The Pleistocene Epoch, often referred to as the Ice Age, spanned from approximately 2.58 million to 11,700 years ago. This period was characterized by significant global cooling, glacial advances, and fluctuating sea levels. Africa, like other continents, experienced profound environmental changes during this epoch, including shifts between arid and humid conditions.
The climate of Africa continued on the trajectory that began in the late Miocene and continued throughout the Pliocene. While the Pleistocene was characterized as a period of global cooling, glacial advances, and dropping sea levels, the cold periods were interspersed with interglacial periods when the ice retreated and sea levels rose. Even within glacial periods, the climate varied.
Global temperature fluctuations from Pliocene (5-1.8 mya), through Pleistocene (1.8-0.1 mya), to present.
The African Humid Period
One of the most significant climatic events in Africa during the late Pleistocene and Holocene epochs was the African Humid Period (AHP). This period, marked by increased precipitation in Northern and Western Africa, resulted from a northward migration of the tropical rainbelt.
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 northern Africa was wetter than today. 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.
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The humid period began 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 dry fluctuations occurred; during the Younger Dryas and the short 8.2 kiloyear event. The African humid period ended 6,000-5,000 years ago during the Piora Oscillation cold period.
The African humid period extended over most of Africa: The Sahara and eastern, southeastern and equatorial Africa. Among the large lakes which may have formed in the Sahara are Lake Megafezzan in Libya and Lake Ptolemy in Sudan.
Lake Chad, one of the lakes that expanded during the African Humid Period.
The Ghosts of the Green Sahara
The Sahara's Transformation
During the preceding Last Glacial Maximum, the Sahara and Sahel had been extremely dry. The Sahara contained extensive dune fields and was mostly uninhabited. It was much larger than today, extending 500-800 kilometres (310-500 mi) farther south to about 12° northern latitude, and its lakes and rivers such as Lake Victoria and the White Nile were either dry or at low levels.
The AHP led to a widespread settlement of the Sahara and the Arabian Desert, and had a profound effect on African cultures, such as 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 ships in the world, and rock paintings such as those in the Cave of Swimmers and in the Acacus Mountains.
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Earlier humid periods in Africa were postulated after the discovery of these rock paintings in now-inhospitable parts of the Sahara. 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, 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. The term "African humid period" (AHP) was coined in 2000 by Peter B.
Humans created rock art such as petroglyphs and rock paintings in the Sahara, perhaps the largest density of such creations in the world. Scenes include animals and everyday life such as swimming which supports the presence of past wetter climates. One well-known such petroglyph location is the Cave of Swimmers in the Gilf Kebir mountains of Egypt; other well known sites are the Gabal El Uweinat mountains also of Egypt, Arabia and the Tassili n'Ajjer in Algeria where rock paintings from this time have been discovered. Humans also left artifacts such as Fesselsteine and ceramics in what today are inhospitable deserts.
Cave of Swimmers in the Gilf Kebir mountains of Egypt.
Glaciation in Africa
Despite its tropical location, Africa experienced glaciation during the Pleistocene, primarily in its high-altitude regions. The current decaying glaciers of Mount Kenya, Mount Kilimanjaro, and the Ruwenzori Range in east and central Africa were larger.
A glacier on Mount Kilimanjaro.
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Extensive glaciers covered the High Atlas mountains in Morocco during the late Pleistocene. On the northern escarpments of the Marrakech High Atlas, a series of cirques perched at ~3000-3500 m above sea level (asl) fed their valley glaciers that, in some cases, extended to as low as 2000 m asl.
Cosmogenic exposure dating with 10Be and 36Cl has shown that at least three phases of glaciation are preserved in glacial deposits over the last glacial cycle at 50, 22, and 12 ka, which appear to correlate with marine isotope stage (MIS) 3, the global Last Glacial Maximum (LGM), and the Younger Dryas chronozone.
The glaciers associated with these three phases of advance had equilibrium line altitudes (ELAs) of 2761 m asl (ca. 50 ka), 2919 m asl (ca. 22 ka), and 3213 m asl (ca. 12 ka). Glacier-climate modeling suggests that all of these phases were driven by both colder temperatures and wetter conditions than today. The dominant moisture supply to these glaciers in all phases would have been sourced from Atlantic depressions.
Climate Variability during the AHP
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, extending over both East Africa, where lake levels dropped in eastern Africa, southern Africa, equatorial Africa and West Africa.
In the 2010s the idea has taken hold that the end of the African humid period occurred from north to south in a stepwise fashion. In northeastern Asia, the western Sahara and east Africa the humid period ended within 500 years with a one-step drying 6,000 - 5,000 years ago north of the present-day monsoon belt.
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.
Other lakes such as Nakuru, Turkana, Lake Chew Bahir, Lake Abbe and Lake Zway also dropped between 5,400 and 4,200 years ago. 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. In Lake Bosumtwi the African humid period ended about 3,000 years ago after a brief moistening between 5,410 ± 80 years ago that ended 3,170 ± 70 years ago.
Pleistocene Fauna and Flora
The severe climatic changes during the Ice Age had major impacts on the fauna and flora. With each advance of the ice, large areas of the continents became depopulated, and plants and animals retreating southwards in front of the advancing glacier faced tremendous stress. The most severe stress resulted from drastic climatic changes, reduced living space, and curtailed food supply.
A major extinction event of large mammals (megafauna), which included mammoths, mastodons, saber-toothed cats, glyptodons, the woolly rhinoceros, various giraffids, such as the Sivatherium; ground sloths, Irish elk, cave lions, cave bears, Gomphotheres, American lions, dire wolves, and short-faced bears, began late in the Pleistocene and continued into the Holocene. Neanderthals also became extinct during this period.
The first zebras, for instance, seem to have entered the continent around 2.5 million years ago, just as the Pleistocene got underway. They were followed by white rhinos, giraffes, elephants, lions, leopards, cheetahs, jackals, and modern hyenas. In many cases, especially among the carnivores, these are the exact same species we have now.
Among the largest were the long-horned buffaloes (Pelorovis spp.), a group of three or four species that include some of the largest bovines there have ever been. Of similar height, but with long legs and a slimmer body, was the giant hartebeest (Megalotragus issaci). Rather more distinctive perhaps was Sivatherium, a sort of short-necked giraffe.
Sivatherium maurusium, a short-necked giraffe that lived during the Pleistocene.
Human Evolution during the Pleistocene
The evolution of anatomically modern humans took place during the Pleistocene. At the beginning of the Pleistocene Paranthropus species were still present, as well as early human ancestors, but during the lower Palaeolithic they disappeared, and the only hominin species found in fossilic records is Homo erectus for much of the Pleistocene.
Acheulean lithics appear along with Homo erectus, some 1.8 million years ago, replacing the more primitive Oldowan industry used by Australopithecus garhi and by the earliest species of Homo.
Elizabeth Vrba claims that there was a pulse-like turnover in faunal populations in East Africa at ~2.5 mya, in response to climatic change. Termed her “Turnover Pulse Hypothesis,” she finds evidence of an increase in bovids (cow-like animals like wildebeests and water buffalo) and their predators and a decrease in forest-dwelling ungulates, corresponding to an increase in dry savanna environment.
The aridification and cooling trends of the preceding Neogene were continued in the Pleistocene. At the end of the preceding Pliocene, the previously isolated North and South American continents were joined by the Isthmus of Panama, causing a faunal interchange between the two regions and changing ocean circulation patterns, with the onset of glaciation in the Northern Hemisphere occurring around 2.7 million years ago.
The End of the Pleistocene and Future Implications
The end of the Pleistocene corresponds with the end of the last glacial period and also with the end of the Paleolithic age used in archaeology. The end of the Younger Dryas is the official start of the current Holocene Epoch. Climate change at high latitudes has been proposed as a cause for the end of the AHP.
Some simulations of global warming and increased carbon dioxide concentrations have shown a substantial increase in precipitation in the Sahel/Sahara. This and the increased plant growth directly induced by carbon dioxide could lead to an expansion of vegetation into present-day desert, although it would be less extensive than during the mid-Holocene and perhaps accompanied by a northward shift of the desert.
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. 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.
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