African humid period

When the 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.

[12] Recently, the hypothesized AHP end point of ~6000 years ago has been used experimentally in the Paleoclimate Modelling Intercomparison Project[13] and the effects of the Sahara's greening on other continents has drawn scientific attention.

[48] The African humid period took place in the late Pleistocene[49] and early-middle Holocene,[50] and saw increased precipitation in Northern and Western Africa due to a northward migration of the tropical rainbelt.

[102] A warming and retreat of sea ice around Antarctica coincides with the start of the African humid period,[123] although the Antarctic Cold Reversal also falls into this time[36] and may relate to a drought interval recorded in the Gulf of Guinea.

[132] 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%,[49] resulting in the African monsoon becoming both stronger and reaching farther north.

[171] Finally, increased greenhouse gas concentrations may have been involved in directing the onset of the AHP in tropical southeastern Africa;[172] there, orbital changes would be expected to lead to climate variations opposite to those in the Northern Hemisphere.

[255] This fauna included antelopes,[49] baboons, cane rats,[256] catfish,[257] clams,[258] cormorants,[259] crocodiles,[49] elephants,[260] frogs,[261] gazelles,[260] giraffes,[49] hartebeest,[262] hares,[260] hippos,[262] molluscs, Nile perches,[263] pelicans,[264] rhinoceroses,[256] snake-eagles,[259] snakes,[261] tilapia,[258] toads,[261] turtles[265] and many more animals,[266] and in Egypt there were African buffaloes, spotted hyenas, warthogs, wildebeest and zebra.

[267] Additional birds include brown-necked raven, coot, common moorhen, crested grebe, glossy ibis, long-legged buzzard, rock dove, spur-winged goose and tufted duck.

[363] Humans were hunting large animals with weapons that have been found in archaeological sites[364] and wild cereals occurring in the Sahara during the AHP such as brachiaria, sorghum and urochloa were an additional source of food.

[391] Genetic and archaeological data indicate that these populations which exploited the resources of the AHP Sahara probably originated in Sub-Saharan Africa and moved north after some time, after the desert got wetter;[392] this may be reflected in the northward spread of Macrohaplogroup L and Haplogroup U6 genomic lineages.

[396] 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,[397] while a possible role in the spread of the Nilo-Saharan languages[253] is debatable.

[472] Wadi Howar was active until 4,500 years ago,[470] and at the time often contained dune-dammed lakes,[473] swamps[474] and wetlands;[473] it was the largest Saharan tributary of the Nile[475] and constituted an important pathway into sub-Saharian Africa.

[486] This overflowing large lake was filled with freshwater and was populated by humans,[487] typically in bays, along capes and protected shorelines;[488] the societies there engaged in fishery[487] but could probably also fall back on other resources in the region.

[515] On Mount Kilimanjaro they may have expanded during the AHP[516] after a phase during the Younger Dryas where the mountain was ice free,[517] but the tree line also rose at that time, accompanied by soil formation.

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,[6] as the ITCZ should shift its average position between the two hemispheres.

[6] More recently obtained paleoclimate data have suggested however that southern Africa was actually wetter during the AHP rather than drier,[585] reaching as far as Rodrigues Island in the Indian Ocean[586] and as far as the catchment of the Orange River.

[588] Conversely, and consistent with the opposite reaction pattern of the Southern Hemisphere, the Zambezi River reached its lowest discharge during the AHP,[589] and precipitation in the Central African Plateau and Zambia decreases in computer simulations of a Green Sahara.

[617] Whether the reduced albedo of the Sahara during the AHP contributed to, or increased cloud cover counteracted, the warming of the Holocene thermal maximum is model-dependent;[618] dust changes did not have a major effect.

[622] Gaetani et al. 2024 found that Green Sahara climate simulations feature warming throughout the Northern Hemisphere[623] and a strengthening of the westerlies and their precipitation in the Atlantic[624] but a decline along the North American West Coast.

[666] 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.

[671] It coincided with cooling in the Northern Atlantic,[672] in surrounding landmasses such as Greenland[673] and around the world;[370] the drought may be related to the 8.2 kiloyear event[674] which separates the Greenlandian and Northgrippian stages of the Holocene[675] and lasted for about one millennium.

[730] A later end in northeast Africa about 4,000 years ago may reflect the different configuration of landmasses and thus monsoon behaviour,[731] while other research has found a westward propagating drying trend.

[752] 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.

[780] The southern Aegean,[781] Libya and the Middle Atlas became gradually more dry,[750] and drying in Morocco took place about 6,000 radiocarbon years ago,[733] Drier conditions in Iberia and the Western Mediterranean 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 ITCZ[782] More complicated changes have been found for the northern margin of the Mediterranean,[783] and winter rainfall increased in the Levant at the end of the AHP.

[835] Conversely, in South America there is evidence that the monsoon behaves in an opposite fashion consistent with precessional forcing;[828] water levels in Lake Titicaca were low during the middle Holocene and began to rise again after the end of the AHP.

[850] The warm episode and coinciding drought may have triggered animal and human migration to less inhospitable areas[779] and the appearance of pastoralists where previously fishery-dependent societies had existed, as happened at Lake Turkana.

[887] On the other hand, the decline of tree cover may have grown the niche available to domestic animals,[888] and modern Afromontane vegetation[889] and some drought-tolerant plant species may have expanded their range.

[895] 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.

[905] 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.

[935] The mechanisms and consequences of the AHP are important context to evaluate such proposals and their ramifications;[919] precipitation may increase[935] but the consumption of carbon dioxide would be small and there could be detrimental impacts on climate and dust fluxes in the far-field.