Sahara desert from space [image credit: NASA]
We’re informed that ‘scientists have identified more than 230 of these greenings occurring about every 21,000 years over the past eight million years.’ (Ice ages may account for the numerical shortfall). Climate models aren’t able to simulate these greenings, or at least not their magnitude. The period they identify is the combined precession cycle, i.e. the beat period of the tropical and anomalistic orbits (years) of the Earth (when the difference between the number of each reaches 1). Inevitably it’s about the energy received from the Sun.
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Algeria’s Tassili N’Ajjer plateau is Africa’s largest national park, says The Conversation (via Phys.org).
Among its vast sandstone formations is perhaps the world’s largest art museum. Over 15,000 etchings and paintings are exhibited there, some as much as 11,000 years old according to scientific dating techniques, representing a unique ethnological and climatological record of the region.
Curiously, however, these images do not depict the arid, barren landscape that is present in the Tassili N’Ajjer today.
Instead, they portray a vibrant savannah inhabited by elephants, giraffes, rhinos and hippos. This rock art is an important record of the past environmental conditions that prevailed in the Sahara, the world’s largest hot desert.
These images depict a period approximately 6,000–11,000 years ago called the Green Sahara or North African Humid Period. There is widespread climatological evidence that during this period the Sahara supported wooded savannah ecosystems and numerous rivers and lakes in what are now Libya, Niger, Chad and Mali.
This greening of the Sahara didn’t happen once. Using marine and lake sediments, scientists have identified more than 230 of these greenings occurring about every 21,000 years over the past eight million years. These greening events provided vegetated corridors which influenced species’ distribution and evolution, including the out-of-Africa migrations of ancient humans.
These dramatic greenings would have required a large-scale reorganization of the atmospheric system to bring rains to this hyper arid region. But most climate models haven’t been able to simulate how dramatic these events were.
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Earth’s changing orbit
The fact that the wetter periods in north Africa have recurred every 21,000 years or so is a big clue about what causes them: variations in Earth’s orbit.
Due to gravitational influences from the moon and other planets in our solar system, the orbit of the Earth around the sun is not constant. It has cyclic variations on multi-thousand year timescales.
These orbital cycles are termed Milankovitch cycles; they influence the amount of energy the Earth receives from the sun.
Full article here.
via Tallbloke’s Talkshop
December 18, 2023 at 01:12PM
Iowa Climate Science Education 