Study shows the Sahara swung between lush and desert conditions every 20,000 years, in sync with monsoon activity

Posted: January 3, 2019 by oldbrew in climate, Cycles, Ice ages, research
Tags: ,

Image credit: BBC


These climatic swings (cycles) were in sync with changes in the Earth’s tilt, say the researchers. They therefore believe ice ages are not the primary factor in these swings.

The Sahara desert is one of the harshest, most inhospitable places on the planet, covering much of North Africa in some 3.6 million square miles of rock and windswept dunes.

But it wasn’t always so desolate and parched, reports Phys.org.

Primitive rock paintings and fossils excavated from the region suggest that the Sahara was once a relatively verdant oasis, where human settlements and a diversity of plants and animals thrived.

Now researchers at MIT have analyzed dust deposited off the coast of west Africa over the last 240,000 years, and found that the Sahara, and North Africa in general, has swung between wet and dry climates every 20,000 years.

They say that this climatic pendulum is mainly driven by changes to the Earth’s axis as the planet orbits the sun, which in turn affect the distribution of sunlight between seasons—every 20,000 years, the Earth swings from more sunlight in summer to less, and back again.

For North Africa, it is likely that, when the Earth is tilted to receive maximum summer sunlight with each orbit around the sun, this increased solar flux intensifies the region’s monsoon activity, which in turn makes for a wetter, “greener” Sahara.

When the planet’s axis swings toward an angle that reduces the amount of incoming summer sunlight, monsoon activity weakens, producing a drier climate similar to what we see today.

“Our results suggest the story of North African climate is dominantly this 20,000-year beat, going back and forth between a green and dry Sahara,” says David McGee, an associate professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “We feel this is a useful time series to examine in order to understand the history of the Sahara desert and what times could have been good for humans to settle the Sahara desert and cross it to disperse out of Africa, versus times that would be inhospitable like today.”

McGee and his colleagues have published their results today in Science Advances.

Continued here.

Comments
  1. JB says:

    Perhaps, on the assumption that obliquity never changed, that present conditions have always existed.

  2. Jim says:

    I believe they have it wrong as to the effect. The sun more or less, and the green desert, are opposite what they say. Less sun, less wind, green grass. More sun, less grass. The longer sun exposed season, the drier, the flat plain. The more seasonal approach, cooler, cloudier, weather inducing the grass to grow. But, an odd cycle, that hasn’t been seen. Some other driver?

  3. interzonkomizar says:

    Greetings from Big Mango (Bkk).
    Article in Nature more interesting, details …

    Green Sahara: African Humid Periods Paced by Earth’s Orbital Changes

    By: Peter B. deMenocal & Jessica E. Tierney © 2012 Nature Education
    Citation: deMenocal, P. B. & Tierney, J. E. (2012) Green Sahara: African Humid Periods Paced by Earth’s Orbital Changes. Nature Education Knowledge 3(10):12

    https://www.nature.com/scitable/knowledge/library/green-sahara-african-humid-periods-paced-by-82884405

    Sandy,
    Minister of Future

  4. oldmanK says:

    interzonkomizar says: January 3, 2019 at 5:09 pm links to a paper by P B deMenocal et al and the heading says ” — slow orbital ‘wobble’ transformed today’s Sahara desert—- “.

    Then look at this https://www.youtube.com/watch?v=LIAkJg8knTI&t=1918s at 33:00, with a clear indication that a change occurred at 5500BP and it was so abrupt that ‘slow orbital change’ is ruled out.

  5. oldbrew says:

    interzonkomizar says: January 3, 2019 at 5:09 pm

    Today, perihelion occurs in northern hemisphere winter but at 10,000 years ago (half of a precession cycle) it occurred in northern hemisphere summer, and summer radiation over North Africa was about 7% higher than it is today (Berger, 1988; Kutzbach, 1981) (Figure 2a). – [link]

    This is the key. It takes around 21,000 years for the number of tropical years to exceed anomalistic years by exactly 1, and that completes one precession cycle (of this type – it’s not the only such cycle). So max to min NH summer insolation, or vice versa, would be half that period.
    – – –
    Another study: Dynamics of Green Sahara Periods and Their Role in Hominin Evolution

    Astronomically forced insolation changes have driven monsoon dynamics and recurrent humid episodes in North Africa, resulting in green Sahara Periods (GSPs) with savannah expansion throughout most of the desert. […] We present a compilation of continental and marine paleoenvironmental records from within and around North Africa, which enables identification of over 230 GSPs within the last 8 million years.[…] Each GSP took 2–3 kyr to develop, peaked over 4–8 kyr, biogeographically connected the African tropics to African and Eurasian mid latitudes, and ended within 2–3 kyr, which resulted in rapid habitat fragmentation.

    https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0076514

  6. Stephen Richards says:

    If the African climate swings every 20000 yrs shouldn’t the climate change for the rest of the planet or at least Europe ?

  7. oldbrew says:

    Possibly the latitude of the Sahara is itself a significant factor.

  8. oldmanK says:

    An abrupt change does not occur in a slow precession cycle.

    That apart from the fact that 5500 years ago that change coincided with several other changes. Which repeated two thousand earlier.

  9. oldbrew says:

    oldmanK – the word ‘abrupt’ appears several times in interzonkomizar’s link, e.g.:

    The supply of terrigenous sediment here is dominated by this dust, and the down-core record of dust flux shows that the AHP can be clearly recognized an interval of low dust fluxes between 11,500 and 5,500 years ago (Figure 2d) (Adkins et al., 2006). A surprising discovery from this high-resolution record was that the onset and termination of the AHP appeared very abrupt, occurring within one to two centuries.

    There’s a section headed: On the Timing and Abruptness of the African Humid Period (AHP) Transitions

  10. interzonkomizar says:

    Greetings from Big Mango (Bkk). No. Africa and So. India are the only large, near equatorial land masses whose monsoon was most affected.

    Sandy,
    Minister of Future

  11. oldmanK says:

    The Jan 2019 paper is by the same author who published this one in 2013 : https://phys.org/news/2013-04-abrupt-widespread-climate-shift-sahara.html#nRlv It also refers to abrupt changes 5000 yrs ago. (Not the 20,000 in the later paper)

    interzonkomizar’s linked paper and the video link are by the same person, Dr P deMenocal. It is in the deMenocal paper of 2012 (interzonkomizer link) were it is mentioned the 20,000 year precession wobble (note that in the last 450k yrs ice-ages followed-somewhat??-the 100k eccentricity). None of that produces abrupt changes/forcings.

    The recent paper mentioned ocean sediment cores. The Med sea-bed sediment was investigated, and what was evident was that there are distinct sapropel deposits the dating of which shows repeated abrupt changes, more frequent than seen from ‘speculating’ on one single piece of evidence. Example here: https://melitamegalithic.wordpress.com/2018/04/24/searching-evidence-2/ where distinct sapropel layers indicate major repeat events. The 5500 (3550bce) deMenocal date for the Sahara is there as well.

    The deMenocal paper makes a link to the earth wobble, liking it to a top. There is a major difference between the dynamics of the two cases. The top is unstable with gravity acting along its axis of precession, acting so to topple; the earth is effected by the sun’s gravity perpendicular to it axis of precession, acting to reduce wobble. Which makes earth wobble due to different reasons, IMO not orbital (see https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/95PA00578 ) The deMenocal paper in fig 2 (A) starts by comparing to insolation – (also using scale that visually misleads). More likely its following events post YD with insolation having nothing to do with it.

  12. Stephen Richards says:

    I think by abrupt they mean relative to geological time.

  13. Richard111 says:

    The mention of geological time makes me think of continental plate movement. Does this have any effect?

  14. dennisambler says:

    DROUGHT IN THE AFRICAN SAHEL: 2004 https://www.researchgate.net/publication/220023784_Drought_in_the_African_Sahel_Long-Term_Perspectives_and_Future_Prospects
    Nick Brooks, Saharan Studies Programme and Tyndall Centre for Climate Change Research, School of Environmental Sciences, University of East Anglia

    “Long-term climatic and environmental change in the Sahel is associated with variations in the strength and position of the African Monsoon. At the last glacial maximum (LGM) some 21 thousand years ago (ka), the Sahara desert covered a much larger area than at present, as apparent from the dating of fossil dunes some 5° south of the present extent of mobile dunes (Talbot, 1983).

    “Over the past 1.65 million years, approximately corresponding to the Quaternary period, there have been some seventeen glacial cycles, each lasting approximately 100ka (Goudie, 1992). Evidence from lake sediments in the central and southern Sahara indicates a succession of arid and humid episodes broadly coincident with glacial and interglacial periods respectively (Kowalski et al., 1989; Szabo et al., 1995; Cremaschi, 1998; Martini et al., 1998).”

    EARTH’S ORBITAL AXIS
    “On multi-millennial timescales, shorter than those represented by the 100ka glacial cycles, monsoon dynamics are modulated by the Earth’s 21ka precessional cycle, which determines the angle at which the Earth’s axis is inclined to the plane of the ecliptic (the plane in which the planets orbit the sun) (Kukla and Gavin, 2004).”

    “When this angle is large, the northern hemisphere is inclined more steeply towards the sun in summer, resulting in increased solar insolation or heating of the Earth’s surface, and a larger differential heating between the northern hemisphere land masses and the oceans, which intensifies the global monsoon system.

    When the angle of inclination of the Earth’s axis is small, boreal summer heating is reduced and the monsoon system is weak. The 21ka and 100ka cycles interact, and an increase in boreal summer insolation is believed to have contributed to the process of deglaciation after the Last Glacial Maximum (LGM) (Goudie, 1992).

    By around 10ka, maximum inclination had been reached, resulting in an increase in incident solar radiation at the Earth’s surface associated with intensified monsoon activity throughout the northern hemisphere subtropics (Tuenter et al., 2003).”

    I suppose we have to ask the question, “what effect does anthropogenic CO2 have on the angle of inclination of the Earth’s axis?”

  15. stpaulchuck says:

    there ya go again, talking about that bright shiny thing in the sky again. As if that has any affect on “climate”. Everyone knows is my backyard grill and the big V8 in my F150 pickup truck that’s turning the Earth into an inferno.
    —————
    “Meanwhile, conservatives are said to have rejected science if they won’t believe that taxes control the weather.” from moonbattery.com

  16. oldbrew says:

    over 230 GSPs within the last 8 million years

    Looks like something is repeating 😉

  17. oldmanK says:

    Maybe cutting across to a different evidence, here is something – well different-. Youtube video at https://www.youtube.com/watch?v=gbq7O0VmZ2M Go to 02:29 to 02:46 and note carefully the dates.

    Then look up this: https://melitamegalithic.wordpress.com/2018/06/29/searching-evidence-3/ and note the dates. There is a lot more. The implications are obvious.

    Changes in the sahara were much faster. If anyone is thinking that this is to do with long-time orbital chronology, I think one is looking for a needle in the wrong haystack.

  18. And where are we in the cycle? Near which end of the dry?

  19. oldbrew says:

    the MD03-2705 record shows a millennial-scale peak in dust fluxes during the most recent stadial, the Younger Dryas (12.7 to 11.9 ka; Fig. 4)
    http://advances.sciencemag.org/content/5/1/eaav1887

  20. oldmanK says:

    Re oldbrew says: January 12, 2019 at 8:37 am a quick reply (part).

    The chronological base-line is too large for effective ‘dissection’ of what has taken place from proxies. On short time-scales things are very different.

    An example. See Med sapropel for the last 10k years and how it correlates with other proxies and other events : https://melitamegalithic.wordpress.com/2018/04/24/searching-evidence-2/ See more correlation at this (my) site in ‘Searching evidence’ series. Those changes are smoothed out in that figure.

    I question the ‘summer insolation’ curve which is based on a study of an “Assumed” very stable earth dynamic. Quote from paper “Spectra and coherence of dust records were calculated against ETP (29) over the past 240 ka to assess the extent to which local and ETP signals are linearly correlated in each frequency band” That paper is here: https://www.aanda.org/articles/aa/pdf/2004/46/aa1335.pdf Instantaneous and transient changes are ignored, -assumed non-existent- but evidence shows they happen.

    There is hard evidence that obliquity is an abruptly changing dynamic on top of the secular, not as assumed. Therefore any assumptions based ONLY on secular orbital changes are wrong. The fact that a signature from a forcing signal is evident does not mean it is ‘the one and only’ regulating force.

  21. oldbrew says:

    Was there a big change in monsoon patterns about 4.2k years ago?

    Differentiating summer and winter rainfall in South Asia around 4.2 ka climatic ‘event’

    https://m.phys.org/news/2019-01-differentiating-summer-winter-rainfall-south.html

  22. oldmanK says:

    Re oldbrew says: January 16, 2019 at 5:20 pm

    Quote from link with additions in brackets: “– between c.4.5-4.3 [ ~2345bce : Dodwell event ] thousand years ago, and after this, winter and summer rainfall both decreased to a minimum at c.4.1 thousand years ago.” Expect that evidence of human abandonment/civilisation collapse will lag by some decades.

    Equally important is that similar evidence was found at other places, including evidence of mass burials. Links:
    https://melitamegalithic.wordpress.com/2018/04/24/searching-evidence-2/
    https://melitamegalithic.wordpress.com/2018/08/12/searching-evidence-4-prehistoric-mass-burials/

    The trends are there, including a bit from before, but the events are not so distinct.

  23. oldbrew says:

    700,000 Square Kilometers Of Added Green Vegetation, Climate Change Shrinks Sahara Desert By Whopping 8%!

    Using satellite images, Venter et al. 2018 found an eight percent increase in woody vegetation in sub-Saharan Africa over the last three decades, underscoring the global “greening trend”.

    http://notrickszone.com/2019/01/16/700000-square-kilometers-of-added-green-vegetation-climate-change-shrinks-sahara-desert-by-whopping-8/

  24. oldmanK says:

    Re oldbrew says: January 16, 2019 at 7:33 pm

    That is actually bad news. The devil is in the detail. One similar instance prior to 3550bce, the Sahara was green and it supported life. Time-wise it coincided with a peak in the Eddy cycle. Archaeology says obliquity was high and changed to low at that point. Today we are at near peak Eddy, high obliquity, greening (somewhat) Sahara, peak human development, similar to RWP and MWP. Is it nearing a switch?

    Read the data in the two links below:
    https://melitamegalithic.wordpress.com/2018/07/24/searching-evidence-update/
    https://melitamegalithic.wordpress.com/2018/08/12/searching-evidence-4-prehistoric-mass-burials/

    To add (to the icing?) a bit of local sunday news meant to educate/entertain linked to another Med island, Lampedusa, for me to discover there similar/additional prehistoric evidence astronomic/tectonic.

    NoTricksZone quotes “”Not here to worship what is known, but to question it” – Jacob Bronowski.”” We have been worshiping what we thought we knew, for too long. A dip in the basics is overdue.

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