African humid period

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(Redirected from
Green Sahara
)

desert
during the African humid period. Instead, most of northern Africa was covered by grass, trees, and lakes.

The African humid period (AHP; also known by

African monsoon; and increased greenhouse gases
. During the preceding
8.2 kiloyear event. The African humid period ended 6,000–5,000 years ago during the Piora Oscillation cold period. While some evidence points to an end 5,500 years ago, in the Sahel, Arabia and East Africa, the end of 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

Nile Valley and Mesopotamia, where they gave rise to early complex societies
.

Research history

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

leaf wax in the sea and wetlands playing an important role.[2][8] Pollen, lake deposits and former levels of lakes have been used to study the ecosystems of the African humid period,[9] and charcoal and leaf impressions have been used to identify vegetation changes.[10] Questions in AHP research are its beginning, cause, intensity, end, land feedbacks and fluctuations during the AHP.[11] The time 6,000 years ago has received particular attention, especially since that period of the AHP has been used as an experiment in the Paleoclimate Modelling Intercomparison Project.[12] Most recently, the effects of the Sahara greening on other continents has drawn scientific attention.[13] The concept of a Sahara significant different than today, and the rich record it left, has driven the imagination of the public and scientists alike.[11]

Research issues

While the precipitation changes since the last glacial cycle are well established, the magnitude and timing of the changes are unclear.

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.[21] Most records of Holocene precipitation in eastern Africa come from low altitudes.[22]

Terminology

The term "African humid period" was coined in 2000 by deMenocal et al.[23] Earlier humid periods are sometimes known as "African humid periods"[24] and a number of dry/wet periods have been defined for the Central Africa region.[25] In general, these types of climate fluctuations between wetter and drier periods are known as "pluvials" and "interpluvials", respectively.[26] Because the AHP did not affect all of Africa, there have been calls to not use the term[27] also because it has supposedly colonial implications,[28] and some researchers have specified "North African humid period" and "Northern African humid period".[29]

Other terms that have been applied to the

before present in the Lake Chad area.[47]

Background and beginning

The African humid period took place in the late

Holocene Thermal Maximum.[54][a] Liu et al. 2017[57] subdivided the humid period into an "AHP I" which lasted until 8,000 years ago, and an "AHP II" from 8,000 years onward,[58] with the former being wetter than the latter.[59]

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,

glacial stages correlate to dry periods;[24] they occur during precession minima, unless large ice sheets or insufficient greenhouse gas concentrations suppress their onset.[66]

The Bølling–Allerød warming appears to be synchronous with the onset of the African humid period[67][68][69] as well as to increased humidity in Arabia.[70] Later, in the Blytt–Sernander sequence the humid period coincides with the Atlantic period.[71]

Conditions before the African humid period

African vegetation during the last glacial maximum

During the

savannah landscapes as temperatures, rainfall, and humidity decreased.[44][81][82]

There is little and often equivocal evidence of human activity in the Sahara or Arabia at that time, reflecting its drier nature;

Before the onset of the AHP, it is thought that Lake Victoria, Lake Albert, Lake Edward,[91] Lake Turkana[92] and the Sudd swamps had dried out.[93] The White Nile had become a seasonal river[93] whose course[94] along with that of the main Nile may have been dammed by dunes.[95] 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[c] farther east.[97] Other lakes across Africa, such as Lake Chad and Lake Tanganyika, also had shrunk[d] during this time,[98] and both the Niger River and Senegal River were stunted.[99]

Early humidity increases

Whether some parts of the desert such as highlands like the

Jebel Marra mountains[103][104] and residual Nile flow may be explained in this way.[105] The highlands of Africa appear to have been less affected by drought during the last glacial maximum.[106]

The end of the glacial drought occurred between 17,000 and 11,000 years ago,[104] with an earlier beginning noted in the Acacus[18] and Saharan mountains[107][81] 26,500 - 22,500[18] and (possibly) 18,500 years ago, respectively.[108] In southern and central Africa earlier starts 17,000 and 17,500 years ago, respectively, may be linked to Antarctic warming,[109][35] while Lake Malawi appears to have been low until about 10,000 years ago.[110]

High lake levels occurred in the Jebel Marra and

glaciation in the High Atlas mountains took place at the same time as the Younger Dryas and early African humid period.[112] Around 14,500 years ago, lakes started to appear in the arid areas.[113]

Onset

The humid period began about 15,000[109][114]-14,500 years ago.[e][48] The onset of the humid period took place almost simultaneously over all of Northern[f] and Tropical Africa,[118] with impacts as far as Santo Antão on Cape Verde.[119][120] In Arabia, wet conditions apparently took about two millennia to advance northward,[117][121] a gradual advance is supported by tephrochronological data.[122] Likewise, in the Sahara there might have been a delay of about a millennium between the onset of the AHP and the full establishment of humid conditions, as vegetation growth and the filling of river systems took time.[123]

Lake Victoria reappeared and overflowed;[113] Lake Albert also overflowed into the White Nile[111] 15,000–14,500 years ago[91] and so did Lake Tana, into the Blue Nile.[111] The White Nile flooded part of its valley[124] and reconnected to the main Nile.[114][g] In Egypt widespread flooding by the "Wild Nile" took place;[111] this "Wild Nile" period[126] led to the largest recorded floods on this river,[95] sedimentation in floodplains,[127] and probably also impacted human populations along the river.[128] 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.[129] In the East African Rift water levels in lakes began to rise by about 15,500/15,000[130]-12,000 years ago;[131] Lake Kivu began overflowing into Lake Tanganyika by about 10,500 years ago.[132]

About the same time that the AHP started, the cold glacial climate in Europe associated with Heinrich event 1 ended[113] with climate changing as far as Australasia.[111] A warming and retreat of sea ice around Antarctica coincides with the start of the African humid period,[133] although the Antarctic Cold Reversal also falls into this time[35] and may relate to a drought interval recorded in the Gulf of Guinea.[134]

Causes

The African humid period was caused by a stronger

North Atlantic and the Mediterranean Sea towards the Mediterranean coasts of Africa.[136][137] There were complex interactions with the atmospheric circulation of the extratropics and between moisture coming from the Atlantic Ocean and the Indian Ocean,[138] and an increased overlap between the areas wetted by the monsoon and those wetted by extratropical cyclones.[139]

Climate models indicate that changes from a dry to a green Sahara and back have threshold behaviour, with the change occurring once a certain level of insolation is exceeded;[140] likewise, a gradual drop of insolation often leads to a sudden transition back to a dry Sahara.[141] This is due to various feedback processes which are at work,[20] and in climate models there is often more than one stable climate-vegetation state.[142] Sea surface temperature and greenhouse gas changes synchronized the beginning of the AHP across Africa.[118]

Orbital changes

Milankovich cycles over the past one million years

The African humid period has been explained by increased

obliquity also decreased during the Holocene[145] but the effect of obliquity changes on the climate is focused on the high latitudes and its influence on the monsoon is unclear.[146]

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[48] from the Atlantic Ocean.[147] This effect was strengthened by the increased summer insolation,[148] leading to a stronger monsoon that also reached farther north.[145] The effects of these circulation changes reached as far as the subtropics.[19]

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[149] but also for monsoon strength variations.[146] Southern Hemisphere monsoons are expected to have the opposite response of Northern Hemisphere monsoons to precession, as the insolation changes are reversed; this observation is borne out by data from South America.[150] The precession change increased seasonality in the Northern Hemisphere while decreasing it in the Southern Hemisphere.[145]

Albedo feedbacks

According to

Lake Megachad[h][19] or the northward expansion of vegetation[154][155][145] unless ocean and land surface changes are factored in.[20]

Decreasing albedo resulting from vegetation changes is an important factor in the precipitation increase.[19] 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 effect.[72] Heat fluxes in the soil and evaporation are also altered by the vegetation.[156]

Reduced dust generation from a wetter Sahara,[157] where major dust-generating regions were submerged by lakes, influences the climate[158] by reducing the amount of light absorbed by dust. Decreased dust emissions also modify cloud properties, making them less reflective and more efficient at inducing precipitation.[1][159][160] In climate models, reduced amounts of dust in the troposphere together with vegetation changes can[161][162] often but not always explain the northward expansion of the monsoon.[163][164] There is not universal agreement on the effects of dust on precipitation in the Sahel, however,[1] in part because the effects of dust on precipitation may be dependent on its size.[165]

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,[166] as well as the fertilizing effects of increased carbon dioxide concentrations in the atmosphere.[156]

Other sources of albedo changes:

  • Changes in soil properties result in changes in the monsoon; replacing desert soils with loamy ones results in increased precipitation,[167] and soils that are wet[156] or contain organic matter reflect less sunlight and accelerate the moistening process.[1] Desert sand changes also modify the albedo.[156]
  • Albedo changes caused by lakes and wetlands[16] can alter precipitation in climate models.[167]

Intertropical Convergence Zone changes

Warmer extratropics during summer may have drawn the Intertropical Convergence Zone (ITCZ) northward[161] by about five or seven degrees latitude,[168] resulting in precipitation changes.[169] Sea surface temperatures off North Africa warmed under orbital effects and through weaker trade winds, leading to a northward movement of the ITCZ and increasing moisture gradients between land and sea.[72] Two temperature gradients, one between a cooler Atlantic during spring and an already warming African continent, the other between warmer temperatures north of 10° latitude and cooler south, may have assisted in this change.[170] In Eastern Africa, ITCZ changes had relatively little effect on precipitation changes.[171][172] The past position of the ITCZ in Arabia is also contentious.[173]

Precipitation changes in East Africa

The African humid period that took place in

Indian Monsoon that draws easterly winds away from East Africa may have occurred.[179]

Changes in the Congo Air Boundary[i][180] or increased convergence along this boundary may have contributed;[177][180] the Congo Air Boundary would have been shifted east by the stronger westerly winds[178] directed by lower atmospheric pressure over Northern Africa,[181] allowing additional moisture from the Atlantic to reach East Africa.[182] The parts of East Africa that were isolated from Atlantic moisture did not become significantly wetter during the AHP[115] although at one site in Somalia the seasonality of precipitation may[183] or may not have decreased.[184]

Various contributing factors may have led to the increased humidity in East Africa, not all of which were necessarily operating simultaneously during the AHP.[185][186] That the "African humid period" reached this part of Africa has been doubted.[187] Finally, increased greenhouse gas concentrations may have been involved in directing the onset of the AHP in tropical southeastern Africa;[188] there, orbital changes would be expected to lead to climate variations opposite to those in the Northern Hemisphere.[189] The pattern of humidity changes in south-eastern Africa are complex.[190]

Additional factors

Effects

Eemian (bottom) and Holocene
(top)

The African humid period extended over most of Africa:

Atacama were lower.[239]

The discharge of the

Ntem,[33] Rufiji,[242] and Sanaga rivers increased.[240] Runoff from Algeria,[243] equatorial Africa, northeastern Africa and the western Sahara was also larger.[244] Changes in the morphology of the river systems and their alluvial plains occurred in response to the increased discharge,[35][33] and the Senegal River expanded its riverbed,[245] breached dunes and re-entered the Atlantic Ocean.[99]

Flora and fauna of the Sahara

During the African humid period, lakes, rivers,

river valleys, organic-rich mats, mudstones, evaporites as well as travertines and tufas deposited in subaqueous environments.[49]

A present-day savannah, Tarangire National Park, Tanzania

The vegetation cover then extended over almost all of the Sahara[48] and consisted of an open grass savannah with shrubs and trees,[147][249] with a moist savanna vegetation getting established in the mountains.[250] In general, the vegetation expanded northward[50] to 2730° northern latitude in West Africa[251][10] with a Sahel boundary at about 23° north,[53] as the Sahara was populated by plants that today often occur about 400–600 kilometres (250–370 mi)[252][253] farther south.[254] The northward movement of vegetation took some time and some plant species moved faster than others.[255] Plants that perform C3 carbon fixation became more common.[256] The fire regime of the vegetation changed;[257] in the desert the expansion of vegetation facilitated fire activity, while in the savanna the increased prevalence of woody vegetation reduced fire activity.[258]

Forests and plants from the humid tropics were concentrated around lakes, rivers[259] and the Atlantic Ocean coast of Senegal;[260] waterbodies were also settled by aquatic and partially aquatic plants[261] and the Senegalese coast by mangroves.[224] The landscape during the AHP has been described as a mosaic between various vegetation types of semi-desert and humid origin[262] rather than a simple northward displacement of plant species,[263] and some brown or yellow vegetation communities persisted.[1] There was no southward displacement of Mediterranean plants during the Holocene[264] and on the Tibesti Mountains cold temperatures may have restricted the expansion of tropical plants.[265] Pollen data often show a dominance of grasses over humid tropics trees.[10] The tree Lophira alata and others may have spread out of the African forests during the AHP,[266] and the Lactuca plants may have split into two species under the effects of the AHP and other climate changes in Africa during the Holocene.[267]

The Sahara climate did not become entirely homogeneous; its central-eastern parts were probably drier than the western and central sectors

sediment cores.[273] In addition, north–south gradations in vegetation patterns have been reconstructed from charcoal and pollen data.[274]

snake-eagles,[280] snakes,[282] tilapia,[279] toads,[282] turtles[277] and many more animals,[286] and in Egypt there were African buffaloes, spotted hyenas, warthogs, wildebeest and zebra.[287] Additional birds include brown-necked raven, coot, common moorhen, crested grebe, glossy ibis, long-legged buzzard, rock dove, spur-winged goose and tufted duck.[288] Large herds of animals lived in the Sahara.[289] Some animals expanded over the whole desert, while others were limited to places with deep water.[284] Earlier humid periods in the Sahara may have allowed species to cross the now-desert.[270] A reduction in open grasslands at the beginning of the AHP may explain the decline of the populations of some mammals during[290] and a population bottleneck in cheetahs at the start of the humid period,[291] while leading to the expansion of the population of other animals such as Hubert's multimammate mouse[292] and Natal multimammate mouse.[293]

Lakes and rivers of the Sahara

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

A number of lakes formed

Mayo Kebbi and the Benue River, eventually reaching the Gulf of Guinea.[302] Older dune systems were submerged by Lake Chad.[307]

Among the large

Great Western Erg[319] and Sebkha Mellala close to Ouargla, both in Algeria,[320] at Wadi Shati and elsewhere in the Fezzan in Libya,[321] at Bilma, Dibella, Fachi[322] and Gobero in the Ténéré,[9] Seeterrassental in Niger[323] and at "Eight Ridges",[324] El Atrun,[325] Lake Gureinat, Merga,[326] "Ridge",[324] Sidigh,[326] at Wadi Mansurab,[4] Selima and Oyo in Sudan.[327] The lakes of Ounianga merged into two large lakes[328] and overflowed, either above surface or underground.[329] Mosaics of small lakes developed in some regions,[310] such as the Grand Erg Occidental.[330] Wetlands also expanded during the AHP, but both their expansion and subsequent retreat were slower than that of lakes.[331] The Niger River, which had been dammed by dunes during the LGM, formed a lake in the Timbuktu region that eventually overflowed and drained at some point during the AHP.[332]

In some parts of the Sahara

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.[343]

Increased precipitation resulted in the formation or reactivation of river systems in the Sahara.

sediment cores[351] and the occurrence of submarine landslides in the area have been related to the activity of these rivers.[352]

Rivers such as the

Tilapia zillii.[273] It is possible that the name Tassili n'Ajjer, which means "plateau of the rivers" in Berber, is a reference to past river flows.[364] On the other hand, intense flows of these rivers may have made their shores dangerous to humans and thus created additional impetus for human movement.[365][366] Now-dry river valleys from the AHP in the eastern Sahara have been used as analogues for former river systems on Mars.[367]

Humans of the Sahara

Conditions and resources were ripe for first

demographic expansion[382] and especially in the Eastern Sahara human occupancy coincides with the AHP.[383] Conversely occupation decreased along the Nile valley, perhaps due to the expansion of wetlands there[384] and frequent large-scale flooding of the Nile delta.[385]

Humans were hunting large animals with weapons that have been found in archaeological sites

Images of swimming people in the Cave of Swimmers

Humans created

BC;[409] the so-called "wavy line" or "dotted wavy-line" motif was widespread across Northern Africa[381] and as far as Lake Turkana.[410]

These populations have been described as

Additional manifestations in the Sahara

The expanded vegetation and

sebkha deposits are known from the Western Sahara.[428] Lightning strikes into soil left lightning-altered rocks in parts of the Central Sahara.[429]

The Lakes of Ounianga are recharged from fossil groundwater that originated partly during the AHP

The increased precipitation also resulted in recharged

Air Mountains, in the Fezzan[432] and elsewhere in Libya[433] and the Sahel.[434] Raised groundwater tables provided water to plants and was discharged in depressions,[435] lakes[127] and valleys, forming widespread carbonate deposits[p] and feeding lakes.[436]

The formation of lakes

marine cores,[437][157] including one core where dust export decreased by almost half,[438] and in Italian lakes.[439] In coastal places, such as in Oman, sea level rise also reduced the production of dust.[79] In the Mediterranean, a decreased dust supply was accompanied by increased sediment input from the Nile, leading to changes in marine sediment composition.[440] Conversely, the increased vegetation may have yielded more volatile organic compounds in the air.[441]

Whether the strengthening of the monsoon enhanced or reduced upwelling off Northwestern Africa is debatable,[442] with some research suggesting that the strengthening in upwelling decreased sea surface temperatures[443][444][445] and increased the biological productivity of the sea,[442] while other research suggests that the opposite occurred; less upwelling with more moisture.[72] However, regardless of whether upwelling increased or decreased, it is possible that the strengthening of the monsoon boosted productivity off the coasts of Northern Africa because the increased river discharge delivered more nutrients to the sea.[443][444][445] The decline of dust input may have caused the cessation of deep-water coral growth in the eastern Atlantic during the AHP by starving them of nutrients.[446]

Arabia

Precipitation in

accumulation terraces.[460] Episodes of increased river discharge occurred in Yemen[461] and increased precipitation is recorded in the caves of Hoti, Qunf in Oman, Mukalla in Yemen and Hoq Cave in Socotra.[462] Increased precipitation resulted in increased groundwater flow, generating groundwater-fed lakes and carbonate deposits.[463]

Forests and wildfire activity expanded across parts of Arabia.[464] Freshwater sources in Arabia during the AHP became focus points of human activity[465] and herding activity between mountains and lowlands occurred.[121] In addition, karstic activity took place on exposed coral reefs in the Red Sea and traces of it are still recognizable today.[466] Increased precipitation has been also invoked to explain decreased salinities in the Red Sea,[467] increased sedimentation[468] and increased river inflow, while dust input decreased.[469] Rock art depicts wildlife that existed in Arabia during the humid period.[470] Archaeological sites such as cairns appeared with the beginning of the humid period.[471]

The humid period in Arabia did not last as long as in Africa,[472] deserts did not retreat as much[233] and precipitation may not have reached the central[473] and northern part of the peninsula[474] past Oman[463] and the Yemen Highlands;[475] northern Arabia remained somewhat drier than southern Arabia,[476] droughts were still common[477] and the land and still produced dust.[478] 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).[479] Whether some former lakes in Arabia were actually marshes is contentious.[480]

East Africa

Fayum Depression[358] to form a deep lake with anoxic bottom waters[482] and reaching 20 metres (66 ft) above sea level,[483] probably once a geomorphic barrier was breached.[484] Wetlands and anastomosing channels developed in the Nile Delta[485] as sediment supply increased.[486] In addition, Nile tributaries in northwestern Sudan[487] such as Wadi Al-Malik,[241] Wadi Howar[r][489] and Valley of the Queens became active during the AHP[490] and contributed sediments to the Nile.[491] Wadi Howar was active until 4,500 years ago,[489] and at the time often contained dune-dammed lakes, swamps and wetlands;[492][222] it was the largest Saharan tributary of the Nile[493] and constituted an important pathway into sub-Saharian Africa.[241] Conversely it appears that Lake Victoria and Lake Albert were not overflowing into the White Nile for all of the AHP,[494] and the White Nile would have been sustained by overflow from Lake Turkana.[489] 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.[495] 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.[241]

Some lakes formed or expanded during the African humid period

freshwater and was populated by humans,[507] typically in bays, along capes and protected shorelines;[508] the societies there engaged in fishery[507] but could probably also fall back on other resources in the region.[509]

The Ethiopian

Lake Abhe expanded to cover an area of 6,000 square kilometres (2,300 sq mi), much larger than the present-day lake, in the "Abhe IV"-"Abhe V" lake cycle.[511] The enlarged lake covered a large area west of the present-day lake, present-day lakes Afambo, Gamari and Tendaho, reducing Borawli, Dama Ale and Kurub to islands.[512] The maximum water level was reached during the early Holocene as river discharge increased, but was later limited by partial overflow and did not rise above 380 metres (1,250 ft) again.[513] Deep thermal groundwater recharge occurred in the region.[514] About 9,000 years of human occupation are documented at the lake.[515] Archaeological sites indicate that people obtained resources from the lake and followed its rise[513] and decline.[516] The cultural traditions at Lake Abhe appear to be unusual by AHP/African standards.[517]

gorges along the way. Lake Naivasha drained south through Lake Siriata[532] into Lake Magadi-Natron.[533] Overflow of several of these lakes allowed animals including Nile crocodiles and fish to propagate to the individual lake basins,[534] but at the same time hindered the propagation of many land-based mammals.[524] River systems in the southern Kenya Rift region became active.[535]

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

Glaciers stopped retreating or briefly expanded in East Africa at the beginning of the AHP before continuing retreat.[536] On Mount Kilimanjaro they may have expanded during the AHP[537] after a phase during the Younger Dryas where the mountain was ice free,[538] but the tree line also rose at that time, accompanied by soil formation.[539] The wetter climate may have destabilized the neighbouring Mount Meru volcano, causing a giant landslide that removed its summit.[540]

atmospheric CO
2
during the AHP.[544]

Surprisingly, and contrary to the patterns expected from precessional changes, the

paleosoil in the Afar region.[557]

In East Africa, the AHP led to improved environmental conditions in terms of food and water supply from large lakes, allowing early human populations to survive and grow in size without requiring major changes in food gathering strategies.

evolution of humans[560] and allowed their spread across the Sahara[561] and into Europe.[562]

Other parts of Africa and the rainforest realm

peatlands of Central Congo started developing during the African humid period and peat continues to accumulate there to this day,[575] albeit with a slowdown in the Cuvette Centrale after the end of the African humid period.[576] In the Gulf of Guinea, increased sedimentation and changed sedimentation patterns from increased river runoff decreased the activity of submarine cold seeps offshore present-day Nigeria.[577]

On

Cape Verde Islands, precipitation and erosion increased.[578] In the Canary Islands, there is evidence of a moister climate on Fuerteventura,[579] the laurel forests changed perhaps as a consequence of the AHP.[120] Recharge of groundwater levels have been inferred from Gran Canaria also in the Canary Islands, followed by a decrease after the end of the AHP.[580] Choughs may have reached the Canary Islands from North Africa when the latter was wetter.[581]

Levant and Mediterranean

High latitude Africa has not undergone large scale changes in the past 11,700 years;[135] the Atlas Mountains may have blocked the monsoon from expanding further north.[582] However, river valley[583] and cave deposits showing a moister climate in southern Morocco,[161] increased precipitation in the Algerian highlands,[584] vegetation changes in the Middle Atlas,[585] several floods in Tunisian rivers[586] and ecosystem changes which impacted steppe-dependent rodents of Northern Africa have been linked to the AHP.[587]

In the

Northern Africa was wetter than today;[590] in Sicily wettening correlates with ITCZ changes in Northern Africa.[591] Mediterranean precipitation is brought by Mediterranean cyclones and the westerlies;[588] either increased precipitation from the westerlies,[592] northward moisture transport from Africa[593] or monsoonal precipitation extending into the Mediterranean may have rendered it wetter.[594] The connection between the African Monsoon and Mediterranean precipitation is unclear[595][588] and it was winter rainfall that increased predominantly,[596][597] although separating monsoonal and non-monsoonal precipitation can be difficult.[598]

The Mediterranean Sea became less saline during the AHP, in part due to increased precipitation from the westerlies[592] 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[u][600][601] and eutrophied,[602] with changes in the main water masses of the sea.[603] The S1 sapropel layer is specifically associated with the AHP[244] and with increased discharge of the Nile and other African rivers.[349] These processes together with decreased dust transport by wind led to changes in the sediment patterns of the Mediterranean,[604] and increased marine nutrient availability[602] and food web productivity in the Mediterranean,[605] which impacted the development of deep-sea corals.[606]

In the

Soreq Cave in Israel,[607] while the Dead Sea has variously been reported to have grown[597] or shrunk during the AHP. Such a decline, if it took place, and a decline of other southern European lakes were low during this period. This is unlike some earlier wet periods in the Sahara; possibly the stronger winter-summer insolation gradient in these earlier wet periods created a different moisture pattern than during the Holocene.[608] The northern Mediterranean may have been drier, with more wildfire activity, during the AHP.[609]

Southern Africa

The effects, if any, of the African humid period on Southern Africa have been unclear. Originally it was proposed that the orbitally driven changes would imply a dry period in Southern Africa which would have given way to moister conditions as the northern AHP ended,[610][611] as the ITCZ should shift its average position between the two hemispheres.[135] 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.[611] More recently obtained paleoclimate data have suggested however that southern Africa was actually wetter during the AHP rather than drier,[612][613] reaching as far as north-northwest Madagascar[614][615] 23° south[177] and as far as the catchment of the Orange River.[616] The area between Lake Tanganyika and Lake Malawi has been interpreted as the limit of the AHP's influence.[617]

Conversely, and consistent with the opposite reaction pattern of the Southern Hemisphere, the

Zambezi River reached its lowest discharge during the AHP,[618] and precipitation in the Central African Plateau and Zambia decreases in computer simulations of a Green Sahara.[619] Thus, the AHP may not have reach southern[620] or southeastern Africa.[621] There may have been opposite changes in precipitation between southeast Africa and tropical East Africa,[622] separated by a "hinge zone".[176] Particular changes occurred in central southern Africa, where a dry period co-occurred with an expansion of Lake Makgadikgadi; presumably the lake during this dry interval was nourished by increased wetness over the Okavango River catchment in the Angolan Highlands due to the AHP;[623] peatlands formed in Angola during the AHP.[624] In general there is little consistency between Northern and Southern Africa in terms of hydrological changes during the Holocene,[625] and nowhere are both the start and end of the AHP apparent.[248] Orbitally-mediated changes in Northern Hemisphere climate affected the Southern Hemisphere through oceanic pathways involving sea surface temperatures.[626] Additionally, wetter periods unrelated to the AHP may have occurred after deglaciation in Southern Africa.[627]

Numerical estimates

Estimates of the exact amount of increased precipitation vary widely.[628] During the African humid period, Saharan rainfall increased to 300–400 millimetres per year (12–16 in/year),[629] and values exceeding 400 millimetres per year (16 in/year) may have spread to 19–21° northern latitude.[630] In the eastern Sahara, a gradient from 200 millimetres per year (7.9 in/year) increment in the north to 500 millimetres per year (20 in/year) in the south has been identified.[338] An area with less than 100 millimetres per year (3.9 in/year) may have remained in the Eastern Sahara however,[631][632] although its driest parts may have received 20-fold more precipitation than today.[435] Precipitation in the Sahara probably reached no more than 500 millimetres per year (20 in/year),[633] with large uncertainty.[223]

Other reconstructed values of the precipitation increase indicate an annual increase of about 150–320 millimetres (5.9–12.6 in) in Africa,

Wahiba Sands of Oman may have reached 250–500 millimetres per year (9.8–19.7 in/year).[640]

Effect on other climate modes

The

Atlantic Niño sea surface temperature patterns develop in the Atlantic Ocean.[647][648]

Remote effects of the AHP on climate have also been studied,

Holocene thermal maximum is model-dependent;[651] dust changes did not have a major effect.[652] The AHP would also influence SSTs in the Indian Ocean, although there is not much evidence about the mid-Holocene sea temperatures there.[650]

The AMOC transports heat from the Southern into the Northern Hemisphere[165] and is implicated in starting the Holocene AHP an earlier AHPs after the end of an ice age.[653] Various studies have been conducted to determine which effects reduced dust supply and the greening of the Sahara would have had on its intensity,[654] with conflicting results on which effects would predominate.[165] Increased heat transport either through the atmosphere or the ocean would result in warming in the Arctic.[655]

Remote precipitation and the AHP

The Sahara greening intensified the Indian and Asian monsoons,

North American Monsoon area may be explained through these remote effects.[666]

Sun et al. 2020 proposed that the greening of the Sahara during the AHP can increase precipitation over the Middle East even if neither the African nor the Indian monsoons reach it.[667] During spring, the increased vegetation forces anomalous atmospheric circulations that direct moisture transport from the Mediterranean, the Red Sea and eastern tropical Africa into the Middle East, increasing precipitation[668] and agricultural productivity there.[669] This could explain increased precipitation in the Middle East during the AHP:[670] A wet climate occurred in the Middle East during the early Holocene, leading to the Ubaid period of settlement in Mesopotamia, followed by dry phases around 5,500 years ago[671] and a concomitant reduction in simulated wheat yield.[672]

Hurricanes and the AHP

One climate model has indicated that a greener Sahara and reduced dust output would have increased

Dry Tortugas of South Florida a decrease of hurricane activity took place during the AHP[684] and dust emission is not always anti-correlated to hurricane activity.[685] Finally, the northward movement of the ITCZ during the AHP may have caused a corresponding northward movement of tropical cyclogenesis areas and storm tracks in the Atlantic Ocean,[686][677] which could also explain decreased hurricane activity in the Bahamas and Dry Tortugas.[684]

Fluctuations

Temperatures in Greenland during the Younger Dryas

Climate variability during the AHP is poorly documented,

Mediterranean[695] where dune activity occurred in the Negev.[696] 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.[697][639]

Another dry phase took place about 8,200 years ago, spanning East Africa

Capsian culture[706] as well as with cultural changes both in the Sahara and the Mediterranean;[377] at the Gobero cemetery a population change occurred after this dry interruption[707] but the occurrence of widespread cultural changes appears to be questionable.[27] This episode appears to have been caused by the draining of ice-dammed lakes in North America [708] although a low latitude origin has also been suggested.[709]

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. [191][219][710] Storm tracks shift north away from the Mediterranean.[711] Earlier Heinrich events were also accompanied by drought in North Africa.[60] Likewise, a weakening of moisture transport and a less eastward position of the Congo Air Boundary contributed to reducing precipitation in East Africa[691] although some parts of southern Africa at Lake Malawi were wetter during the Younger Dryas.[712]

Many humidity fluctuations in the early Holocene appear to be caused by the discharge of

solar activity changes,[15] water levels of Lake Turkana for example appear to reflect the 11-year solar cycle.[714]

In

tectonic phenomena occur at Lake Turkana, but do not have the magnitude required to explain large changes in lake level.[718] Water level fluctuations have also been inferred for Lake Chad on the basis of pollen data, especially towards the end of the AHP.[719] In the Taoudenni lake fluctuations of about a quarter-millennium have been recorded[720] and frequent droughts occurred in the Eastern Sahara.[721]

Other variations appear to have occurred 9,500–9,000 and 7,400–6,800[309] 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,[711] 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.[722] The duration and severity of dry events is difficult to reconstruct[392] and the impact of events like the Younger Dryas is heterogeneous even between neighbouring areas.[723] During dry episodes, humans might have headed to waterbodies which still had resources,[381] and cultural changes in the central Sahara have been linked to some dry episodes.[724] Aside from fluctuations, a southward retreat of the humid period may have been underway after 8,000 years ago[725] with a major drought around 7,800 years ago.[726]

End

The African humid period ended about 6,000–5,000 years ago;

Wind erosion increased in northern Africa,[729] and dust export from the now-desert[711] and from dried up lakes[730] such as the Bodélé Basin grew; Bodélé today is the largest single source of dust on Earth.[731] The lakes dried up, mesic vegetation disappeared, and sedentary human populations were replaced by more mobile cultures.[19] 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;[732] today almost no precipitation falls in the region.[48] The end of the AHP but also its beginning could be considered a "climate crisis" given the strong and extended impact.[703] Drying extended as far as the Canary Islands[733][734] and southeastern Iran,[735] and there is evidence of climate change on São Nicolau, Cape Verde.[736]

The

A major pan-tropical environmental change took place about 4,000 years ago.

Chronology

Whether the drying happened everywhere at the same time and whether it took place in centuries or millennia is unclear

Prussian naturalist Alexander von Humboldt suggested that only a quick drying could form the desert.[752]

Most recently, the idea has taken hold that the end of the African humid period occurred from north to south in a stepwise fashion.

Old Kingdom was still wetter than today.[759] A later end in northeast Africa about 4,000 years ago may reflect the different configuration of landmasses and thus monsoon behaviour,[760] while other research has found a westward propagating drying trend.[117]

Some evidence points to a two-phase change in climate with two distinct dry transitions[761] caused by the existence of two different steps of insolation decrease at which climate changes.[762] Distinct environmental changes may have occurred in Central Africa, Western Africa and East Africa.

4.2 kiloyear event - the transition from the Northgrippian to the Meghalayan stage of the Holocene -[705] is considered to be the true end of the AHP,[709] especially in central Africa.[763]

Increased variability in precipitation may have preceded the end of the AHP; this is commonly observed before a sudden change in climate.

BCE – 300 CE in Roman Northern Africa and along the Dead Sea[766] and an earlier one 2,100 years before present in the western Sahel.[120] By 2,700 years ago the central Sahara had become a desert and remained one until the present-day.[767]

Sahara and Sahel

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,

Bodele basin drying up entirely about 2,000[303]-1,000 years ago[771] as it was disconnected from the southern basin where its major tributary, the Chari River, enters Lake Chad.[296] The dried out basin was now exposed to the Harmattan winds, which blow dust out of the dry lake bed,[772] making it the single largest source of dust in the world.[773] Dunes formed in the dried-up Sahara[774] and Sahel[775] or began moving again after stabilizing during the AHP.[776]

The tropical vegetation was replaced by desert vegetation, in some places suddenly and in others more gradually.

Terrace".[782] The Egyptian Sahara might still have been vegetated until 4,200 years ago, based on depictions of savanna environments in Fifth Dynasty tombs in Egypt.[783]

At

hypersaline 4,000 years ago.[784][785][786] Lake Teli dried out completely about 4,200 years ago.[787] However, the climate of the Ounianga lakes may have been affected by the Tibesti Mountains and the end of the AHP thus delayed,[770] and fossil groundwater left by the AHP nourishes the lake to this day.[788] In the central Sahara, water resources in the mountains persisted longer.[789]

East Africa and Arabia

In northern East Africa, water levels dropped rapidly about 5,500 years ago

Indian Monsoon took place about 5,900 years ago.[121] Drying is also documented from Oman,[129] and rivers and lakes of Arabia became intermittent or entirely dry.[790] The Blue Nile basin became less moist[129] with a noticeable decrease of Nile discharge about 4,000 years ago.[604] Decreased discharge of the Nile led to the cessation of sapropel deposition and turbidite activity off its delta,[111] the abandonment of river channels in its delta and upstream[791] and increased seawater influence in the delta.[792]

Some data from

arid climate between 6,000 and 5,000 years ago has been observed[795][796] which continued until 2,700 years ago.[452] In the Bale Mountains and the Sanetti Plateau of Ethiopia vegetation changes signalling a drier climate took place around 4,600 years ago.[797]

Forest cover in the area of the African Great Lakes decreased between 4,700 and 3,700 years ago,

Lake Zway also dropped between 5,400 and 4,200 years ago.[799] 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.[800]

The end of the AHP at

Predynastic societies dependent on it.[805]

Mediterranean

The southern

Mediterranean[812] and might have been caused by changes in the circulation of the Atlantic Ocean.[193]

Tropical West Africa

In Lake Bosumtwi the African humid period ended about 3,000 years ago[144] after 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.[710] Some drying occurred simultaneously between the Sahel and the Gulf of Guinea.[219] Some lakes in the Guineo-Congolian region dried out, while others were relatively unaffected.[778]

A general tendency towards a drier climate is observed in West Africa at the end of the AHP.[813] There, dense vegetation became progressively thinner between 5,000 and 3,000 years ago,[798] and major perturbations of the vegetation took place around 4,200 and 3,000–2,500[814][815] /2,400 calibrated years ago.[816] A brief return of moister conditions took place 4,000 years ago[703] while a substantial dry phase occurred between 3,500 and 1,700 years ago.[813] Aridity became established between 5,200 and 3,600 years ago in the Sahara.[817] In Senegal mangroves collapsed 2,500 years ago[224] and modern-type vegetation arose about 2,000 years ago.[818]

Central Africa

Farther south at the

Sao Tome.[570] In the Congo Basin, there were changes in the composition and density of the forests rather than their extent,[821] and along the equator precipitation may have increased around 4.2 ka.[822] Many vegetation changes in the tropical regions were probably caused by a longer dry season[823] and perhaps a smaller latitudinal range of the ITCZ.[816]

Southern Hemisphere Africa

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.[803] Contrarily, increased water levels in Etosha Pan (Namibia) appear to relate to a southward movement of the ITCZ at the end of the AHP[824] although stalagmite growth data in Dante Cave also in Namibia has been interpreted as indicating a wetter climate during the AHP.[612] Several records indicate that 5,500 years ago, precipitation changed in an east-west dipole-like way[825] with drying in the west and moistening in the east.[826] This pattern was probably driven by shifts in atmospheric moisture transport and of rain belt width.[827]

Mechanisms

The end of the humid period appears to reflect the changes in

insolation during the Holocene,[118] as a progressive decrease of summer insolation caused the insolation gradients between Earth's hemispheres to decrease.[828] However, the drying appears to have been much more abrupt than the insolation changes;[141] it is not clear whether non-linear feedbacks led to abrupt changes in climate and it is also unclear whether the process, driven by orbital changes, was abrupt.[144] Also, the Southern Hemisphere warmed and this resulted in a southward shift of the ITCZ;[829] orbitally-driven insolation has increased over the Holocene in the Southern Hemisphere.[133]

As precipitation decreased, so did vegetation, in turn increasing the albedo and further decreasing precipitation.[148] Furthermore, vegetation may have responded to increased variations in precipitation towards the end of the AHP[145] although this view has been challenged.[830] 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.[831] Plants at higher and lower latitudes might respond differently to climate change; for example more diverse plant communities might have slowed down the end of the AHP.[90]

Other proposed mechanisms:

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 water levels of Lake Turkana.[841] At Lake Victoria on the other hand, solar variations appear to sometimes lead to drought and sometimes lead to wetness, probably due to changes in the ITCZ.[829]

Potentially human-mediated changes

Major changes in vegetation in East Africa about 2,000 years ago may have been caused by human activity, including large-scale deforestation for iron production during the Iron Age.[842] Similar changes have been observed on the Adamawa Plateau[843] (Cameroon[819]) but later dating of archaeological sites has found no correlation between human expansion in Cameroon and environmental degradation.[844] Similar rainforest degradation across Western African took place between 3,000 and 2,000 years ago[845] and the degradation is also known as "third millennium rainforest crisis".[846] Climate-mediated processes may have increased the impact of land use changes in East Africa.[551] In the Sudanian and Sahelian savannah on the other hand human activity seems to have had little impact,[296] and in Central Africa forest changes were clearly triggered by climate change with little or no evidence of anthropogenic changes.[847] The question has led to intense debate among paleoecologists and archaeologists.[848]

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[849] although vegetation changes may have been induced by human activity[254] and grazing.[850] Later it was suggested that overgrazing may have triggered the end of the AHP around 5,500 years ago;[368] 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.[424] Later research has on the contrary suggested that human pastoralism may have actually delayed the end of the AHP by half a millennium[851] 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.[852][853] Which effects prevailed is still controversial.[395] Increased grazing has been invoked to explain the increase in dust emissions after the end of the AHP.[854] The effects of grazing on vegetation cover are context-dependent and hard to generalize over wider regions.[855]

Global

A general drying tendency is observed in the northern tropics

Central Atlantic.[862] The final retreat of vegetation from the Sahara may have helped cause the 4.2 kiloyear event.[863]

Conversely, in South America there is evidence that the monsoon behaves in an opposite fashion consistent with precessional forcing;[856] water levels in Lake Titicaca were low during the middle Holocene and began to rise again after the end of the AHP.[864] Likewise, a trend towards increased wetness took place in the Rocky Mountains at this time[865] although it was accompanied by a drier phase around Lake Tahoe, California and in the Western United States.[866]

Consequences

Humans

As observed in archaeological sites, settlement activity decreased in the Sahara after the AHP.[867] Population in Northern Africa decreased between 6,300 and 5,200 years ago[144][375] over less than a millennium,[835] beginning from the north.[868] In inner Arabia many settlements were abandoned about 5,300 years ago.[152] Some Neolithic people in the desert persisted for longer thanks to the exploitation of groundwater.[761]

Different human populations responded to the drying in diverse manners,

Red Sea Hills in response to the end of the AHP.[872] There was a shift in domestic animal use from cattle to sheep and goats as these are more suited in arid climates, a change reflected in rock art from which cattle disappeared at this time.[873]

The development of irrigation systems in Arabia may have been an adaptation to the drying tendency.[465] The decreased availability of resources forced human populations to adapt,[874] in general fishing and hunting declined in favour of farming and herding.[875] However, the effects of the end of the AHP on human food production have been subject to controversy.[876]

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

oases[886][765] and hunter-gatherers also stayed in the Horn of Africa.[185]

The Nile itself was not totally unaffected however;

Garamantian civilization may also relate to climate change although other historical events were probably more important;[891] at Tanezzuft oasis after 1,600 years ago it certainly relates to the drying trend.[886]

In Central Africa, forests became discontinuous and

savannahs formed in some places, facilitating the movement and growth of Bantu speaking populations;[831] these in turn may have affected the ecosystem.[892] The vegetation changes may have aided in the establishment of agriculture.[847] The relatively slow decline of precipitation gave humans more time to adapt to the changing climate conditions.[558] In East Africa, the beginning of the "Pastoral Neolithic" and the appearance of Nderit pottery have been attributed to the climatic changes at the end of the AHP.[893]

Cultural changes may also have occurred as a consequence of climate change, such as

human burials where formerly cattle burials predominated,[896] as well as an increase of monumental architecture in the Sahara may have also been a response to increasingly adverse climates.[869] A spread in cattle domestication at the time of climate change[389] and as herders escaped the drying Sahara southwards[897][898] may also relate to these events, although the details of the exact process by which cattle domestication spread are still controversial.[894][899] 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.[900]

Non-human

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.

domestic animals[914] and some drought-tolerant plant species may have expanded their range.[915]

The

oligotrophic conditions as discharge from African rivers decreased.[605] Desert varnish formed on exposed rocks in the Sahara[918] and at Lake Turkana in East Africa.[717]

Global climate

The shrinkage of subtropical wetlands probably led to a drop in atmospheric

atmospheric carbon dioxide decreases in the early Holocene may relate to the vegetation expansion caused by the AHP.[919] Carbon dioxide concentration then increased after about 7,000 years as the biosphere began releasing carbon in response to increasing aridity.[890]

Dust originating in the Bodele depression

A sudden increase in the amount of land-originating dust in an oceanic

alluvial[aa] sediments emplaced during the AHP[925] and dried up lake basins became an important source for dust[786][134] and silt-sized particles.[926] Today, the Sahara is the single largest source of dust in the world[ab], with far ranging effects on climate and ecosystems,[928] such as the growth of the Amazon rainforest.[929]

In one climate model, the

cold air outbreaks in much of Europe; such changes have also been observed in paleoclimate data.[931] These findings imply that the vegetation state of the Sahara influences the Northern Hemisphere climate.[932] In turn, this high latitude cooling may have further reduced precipitation over Africa.[840]

Present-day situation

Presently, the African

AD has been explained with changed agricultural practices.[939]

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[940] when the ITCZ moves northward and southward over the region, respectively;[941] in addition to the Indian Ocean-sourced precipitation there is also Atlantic[ad]- and Congo-sourced precipitation west of the Congo Air Boundary.[933][940] 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.[942] East Africa is also under the influence of monsoon circulations.[943] South Africa has both monsoonal climates, winter precipitation climates and climates without clear precipitation seasonality.[610]

Implications for future global warming

Greening of the Sahel between 1982 and 1999

Some simulations of

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.[679]

The

drying trend may be due to global warming.[947] On the other hand, West Africa[948] and parts of East Africa may become drier during given seasons and months.[948][947] Currently, the Sahel is becoming greener but precipitation has not fully recovered to levels reached in the mid-20th century.[944]

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,[949] in particular through increased inter-hemispheric temperature gradients.[676] The direct effect of heat on plants may be detrimental.[950] Non-linear increases in vegetation cover are also possible,[676] with several climate models showing abrupt increases when global temperatures rise by 2–4 °C (3.6–7.2 °F).[951] One study in 2003 showed that vegetation intrusions in the Sahara can occur within decades after strong rises in atmospheric carbon dioxide[952] but would not cover more than about 45% of the Sahara.[53] That climate study also indicated that vegetation expansion can only occur if grazing or other perturbations to vegetation growth do not hamper it.[953] On the other hand, increased irrigation and other measures to increase vegetation growth such as the Great Green Wall could enhance it.[950] A 2022 study indicated that while increased greenhouse gas concentrations by themselves are not sufficient to start an AHP if greenhouse gas-vegetation feedbacks are ignored, they lower the threshold for orbital changes to induce Sahara greening.[954]

Plans to

solar farms in the Sahara desert would also act to decrease its albedo and may trigger similar climate responses.[956]

A greening of the Sahara on the one hand may allow

water borne diseases and flooding.[957] Expanded human activity resulting from a wetter climate may be vulnerable to climate reversals as demonstrated by the droughts that followed the mid-20th century wet period.[958]

See also

Notes

  1. ^ The end of the AHP coincides with the maximum temperatures.[18] At Lake Ashenge, the onset of the AHP was accompanied by climatic warming[55] while in Senegal, temperatures during the AHP were 1 °C (1.8 °F) lower than today there.[56]
  2. Arabia, Israel[79] and the exposed seafloor of the Persian Gulf[80] where dust generation increased.[70]
  3. ^ Dune-covered areas.[96]
  4. ^ However, some lakes persisted in areas where colder temperatures had decreased evaporation.[46]
  5. ^ 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;[72] the older hypothesis has not been entirely abandoned.[115] 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.[116]
  6. ^ Whether it commenced first in the eastern Sahara is unclear.[117]
  7. ^ This was originally believed to have occurred 7,000 or 13,000 years before present,[114] but a more recent suggestion indicates a reconnection of the Nile 14,000–15,000 years ago.[125]
  8. ^ Lake Megachad is an expanded Lake Chad[151] which had a size comparable to the Caspian Sea[152] which is today's largest lake.[153]
  9. ^ The Congo Air Boundary is the point at which moisture bearing winds from the Indian Ocean collide with those from the Atlantic Ocean.[177]
  10. ^ 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.[231]
  11. ^ Where the Monsoon of South Asia penetrated farther inland[15] and was more intense starting about 14,800 years ago.[109]
  12. ^ Salt deposits left there were mined beginning in the 16th century.[316]
  13. ^ Both Bir Kiseiba and Nabta Playa feature archaeological sites;[333] Nabta may have been a religious centre of regional importance.[334]
  14. ^ Which may have expanded into previously dry areas at the beginning of the AHP.[373]
  15. ^ Fesselsteine are stony artifacts, that are interpreted as tools for restraining animals.[408]
  16. calcretes, "lake chalks", rhizoliths, travertines and tufa.[436]
  17. ^ Local runoff contributed to the filling of the Fayum Depression.[201]
  18. Yellow Nile[488]
  19. ^ Assuming that they merged, which is not clearly established.[529] Tectonic or geographic changes may have been necessary to establish a connection.[530] Alternatively, water might have bypassed Baringo.[531]
  20. ^ A lake level drop 8,000 years ago has been related to the northward movement of the rainbelt.[564]
  21. ^ Which would prevent oxygen-rich waters from sinking to the deep ocean in winter, suffocating organisms on the seafloor.[599]
  22. ^ Except for most of the Western North Pacific according to Pausata et al. 2017.[673]
  23. ^ There is conflicting evidence on whether the Younger Dryas was wetter or drier in tropical southeastern Africa.[690]
  24. ^ Whether it also took place in Asia is unclear; perhaps it was too short to trigger climate changes recognizable in records[699] but some evidence has been found.[700]
  25. Predynastic.[878] In upper Egypt, the Badarian culture developed when the AHP ended.[879]
  26. ^ The Dahomey Gap is a region without forests in southern Benin, Ghana and Togo[916] that forms a gap in the Guineo-Congolian forest belt.[778]
  27. ^ Alluvium refers to sediments deposited by running water, which have not solidified into rocks.[924]
  28. ^ Roughly five times larger than during the AHP.[927]
  29. ^ The main area of monsoon rains does not coincide with the ITCZ.[933]
  30. ^ The Atlantic Ocean is also the source of monsoon rainfall for the Sahel.[3]

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External links