The African humid period (AHP; also known by other names) was a climate period in Africa during the late Pleistocene and Holocene geologic epochs, when North Africa was wetter than it is today. The covering of much of the Sahara by grasses, trees, and lakes was caused by changes in the Earth's axial tilt, changes in vegetation and dust in the Sahara which strengthened the African monsoon, and increased greenhouse gases.
During the preceding Last Glacial Maximum, the Sahara contained extensive dune fields and was mostly uninhabited. It was much larger than it is today, and its lakes and rivers, such as Lake Victoria and the White Nile, were either dry or at low levels. The humid period began about 14,600–14,500 years ago at the end of Heinrich event 1, simultaneously with 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 dry fluctuations occurred during the Younger Dryas and the short 8.2-kiloyear event. The AHP 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 occurred in several steps, such as the 4.2-kiloyear event.
The AHP led to the widespread settlement of the Sahara and the Arabian Desert and had a profound effect on African cultures, including the birth of the Ancient Egyptian civilization. People in the Sahara lived as hunter-gatherers and domesticated cattle, goats, and sheep. They left archaeological sites and artifacts, such as one of the oldest canoes ever discovered 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. 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.
Research
Discovery and hypotheses In 1850 researcher Heinrich Barth discussed the possibility of past climate change leading to increased wetness in the Sahara after discovering petroglyphs in the Murzuq Desert, as did Ahmed Hassanein following his 1923 exploration of the Libyan Desert when he saw depictions of savanna animals at Gabal El Uweinat. 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 and higher Nile flow levels was increasingly reported, and it was recognized that the Holocene featured a humid period in the Sahara.
The idea that changes in Earth's orbit around the Sun influence the strength of the monsoons was advanced as early as 1921; while the original description was partly inaccurate, later, widespread evidence for such orbital controls on climate was found. At first it was believed that humid periods in Africa correlated with glacial stages ("pluvial hypothesis") before radiocarbon dating became widespread. Beginning in the 1970s, the humidification was attributed to precessional changes.
The development and existence of the AHP has been investigated with archaeology, climate modelling and paleoproxies. Archaeological sites, deposits left by wind, vegetation (e.g., leaf wax), and lakes and wetlands also played an important role. Pollen, lake deposits and former levels of lakes have been used to study the ecosystems of the AHP, and charcoal and leaf impressions have been used to identify vegetation changes. Many unresolved questions concerning the AHP remain: its beginning, cause, intensity, end, land feedbacks, and the fluctuations during the period.
Recently, the hypothesized AHP endpoint of ~6,000 years ago has been experimentally tested in the Paleoclimate Modelling Intercomparison Project, and the effects of the Sahara's greening on other continents have attracted scientific attention. The concept of the Sahara is significantly different than today, and the rich record it left has driven the imagination of the public and scientists alike. It has been used as an analogue for the drying of Mars after the Amazonian-Hesperian.
Research issues
While the precipitation changes since the Last Glacial Period are well established, the magnitude and timing of the changes are unclear. Depending on how and where measurements and reconstructions are made, different beginning dates, ending dates, durations and precipitation levels have been determined for the AHP. The amounts of precipitation reconstructed from paleoclimate records and simulated by climate modelling are often inconsistent with each other; in general, the simulation of the Green Sahara is considered a problem for Earth system models. There is more evidence of the late phase of the AHP than its beginning. Erosion of lake sediments and carbon reservoir effects make it difficult to date when they dried up. Vegetation changes by themselves do not necessarily indicate precipitation changes, as changes in seasonality, plant species composition, and changes in land use also play a role in vegetation changes. 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. Most records of Holocene precipitation in eastern Africa come from low altitudes.
Terminology
The term "African humid period" was coined in 2000 by Peter B. de Menocal et al. Earlier humid periods are sometimes known as "African humid periods", and several dry/wet periods have been defined for Central Africa. In general, these types of climate fluctuations between wetter and drier periods are known as "pluvials" and "interpluvials", respectively. The term "Green Sahara" is frequently used to describe the AHPs. Because the AHP did not affect all of Africa, some scientists have instead used and recommended "North African humid period" and "Northern African humid period".
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; "Early Holocene Humid Period"; "early to mid-Holocene humid episode"; "African Holocene Humid Period"; " "Holocene Pluvial"; "Holocene Wet Phase"; "" in Central Africa; "Makalian" for the Neolithic period of northern Sudan; "Nabtian Pluvial", "Nabtian Wet Phase" or "Nabtian period" for the 14,000–6,000 humid period over the eastern Mediterranean and Levant; "Neolithic pluvial"; "Neolithic Subpluvial"; "Neolithic wet phase"; "" of the western Sahara 6,500 – 4,000 years before present; "Subpluvial II" and "" in the central Sahara 14,000 – 7,500 years before present. The terms "Big Dry", "" and have been applied to the dry period in the Last Glacial Maximum (LGM), the latter is equivalent to the "Kanemian"; "Kanemian dry period" refers to a dry period between 20,000 and 13,000 years before present (BP) in the Lake Chad area.
Background and beginning
The African humid period took place in the late Pleistocene and early-middle Holocene, with increased precipitation in northern and western Africa due to a northward migration of the tropical rain belt. The AHP stands out within the otherwise relatively climatically stable Holocene. It is part of the so-called Holocene climatic optimum and coincides with a global warm phase, the Holocene Thermal Maximum. Liu et al. 2017 subdivide the humid period into an "AHP I" which lasted until 8,000 years ago, and an "AHP II" from 8,000 years onward, with the former being wetter than the latter.
The AHP was not the first such phase; some evidence exists for as many as 230 older "green Sahara"/wet periods, dating back perhaps to the first appearance of the Sahara 7–8 million BP. Earlier humid periods appear to have been more intense than the AHP of the Holocene, including the exceptionally intense Eemian humid period. This humid period provided the pathways for early humans to cross Arabia and northern Africa and which, together with later moist periods, has been linked to expansions of the Aterian populations and the speciation of insect species. Such humid periods are usually associated with interglacials, while glacial stages correlate to dry periods; they occur during precession minima, unless large ice sheets or insufficient greenhouse gas concentrations suppress their onset.
The Bølling–Allerød warming appears to be synchronous with the onset of the AHP and with increased humidity in Arabia. Later, in the Blytt–Sernander sequence the humid period coincides with the Atlantic period.
Conditions before the African humid period
During the LGM, the Sahara and Sahel were extremely dry. The extent of dune sheets and water levels in closed lakes indicate that less precipitation fell than today. The Sahara was much larger, extending farther south to about 12° northern latitude. Dunes were active much closer to the equator, and rainforests had retreated in favour of afromontane and savanna landscapes as temperatures, rainfall, and humidity decreased.
There is little and often equivocal evidence of human activity in the Sahara or Arabia at that time, reflecting their drier nature; in the Acacus Mountains the last human presence was recorded 70,000–61,000 BP, and by then the LGM humans had largely retreated to the Mediterranean coast and the Nile valley. The aridity during the LGM 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. Exceptional dry phases are linked to Heinrich events when there are a large number of icebergs in the North Atlantic; the discharge of large amounts of such icebergs between 11,500 and 21,000 BP coincided with droughts in the subtropics.
Before the onset of the AHP, it is thought that Lake Victoria, Lake Albert, Lake Edward, Lake Turkana and the Sudd swamps had dried out. The White Nile had become a seasonal river whose course along with that of the main Nile may have been dammed by dunes. 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 farther east. Other lakes across Africa, such as Lake Chad and Lake Tanganyika, also had shrunk during this time, and both the Niger River and Senegal River were stunted.
Early humidity increases
Whether some parts of the desert, such as highlands like the Red Sea Hills, were reached by the westerlies or weather systems associated with the subtropical jet stream—and thus received precipitation—is contentious. It is only clearly supported for the Maghreb in northwestern Africa and parts of northeastern Africa, though river flow/terrace formation and lake development in the Tibesti and Jebel Marra mountains. Residual Nile flow may be explained in this way. The highlands of Africa appear to have been less affected by drought during the LGM.
The end of the glacial drought occurred between 17,000 and 11,000 BP, with an earlier beginning noted in the Acacus, Sinai and Saharan mountains 26,500–22,500 BP and 18,500 BP, respectively. In southern and central Africa, earlier starts 17,000 and 17,500 BP, respectively, may be linked to Antarctic warming, while Lake Malawi appears to have been low until about 10,000 BP.
High lake levels occurred in the Jebel Marra and Tibesti Mountains between 15,000 and 14,000 BP and the earliest stage of glaciation in the High Atlas mountains took place at the same time as the Younger Dryas and early AHP. Around 14,500 BP, lakes started to appear in the arid areas.
Onset