Timothy Birdnow

So if the Sahara periodically turns green because of orbital precession, why do we dismiss such a forcing for the modest planetary warming we saw in the late 20th and early 21st centuries?

The Sahara’s Secret Past: When Deserts Turned Green

FTA:

A pioneering study has unveiled new insights into the humid phases in North Africa over the last 800,000 years, explaining why the Sahara Desert periodically transformed green.

The study, recently published in the journal Nature Communications, showed periodic wet phases in the Sahara were driven by changes in Earth’s orbit around the Sun and were suppressed during the ice ages.

For the first time, climate scientists simulated the historic intervals of ‘greening’ of the Sahara, offering evidence for how the timing and intensity of these humid events were also influenced remotely by the effects of large, distant, high-latitude ice sheets in the Northern Hemisphere.

Lead author Dr Edward Armstrong, a climate scientist at the University of Helsinki and the University of Bristol, said: "The cyclic transformation of the Sahara Desert into savannah and woodland ecosystems is one of the most remarkable environmental changes on the planet.

[...]


A recent study reveals that North African humid periods, when the Sahara Desert turned green, occurred due to Earth’s orbital precession around the Sun and were suppressed during ice ages. This research, utilizing a new climate model, advances the understanding of these humid periods and the significant environmental changes in the Sahara.

A pioneering study has unveiled new insights into the humid phases in North Africa over the last 800,000 years, explaining why the Sahara Desert periodically transformed green.

The study, recently published in the journal Nature Communications, showed periodic wet phases in the Sahara were driven by changes in Earth’s orbit around the Sun and were suppressed during the ice ages.

For the first time, climate scientists simulated the historic intervals of ‘greening’ of the Sahara, offering evidence for how the timing and intensity of these humid events were also influenced remotely by the effects of large, distant, high-latitude ice sheets in the Northern Hemisphere.

Lead author Dr Edward Armstrong, a climate scientist at the University of Helsinki and the University of Bristol, said: "The cyclic transformation of the Sahara Desert into savannah and woodland ecosystems is one of the most remarkable environmental changes on the planet.

"Our study is one of the first climate modeling studies to simulate the African Humid Periods with comparable magnitude to what the palaeoclimate observations indicate, revealing why and when these events occurred.”

There is widespread evidence that the Sahara was periodically vegetated in the past, with the proliferation of rivers, lakes, and water-dependent animals such as hippos, before it became what is now desert. These North African Humid Periods may have been crucial in providing vegetated corridors out of Africa, allowing the dispersal of various species, including early humans, around the world.

The so-called ‘greenings’ are thought to have been driven by changes in Earth’s orbital conditions, specifically Earth’s orbital precession. Precession refers to how Earth wobbles on its axis, which influences seasonality (i.e. the seasonal contrast) over an approximate 21,000-year cycle. These changes in precession determine the amount of energy received by the Earth in different seasons, which in turn controls the strength of the African Monsoon and the spread of vegetation across this vast region.
Changes of Vegetation Between Humid and Arid Phases in North Africa

Changes of vegetation between humid and arid phases in North Africa. Vegetation zones are based on the minimum precipitation requirements of each vegetation type. Credit: Jani Närhi / University of Helsinki

A major barrier to understanding these events is that the majority of climate models have been unable to simulate the amplitude of these humid periods, so the specific mechanisms driving them have remained uncertain.

[...]

The results confirm the North African Humid Periods occurred every 21,000 years and were determined by changes in Earth’s orbital precession. This caused warmer summers in the Northern Hemisphere, which intensified the strength of the West African Monsoon system and increased Saharan precipitation, resulting in the spread of savannah-type vegetation across the desert.

The findings also show the humid periods did not occur during the ice ages when there were large glacial ice sheets covering much of the high latitudes. This is because these vast ice sheets cooled the atmosphere and suppressed the tendency for the African monsoon system to expand. This highlights a major teleconnection between these distant regions, which may have restricted the dispersal of species, including humans, out of Africa during the glacial periods of the last 800,000 years.

End excerpts.

Both Herodotus in 440 BC and Strabo in 23 AD discussed this greening in their writings, which suggests that there is cyclical increases in rainfall in the region.

So, warmer summers in the Northern Hemisphere led to this greening. And all without carbon dioxide. Amazing! This warming related to the Earth's orbit.

But, but, but...it's all industrial emissions and we're all going to DIEEEEEEE.....

BTW this directly contradicts the ridiculous claims that it is industrial emissions causing the greening in the Sahara.

Posted by: Timothy Birdnow at 09:32 AM | Comments (45) | Add Comment
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