Tassili N’Ajjer plateau, Algeria.
A once fertile savannah with lakes and rivers.
A once fertile savannah with lakes and rivers.
Having visited the Sahara Desert in April about 10 years ago, I can assure readers that it is not the hot, dry place of repute but can be cold and wet, at least in the Tunisian part. It was so cold with a fine drizzle, that, shivering in a t-shirt, I offered to buy the thick, hooded duffle coat a local troglodyte guide was wearing, but he quoted me 5000 dinars (about £400) with a knowing twinkle in his eye. I elected to shiver until I got back on the coach and the driver turned the heating on. Yes, there are troglodytes in Tunisia!
There have been times in the past when rain in the Sahara was not just freak weather in Spring to annoy tourists, but the norm in much of the year, so much so that the Sahara was mixed savannah and scrub with lakes and rivers, especially the western part.
Readers may recall how I mentioned the periodic greening of the Sahara in my article about the evolution of rock doves and feral pigeons. Briefly, a species of dove resident in West Africa crossed the Sahara during one such period when there was forest, grassland and water in place of sand. Then when the Sahara became desert the two populations diverged and the one which had made it as far as the Middle East hybridized with a resident related dove. This hybrid quickly became the normal form of the rock dove north and east of the Sahara and diversified further into several subspecies, one of which was domesticated and selectively bred to produce lots of different varieties. Some of these eventually reverted to a feral existence and became the ubiquitous town pigeon with a very different lifestyle and habitat to the original rock dove.
This process of African species moving into and across the Sahara during these periods of greening, and then becoming isolated from the African population, is known as the Saharah Pump and accounts for some of the sub-Saharan African species having a closely related counterpart in North Africa and Eurasia.
And this process has been going on since about 8 million years before creationists think Earth was created and may account for the migration of humans out of Africa some 40-50,000 years ago. More recently, however, there was certainly a population of humans living in a green and fertile Sahara up to at least 11,000 years ago (i.e., at least 1,000 years before 'Creation Week'. We know this because they left a record in rock carvings at Tassili N’Ajjer plateau in present-day Algeria, which show us some of the animals that lived there too.
These periods of greening have occurred approximately every 21,000 years and now two geoscientists from Helsinki University, Finland together with colleagues at Birmingham and Bristol universities, UK., have developed a climate model that explains how the climate changed so regularly and so radically. They have published their findings, open access, in Nature Communications. One of them, Edward Armstrong of Helsinki University, has also written an article about their research in The Conversation. His article is reprinted here under a Creative Commons licence, reformatted for stylistic consistency:
The Sahara Desert used to be a green savannah – new research explains why
Rock carving of an elephant at the Tassili N'Ajjer National Park, Tadrart Rouge, in Algeria.
Eric Lafforgue/Getty Images
Edward Armstrong, University of Helsinki
Algeria’s Tassili N’Ajjer plateau is Africa’s largest national park. 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 over 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 reorganisation 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.
As a team of climate modellers and anthropologists, we have overcome this obstacle. We developed a climate model that more accurately simulates atmospheric circulation over the Sahara and the impacts of vegetation on rainfall.
We identified why north Africa greened approximately every 21,000 years over the past eight million years. It was caused by changes in the Earth’s orbital precession - the slight wobbling of the planet while rotating. This moves the Northern Hemisphere closer to the sun during the summer months.
This caused warmer summers in the Northern Hemisphere, and warmer air is able to hold more moisture. This intensified the strength of the West African Monsoon system and shifted the African rainbelt northwards. This increased Saharan rainfall, resulting in the spread of savannah and wooded grassland across the desert from the tropics to the Mediterranean, providing a vast habitat for plants and animals.
Our results demonstrate the sensitivity of the Sahara Desert to changes in past climate. They explain how this sensitivity affects rainfall across north Africa. This is important for understanding the implications of present-day climate change (driven by human activities). Warmer temperatures in the future may also enhance monsoon strength, with both local and global impacts.
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.
On 100,000-year cycles, the shape of Earth’s orbit (or eccentricity) shifts between circular and oval, and on 41,000 year cycles the tilt of Earth’s axis varies (termed obliquity). Eccentricity and obliquity cycles are responsible for driving the ice ages of the past 2.4 million years.
The third Milankovitch cycle is precession. This concerns Earth’s wobble on its axis, which varies on a 21,000 year timescale. The similarity between the precession cycle and the timing of the humid periods indicates that precession is their dominant driver. Precession influences seasonal contrasts, increasing them in one hemisphere and reducing them in another. During warmer Northern Hemisphere summers, a consequent increase in north African summer rainfall would have initiated a humid phase, resulting in the spread of vegetation across the region.
Eccentricity and the ice sheets
In our study we also identified that the humid periods did not occur during the ice ages, when large glacial ice sheets covered much of the polar regions. This is because these vast ice sheets cooled the atmosphere. The cooling countered the influence of precession and suppressed the expansion of the African monsoon system.
The ice ages are driven by the eccentricity cycle, which determines how circular Earth’s orbit is around the sun. So our findings show that eccentricity indirectly influences the magnitude of the humid periods via its influence on the ice sheets. This highlights, for the first time, a major connection between these distant high latitude and tropical regions.
The Sahara acts as a gate. It controls the dispersal of species between north and sub-Saharan Africa, and in and out of the continent. The gate was open when the Sahara was green and closed when deserts prevailed. Our results reveal the sensitivity of this gate to Earth’s orbit around the sun. They also show that high latitude ice sheets may have restricted the dispersal of species during the glacial periods of the last 800,000 years.
Our ability to model the African humid periods helps us understand the alternation of humid and arid phases. This had major consequences for the dispersal and evolution of species, including humans, within and out of Africa. Furthermore, it provides a tool for understanding future greening in response to climate change and its environmental impact.
Refined models may, in the future, be able to identify how climate warming will influence rainfall and vegetation in the Sahara region, and the wider implications for society.
Edward Armstrong, Postdoctoral research fellow, University of Helsinki
This article is republished from The Conversation under a Creative Commons license. Read the original article.
AbstractSo much history that is part of the real story of life on Earth, and creationists frauds fool their dupes into believing in magic instead. And all for money and the political power controlling what other people think gives them.
The Sahara region has experienced periodic wet periods over the Quaternary and beyond. These North African Humid Periods (NAHPs) are astronomically paced by precession which controls the intensity of the African monsoon system. However, most climate models cannot reconcile the magnitude of these events and so the driving mechanisms remain poorly constrained. Here, we utilise a recently developed version of the HadCM3B coupled climate model that simulates 20 NAHPs over the past 800 kyr which have good agreement with NAHPs identified in proxy data. Our results show that precession determines NAHP pacing, but we identify that their amplitude is strongly linked to eccentricity via its control over ice sheet extent. During glacial periods, enhanced ice-albedo driven cooling suppresses NAHP amplitude at precession minima, when humid conditions would otherwise be expected. This highlights the importance of both precession and eccentricity, and the role of high latitude processes in determining the timing and amplitude of the NAHPs. This may have implications for the out of Africa dispersal of plants and animals throughout the Quaternary.
Introduction
There is widespread palaeoclimatological evidence indicating that the Sahara has experienced wetter periods in the past, with proliferation of vegetation, rivers and lakes into what is now desert1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16. The alternation of arid and humid phases and consequent dramatic environmental changes in this vast region forms a key biogeographic factor influencing species’ distributions and evolution17,18,19. It has also been suggested that changes in the precipitation and vegetation cover of the Sahara have consequences beyond Africa, influencing the climate system from the tropics to the Arctic11. Currently, 230 of these North African humid periods (NAHPs) have been identified in proxy records over the past 8 myr8. They were ultimately driven by the precession cycle, which governs the seasonal insolation contrast and is modulated by eccentricity, with periods of increased boreal summer insolation (i.e., precession minima) intensifying the African monsoon system6,20,21,22,23,24. Internal biogeophysical feedbacks then amplified the external orbital forcing and further enhanced precipitation15,25,26,27,28.
Fig. 3: Vegetation reconstructions during composite periods of precession maxima (PMax) and the 20 modelled North African Humid Periods.
a Output from the HadCM3BB-v1.0 dynamic global vegetation model (TRIFFID), showing only the most abundant plant function type in each grid square. b Vegetation reconstruction utilising the North African vegetation classification of Larrasoaña, et al.8 which demonstrates regional vegetation types.Armstrong, E., Tallavaara, M., Hopcroft, P.O., Valdes P.J.
North African humid periods over the past 800,000 years.
Nat Commun 14, 5549 (2023). https://doi.org/10.1038/s41467-023-41219-4
Copyright: © 2023 The authors.
Published by Springer Nature Ltd. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
Like so much else, religion provides the excuses for these criminals to abuse people in the name of greed, selfishness and self-aggrandizement, and for the ego boost they get from fooling people into believing things they know to be false.
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