The African humid period (AHP; also known by other names) is a climate period in Africa during the late Pleistocene and Holocene geologic epochs, when northern Africa was wetter than today. The covering of much of the Sahara desert 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 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 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. 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 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. 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 history
Herodotus in 440 BC and Strabo in 23 AD discussed the existence of a greener Sahara, although their reports were at first questioned owing to their anecdotal nature. In 1850 the researcher Heinrich Barth discussed the possibility of past climate change leading to increased wetness in the Sahara after discovering petroglyphs in the Murzuq Desert, and further discoveries of petroglyphs led desert explorer László Almásy to coin the concept of a Green Sahara in the 1930s. Later in the 20th century, conclusive evidence of a past greener Sahara, the existence of lakes 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 already advanced in 1921, and 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 correlate with glacial stages ("pluvial hypothesis") before radiocarbon dating became widespread.
The development and existence of the African humid period has been investigated with archaeology, climate modelling and paleoproxies, with archaeological sites, dunes and deposits left by lakes, aeolian deposits and leaf wax in the sea and wetlands playing an important role. Pollen, lake deposits and former levels of lakes have been used to study the ecosystems of the African humid period, and charcoal and leaf impressions have been used to identify vegetation changes. 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. Most recently, the effects of the Sahara greening on other continents has drawn scientific attention.