Green Sahara Periods

I subscribed to The Economist magazine for awhile.  It’s an excellent magazine for news about world affairs.  I’m interested in world affairs, but not to the depth it gets covered in this periodical, and I recently decided not to renew my subscription.  Some of their articles are redundant because they’ll often have 2 articles in  1 issue about the same subject that say the same thing.  Their articles are also far too wordy.  The editors of this magazine need to learn how to be more succinct.  They could probably cut the word count of their articles by 75% and not lose anything in the translation.  In 1 of the last issues I read there was an article (actually 2) about the expansion of the Sahara desert.  The unnamed author of this article assumed the expansion of the Sahara desert was caused by man-made climate change.  His assumption was just plain ignorant.  Astronomically forced insolation can entirely explain the expansion and retraction of the Sahara desert.

Image result for Green Sahara Periods

Map comparing vegetation of North Africa during dry and humid climate cycles.  The Sahara desert becomes a lush environment at regular intervals that last for about 6,000 years.  The current natural cycle causes the present day arid conditions.

Scientists have determined  the Sahara desert becomes a lush environment with lightly wooded grasslands, lakes, and rivers at cyclical intervals.  They refer to these times as Green Sahara Periods.  Animal life colonizes the region during Green Sahara Periods, and the environment resembles the Serengeti Plain rather than the desert it is today.  The increase in moisture that transforms the desert into a rich natural community is caused by the 23,000 year variation in the earth’s wobble.  The earth normally spins like a top, and like the child’s toy this spin can wobble.  The wobble leads to a seasonal variation when the earth is closest to the sun (perihelion).  At the point in the cycle when earth is closest to the sun during summer, the amount of solar heat increases in this region.  This differential heating of the atmosphere causes low pressure systems to form over the Sahara, drawing in monsoonal precipitation from the Atlantic Ocean.  10 times more rain falls on the Sahara during humid periods than during present day conditions.  This transforms the region into a much more inhabitable environment.

Evidence of Green Sahara Periods dates to the late Miocene ~9 million years BP.  The last 4 Green Sahara Periods occurred from 6,000-10,000 years BP; 77,000-81,000 years BP; 102,000-108,000 years BP; and 122,000-128,000 years BP.  Notice 1 cycle was skipped.  This was during the Last Glacial Maximum when the earth was particularly arid.  Perhaps, other factors outweighed the 23,000 year cycle.  Scientists have noticed Green Sahara Periods have become less frequent since the mid-Pleistocene.

Evidence for Green Sahara Periods can be found on the land and in the ocean.  Explorers crossing the Sahara desert find dry lake beds and river drainages; and there are many rock paintings depicting scenes rich in wildlife that no longer occur in the region.  Samples of cores drilled from the ocean bottom find fluctuations in dust levels at regular intervals.  Sediment dated to dry periods contains high amounts of dust blown from land in sand storms.  Of course, the amount of sand greatly decreases during humid periods.  The shells of microscopic sea creatures, known as foraminifera, excavated from ocean cores also show isotopic variations that relate to changes in precipitation.

Image result for Sahara desert cave paintings

Rock painting in the Sahara desert depicting giraffes, goats, dogs, and people.  Giraffes no longer occur in this region.

The Green Sahara Periods influenced human history.  The rich environment allowed humans to expand from Africa into Asia across the Levantine gateway.  During desert cycles hunter-gatherers could not cross from 1 continent to the other.  1 study suggests humans may be hastening the expansion of the desert by grazing their livestock on the edges of the desert.  I don’t buy this.  Humans likely play a minor role compared to the natural cycle.  If precipitation increased, the desert would begin retracting, regardless of human activity.

Reference:

Larrasoara, J; A. Roberts, E. Rohlirn

“Dynamics of Green Sahara Periods and their role in Hominim Expansion”

Plos One 2013

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2 Responses to “Green Sahara Periods”

  1. ina puustinen-westerholm Says:

    Terrific way to start moi day here..thank you. I can drink my coffee..and ‘think’..about a 4 mile distance away..to a nice Baptist family..who are totally..on board with a ..6 thousand year old earth. The reading of your ..two articles..the same..routine. I am guessing that they are from two diff. life styles /cultureal areas..so..the ..too much wordiness..may have..a decent value..if ..reading between the lines..so to speak. Sometimes..nuance..or..slight ..variation..may make..a stark difference. Jus saying. again..thanks much. ina

  2. indianeskitchen Says:

    Fascinating information about the Sahara Desert! Thanks for sharing it!

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