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Dust and Monsoons

3 February 2021

William sent in a link to this story. It is about scientists trying to understand why it was greener in the Sahara during the early to mid Holocene – quite unlike the desert conditions of today. The study, in Science Advances, can be seen at https://advances.sciencemag.org/content/2/11/e160045 … They first tried to model it into orbital changes of the earth as it circles the sun, in order to see why the monsoon track may have shifted, or progressively became less abundant. We are told that evidence of substantial human settlement occupying the Sahara during phases of strong monsoon activity, is a result of the wobble in earth's axis as it moved on its orbit aroound the sun. Then, in one sentence, we learn, the mysterious thing is, if you try to simulate all these changes in early to mid Holocene climate, the models intensify the monsoon, as expected – but nowhere near the amounts suggested by the paleo data. You don't get that idea from reading a textbook on the subject, or in a school curriculum. Yet, this has been recognised for a number of years. Hence, the switch in climate cannot be put down solely to the Milankovitch effect. Something else was going on. The researchers decided to look at the role of dust blown from the Sahara across the pond to the Americas, much of it falling into the Atlantic along the way. They focussed on soil and sediment loading in the Bahamas and discovered the plume of Sahara dust in the early Holocene was half of what it is in the modern world. This is not surprising if the Sahara was much wetter, with rivers and lakes. We may also wonder how dust was raised in that kind of environment. One can see why in a desert, that is bone dry, but in a wet environment. Well. Where would that dust actually come from? Was it dry and arid somewhere else?

The idea of course is to prop up the mainstream view that orbital changes are responsible not just for greening the Sahara but for the Ice Age ebb and flow. It is important to keep the mantra going, I suppose, as it has become a fundamental of mainstream geology and paleo climate research. They concluded that by reducing the amount of dust in the atmosphere this would allow greater warming of the planet and therefore affect water vapour levels, and precipitation. When added to the orbital change calculation they hope to demonstrate the two things, combined, can explain away the mystery. On the other hand, something else might be going on – as yet not dreamed up. We should also bear in mind that changes to the actual axis of rotation, could more easily explain the conundrum – but playing around with the poles and axis of rotation is not allowed in mainstream theories.


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