page_title: African humid period
section_title: Orbital changes
hierarchical_section_title: African humid period / Causes / Orbital changes
caption_attribution_description: English: This figure shows the variations in Earth's orbit, the resulting changes in solar energy flux at high latitude, and the observed glacial cycles. According to Milankovitch Theory, the precession of the equinoxes and the apsides, variations in the tilt of the Earth's axis (obliquity) and changes in the eccentricity of the Earth's orbit are responsible for causing the observed 100 kyr cycle in ice ages by varying the amount of sunlight received by the Earth at different times and locations, particularly high northern latitude summer. These changes in the Earth's orbit are the predictable consequence of interactions between the Earth, its moon, and the other planets. The orbital data shown are from Quinn et al. (1991). Principal frequencies for each of the three kinds of variations are labeled. The solar forcing curve (aka "insolation") is derived from July 1st sunlight at 65 °N latitude according to Jonathan Levine's insolation calculator [1]. The glacial data are from Lisiecki and Raymo (2005) and gray bars indicate interglacial periods, defined here as deviations in the 5 kyr average of at least 0.8 standard deviations above the mean.
context_page_description: The African humid period 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 Earth's orbit around the Sun; changes in vegetation and dust in the Sahara which strengthened the African monsoon; and increased greenhouse gases, which may, or may not, imply that anthropogenic global warming could result in a shrinkage of the Sahara desert.
  During the preceding last glacial maximum, the Sahara contained extensive dune fields and was mostly uninhabited. It was much larger than today, but 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.
context_section_description: The African humid period has been explained by increased insolation during Northern Hemisphere summer. Due to precession, the season at which Earth passes closest to the Sun on its elliptical orbit – the perihelion – changes, with maximum summer insolation occurring when this happens during Northern Hemisphere summer. 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%, resulting in the African monsoon becoming both stronger and reaching farther north. Between 15,000 and 5,000 years ago, summer insolation was at least 4% higher than today. The obliquity also decreased during the Holocene but the effect of obliquity changes on the climate is focused on the high latitudes and its influence on the monsoon is unclear.
  During summer, solar heating is stronger over the North African land than over the ocean, forming a low pressure area that draws moist air and precipitation in from the Atlantic Ocean. This effect was strengthened by the increased summer insolation, leading to a stronger monsoon that also reached farther north. The effects of these circulation changes reached as far as the subtropics.
  Obliquity and precession are responsible for two of the foremost Milankovich cycles and are responsible not only for the onset and cessation of ice ages but also for monsoon strength variations. Southern Hemisphere monsoons are expected to have the opposite response of Northern Hemisphere monsoons to precession, as the insolation changes are reversed; this observation is borne out by data from South America. The precession change increased seasonality in the Northern Hemisphere while decreasing it in the Southern Hemisphere.
caption: Milankovich cycles over the past one million years
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