The Sahara may flip from desert to grass every 20,000 years. Blame The Sun.

MIT researchers think they have solved a bit of a mystery regarding Sahara dust, but if they’re right it means the Sahara Desert has already come and gone 3 – 5 times since humans walked the Earth. The Sahara is the largest desert on Earth, and this would be the largest and longest drought “ever” on the planet (as far as we know).

UPDATED: Commenter Javier points out these drying cycles were known years ago. (See below)

This would rather redefine the whole idea of “climate change” — 3.5 million square miles of Green Sahara turns into Dust-bowl Sahara — and it’s all thanks to sunlight. The drought doesn’t just last 7 years, but more like 7,000. And it’s happening over 9 million square kilometers, an area larger than Australia. The major climate models leaned towards the monsoonal cycle, rather than the longer ice age one. So this theory may have resolved one of the 495 contradictions in climate models. Or not. But the bigger message here is that the sun causes climate change and on a massive scale.

h/t to Roger Tallbloke.

The Sahara is the largest dust bowl in the world, dumping 10 million trucks of dust across North Africa and the Atlantic Ocean. The dust can end up as far away as Florida. People tracked the layers of this dust before but thought the highs and lows were driven by ice-ages on a 100,000 year cycle. This new work took a closer look at the layers on the ocean and say they see 20,000 year cycles, which fit with cycles of solar insolation (meaning the strength of sunlight as the Earth rolls around on its axis). It sounds like a long bow figuring out whether there were trees and rain in the Sahara based on layers of dirt on the ocean floor, so keep our skeptical hats on. It could turn out to be wrong. But then again, they are looking at 240,000 years of mud and it fits better with other proxies too, so it has some redeeming features.

Monsoons vary with the solar insolation, as does the level of the Nile River and other stuff:

This finding is consistent with a wide range of other monsoon proxies from North and East Africa that show high-amplitude precessional variability (Fig. 4). Sapropels in the eastern Mediterranean (Fig. 1), a marker of high Nile River discharge, occur during each of the dust flux minima in our record over the past 240 ka and show clear precessional pacing (Fig. 4) (28). Similarly, δD values in leaf waxes from the Gulf of Aden (Fig. 1), a tracer of monsoon intensity in East Africa (17), show strong similarities with our record (Fig. 4). Records of the accumulation of windblown freshwater diatoms in Tropical Atlantic sediments, a tracer of the desiccation of Saharan lakes at the end of wet periods, also show precessional variability with little power at 41- and 100-ka periods (18).

It always makes me suspicious when the researchers cite failed climate models as another reason to believe:

But McGee says this [older] interpretation of the sediment cores chafes against climate models, which show that Saharan climate should be driven by the region’s monsoon season, the strength of which is determined by the tilt of the Earth’s axis and the amount of sunlight that can fuel monsoons in the summer.

The researchers think previous teams missed the 20k cycle because some dust spikes were there for a different reason:

“What we found was that some of the peaks of dust in the cores were due to increases in dust deposition in the ocean, but other peaks were simply because of carbonate dissolution and the fact that during ice ages, in this region of the ocean, the ocean was more acidic and corrosive to calcium carbonate,” McGee says. “It might look like there’s more dust deposited in the ocean, when really, there isn’t.”

Wait — so cold ice-ages with low CO2 levels make that part of the ocean acidic? Seriously?

Is that the sound of another sacred cow dying?

UPDATE: Javier points out that the 20,000 year cycle was reported 30 years ago

OK, this is anything but new. It has been known since the mid-1980′s that the Sahara greening and drying follows the 23 kyr precession cycle because precession-linked insolation sets the position of the Inter-Tropical Convergence Zone, the climatic equator that determines how far North the West-African Monsoon reaches.

Martine Rossignol-Strick worked out this relationship in 1985 in her article:

Rossignol-Strick, M. (1985). Mediterranean Quaternary sapropels, an immediate response of the African monsoon to variation of insolation. Palaeogeography, palaeoclimatology, palaeoecology, 49(3-4), 237-263.

In this article from 2012: Green Sahara: African Humid Periods Paced by Earth’s Orbital Changes, Nature, 2012

We can read:

“Longer stratigraphic sections extending as far back as the Miocene (Hilgen et al., 1995) contain hundreds of these sapropel layers, each representative of an earlier African humid phase and each paced by the characteristic orbital precession beat of 20,000 years (Figure 4). These sapropel layers are packaged into groups of 4–5 (Figure 4), reflecting the influence of slight changes in the shape of the Earth’s orbit around the sun (eccentricity) on modulating the strength of the precessional beat.”

I hate this press releases that make it look like they have discovered something new, when in reality they just got more evidence for what is already known.

MIT press release | Eureka Alert Press Release. | Science Daily

REFERENCE

Skonieczvy, C, McGee, D, other people (2019) Science Advances Vol. 5, no. 1, eaav1887 DOI: 10.1126/sciadv.aav1887

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