovering petroglyphs in the Murzuq Desert, and further discoveries of petroglyphs led desert explorer László Almásy to coin the concept of a Green Sahara in the 1930s. Later in the 20th century, conclusive evidence of a past greener Sahara, the existence of lakes[1][2] and higher Nile flow levels was increasingly reported[3] and it was recognized that the Holocene featured a humid period in the Sahara.[4]

The idea that changes in Earth's orbit around the Sun influence the strength of the monsoons was already advanced in 1921, and while the original description was partly inaccurate, later widespread evidence for such orbital controls on climate was found.[1] At first it was believed that humid periods in Africa correlate with glacial stages ("pluvial hypothesis") before radiocarbon dating became widespread.[5]

The development and existence of the African humid period has been investigated with archeology, climate modelling and paleoproxies,[6] with archeological sites,[7] dunes as well as lake, marine and wetland deposits playing an important role.[2] Pollen, lake deposits and former levels of lakes have been used to study its ecosystems,[8] and charcoal and leaf impressions have been used to identify vegetation changes.[9]

Research issuesEdit

While the precipitation changes since the last glacial cycle are well established, the magnitude and timing of the changes are unclear.[10] Depending on how and where measurements and reconstructions are made, different durations[3] and precipitation levels[11] have been determined for the African humid period.[3] The amounts of precipitation reconstructed from paleoclimate records and simulated by climate modelling are often inconsistent with each other;[12] the simulation of the Green Sahara in general is considered a problem for earth system models.[13] Erosion of lake sediments and carbon reservoir effects make it difficult to date when they dried up.[14] Vegetation changes by themselves do not necessarily indicate precipitation changes, as changes in seasonality, plant species composition and changes in land use also influence the vegetation records.[15] Isotope ratios such as the hydrogen/deuterium ratio that have been used to reconstruct past precipitation values likewise are under the influence of various physical effects, which complicates their interpretation.[16]

TerminologyEdit

Earlier humid periods are sometimes known as "African humid periods"[17] and a number of dry/wet periods have been defined for the Central Africa region.[18] In general, these types of climate fluctuations between wetter and drier periods are known as "pluvials" and "interpluvials", respectively.[19][a]

The terms "Léopoldvillien"[29] and Ogolien [fr] have been applied to the dry period in the last glacial maximum,[30] the latter is equivalent to the "Kanemian";[31] "Kanemian dry period" refers to a dry period between 20,000–13,000 years before present in the Lake Chad area.[32]

Background and beginningEdit

The African humid period took place in the late Pleistocene[33] and early-middle Holocene[34] and was characterized by increased precipitation in Northern and Western Africa relative to today[15] due to a northward migration of the tropical rainbelt.[35] Within the otherwise relatively climatically stable Holocene, the AHP is considered to be a major fluctuation.[36] It is also part of the so-called Holocene climatic optimum which featured warmer summers than present-day in the Northern Hemisphere.[37][b] Sometimes the humid period is subdivided into an "AHP I" which lasted until 8,000 years ago, and an "AHP II" from 8,000 years onward[42] the former being wetter than the latter.[43]

The African humid period was not the first such phase; evidence for about 230 older such "green Sahara"/wet periods exist going back perhaps to the first appearance of the Sahara 7-8 million years ago,[1] for example during Marine Isotope Stage 5 a and c.[44] Such humid periods are usually associated with interglacials, while glacial stages correlated to dry periods.[17] For example, the Bølling-Allerød warming appears to be synchronous to the onset of the African humid period[45][46][47] as well as to increased humidity in Arabia;[48] later, the humid period coincides with the Atlantic period of the Blytt–Sernander sequence.[49]

Earlier humid periods appear to have been more intense than the AHP of the Holocene,[50][51] including the exceptionally intense Eemian humid period which provided the pathways for early humans to cross Arabia and Northern Africa[52] and which, together with later moist periods, has been linked to expansions of the Aterian populations.[53]

Conditions before the African humid periodEdit

 
African vegetation during the last glacial maximum

During the Last Glacial Maximum, before the AHP, the Sahara and Sahel had been extremely dry[54] with less precipitation occurring than today[55][56] as reflected by the extent of dune sheets and water levels in closed lakes.[54] The Sahara was much larger than today,[57] extending 500–800 kilometres (310–500 mi) farther south,[58] a difference of 5° latitude.[59] Dunes were active much closer to the equator,[58][60][c] and rainforests retreated in favor of afromontane and savannah landscapes as temperatures, rainfall, and humidity decreased.[29][63]

There is little and often equivocal evidence of human activity in the Sahara and also in Arabia at that time, reflecting its drier nature.[64][65][66] The aridity during the Last Glacial Maximum appears to have been the consequence of the colder climate and larger polar ice sheets, which squeezed the monsoon belt to the equator and weakened the West African Monsoon. The atmospheric water cycle and the Walker and Hadley circulations were weaker as well.[67] Exceptional dry phases are linked to Heinrich events,[68] which occur when a large number of icebergs are present in the North Atlantic;[69] the discharge of large amounts of such icebergs between 11,500 and 21,000 years before present coincided with droughts in the subtropics.[70]

Before the onset of the AHP, Lake Victoria, Albert, Edward,[71] Turkana[72] and the Sudd swamps apparently had dried out.[73] The White Nile had become a seasonal river[73] whose course[74] along with that of the main Nile may have been dammed by dunes.[75] The Nile Delta was partially dry, with sandy plains extending between ephemeral channels and exposed seafloor, and it became a source of sand for ergs[d] farther east.[77] Other lakes across Africa, such as Lake Chad and Lake Tanganyika, also had shrunk[e] during this time,[78] and both the Niger River and Senegal River were stunted.[79]

Early humidity increasesEdit

Whether some parts of the desert such as highlands like the Red Sea Hills were reached by the westerlies[80] or weather systems associated with the subtropical jet stream[81]—and thus received precipitation—is contentious. It is only clearly supported for the Maghreb in northwestern Africa,[80] though river flow[60]/terrace formation[82] and lake development in the Tibesti and Jebel Marra mountains[83][84] and residual Nile flow may be explained in this way.[85] The highlands of Africa appear to have been less affected by drought during the last glacial maximum.[86]

The end of the glacial drought occurred between 17,000 and 11,000 years ago,[84] with an earlier beginning noted in the Saharan mountains[87][63] where it may have begun 18,500 years ago.[88] In southern and central Africa earlier starts 17,000 and 17,500 years ago, respectively, are possibly linked to Antarctic warming,[89][23] while Lake Malawi appears to have been low until about 10,000 years ago.[90]

High lake levels occurred in the Jebel Marra and Tibesti Mountains between 15,000 and 14,000 years ago[91] and the youngest stage of glaciation in the High Atlas mountains took place at the same time as the early African humid period.[92] 14,500 years ago lakes started to appear in the arid areas.[93]

OnsetEdit

The humid period began about 15,000[89][94]-14,500 years ago,[f][33] The onset of the humid period took place almost simultaneously over all of Northern[g] and Tropical Africa[98] with impacts as far as Santo Antão on Cape Verde.[99][100] In Arabia, wet conditions apparently took about two millennia to advance northward[97][101] with a gradual advance supported by tephrochronological data.[102]

Lake Victoria reappeared and overflowed;[93] Lake Albert also overflowed into the White Nile[91] 15,000–14,500 years ago[71] and so did Lake Tana, into the Blue Nile.[91] The White Nile flooded part of its valley[103] and reconnected to the main Nile.[94][h] In Egypt widespread flooding took place as part of the "Wild Nile" period[91] which led to the largest recorded floods on this river,[75] sedimentation in floodplains[105] and probably also impacted human populations along the river.[106] Even earlier, 17,000–16,800 years ago, meltwater from glaciers in Ethiopia – which were retreating at that time – may have begun to increase the flow of water and sediment in the Nile.[107] In the East African Rift water levels in lakes began to rise by about 15,500/15,000[108]-12,000 years ago;[109] Lake Kivu began overflowing into Lake Tanganyika by about 10,500 years ago.[110]

About the same time that the AHP started, the cold glacial climate in Europe associated with Heinrich event 1 ended[93] with climate changing as far as Australasia.[91] A warming and retreat of sea ice around Antarctica coincides with the start of the African humid period,[111] although the Antarctic Cold Reversal also falls into this time.[23]

CausesEdit

The African humid period was caused by a stronger West African Monsoon[112] directed by insolation changes and changes in albedo feedbacks,[12] leading to increased moisture import from the equatorial Atlantic to West Africa, but also from the North Atlantic and the Mediterranean Sea towards Mediterranean Africa.[113][114] There also were complex interactions with the atmospheric circulation of the extratropics and between moisture coming from the Atlantic Ocean and the Indian Ocean,[115] and an increased overlap between the areas wetted by the monsoon and by extratropical cyclones.[116]

Climate models indicate that changes from a dry to a green Sahara and back have threshold behaviour, with the flip occurring once a certain level of insolation is exceeded;[117] likewise, a gradual drop of insolation often leads to a sudden transition back to a dry Sahara.[118] This is due to various feedback processes which are at work[15] and in climate models there is often more than one stable climate-vegetation state.[119] As an example, sea surface temperature and greenhouse gas changes synchronized the beginning of the AHP across Africa.[98]

Orbital changesEdit

 
Milankovich cycles over the past one million years

The African humid period has been explained by increased insolation during Northern Hemisphere summer.[15] Due to precession, the season at which Earth passes closest to the Sun on its elliptical orbit – the perihelion – changes, with maximum summer insolation occurring when this happens during Northern Hemisphere summer.[120] Between 11,000 and 10,000 years ago, Earth passed through the perihelion at the time of summer solstice increasing the amount of solar radiation by about 8%,[33] resulting in the African monsoon becoming both stronger and reaching farther north.[121] The obliquity also decreased during the Holocene[122] but the effect of obliquity changes on the climate is focused on the high latitudes and its influence on the monsoon is unclear.[123]

During summer, solar heating is stronger over the North African land than over the ocean, forming a low pressure area that draws moist air and precipitation in during the summer months[33] from the Atlantic Ocean.[124] This effect was strengthened by the increased summer insolation[125] thus leading to a stronger monsoon that also reached farther north.[122] The strength of this circulation and resulting precipitation can change strongly in response to changes in summer insolation, with effects as far as the subtropics.[14]

Obliquity and precession are responsible for two of the foremost Milankovich cycles and are responsible not only for the onset and cessation of ice ages[126] but also for monsoon strength variations.[123] Southern Hemisphere monsoons are expected to have the opposite response of Northern Hemisphere monsoons to precession, seeing as the insolation changes are reversed; this observation is borne out by data from South America.[127] The precession change increased seasonality in the Northern Hemisphere while decreasing it in the Southern Hemisphere.[122]

Albedo feedbacksEdit

According to climate modelling,[1] orbital changes by themselves cannot increase precipitation over Africa enough to explain the formation of the large desert lakes such as 330,000 square kilometres (130,000 sq mi) Lake Megachad[14] – an expanded Lake Chad[128] which had a size comparable to the Caspian Sea[i][130] – or the northward expansion of vegetation[131][132][122] unless ocean and land surface changes are factored in.[15]

Decreasing albedo resulting from vegetation changes is an important factor in the precipitation increase.[14] Specifically, increased precipitation increases the amount of vegetation; vegetation absorbs more sunlight and thus more energy is available for the monsoon. In addition, evapotranspiration from vegetation adds more moisture, although this effect is less pronounced than the albedo one.[54] Heat fluxes in the soil and evaporation are also altered by the vegetation.[133]

In addition to raw precipitation changes, changes in precipitation seasonality such as the length of dry seasons need to be considered when assessing the effects of climate change on vegetation,[134] as well as the fertilizing effects of increased carbon dioxide concentrations in the atmosphere.[133]

Other sources of albedo changes:

  • Changes in soil properties also result in changes in the monsoon; replacing desert soils with loamy ones results in increased precipitation,[135] and soils that are wet[133] or contain organic matter reflect less sunlight and accelerate the moistening process.[1] Wet soil[136] and desert sand changes also modify the albedo.[133]
  • Albedo changes caused by lakes and wetlands[12] in modelling can alter precipitation.[135]
  • Reduced dust generation from a wetter Sahara influences the climate[137] by reducing the amount of light absorbed by dust and also modifying cloud properties, making them less reflective and more efficient at inducing precipitation.[1][138][139] In climate models reduced amounts of dust in the troposphere together with vegetation changes can[140] often but not always explain the northward expansion of the monsoon;[141] there is not universal agreement on the effects of dust on precipitation in the Sahel, however.[1]

Intertropical Convergence Zone changesEdit

Warmer extratropics during summer may have drawn the Intertropical Convergence Zone northward,[140] and shifts in the Intertropical Convergence Zone may have enhanced precipitation in the sub-Saharan space.[142] Sea surface temperatures off North Africa warmed under orbital effects and also due to weaker trade winds and direct a northward movement of the Intertropical Convergence Zone and increased moisture gradients between land and sea.[143][54] Temperature gradients between a cooler Atlantic in springs and the African continent and between warmer temperatures north of 10° latitude and cooler south of it may have assisted in this change.[144] In Eastern Africa on the other hand Intertropical Convergence Zone changes had relatively little effect on precipitation changes.[145][146] The position of the Intertropical Convergence Zone is also contentious in Arabia.[147]

Precipitation changes in East AfricaEdit

The African humid period that took place in East Africa on the other hand appears to have been caused by different mechanisms,[148] namely decreased seasonality of precipitation[149] due to increased dry season precipitation,[150] increased precipitation or shorter dry seasons[151] and increased inflow of moisture from the Atlantic and Indian Oceans. The Atlantic moisture inflow was in part triggered by a stronger West African and Indian monsoon, perhaps explaining why the effects of the AHP extended into the Southern Hemisphere.[145][152] The effect of the easterly trade winds is unclear; increased moisture transport by easterly trade winds may have aided in the development of the AHP[112] but alternatively a stronger Indian Monsoon that draws easterly winds away from East Africa may have occurred.[153]

Changes in the Congo Air Boundary[j][154] or increased convergence along this boundary may be involved here;[151][154] the Congo Air Boundary would have been shifted east by the stronger westerly winds[152] that are directed by lower atmospheric pressure over Northern Africa[155] and thus allow additional moisture from the Atlantic to reach East Africa.[156] The parts of East Africa that were isolated from Atlantic moisture did not become much wetter during the AHP[95] although at one site of Somalia the seasonality of precipitation decreased.[157]

Different causes for increased humidity in East Africa might have dominated in the early and late AHP,[158][159] and the concept of an "African humid period" extending into this part of Africa has raised criticism.[160] Finally, increased greenhouse gas concentrations may have been involved in directing the onset of the AHP in tropical southeastern Africa;[161] there, orbital changes would be expected to lead to climate variations opposite to these in the Northern Hemisphere[162] but the sparse evidence of the former climate does not entirely match this theory.[163]

Additional factorsEdit

  • Climate change in the far northern latitudes may have contributed to the onset of the AHP.[112] The shrinkage of the Scandinavian and the Laurentide Ice Sheets occurred at the beginning of the AHP.[133] In climate models, a retreat of the ice sheets is often required to simulate the humid period.[164] Their existence might also explain why the AHP did not start immediately with the early insolation peak, as still existing ice sheets would have cooled the climate.[165]
  • Sea surface temperature changes in the Atlantic influence the African monsoon[112] and may have influenced the onset of the AHP. Weaker trade winds and higher insolation would lead to warmer sea surface temperatures, increasing precipitation by increasing moisture gradients between land and sea.[54] Changes in North Atlantic temperature gradients were also involved.[124]
  • Warming of the Mediterranean Sea increases the amount of Sahel precipitation; this effect is responsible for the recent anthropogenic global warming mediated increase in Sahel precipitation.[1] Warmer sea surface temperatures there might also explain the increased precipitation recorded in the Mediterranean during the AHP.[147]
  • Increased precipitation during winter is correlated with a larger extent of Mediterranean precipitation and might have aided in the establishment of the AHP, especially in North Africa,[166][167][168] around the northern Red Sea,[169] in the Tibesti[170][171] and in northern Arabia[147] and generally at higher latitudes where the monsoon did not arrive.[144] This precipitation may have extended to other parts of the Sahara; such would have led to the areas of summer and winter precipitation overlapping[172] and the dry area between the monsoonal and westerlies-influenced climate zones becoming wetter or disappearing altogether.[173] Such changes in Mediterranean-derived precipitation may correlate with changes in the North Atlantic and Arctic Oscillations.[166]
  • Trough-mediated northward transport of moisture during autumn and spring has also been proposed to explain the increased precipitation and its underestimation by climate models.[12] In one climate model, increased northward moisture transport by such troughs increases autumn rainfall in the Sahara, especially in the mid-Holocene and when the climate is already moister than usual there.[174]
  • Weaker subtropical anticyclones were proposed as an explanation during the 1970s-1980s.[175]
  • In montane regions such as the Meidob volcanic field cold temperatures after the last glacial maximum may have reduced evaporation and thus allowed an early onset of humidity.[176]
  • Changes in the Earth's geomagnetic field may be linked to the humidity changes.[177]
  • Increased moisture supply from larger lakes like Lake Megachad may have increased the precipitation, although this effect is probably not adequate to explain the entire AHP wettening.[136] A similar role has been attributed to the extensive wetlands, drainages and lakes in the Eastern Sahara[178] and to the ecosystem in general.[179]
  • Two high elevation winds, the African Easterly Jet and the Tropical Easterly Jet modulate atmospheric air flows over Africa and thus also the amount of precipitation; the Tropical Easterly Jet comes from India and is powered by temperature gradients between the tropics[55] and the subtropics while the African Easterly Jet is powered by temperature gradients in the Sahel.[180] A stronger West African Monsoon resulted in a weaker African Easterly Jet and thus decreased transport of moisture out of Africa.[152]
  • Increased atmospheric carbon dioxide concentrations may have played a role in triggering the AHP,[133] especially its extension across the equator,[181] as well as its resumption after the Younger Dryas and Heinrich event 1 through increased sea surface temperatures.[182]
  • In some parts of the Sahara increased water supply from montane regions may have assisted in the development of moist conditions.[183][184]
  • Larger forests in Eurasia may have led to a northward shift of the ITCZ.[185]
  • Other proposed mechanisms involve convection occurring above the atmospheric boundary layer,[186] increased latent heat fluxes,[138] low pressure in northwestern Africa drawing moisture into the Sahara,[187] changes in the solar cycles[188] and complex atmospheric flow phenomena.[189]

EffectsEdit

 
Vegetation and water bodies in the Eemian (bottom) and Holocene (top)

The African humid period extended over the Sahara as well as eastern,[190][k] southeastern and equatorial Africa. In general, forests and woodlands expanded through the continent.[192] A similar wet episode took place in the tropical Americas,[l] China, Asia,[m][193][194][35][54][195] India,[196] the Middle East and the Arabian Peninsula[193][194][35][54][195] and appears to relate to the same orbital forcing as the AHP.[193] An early Holocene monsoonal episode extended as far as the Mojave Desert in North America.[197] Conversely, 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.[198]

The discharge of the Sanaga River[199] and other rivers[200] in Cameroon,[201] Niger River, Congo River,[199] Nile River,[202] Ntem River[21] and Rufiji River increased[203] and runoff from equatorial Africa, northeastern Africa and the western Sahara was also larger.[204] Changes in the morphology of the river systems and their alluvial plains occurred in response to the increased discharge,[23][21] and the Senegal River breached dunes and re-entered the Atlantic Ocean.[79]

Flora and fauna of the SaharaEdit

During the African humid period, lakes, rivers, wetlands and vegetation including grass and trees covered the Sahara and Sahel[125][205][121] and into the Red Sea Hills,[206] creating a "Green Sahara".[207] Evidence includes pollen data, archeological 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.[34]

 
A present-day savannah, Tarangire National Park, Tanzania

The vegetation cover then extended over almost all of the Sahara[33] and consisted of an open grass savannah with shrubs and trees.[124][208] In general, the vegetation expanded northward[35] to 27-30° northern latitude in West Africa[209][9] with a Sahel boundary at about 23° north,[37] as the Sahara was populated by plants that today often occur about 400 kilometres (250 mi)[210][211]-600 kilometres (370 mi) farther south.[212] The northward movement of vegetation took some time and some plant species moved faster than others.[213]

Forests[214] and plants from the humid tropics were concentrated around lakes and rivers.[215] The landscape during the AHP has been described as a mosaic between various vegetation types of semi-desert and humid origin[216] rather than a simple northward displacement of plant species,[217] and some brown or yellow vegetation communities persisted.[1] Pollen data often show a dominance of grasses over humid tropics trees.[9]

Nevertheless, the climate of the Sahara did not become entirely homogeneous; the central-eastern parts of the desert were probably drier than the western and central sectors[218] and the Libyan sand sea was still a desert[1] although pure desert areas retreated or became arid/semiarid.[219] An arid belt may have existed north of 22° latitude,[220] or the vegetation[131] and the African monsoon might have reached 28-31° northern latitude;[221] in general conditions between 21° and 28° northern latitude are poorly known.[222] 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.[223] In addition, north-south gradations in vegetation patterns have been reconstructed from charcoal and pollen data.[224]

Fossils record changes in the animal fauna of the Sahara.[225] This fauna included antelopes,[33] catfish,[226] clams,[227] crocodiles,[33] elephants, gazelles,[228] giraffes,[33] hartebeest,[226][229] hares,[228] hippos,[226][229] molluscs, Nile perchs,[230] tilapia,[227] turtles[226] and many more animals,[231] and in Egypt spotted hyenas, warthogs, water buffaloes, wildebeest and zebra occurred.[232] Some animals expanded over the whole desert, while others were limited to places with deep water.[230] Earlier humid periods in the Sahara may have allowed species to cross the now-desert.[220] A reduction in open grasslands at the beginning of the AHP may explain a population bottleneck in cheetahs at the start of the humid period,[233] while the humid period led to the expansion of some animal populations such as Hubert's multimammate mouse.[234]

Lakes and rivers of the SaharaEdit

 
Lake Megachad, with present-day Lake Chad highlighted in green

A number of lakes formed[225] or expanded in the Sahara[175] including both mosaic-like lakes and large lakes; the largest of which was Lake Chad[235]/Lake Megachad[128] which increased to at least ten times its present-day size[236] and overflowed into the Niger River[237] during highstand through the Mayo Kebbi and the Benue River, eventually reaching the Gulf of Guinea.[238] This enlarged Lake Chad reached dimensions of 1,000 by 600 kilometres (620 mi × 370 mi) in north-south and east-west direction respectively,[214] covering the Bodélé Depression[239] and perhaps as much as 8% of the present-day Sahara desert.[240] It influenced the climate itself;[241] for example rainfall would be reduced at the centre of the lake and increased at its margins.[1] Lake Chad was possibly fed from the north by rivers draining the Hoggar (Taffassasset drainage)[242] and Tibesti Mountains and from the south by the Chari River-Logone and Komadugu;[238] the Chari River was the main tributary[243] while the rivers draining the Tibesti formed alluvial fans[244]/the Angamma river delta at their entry into the Lake Chad basin.[245] Skeletons of elephants, hippos and hominins have been found in the Angamma river delta on the northern side of then-Lake Megachad, the dominant shoreline feature on its northern side.[214] Older dune systems were submerged by the lake.[246]

Among the lakes which may have formed in the Sahara are Lake Megafezzan in Libya[247] and Lake Ptolemy in Sudan.[143][240][235] In 2018, some doubts have been raised about the size and existence of some of these lakes, however,[248] especially for Lake Megafezzan.[249] Other lakes are known from Adrar Bous in Niger,[79] I-n-Atei in the Hoggar, at Ine Sakane[250] and in Taoudenni[n] in Mali,[252] Chemchane in Mauretania,[253] at Sebkha Mellala close to Ouargla in Algeria,[254] at Bilma, Dibella, Fachi[255] and Gobero in the Ténéré[8] and at "Eight Ridges",[256] El Atrun,[257] Lake Gureinat, Merga,[258] "Ridge",[256] Sidigh,[258] at Wadi Mansurab,[4] Selima and Oyo in Sudan.[259]

In some parts of the Sahara ephemeral lakes formed such as at Bir Kiseiba and Nabta Playa, both in Egypt and both featuring archeological sites,[260] Abu Ballas, Bir Sahara and Bir Tarfawi also in Egypt,[258] which may relate to later Egyptian religions,[261] or swamp-lakes such as at Adrar Bous close to the Air Mountains.[255] In addition, dune-contained ephemeral lakes formed in some areas of the Sahara desert[262] and among the dunes of the last glacial maximum,[79] and a "freshwater archipelago" appears to have existed in the Murzuq basin.[263] All these lake systems left fossils such as fish and limnic sediments.[264] Finally, crater lakes formed in volcanic fields[265] and sometimes survive to this day as smaller remnant lakes such as Malha crater[266] in the Meidob volcanic field.[265] 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.[267]

In addition to lakes, rivers such as the Irharhar in Algeria, Libya and Tunisia[268] and the Sahabi and Kufra rivers in Libya were active during this time[269] although there is some doubt that they carried perennial water;[270] they appear to have been more important in earlier humid periods.[271] Flow also took place in small watersheds,[272] wadis[273] and in rivers discharging into endorheic basins such as Wadi Tanezzuft.[274] In the Air, Hoggar and Tibesti Mountains, the so-called "Middle Terrace" was emplaced at this time.[275] The rivers of the Sahara,[269] lakes and their watersheds may have acted as pathways for the spread of humans and animals;[276] the rivers were often connected between each other through alluvial fans.[269] Proposed examples of animals that spread through rivers are the Nile crocodile and the fish Clarias gariepinus and Tilapia zillii.[223] Some rivers discharging through the Bay of Arguin in Mauretania formed estuaries and mangroves there,[277] such as the large Tamanrasset River[278] which flowing from the Atlas Mountains and Hoggar westward towards the Atlantic[279] once formed the 12th largest watershed in the world[280] and has left a submarine canyon and riverine sediments.[271] Other rivers in the same area also formed submarine canyons,[281] and sediment patterns in marine sediment cores[282] and the occurrence of submarine landslides in the area have been related to the activity of these rivers.[283]

Humans of the SaharaEdit

Conditions and resources were ripe for first hunter-gatherers, fishermen[284] and, later, pastoralists,[285] whose arrival in the Sahara coincided with a time of developing lakes[286] and may have occurred as a migration from either the north (Maghreb or Cyrenaica)[287][288] where the Capsian culture was located,[289] the south (Sub-Saharan Africa), or the east (Nile Valley).[287] Traces of human activity have been found in the Acacus Mountains[290] where caves and rock shelters were used as basecamps for humans,[291] such as the Uan Afuda cave[290] and the Uan Tabu and Takarkori rock shelters.[292] In Takarkori the first occupation took place in the 9th and 10th millennium before present;[293] about five millennia of human cultural evolution are recorded there.[285] 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,[8] and at Lake Ptolemy in Nubia humans settled close to the lake shore, using its resources and perhaps even engaging in leisure activities.[294] Many humans at that time 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"[175][205] although substantial differences between the cultures of various places have been found.[295] The greening of the Sahara led to a demographic expansion[38] and especially in the Eastern Sahara human occupancy coincides with the AHP.[296] Conversely occupation decreased along the Nile valley, perhaps due to expanding wetlands.[297]

Humans were hunting large animals with weapons that have been found in archeological sites[298] and wild cereals occurring in the Sahara during the AHP such as brachiaria, sorghum and urochlea were an additional source of food.[299] Humans also domesticated cattle[49] especially in the more environmentally variable Eastern Sahara,[300] goats and sheep.[301] 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;[284] cattle and goat keeping spread southwestwards from northeasternmost Africa from 8,000 years before present.[302] Dairying has been demonstrated in some locations[303] and cattle-husbandry is supported by the frequent depiction of cattle in rock paintings.[304] The Dufuna canoe, one of the oldest known ships in the world,[305] appears to date to the Holocene humid period and implies that the waterbodies of that time were navigated by humans.[306] In the Acacus Mountains, several cultural horizons known as Early and Late Acacus and Early, Middle, Late and Final Pastoral have been identified[307] while in Niger the Kiffian culture has been related to the beginning of the AHP.[308] Ancient civilizations thrived,[35] with farming and animal husbandry taking place in Neolithic settlements.[253][309] Possibly, the domestication of plants in Africa was delayed by the increased food availability during the AHP, only taking place around 2,500 BC.[310][311]

 
Images of swimming people in the Cave of Swimmers

Humans created rock art such as a number of petroglyphs and rock paintings in the Sahara, perhaps the largest density of such creations in the world.[312] Scenes include animals[121] and everyday life[312] such as swimming which supports the presence of past wetter climates.[267] One well-known such petroglyph location is the Cave of Swimmers in the Gilf Kebir mountains of Egypt;[313] other well known sites are the Gabal El Uweinat mountains also of Egypt,[49] Arabia[314] and the Tassili n'Ajjer in Algeria where a number of rock paintings from this time have been discovered.[315] Humans also left artifacts such as Fesselsteine[o] and ceramics in what today are inhospitable deserts;[49] North Africa together with East Asia is one of the first places where pottery was developed[285] 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;[317] the so-called "wavy line" or "dotted wavy-line" motif was widespread across Northern Africa.[295]

These populations have been described as Epipaleolithic, Mesolithic and Neolithic[318] and produced a variety of lithic tools and other assemblages.[319] Genetic and archeological 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;[320] this may be reflected in the northward spread of Macrohaplogroup L and Haplogroup U6 genomic lineages.[321] In return, the AHP facilitated the movement of some Eurasian populations into Africa.[322] 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,[323] and in the spread of the Nilo-Saharan languages.[223][295]

Additional manifestations in the SaharaEdit

The expanded vegetation stabilized previously active dunes, eventually giving rise to the present-day draa dunes in the Great Sand Sea of Egypt for example,[262] although there is uncertainty about whether this stabilization was widespread.[324] Soil development and biological activity in soils are attested in the Acacus Mountains[325] and the Mesak Settafet area of Libya,[326] but evidence of soil formation[327]/pedogenesis[50] such as the development of bog iron[328] are described from other parts of the Sahara as well.[50] The Central and Southern Sahara saw the development of alluvial deposits[175] while sebkha deposits are known from the Western Sahara.[329] In Northern Africa and in the Sinai Peninsula, deposition of tufa and carbonate sediments within groundwater-fed lakes have been associated with the AHP.[105] Finally, lightning strikes into soil left altered rocks in parts of the Central Sahara.[330]

The increased precipitation also resulted in aquifer recharge[331][318] such as the Nubian Sandstone Aquifer; presently, water from this aquifer maintains several lakes in the Sahara, such as the Lakes of Ounianga.[332] Other groundwater systems were active at that time in the Acacus Mountains, Air Mountains, in the Fezzan[333] and elsewhere in Libya[334] and the Sahel.[335] Raised groundwater tables provided water to plants and was discharged in depressions[336] and valleys, forming widespread carbonate deposits[p] and feeding lakes.[337]

The formation of lakes[61] and vegetation reduced the export of dust from the Sahara, which is reflected in marine cores[338][137] including one core where dust export decreased by almost half.[339] In coastal places, such as in Oman, sea level rise also reduced the production of dust.[61] In the Mediterranean, a decreased dust supply was accompanied by increased sediment input from the Nile, leading to changes in marine sediment composition.[340]

Whether the strengthening of the monsoon enhanced or reduced upwelling off Northwestern Africa is debatable,[341] with some research suggesting that the strengthening in upwelling decreased sea surface temperatures[342][343][344] and increased the biological productivity of the sea,[341] while other research suggests that the opposite occurred; less upwelling with more moisture.[54] However, regardless of whether upwelling increased or decreased, it is possible that either way, the strengthening of the monsoon still provided a boost of productivity to the coasts of Northern Africa because the increased river discharge delivered more nutrients to the sea.[342][343][344]

ArabiaEdit

Precipitation in Dhofar and southwestern Arabia is brought by the African monsoon,[345] and a change to a wetter climate resembling Africa has been noted in southern Arabia[346] and Socotra from cave and river deposits.[347] Paleolakes are recorded at Tayma, Jubbah,[348] in the Wahiba Sands of Oman[349] and Mundafan for the Holocene, and the Wadi ad-Dawasir river system became active again[350][351] with increased river runoff into the Persian Gulf.[352] In the Rub al-Khali lakes formed between 9,000 and 7,000 years ago[353] and dunes were stabilized by vegetation,[101] although the formation of lakes there was less pronounced than in the Pleistocene.[354] Episodes of increased river discharge occurred in Yemen[355] and increased precipitation is recorded in the caves of Hoti, Qunf in Oman, Mukalla in Yemen and Hoq cave in Socotra.[356] Freshwater sources in Arabia during the AHP became focus points of human activity[357] and herding activity between mountains and lowlands occurred.[101] In addition, karstic activity took place on exposed coral reefs in the Red Sea and has left traces to this day.[358] Increased precipitation has been also invoked to explain decreased salinities in the Red Sea.[359]

The humid period in Arabia did not last as long as in Africa,[360] deserts did not retreat as much[194] and precipitation may not have reached the central[361] and northern part of the peninsula[362] past the Yemen Highlands;[191] northern Arabia remained somewhat drier than southern Arabia[363] and still produced dust.[364] One study has estimated that the amount of rainfall in the Red Sea did increase to no more than 1 metre per year (39 in/year).[365] Some former lakes in Arabia have been interpreted to be marshes but these interpretations have been contested.[366]

East AfricaEdit

Nile discharge was higher than today[202] and during the early African humid period, the Nile in Egypt flooded up to 3–5 metres (9.8–16.4 ft)[202] higher than it did recently before flood control;[91] the increased flooding may explain why many archeological sites along the Nile were abandoned.[74][106] Waters from the Nile filled depressions like the Fayum Depression.[274] In addition, Nile tributaries in northwestern Sudan[367] such as Wadi Al-Malik,[202] Wadi Howar[q][369] and Valley of the Queens became active during the AHP.[370] Wadi Howar was active until 4,500 years ago,[369] and at the time often contained dune-dammed lakes, swamps and wetlands;[371][184] it was the largest Saharan tributary of the Nile[372] and constituted an important pathway into sub-Saharian Africa.[202] Conversely it appears that Lake Victoria and Lake Albert were not overflowing into the White Nile for all of the AHP,[373] and the White Nile would have then been sustained by overflow from Lake Turkana instead.[369] 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.[374] 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.[202]

 
Some lakes formed or expanded during the African humid period

Closed lakes in East Africa rose, sometimes by hundreds of metres.[375] Lake Suguta developed in the Suguta Valley, accompanied by the formation of river deltas where rivers such as the Baragoi River entered the lake,[376] and overflowed into the Kerio River, this adding water to Lake Turkana[377] where increased discharge by the Turkwel River led to the formation of a large river delta.[378] In turn, Lake Turkana overflowed on its northwestern side through the Lotikipi Swamp into the White Nile.[379][380] This overflowing large lake was filled with freshwater and was populated by societies that engaged in fishery[381] but probably could also fall back on other resources in the region.[382] Deposits from this lake highstand form the Galana Boi Formation.[295] Other lakes that expanded include Lake Zway and Lake Shala in Ethiopia which joined with Lake Abiyata and Lake Langano to a large waterbody[383] and began overflowing into the Awash River,[384] Lake Hayq also in Ethiopia,[385] and Lake Bogoria, Lake Naivasha,[175] Lake Nakuru/Lake Elmenteita all in Kenya[386] where a lake formed in the caldera of the Menengai volcano.[387] A 1,600 square kilometres (620 sq mi) large and 50 metres (160 ft) deep Lake Magadi formed in the early Holocene,[130] and in the Danakil Depression of Ethiopia freshwater conditions became established.[175] Finally, lakes formed in depressions on the mountains around Lake Kivu.[388]

 
A glacier on Mount Kilimanjaro. The oldest now present ice of Kilimanjaro formed during the African humid period

Glaciers on Mount Kilimanjaro expanded during the AHP[389] after a phase during the Younger Dryas where the mountain was ice free,[390] but the tree line also rose at that time, accompanied by soil formation.[391] The wetter climate may have destabilized the neighbouring Mount Meru volcano, causing a giant landslide that removed its summit.[392]

Erosion in catchments of East Africa increased with the beginning of the humid period but then decreased even before its end,[393] 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.[394] Increased weathering resulted in the increased consumption of atmospheric CO2 during the AHP.[395]

Surprisingly, and contrary to the patterns expected from precessional changes, the East African Rift also featured wetter climates during the AHP,[124] which extended at least as far south as Lake Rukwa and Lake Cheshi into the Southern Hemisphere.[396][397] In the region of the African Great Lakes, pollen evidence points to the occurrence of forests including rainforest vegetation, while today they occur only in limited areas,[398] due to the increased precipitation.[399] Denser vegetation also occurred at Lake Turkana,[400] with wooden vegetation covering almost half of the surface.[401] Different types of vegetation, including dryland vegetation, existed at Lake Malawi and Lake Tanganyika however,[402] and vegetation changes there were not overly strong.[403] 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.[404] Vegetation cover also increased in the Afar region.[405]

In East Africa, the AHP led to improved environmental conditions in terms of food and water supply from large lakes that allowed early human populations to grow in size and survive without requiring major changes in food gathering strategies.[406] Earlier wet and dry periods in East Africa may have influenced the evolution of humans[407] and allowed their spread across the Sahara[408] and into Europe.[409]

Other parts of Africa and the rainforest realmEdit

Lake Bosumtwi in Ghana rose during the AHP,[410] and tropical forests expanded in the Adamawa Plateau of Cameroon[411][412] with an upward migration noted at Lake Bambili also in Cameroon.[413] On Fuerteventura in the Canary Islands there is evidence of a moister climate during the AHP,[414] and recharge groundwater levels have been inferred from Gran Canaria also in the Canary Islands followed by a decrease after its end,[415] and changes in the laurel forests of the Canary Islands have been attributed to the African humid period.[100] The core of the rainforest was probably unaltered by the African humid period, perhaps with some changes in species[416][417] and an expansion of their area,[47] although the peatlands of Central Congo started developing during the African humid period and peat continues to accumulate there to this day,[418] albeit with a slowdown in the Cuvette Centrale after the end of the African humid period.[419]

Levant and MediterraneanEdit

High latitude Africa did not undergo large scale changes in the past 11,700 years,[112] and the Atlas Mountains may have blocked the monsoon from expanding further north.[420] However, cave deposits in southern Morocco,[140] vegetation changes in the Middle Atlas,[421] several floods in Tunisian rivers[422] and ecosystem changes which impacted steppe-dependent rodents of Northern Africa have been linked to the AHP.[423]

In the Pleistocene and Holocene humidity in the Mediterranean is often correlated to humidity in the Sahara,[424][425] and the early-mid Holocene climate of Iberia, Italy, Negev and Northern Africa was wetter than today[426] which in Sicily correlates with Intertropical Convergence Zone changes in Northern Africa.[427] Mediterranean precipitation is brought by mid-latitude cyclones[424] and either increased precipitation from the westerlies[428] or monsoonal precipitation extending into the Mediterranean may have rendered it wetter,[41] although the connection between the African Monsoon and Mediterranean precipitation is unclear.[429][424]

The Mediterranean Sea became less saline during the AHP, in part due to increased precipitation from the westerlies[428] 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.[430][431] The S1 sapropel layer is specifically associated with the AHP[204] and with increased discharge of the Nile and other African rivers.[271] This together with decreased dust transport by wind led to changes in the sediment patterns[432] and an increased marine food web productivity in the Mediterranean,[433] which impacted the development of deep-sea corals there.[434]

In the Levant, wetter conditions during the AHP are recorded from Jeita Cave in Lebanon and Soreq Cave in Israel[435] while the Dead Sea and other southern European lakes were low during this period, unlike some earlier wet periods in the Sahara; possibly the stronger winter-summer insolation gradient in these earlier wet periods played a role in making the behaviour of these water bodies distinct.[436]

Southern AfricaEdit

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,[437] as the Intertropical Convergence Zone should shift its average position between the two hemispheres.[112] 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.[437] More recently obtained paleoclimate data have suggested however that southern Africa was actually wetter during the AHP rather than dryer,[438][439] perhaps reaching as far as north[150] and northwest Madagascar[397] and 23° south[151] and as far as the catchment of the Orange River.[440] Orbitally-mediated changes in Northern Hemisphere climate affected the Southern Hemisphere through oceanic pathways involving sea surface temperatures.[441] Additionally, wetter periods unrelated to the AHP may have occurred after deglaciation in Southern Africa.[442] Particular changes occurred in central southern Africa, where a dry period co-occurred with an expansion of Lake Makgadikgadi; presumably increased wetness over the Okavango River catchment in the Angolan Highlands due to the AHP nourished the lake during a dry interval.[443] Conversely, and consistent with the opposite reaction pattern of the Southern Hemisphere, the Zambezi River reached its lowest discharge during the AHP,[444] and the AHP did not reach southeastern Africa.[445] There may have been opposite changes in precipitation between southeast Africa and tropical East Africa,[446] separated by a "hinge zone".[150] In general there is little consistency between Northern and Southern Africa in terms of hydrological changes during the Holocene.[447]

Numerical estimatesEdit

During the African humid period, Saharan rainfall increased to no more than 500 millimetres per year (20 in/year),[448] with large uncertainty;[185] the increases have been estimated to amount to 300–400 millimetres per year (12–16 in/year),[449] and values exceeding 400 millimetres per year (16 in/year) may have spread to 19-21° northern latitude.[450] In the eastern Sahara, a gradient from 200 millimetres per year (7.9 in/year) to 500 millimetres per year (20 in/year) increment has been identified.[264] An area with less than 100 millimetres per year (3.9 in/year) may have remained in the Eastern Sahara however,[451][452] although its driest parts may have received 20-fold more precipitation.[336]

Other reconstructed values of the precipitation increase indicate an annual increase of about 150–320 millimetres (5.9–12.6 in) in Africa,[453] with strong regional variation.[454] From lake levels precipitation increases of 20-33%[455] or 50-100%[175]/40-150% have been inferred for East Africa,[395] with an increase of 40% reconstructed for Northern Africa.[456] In the early Holocene, there appears to have been an eastward- and northward-decreasing trend of humidity.[457] Additionally, at Tayma in Arabia a threefold increase appears to have occurred[458] while precipitation in the Wahiba Sands of Oman may have reached 250–500 millimetres per year (9.8–19.7 in/year).[459]

GlobalEdit

An increase in atmospheric methane concentrations, detected in Greenland ice cores about 14,700 years ago, was probably a consequence of growing tropical wetlands.[93]

Effect on other climate modesEdit

One climate model has indicated that a greener Sahara and reduced dust output would induce increases tropical cyclone activity, especially over the Atlantic but also in most other tropical cyclone basins due to 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,[460] despite an expected decrease of tropical wave activity over the Atlantic in climate models.[461] While there are no good paleotempestology data for the time of the African humid period that could confirm or refute this theory[462] and many of these records are specific for particular locations,[463] hurricane activity[464] including past strikes in Puerto Rico[465] and in Vieques appear to correlate with the strength of the West African Monsoon.[466] 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[467] and dust emission is not always anti-correlated to hurricane activity.[468] Finally, the northward movement of the Intertropical Convergence Zone during the AHP may have caused a corresponding northward movement of tropical cyclogenesis areas and storm tracks in the Atlantic Ocean,[469] which could also explain decreased hurricane activity in the Bahamas and Dry Tortugas.[467]

The El Nino-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.[470][471] The Green Sahara may have suppressed ENSO activity, forcing a La Nina-like climate state,[465][471] in a climate model this is accompanied by decreased upwelling and deepening of the thermocline in the Eastern Pacific as the Walker circulation shifts westward.[472] In addition Atlantic Nino sea surface temperature patterns develop in the Atlantic Ocean.[473][474]

Remote effects of the AHP on the monsoons of the Northern Hemisphere have also been studied.[475] In climate models, the strengthened and expanding monsoons of Africa and Asia alter the atmospheric circulation of the planet, inducing a wetter East Asian Monsoon and drying across tropical South America and central-eastern North America,[476][477] while the reduced dust emission warms the North Atlantic and increases westerly flow into the North American Monsoon, strengthening it.[475] The far-field precipitation changes reach as far as Europe and Australia.[478] Discrepancies between modelled and reconstructed northward extension[479] and precipitation in the Asian monsoon regions and the North American Monsoon area may be explained through these remote effects.[480]

FluctuationsEdit

 
Temperatures in Greenland during the Younger Dryas

Some gaps with less precipitation took place during the late glacial and the Holocene.[208] During the Younger Dryas 12,500-11,500 years ago, the North Atlantic and Europe became much colder again and there was a phase of drought in the area of the African humid period,[481][482] extending over both East Africa[r] and West Africa and also to India[484] and the Mediterranean[485] where dune activity occurred in the Negev,[486] with lake water levels dropping in much of East Africa[487][488] and drought extending over southern Africa.[489] At the end of the Younger Dryas, precipitation, lake levels and river runoff increased again, although south of the equator the return of humid conditions was slower than the relatively abrupt change to its north.[490][458]

Another dry phase took place about 8,200 years ago, spanning East Africa[159] and Northern Africa[s] as documented by various lines of evidence[492] such as decreased water levels in lakes,[493] accompanied by cooling in the Northern Atlantic,[494] in surrounding landmasses such as Greenland[495] and around the world;[302] it may coincide with the 8.2 kiloyear event.[481] Such abrupt changes have been particularly noticeable in dust records off northwestern Africa, where they took decades to centuries.[496] The 8,200 year event has also been noted in the Maghreb, where it is associated with a transition of the Capsian culture[497] as well as with cultural changes both in the Sahara and the Mediterranean;[291] at the Gobero cemetery a population change occurred after this dry interruption.[498] This episode appears to have been the consequence of ice-dammed lakes in North America draining[499] although a low latitude origin has also been suggested.[500]

Cooling of the Northern Atlantic during Heinrich event 1 and the Younger Dryas associated with a weaker Atlantic meridional overturning circulation leads to atmospheric pressure anomalies that shift the Tropical Easterly Jet and precipitation belts south, making Northern Africa drier. [164][182][501] Earlier Heinrich events were also accompanied by North Africa drought.[44] Likewise, a weakening of moisture transport and a less eastward position of the Congo Air Boundary contributed to reducing precipitation in East Africa[484] although some parts of southern Africa at Lake Malawi were wetter during the Younger Dryas.[502]

Many of these variations in the early Holocene appear to be caused by the discharge of meltwater from the Laurentide Ice Sheet into the Atlantic, which weakens the Atlantic meridional overturning circulation and shift storm tracks north away from the Mediterranean.[503] Some dry periods in marine cores in the Gulf of Guinea appear to coincide with events recorded in Greenland ice cores.[504] Other variations in precipitation observed in records have been attributed to solar activity changes,[11] water levels of Lake Turkana for example appear to reflect the 11-year solar cycle.[505]

In Lake Turkana, water level fluctuations took place between 8,500 and 4,500 years before present, with highstands before 8,400, around 7,000 and between 5,500 and 5,000[506] and lowstands around 8,000, 10,000 and 12,000 years before present.[507] The highstands appear to be controlled by sea surface temperature patterns in the Atlantic and Indian Oceans, but also by overflow of water from Lake Suguta and the Chew Bahir basins into Lake Turkana,[506] which themselves received water from additional lakes.[380] Volcanic and tectonic phenomena occur at Lake Turkana, but do not have the magnitude required to explain large changes in lake level.[508] Water level fluctuations have also been inferred for Lake Chad on the basis of pollen data, especially towards the end of the AHP.[509] In the Taoudenni lake fluctuations of about a quarter-millennium have been recorded[510] and frequent droughts occurred in the Eastern Sahara.[511]

Other variations appear to have occurred 9,500 - 9,000 and 7,400 - 6,800[247] as well as 10,200, 8,200, 6,600 and 6,000 years before present; they were accompanied by decreased population density in parts of the Sahara,[503] and other dry interludes in Egypt have been noted 9,400 - 9,300, 8,800 - 8,600, 7,100 - 6,900 and 6,100 - 5,900 years ago.[512] The duration and severity of dry events is difficult to reconstruct.[302] During dry episodes, humans might have headed to waterbodies which still had resources,[295] and cultural changes in the central Sahara have been linked to some dry episodes.[513] Aside from fluctuations, a southward retreat of the humid period may have been underway after 8,000 years ago[514] with a major drought around 7,800 years ago.[515]

EndEdit

The African humid period ended about 6,000-5,000 years ago[14][516] around 5,500 years before present.[517] After vegetation declined[55] sand claimed the Sahara which became barren,[121] accompanied by increases by wind erosion in northern Africa[518] in dust export from the now-desert[503] and from dried up lakes[519] such as the Bodélé Basin, which today is the largest single source of dust on Earth.[520] The transition from the "green Sahara" to the present-day dry Sahara is considered to be the greatest environmental transition of the Holocene in northern Africa;[521] today almost no precipitation falls in the region.[33] The lakes dried up, mesic vegetation disappeared, and sedentary human populations were replaced by more mobile cultures.[14] The end of the AHP but also its beginning could be considered a "climate crisis" given the strong and extended impact[494] and the drying extended as far as the Canary Islands[522] and southeastern Iran.[523]

The Piora Oscillation cold period in the Alps[524] coincides with the end of the AHP;[323][525] the 5,600–5,000 calibrated years ago period was characterized by widespread cooling and more variable precipitation changes around the world[525] including a cooling of sea surface temperatures on both sides of the North Atlantic,[526] with some climate changes possibly extending into southeastern Australia,[527] Central America[528] and into South America where the neoglacial began.[529]

A major pan-tropical environmental change took place about 4,000 calibrated years ago.[530] This change was accompanied by the collapse of ancient civilizations, severe drought in Africa, Asia and the Middle East and the retreat of glaciers on Mount Kilimanjaro[531] and Mount Kenya.[532]

ChronologyEdit

Whether the drying happened everywhere at the same time and whether it took place in centuries or millennia is unclear[228][35][121] in part due to disagreeing records[221][533] and has led to controversy,[190][210] and such a disagreement on timing also exists with respect to the expected vegetation changes.[154][192] Most recently, the idea has taken hold that the end of the African humid period occurred from north to south in a stepwise fashion[534][535][284] with its end closer to the equator occurring between 4,000 and 2,500 years ago,[98][12] while it suddenly ceased in the western Sahara and east Africa.[536]

A later end in northeast Africa about 4,000 years ago may reflect the different configuration of landmasses and thus monsoon behaviour,[537] while other research has found an east-west moving drying trend.[97] North of the present-day monsoon belt and in the (western) Sahara the drying occurred in one step 6,000-5,000 years ago while south of it precipitation decreased in a more protracted fashion.[12][98][538] Distinct environmental changes may have occurred in Central Africa, Western Africa and East Africa.[210] Later fluctuations in climate that produced brief humid spells also took place,[539] such as a moister period between 500 BCE - 300 CE in Roman Northern Africa and along the Dead Sea[540] and an earlier one 2,100 years before present in the western Sahel.[100]

Some evidence points to a two-phase change in climate with two distinct dry transitions[541] caused by the existence of two different steps of insolation decrease at which climate changes.[542] Finally, sometimes the 4.2 kiloyear event is considered to be the true end of the AHP[500] especially in central Africa.[543]

Marine cores usually indicate an abrupt change[544][118] but not without exceptions[190] while pollen data do not, perhaps due to regional and local differences in vegetation.[545] Furthermore, groundwater and local vegetation can modify local conditions;[284] groundwater-fed water bodies for example persisted longer than those nourished by rain.[230]

Increased variability in precipitation may have preceded the end of the AHP; this is a commonly observed pattern before a sudden change in climate.[546] In Gilf Kebir, between 6,300 and 5,200 years ago apparently a winter rainfall regime became established as the AHP ended.[169]

Sahara and SahelEdit

After a first brief lake level drop between 5,700 and 4,700 calibrated years ago that might reflect climate variability towards the end of the African humid period,[547] water levels in Lake Megachad decreased quickly after 5,200 years before present[548] with a water body persisting in the Bodele depression until 1,000 calibrated years ago at the least.[549]

Lake Chad shrank to about 5% of its former size,[214] with the deeper northern Bodele basin drying up entirely about 2,000 years ago[243] as it was disconnected from the southern basin where its major tributary, the Chari River, enters the lake.[214] The dried out basin was now exposed to the Harmattan winds, which blow dust out of the dry lake bed[550] which is the single largest source of dust in the world.[551] Dunes formed in the dried-up Sahara.[552]

The tropical vegetation was replaced by desert vegetation, in some places suddenly and in others the change was more gradual.[553] Along the Atlantic coast, the vegetation retreat was slowed by a stage of sea level rise that increased soil moisture levels, delaying the retreat by about two millennia.[554][555] In Libya at Wadi Tanezzuft the end of the humid period was also delayed by leftover water in dune systems and in the Tassili mountains until 2,700 years ago, when river activity finally ceased.[64][556] A brief moist pulse between 5,000 - 4,000 years ago in the Tibesti led to the development of the so-called "Lower Terrace".[557] Until 4,200 years ago the Egyptian Sahara might still have been vegetated, based on depictions of savanna environments in Fifth Dynasty tombs in Egypt.[558]

At Lake Yoa, which is groundwater-fed, vegetation decreased and became desert vegetation between 4,700-4,300 and 2,700 years ago, while the lake became hypersaline 4,000 years ago;[559][560][561] however the climate there may have been affected by the Tibesti Mountains and the end of the AHP thus delayed,[548] and fossil groundwater left by the AHP nourishes the lake to this day.[562] In the central Sahara, water resources persisted longer in the mountains.[563]

East Africa and ArabiaEdit

In northern East Africa, water levels dropped rapidly about 5,500 years before present[136] while in Hoti cave in Arabia a southward retreat of the Indian Monsoon took place about 5,900 years ago.[101] Reconstructions from Lake Abiyata in Ethiopia suggest that the end of the African humid period took the form of severe droughts rather than a gradual decrease of precipitation.[564] Drying is also documented from Oman and the Blue Nile basin[107] with a noticeable decrease of Nile discharge about 4,000 calibrated years ago.[432] Decreased discharge of the Nile led to the cessation of sapropel deposition and turbidite activity off its delta,[91] and rivers and lakes of Arabia became intermittent or entirely dry.[565]

Some data from Ethiopia and the Horn of Africa indicate that drying there may have begun already 7,000–8,000 years ago or earlier.[488][347] Drying in Arabia commenced about 7,000 calibrated years ago[357] and there are large disparities in the timing between various parts of Arabia[39] but a tendency towards an arid climate between 6,000 and 5,000 years ago has been observed[566] which continued until 2,700 years ago.[349]

Forest cover in the area of the African Great Lakes decreased between 4,700 and 3,700 years ago,[398] although drying at Lake Victoria began already 8,000 years ago,[402] at Lake Rukwa 6,700 years ago,[396] at Lake Tanganyika about 6,000 years ago[402] and at Lake Edward major changes in lake chemistry consistent with drying are noted 5,200 years ago. There a minor recovery in vegetation took place between 2,500 and 2,000 years ago, followed by a much more rapid appearance of grasses accompanied also by substantial wildfire activity. This might have been the most severe drought of the Lake Edward region in the Holocene, with many lakes such as Lake George dropping significantly or drying up altogether.[567] Other lakes such as Nakuru, Turkana, Lake Chew Bahir, Lake Abbe and Lake Zway also dropped between 5,400–4,200 years ago.[568] Decreased vegetation cover in the catchment of the Blue Nile has been correlated with increased sediment transport in the river beginning 3,600 - 4,000 years ago.[569]

The end of the AHP at Lake Turkana occurred about 5,300 years before present,[t] accompanied by a lake level decline[571] and the cessation of overflow from other lakes in its area into Lake Turkana.[378] Between 5,000 and 4,200, Lake Turkana became more saline and its water levels decreased below the level of outflow to the Nile.[572] Towards the end of the AHP water temperatures in the lake and in other regional lakes appear to have increased, followed by a drop after its end[573] possibly resulting from the insolation seasonality pattern that was in force at the time of the end of the AHP.[574] The decrease of water levels in Lake Turkana also impacted the Nile and the Predynastic societies dependent on it,[575] with Nile flow decreasing.[576]

MediterraneanEdit

Libya and the Middle Atlas became gradually more dry.[553] In Morocco, drying is observed about 6,000 radiocarbon years ago[541] while drier conditions in Iberia accompanied the end of the African humid period between 6,000 and 4,000 years ago perhaps as a consequence of increasingly frequent positive North Atlantic Oscillation episodes and the shift of the Intertropical Convergence Zone.[577][578] More complicated changes have been found for the northern margin of the Mediterranean.[579] A 4.2 kiloyear event is recorded in dust records from the Mediterranean[580] and might have been caused by changes in the Atlantic Ocean circulations.[581]

Tropical West AfricaEdit

In Lake Bosumtwi the African humid period ended about 3,000 years ago, farther north between 5,000 and 6,000 years ago,[121] and at Lake Bosumtwi there was a brief moistening between 5,410 ± 80 years ago that ended 3,170 ± 70 years ago. This, earlier but similar changes off western Senegal and later but similar changes in the Congo Fan appear to reflect a southward shift of the precipitation zone over time.[501] Some drying occurred simultaneously between the Sahel and the Gulf of Guinea.[182] Some lakes in the Guineo-Congolian region dried out, while others were relatively unaffected.[554]

A general tendency towards a drier climate is observed in West Africa when the AHP ended.[582] There, dense vegetation became progressively thinner between 5,000 and 3,000 years ago,[567] with the establishment of aridity between 5,200–3,600 years ago in the Sahara[583] and major perturbations around 4,200 and 3,000–2,500[584]/2,400 calibrated years ago.[585] A brief return of moister conditions took place 4,000 years ago[494] while a substantial dry phase occurred between 3,500 and 1,700 years ago.[582] In Senegal modern-type vegetation arose about 2,000 years ago.[586]

Central AfricaEdit

Farther south at the equator between 6,100 and 3,000 calibrated years before present savannah expanded at the expense of forests, with the transition possibly lasting until 2,500 calibrated years before present;[530] a different time course estimate for the area between 4° southern and 7° northern latitude states that forest cover decreased between 4,500–1,300 years ago.[554] In the Adamawa Plateau (Cameroon[587]), the Ubangui Plateau (Central African Republic[587]) and the Cameroon Volcanic Line montane forests disappeared at the end of the African humid period.[588] In the Adamawa Plateau savannah has continuously expanded since 4,000 calibrated years ago due to an increased establishment of an annual dry season.[588] Such a change took also place in Benin and Nigeria between 4,500 and 3,400 calibrated years ago.[554] Many vegetation changes in the tropical regions were probably caused by a longer dry season[589] and perhaps a smaller latitudinal range of the Intertropical Convergence Zone.[585]

Southern Hemisphere AfricaEdit

In the Southern Hemisphere at Lake Malawi drying began later – 1,000 years before present – as did the African humid period which there began only about 8,000 years ago.[573] Contrarily, increased water levels in Etosha Pan (Namibia) appear to relate to a southward movement of the Intertropical Convergence Zone at the end of the AHP[590] although stalagmite growth data in Dante Cave also in Namibia has been interpreted as indicating a wetter climate during the AHP.[438]

MechanismsEdit

The end of the humid period appears to reflect the changes in insolation during the Holocene,[98] as a progressive decrease of summer insolation caused the insolation gradients between Earth's hemispheres to decrease.[591] However, the drying appears to have been much more abrupt than the insolation changes;[118] It is not clear whether non-linear feedbacks led to abrupt changes in climate or whether the process, driven by orbital changes, was abrupt.[121] Also, the Southern Hemisphere warmed and this resulted in a southward shift of the Intertropical Convergence Zone;[592] orbitally-driven insolation has increased over the Holocene in the Southern Hemisphere.[111]

As precipitation decreased, so did vegetation, in turn increasing the albedo and further decreasing precipitation.[125] Furthermore, vegetation may have responded to increased variations in precipitation towards the end of the AHP[122] although this view has been challenged.[593] This could have directed sudden changes in precipitation, although this view has been cast in doubt by the observation that in many places the end of the African humid period was gradual rather than sudden.[594] There might be differences between plants at higher and lower latitudes, in how they respond to climate change; for example more diverse plant communities might have slowed down the end of the AHP.[70]

Other proposed mechanisms:

  • Decreases in polar insolation through altered cosmic ray fluxes might promote the growth of sea ice and cooling at high latitudes, which in turn result in stronger equator-to-pole temperature gradients, stronger subtropical anticyclones and more intense upwelling in for example the Benguela current.[177]
  • Changes in the circulation of high latitude oceans may have played a role as well,[591] such as the potential occurrence of another meltwater/ice rafting pulse around 5,700 years before present.[592] The decreased insolation during the mid-Holocene may have made the climate system more sensitive to changes, explaining why earlier comparable pulses did not terminate the humid period for good.[595]
  • In climate models, increased snow and ice on the Tibetan Plateau can lead to a weakening of the Indian and African monsoons, with the weakening of the former preceding that of the latter by 1,500–2,000 years.[596] There is evidence that glaciers in Tibet such as at Nanga Parbat expanded during the Holocene, especially towards the end of the AHP.[597]
  • Decreases in sea surface temperatures of the Indian Ocean may be involved in the drying of East Africa, however the temperature records from that ocean are not agreed upon.[156] Moreover, there is no evidence of temperature changes in the Gulf of Guinea at the critical time that might explain the end of the AHP.[136]
  • Additional feedback processes may have included the drying of soils and loss of vegetation after decreased rainfall,[121] which would have led to wind-driven deflation of the soils.[598]
  • An expansion of sea ice around Antarctica about 5,000 calibrated years ago may have provided another positive feedback.[599]
  • The expanding dry belt of the Sahara pushed the regions of cyclogenesis in the Mediterranean northwest-northward, resulting in changes of the wind[600] and precipitation regime in parts of Italy.[601]
  • High latitude climate changes have been proposed as a cause for the end of the AHP. Specifically, about 6,000–5,000 years ago the Arctic became colder, with sea ice expanding, temperatures in Europe and off Northern Africa decreasing and the Atlantic meridional overturning circulation weakening.[136] This cooling tendency may have weakened the Tropical Easterly Jet and thus reduced the amount of precipitation falling over Africa.[602]

The orbitally-induced changes of precipitation may have been modified by the solar cycle; specifically, solar activity maxima during the ending phase of the AHP may have offset the orbital effect and thus stabilized precipitation levels, while solar activity minima compounded the orbital effects and thus induced rapid decreases in lake water levels of Lake Turkana.[603] At Lake Victoria on the other hand, solar variations appear to sometimes lead to drough and sometimes lead to wetness, probably due to changes in the Intertropical Convergence Zone.[592]

Potentially human-mediated changesEdit

About 2,000 years ago, major changes in vegetation in East Africa may have been caused by human activity, including large-scale deforestation for iron production during the Iron Age.[604] Similar changes have been observed on the Adamawa Plateau[605] (Cameroon[587]) but later dating of archeological sites has found no correlation between human expansion in Cameroon and environmental degradation.[606] Similar rainforest degradation across Western African took place between 3,000 and 2,000 years ago[607] and the degradation is also known as "third millennium rainforest crisis".[608] Climate-mediated processes may have increased the impact of land use changes in East Africa.[404] In the Sudanian and Sahelian savannah on the other hand human activity seems to have had little impact,[214] and in Central Africa forest changes were clearly triggered by climate change with little or no evidence of anthropogenic changes.[609] The question has led to intense debate among paleoecologists and archeologists.[610]

While humans were active in Africa during the end of the African humid period, climate models analyzed by Claussen and colleagues 1999 indicate that its end does not need any human activity as an explanation[611] although vegetation changes may have been induced by human activity.[212] Later it was suggested that overgrazing may have triggered the end of the AHP around 5,500 years ago;[284] human influence might explain why the Sahara became a desert without the accompanying onset of an ice age; usually the existence of a Sahara desert is associated with the expansion of high latitude glaciers.[326] Later research has on the contrary suggested that human pastoralism may have actually delayed the end of the AHP by half a millennium[612] as moving herds of animals driven by humans seeking good pasture conditions may lead to more balanced impacts of pastures on the vegetation and thus to greater vegetation quality.[613][614] On the other hand, increased grazing has been invoked to explain the increase in dust emissions after the end of the AHP.[615]

GlobalEdit

A general drying tendency is observed in the northern tropics[616] such as in Asia more generally between 5,000 and 4,500 calibrated years ago,[617] with Asian monsoon precipitation declining between 5,000 and 4,000 years ago.[20] A drought 5,500 years ago is recorded in Mongolia[618] and eastern America, where drought conditions around 5,500–5,000 years ago occurred in places like Florida, New Hampshire and Ontario.[619][620] A drying tendency is also noted in the Caribbean and the Central Atlantic.[621]

Conversely, in South America there is evidence that the monsoon behaves in an opposite fashion consistent with precessional forcing;[616] water levels in Lake Titicaca were low during the middle Holocene and began to rise again after the end of the AHP.[622] Likewise, a trend towards increased wetness took place in the Rocky Mountains at this time[623] although it was accompanied by a drier phase around Lake Tahoe, California and in the Western United States.[624]

ConsequencesEdit

HumansEdit

As observed in archeological sites, population in Northern Africa decreased between 6,300–5,200 years ago[121] over less than a millennium,[598] and in inner Arabia many settlements were abandoned about 5,300 years ago.[130] Some Neolithic people in the desert persisted for longer thanks to the exploitation of groundwater.[541]

Different human populations responded to the drying in diverse manners,[318] with human responses in the Western Sahara being distinct from the reactions in the Central Sahara.[8] In the Central Sahara, pastoralism replaced hunter-gatherer activity[625] and a more nomadic lifestyle replaced semi-sedentary lifestyles[626] as observed in the Acacus Mountains of Libya.[300] Nomadic lifestyles also developed in the Eastern Sahara/Red Sea Hills in response to the end of the AHP.[627] Finally, there was a shift in domestic animal use from cattle to sheep and goats as these are more suitable for arid climates, a change reflected in rock art from which cattle disappeared at this time.[628]

The development of irrigation systems in Arabia may have been an adaptation to the drying tendency.[357] The decreased availability of resources forced human populations to adapt,[629] in general fishing and hunting declined in favour of farming and herding.[630] However, the effects of the end of the AHP on human food production have been subject to controversy.[631]

 
The pyramids of Giza, the most recognizable trace left by the Egyptian civilization

The warm episode and coinciding drought may have triggered animal and human migration to less inhospitable areas[574] and the appearance of pastoralists where previously fishery-dependent societies had existed as happened at Lake Turkana.[381] Humans moved to the Nile, where the society of Ancient Egypt with pharaohs and pyramids was eventually forged by these climate refugees[632][598][633] perhaps reflecting renewed exuberance;[323] thus the end of the AHP can be considered responsible for the birth of Ancient Egypt.[633][1] Lower water levels in the Nile also aided the settlement of its valley as has been observed at Kerma.[634] A similar process may have led to the development of the Garamantian civilization.[635] Such human migrations towards more hospitable conditions along rivers and the development of irrigation also took place along the Euphrates River, Tigris River and Indus River, leading to the development of the Sumerian and Harappan civilizations.[636][70] Population shifts into mountain areas have also been reported for the Air Mountains, Hoggar and Tibesti.[456] In other places, such as the Acacus Mountains populations conversely remained in oases[637][539] and hunter-gatherers also stayed in the Horn of Africa.[158] Herders spread into sub-Saharan Africa while escaping the drying desert.[638]

The Nile itself was not totally unaffected however;[373] the 4.2 kiloyear event[639] and end of the AHP may also be linked to the collapse of the Old Kingdom in Egypt[35] when the Nile floods failed for three decades around 4,160 years before present.[640] The ongoing decrease of precipitation after the end of the AHP may also be the cause of the end of the Akkadian Kingdom in Mesopotamia.[641] The end of the Garamantian civilization may also relate to climate change although other historical events were probably more important;[642] at Tanezzuft oasis it certainly relates to the drying trend after 1,600 years ago.[637]

In Central Africa, forests fragmented and savannahs formed in some places, which facilitated the movement and growth of Bantu speaking populations;[594] these in turn may have affected the ecosystem.[643] The vegetation changes may have aided in the establishment of agriculture.[609] The relatively slow decline of precipitation gave humans more time to adapt to the changing climate conditions.[406]

Cultural changes may also have occurred as a consequence of climate change, such as[644] changes in gender roles, the development of elites,[645] the increased presence of human burials where formerly cattle burials predominated,[646] as well as an increase of monumental architecture in the Sahara may have also been a response to increasingly adverse climates.[625] A spread in cattle domestication at the time of climate change[300] and as herders escaped the drying Sahara southwards[647] may also relate to these events, although its role is controversial.[644] Finally, changes in agricultural practices at the end of the AHP may be associated with the propagation of malaria and one of its causative pathogens Plasmodium falciparum; in turn these may correlate with the origin of human genome variants such as sickle cell disease that are linked to malaria resistance.[648]

Non-humanEdit

In the Sahara, animal and plant populations were fragmented and restricted to certain favoured areas such as moist areas of mountain ranges; this happened for example to fish and crocodiles which only persist in isolated water bodies and to Mediterranean plants[649][650] such as cypresses which persist only in mountains,[651] along with some reptiles that may have also been stranded there by the drying.[652] The buffalo species Syncerus antiquus probably went extinct from the increased competition of pastoralists triggered by the climate drying.[653] Gorilla populations became split in West African and East African populations by the drying of the African Great Lakes region,[399] and a similar population split between the insect species Chalinus albitibialis and Chalinus timnaensis in Northern Africa and the Middle East may have also been caused by the expansion of deserts there.[654] Giraffes, widespread in the Sahara during the AHP, may have been forced to migrate into the Sahel; this together with the separating effect of Lake Megachad may also have influenced the development of giraffe subspecies.[655] Climate change together with human impacts may have led to the extinction of a number of large mammals in Egypt.[656]

The Dahomey Gap[u] formed 4,500–3,200 years before present, correlative to the end of the AHP.[658] The harbour porpoise declined in the Mediterranean due to a switch to oligotrophic conditions as discharge from African rivers decreased.[433] Desert varnish formed on some exposed rocks in the Sahara.[659]

Global climateEdit

The shrinkage of subtropical wetlands probably led to a drop in atmospheric methane concentrations between 5,500 and 5,000 years ago, before boreal wetlands expanded and offset the loss of subtropical wetlands, leading to a return of higher atmospheric methane concentrations.[494] Conversely, atmospheric carbon dioxide decreases in the early Holocene may relate to the vegetation expansion caused by the AHP,[660] its values then increased after about 7,000 years as the biosphere began releasing carbon in response to increasing aridity.[641]

 
Dust originating in the Bodele depression

A sudden increase in the amount of land-originating dust in an oceanic drill core off Cape Blanc, Mauritania, has been interpreted as reflecting the end of the AHP 5,500 years ago occurring in only a few centuries.[661] Potentially, dried up lake basins became an important source for dust.[561] Today, the Sahara is the single largest source of dust in the world, with far ranging effects on climate and ecosystems,[662] such as the growth of the Amazon rainforest.[663]

The period 5,500–5,000 years ago also witnessed major changes in global climate, including the onset of global cooling in the form of the Neoglacial.[664] In one climate model, the desertification of the Sahara at the end of the AHP reduces the amount of heat transported in the atmosphere and ocean towards the poles, inducing cooling of 1–2 °C (1.8–3.6 °F) especially in winter in the Arctic and an expansion of sea ice. Reconstructed temperatures in the Arctic indeed show a cooling, although less pronounced than in the climate model.[665] Further, this climate transition in the climate model is accompanied by increased negative Arctic Oscillation states, a weaker subpolar gyre and increased precipitation and cold air outbreaks in much of Europe; such changes have also been observed in paleoclimate data.[666] These findings imply that the vegetation state of the Sahara influences the Northern Hemisphere climate.[667] In turn, this high latitude cooling may have further reduced precipitation over Africa.[602]

Present-day situationEdit

Presently, the African Monsoon still influences the climate between 5° south and 25° north latitude; the latitudes around 10° north receive the bulk of their precipitation from the monsoon[v] during summer, with smaller amounts of rainfall occurring farther north. Thus farther north deserts can be found while the moister areas are vegetated.[122] In the Central Sahara, annual precipitation reaches no more than 50–100 millimetres per year (2.0–3.9 in/year).[669] Even farther north, the margin of the desert coincides with the area where the westerlies bring precipitation;[2] they also influence southernmost Africa.[670] The existence of the deserts is because of the subsidence of air over parts of Northern Africa, which is further increased by the radiative cooling over the desert.[1] In East Africa the monsoon leads to two rain seasons in the equatorial area, the so-called "long rains" in March–May and the "short rains" in October–November[671] when the Intertropical Convergence Zone moves northward and southward over the region, respectively;[672] in addition to the Indian Ocean-sourced precipitation there is also Atlantic- and Congo-sourced precipitation west of the Congo Air Boundary.[668][671] The Atlantic Ocean is also the source of monsoon rainfall for the Sahel.[3] Climate variability exists to this day, with the Sahel suffering from droughts in the 1970s and 1980s when precipitation decreased by 30% and the flow of the Niger River and Senegal River even more,[673] followed by an increase of precipitation.[1] In Arabia, the monsoon does not penetrate far from the Arabian Sea and some areas are under the influence of winter precipitation brought by cyclones from the Mediterranean Sea.[674] East Africa is also under the influence of monsoon circulations.[675]

Implications for future global warmingEdit

 
Greening of the Sahel between 1982 and 1999

Some simulations of global warming and increased carbon dioxide concentrations have shown a substantial increase in precipitation in the Sahel/Sahara. This could lead to an expansion of vegetation into present-day desert, although it would be less extensive than during the mid-Holocene[119] and perhaps accompanied by a northward shift of the desert, i.e. a drying of northernmost Africa.[676] Such a precipitation increase may also reduce the amount of dust originating in Northern Africa,[677] with effects on hurricane activity in the Atlantic and increased threats of hurricane strikes in the Caribbean, the Gulf of Mexico and the East Coast of the United States of America.[462]

The Special Report on Global Warming of 1.5 °C and the IPCC Fifth Assessment Report indicate that global warming will likely result in increased precipitation across most of East Africa, parts of Central Africa and the principal wet season of West Africa, although there is significant uncertainty related to these projections especially for West Africa.[678] On the other hand, West Africa[679] and parts of East Africa may become drier during given seasons and months.[679][680] In addition, the end of the 20th century drying trend may be due to global warming.[680] Currently, the Sahel is becoming greener but precipitation has not fully recovered to levels reached in the mid-20th century.[676]

Climate models have yielded equivocal results about the effects of anthropogenic global warming on the Sahara/Sahel precipitation. Human-caused climate change occurs through different mechanisms than the natural climate change that led to the AHP.[681] One study in 2003 showed that vegetation intrusions in the Sahara can occur within decades after strong rises in atmospheric carbon dioxide[682] but would not cover more than about 45% of the Sahara.[37] That climate study also indicated that vegetation expansion can only occur if grazing or other perturbations to vegetation growth do not hamper it.[683]

A greening of the Sahara on the one hand may allow agriculture and pastoralism to expand into hitherto unsuitable areas, but increased precipitation can also lead to increased water borne diseases and flooding.[684] Also, expanded human activity may be vulnerable to climate reversals as demonstrated by the droughts that followed the mid-20th century wet period.[685]

See alsoEdit

NotesEdit

  1. ^ Other terms that have been applied to the Holocene AHP or correlative climate phases are "Holocene humid period", which also covers an analogous episode in Arabia and Asia;[20] "Holocene Pluvial";[21] "Holocene Wet Phase";[22] "Kibangien A" in Central Africa;[23] "Makalian" for the Neolithic period of northern Sudan;[24] "Nabtian Wet Phase"[25] or "Nabtian period" for the 14,000–6,000 humid period over the Eastern Mediterranean and Levant;[26] "Neolithic pluvial";[27] "Neolithic Subpluvial";[22] "Nouakchottien" of the Western Sahara 6,500 - 4,000 years before present;[28] and "Tchadien" in the Central Sahara 14,000 - 7,500 years before present.[28]
  2. ^ The hypsithermal, which coincides with the AHP,[38] has been recorded from Arabia,[39] the Caribbean[40] and the Mediterranean.[41]
  3. ^ Active dunes also formed in Arabia, Israel[61] and the exposed seafloor of the Persian Gulf[62] where dust generation increased.[48]
  4. ^ Dune-covered areas.[76]
  5. ^ However, some lakes persisted in areas where colder temperatures had decreased evaporation.[31]
  6. ^ Earlier it was thought that it had started about 9,000 years ago, before it was found that it probably began earlier and was interrupted by the Younger Dryas,[54] an interpretation that is subject to controversy.[95] Some lake level curves indicate a stepwise increase of lake levels 15,000 ± 500 and 11,500–10,800 years ago, before and after the Younger Dryas.[96]
  7. ^ Whether it first commenced in the eastern Sahara or not is unclear.[97]
  8. ^ This was originally believed to have occurred 7,000 or 13,000 years before present,[94] but a more recent suggestion indicates a reconnection of the Nile 14,000–15,000 years ago.[104]
  9. ^ Today's largest lake.[129]
  10. ^ The Congo Air Boundary is the point at which moisture bearing winds from the Indian Ocean collide with these from the Atlantic Ocean.[151]
  11. ^ Including the Afar region.[191]
  12. ^ In the Caribbean, a wet period has been identified in the mid-Holocene which correlated with the African wet period and was preceded and followed by drier conditions.[40]
  13. ^ Where the Indian Summer Monsoon penetrated farther inland[11] and was more intense starting about 14,800 years ago.[89]
  14. ^ Salt deposits left there were mined beginning in the 16th century.[251]
  15. ^ Fesselsteine are stony artifacts, that are interpreted as tools for restraining animals.[316]
  16. ^ In the form of calcretes, "lake chalks", rhizoliths, travertines and tufa.[337]
  17. ^ Also known as the Yellow Nile[368]
  18. ^ There is conflicting evidence on whether the Younger Dryas was wetter or drier in tropical southeastern Africa.[483]
  19. ^ Whether it also took place in Asia is unclear; perhaps it was too short to trigger climate changes recognizable in records.[491]
  20. ^ Three different conceptual ideas for its termination exist.[570]
  21. ^ The Dahomey Gap is a region without forests in southern Benin, Ghana and Togo[657] that forms a gap in the Guineo-Congolian forest belt.[554]
  22. ^ The main area of monsoon rains does not coincide with the Intertropical Convergence Zone.[668]

ReferencesEdit

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  2. ^ a b c Hoelzmann & Holmes 2017, p. 3.
  3. ^ a b c d McCool 2019, p. 5.
  4. ^ a b Dawelbeit, Jaillard & Eisawi 2019, p. 12.
  5. ^ Wendorf, Karlén & Schild 2007, p. 190.
  6. ^ Timm et al. 2010, p. 2612.
  7. ^ Hoelzmann et al. 2001, p. 193.
  8. ^ a b c d Stivers et al. 2008, p. 2.
  9. ^ a b c Watrin, Lézine & Hély 2009, p. 657.
  10. ^ Lézine, Duplessy & Cazet 2005, p. 227.
  11. ^ a b c Junginger et al. 2014, p. 1.
  12. ^ a b c d e f Skinner & Poulsen 2016, p. 349.
  13. ^ Hopcroft et al. 2017, p. 6805.
  14. ^ a b c d e f Menocal et al. 2000, p. 348.
  15. ^ a b c d e Peck et al. 2015, p. 140.
  16. ^ Hoelzmann & Holmes 2017, p. 11.
  17. ^ a b Krüger et al. 2017, p. 1.
  18. ^ Sangen 2012, p. 144.
  19. ^ Médail et al. 2013, p. 1.
  20. ^ a b Lézine et al. 2017, p. 68.
  21. ^ a b c Runge 2013, p. 81.
  22. ^ a b Olsen 2017, p. 90.
  23. ^ a b c d Sangen 2012, p. 213.
  24. ^ Spinage 2012, p. 71.
  25. ^ Said 1993, p. 128.
  26. ^ Revel et al. 2010, p. 1357.
  27. ^ Brass, Michael (1 March 2018). "Early North African Cattle Domestication and Its Ecological Setting: A Reassessment". Journal of World Prehistory. 31 (1): 86. doi:10.1007/s10963-017-9112-9. ISSN 1573-7802.
  28. ^ a b Baumhauer & Runge 2009, p. 10.
  29. ^ a b Sangen 2012, p. 211.
  30. ^ Soriano et al. 2009, p. 2.
  31. ^ a b Pachur & Altmann 2006, p. 32.
  32. ^ Sepulchre et al. 2008, p. 42.
  33. ^ a b c d e f g h i Menocal et al. 2000, p. 347.
  34. ^ a b Quade et al. 2018, p. 1.
  35. ^ a b c d e f g Costa et al. 2014, p. 58.
  36. ^ Blanchet et al. 2013, p. 98.
  37. ^ a b c Petoukhov et al. 2003, p. 99.
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