用户:DrizzleD/非洲湿润时期
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The African humid period (AHP) is a climate period in Africa during the Holocene during which northern Africa was wetter than today. It was caused by changes in Earth's orbit around the Sun, and involved changes in vegetation and dust in the Sahara that altered the African monsoon, the disappearance of much of the Sahara desert which was replaced by grassy vegetation, trees and lakes and the settlement of the former desert by various animals and humans, who lived as hunter-gatherers. It has had profound effects on present-day Africa such as the birth of the Pharaonic civilization and the pyramids and potentially also the development of widespread Golden Age myths.
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Before the African humid period, during the last glacial maximum, the Sahara was much larger than today and had extensive dune fields; many lakes and rivers such as Lake Victoria and the White Nile were either dry or at low levels and the Sahara mostly unhabitated. The African humid period commenced about 14,600–14,500 years ago at the end of Heinrich event 1 and concomitant 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 fluctuations occurred, one during the Younger Dryas and the other during the 8.2 kiloyear event during both of which temporarily drier conditions returned across Africa. The end of the African humid period came about 6,000–5,000 years ago during the Piora Oscillation cold period when the Sahara occupied its present position. While some evidence points to an end 5,500 years ago in the Sahara, in the Sahel, Arabia and East Africa the end of humidity appears to have taken place in several steps such as the 4.2 kiloyear event.
Earlier than the African humid period, humid periods in Africa had influenced the evolution of modern humans; the African humid period now led to a widespread settlement of the Sahara and the Arabian Deserts by humans. These at first lived on animals and plants naturally occurring in the region; later they started domesticating animals such as cattle, goats and sheep. They have left archeological sites and artifacts such as one of the oldest ships in the world; but in particular they created rock paintings such as those in the Cave of Swimmers and in the Acacus Mountains; in fact the existence of earlier wet periods was postulated after the discovery of these rock paintings in now-inhospitable parts of the Sahara. When the African humid 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.
The African humid period was part of a phase where monsoon activities were stronger across the Northern Hemisphere, from the Mojave Desert in North America over Africa and the Middle East to India and China. The ultimate reason was the precession of Earth's orbit around the Sun, which shifted the season during which Earth is closest to the Sun towards Northern Hemisphere summer, increasing summer insolation and the strength of the monsoons that depend on it. This alone was not enough to make the Sahara disappear; other processes, among others the ability of vegetation and large lakes in the desert to reduce the emission of reflecting dust and to increase the amount of sunlight absorbed by the ground, did play a role in its onset. Decreased summer insolation as the Holocene progressed also brought the African humid period to an end. Increased greenhouse gases during the Holocene appear to have aided in the onset of the African humid period; this may imply that anthropogenic global warming will also result in a shrinkage of the Sahara desert.