Scientists find huge reduction in African dust plume led to more Saharan monsoons 11,000 years ago

Posted: November 23, 2016 by oldbrew in climate, modelling, Natural Variation, research
Tags: , ,

Saharan dust storm [image credit: BBC]

Saharan dust storm [image credit: BBC]


Last year Ralph Ellis proposed a ‘dust theory of ice ages’ which we featured at the Talkshop. This research looks interesting in that context, and in its own right too.

Every year, trade winds over the Sahara Desert sweep up huge plumes of mineral dust, transporting hundreds of teragrams—enough to fill 10 million dump trucks—across North Africa and over the Atlantic Ocean.

This dust can be blown for thousands of kilometers and settle in places as far away as Florida and the Bahamas. The Sahara is the largest source of windblown dust to the Earth’s atmosphere.

But researchers from MIT, Yale University, and elsewhere now report that the African plume was far less dusty between 5,000 and 11,000 years ago, containing only half the amount of dust that is transported today.

In a paper published today in Science Advances, the researchers have reconstructed the African dust plume over the last 23,000 years and observed a dramatic reduction in dust beginning around 11,000 years ago. They say this weakened plume may have allowed more sunlight to reach the ocean, increasing its temperature by 0.15 degrees Celsius—a small but significant spike that likely helped whip up monsoons over North Africa, where climate at the time was far more temperate and hospitable than it is today.

“In the tropical ocean, fractions of a degree can cause big differences in precipitation patterns and winds,” says co-author David McGee, the Kerr-McGee Career Development Assistant Professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “It does seem like dust variations may have large enough effects that it’s important to know how big those impacts were in past and future climates.”

Talkshop note: the above is just the first part of the Phys.org report. The report concludes:

Dust’s climate role

McGee and his colleagues obtained sediment core samples from the Bahamas that were collected in the 1980s by scientists from the Woods Hole Oceanographic Institution. They brought the samples back to the lab and analyzed their chemical composition, including isotopes of thorium—an element that exists in windblown dust worldwide, at known concentrations.

They determined how much dust was in each sediment layer by measuring the primary isotope of thorium, and determined how fast it was accumulating by measuring the amount of a rare thorium isotope in each layer.

In this way, the team analyzed sediment layers from the last 23,000 years, and showed that around 16,000 years ago, toward the end of the last ice age, the dust plume was at its highest, lofting at least twice the amount of dust over the Atlantic, compared to today. However, between 5,000 and 11,000 years ago, this plume weakened significantly, with just half the amount of today’s windblown dust.

Colleagues at Yale University then plugged their estimates into a climate model to see how such changes in the African dust plume would affect both ocean temperatures in the North Atlantic and overall climate in North Africa. The simulations showed that a drop in long-range windblown dust would raise sea surface temperatures by 0.15 degrees Celsius, drawing more water vapor over the Sahara, which would have helped to drive more intense monsoon rains in the region.

“The modeling showed that if dust had even relatively small impacts on sea surface temperatures, this could have pronounced impacts on precipitation and winds both in the north Atlantic and over North Africa,” McGee says. Noting that the next key step is to reduce uncertainties in the modeling of dust’s climate impacts, he adds: “We’re not saying, the expansion of monsoon rains into the Sahara was caused solely by dust impacts. We’re saying we need to figure out how big those dust impacts are, to understand both past and future climates.”

Full report: Scientists find huge reduction in African dust plume led to more Saharan monsoons 11,000 years ago | Phys.org

See also: Modulation of ice ages via precession and dust-albedo feedbacks
– Ralph Ellis, Michael Palmer
http://www.sciencedirect.com/science/article/pii/S1674987116300305

Comments
  1. oldbrew says:

    MIT News adds this:

    Ina Tegen, a professor at the Leibniz Institute for Tropospheric Research in Germany, says the group’s results suggest that “dust effects today may be considerable as well.” [bold added]

    “Dust loads vary with changing climate, and due to the effects of dust on [solar] radiation, ice formation in clouds, and the carbon cycle, this may cause important climate feedbacks,” says Tegen, who was not involved in the research. “The changing climate since the last ice age can be considered a ‘natural laboratory’ to study such effects. Understanding the past is the basis for predicting future changes with any confidence.”

    http://news.mit.edu/2016/saharan-dust-monsoons-1123

  2. […] Source: Scientists find huge reduction in African dust plume led to more Saharan monsoons 11,000 years ago |… […]

  3. rishrac says:

    So the dust blowing off North Africa has no effect on the formation of hurricanes in the Atlantic basin ?

  4. oldbrew says:

    rishrac: Saharan dust can certainly be a significant factor in climate e.g.:

    It is known that one of the major factors that create hurricanes is warm water temperatures on the surface of the ocean. Evidence shows that dust from the Sahara desert caused surface temperatures to be cooler in 2006 than in 2005.
    http://en.wikipedia.org/wiki/Mineral_dust#Saharan_dust

  5. DB says:

    “So the dust blowing off North Africa has no effect on the formation of hurricanes in the Atlantic basin?”

    From 2006:
    Dust ‘affects hurricane activity’
    http://news.bbc.co.uk/2/hi/science/nature/6038296.stm
    US researchers have discovered a link between Atlantic hurricane activity and thick clouds of dust that periodically rise up from the Sahara Desert. At times of intense hurricane activity, dust clouds were scarce, but in years with stronger dust storms, fewer hurricanes swept across the Atlantic.

    From 2008:
    Trends in Saharan dust and tropical Atlantic climate during 1980–2006
    http://www.aoml.noaa.gov/phod/docs/2008GL035042.pdf

    Foltz and McPhaden write:
    “During the past century tropical North Atlantic sea surface temperatures (SST) have fluctuated strongly with a period of ~70 years. The oscillations are part of a basin-scale Atlantic multidecadal oscillation (AMO) thought to be driven by changes in the strength of the Atlantic thermohaline circulation. The AMO exerts a significant influence on weather and climate in the tropical Atlantic sector, with positive phases of the AMO contributing to enhanced rainfall in the Sahel region of Africa and above-normal hurricane activity in the Atlantic basin. Superimposed on the AMO is a strong warming trend that has been attributed to anthropogenic greenhouse gas forcing.”

    From satellite data they pieced together a dust record for the tropical North Atlantic beginning in 1980. A decline in dustiness means more sunshine can reach the surface and heat things up. They calculated that the decline in dustiness from 1980 to 2006 could have warmed the underlying sea surface temperature by 3ºC – an amount that is five times the observed increase during that period.

  6. oldbrew says:

    The AMO (the Atlantic Multidecadal Oscillation) is a cyclic variation of the Oceanic and atmospheric current in large time scale in the North Atlantic Ocean, that combines to alternately increase and drop the temperature of the sea surface Atlantic (SST)…The causes of the AMO are currently not well known because this oscillation is long.
    http://la.climatologie.free.fr/amo/amo-english.htm

  7. oldbrew says:

    The Ellis/Palmer paper says:
    ‘…it is proposed here that during the glacial maximum, CO2 depletion starves terrestrial plant life of a vital nutrient and causes a die-back of upland forests and savannahs, resulting in widespread desertification and soil erosion. The resulting dust storms deposit large amounts of dust upon the ice sheets and thereby reduce their albedo, allowing a much greater absorption of insolation.’

    http://www.sciencedirect.com/science/article/pii/S1674987116300305

  8. JKrob says:

    As for that AMO/Hurricane frequency chart posted by oldbrew, not sure I’d take the hurricane count before the 1950’s as ‘gospel’. Storms back there were ‘counted’ if a ship happened to come across them. If no ship reported a storm, it was presumed that no hurricane/tropical cyclone existed. Since the era of geostationary wx satellite, *every* tropical cyclone is accounted for. Those earlier numbers are most likely too low…just my opinion of course.

    [reply] fair point JK

  9. oldmanK says:

    On this matter the researchers are only confirming something that is already well known. But here dust is not a primary agent but rather an after-effect of something else, perhaps even twice removed.

    If one has time this video tells a very good story re Sahara weather changes : https://www.youtube.com/watch?v=ICSaQcgVdXU&t=2039s

    Between 11,000 years and 5000 yrs ago the Sahara was greener (as the video explains very well). It also supported some sedentary habitation –see this: https://www.youtube.com/watch?v=S3g-93t4uRw&list=PLntMub8_LlVvQwQWftMgf0IJbFZaVza90

    The dates are very important. 10,000 is about the end of the YD. 5000 – or some centuries before- is the time when the Earth went through a drastic change. The main question ought to be what caused the changes in Sahara climate and dust cloud formation at those two dates.

  10. oldbrew says:

    oldmanK – Ralph Ellis says in his paper:

    Precession describes the rotational motion of the Earth’s axial orientation. Axial precession has a roughly 25.7 kyr cycle, and it was known to the ancient Egyptians, Greeks and Chinese as the Great Year (Yoke, 1985 and Campion and Dally, 1997). And its comparison to an annual year is quite valid, because the Celestial Great Year combines with orbital eccentricity to produce warm and cool seasons in each hemisphere. However, apsidal precession reduces the approximate 25.7 kyr Celestial Great Year down to an approximate 23 kyr cycle, which will be called herein the ‘Seasonal Great Year’ (SGY). So each Great Season of this Seasonal Great Year is approximately 5700 years long, and this is a significant periodicity because most of the interglacial warming events last about 5000 years. These are average figures as the SGY has ranged from 15 kyr to 27 kyr over the last 400 kyr (Laskar et al., 2004), a variable cycle that is likely to generate confusion if specific numbers or averaged lengths of precessional cycles are assumed to be controlling the interglacial cycle.

  11. oldmanK says:

    oldbrew – the findings in the above subject appear to be contrary to the main thrust of the paper.

    The video on the Sahara tells from ocean deposits that the Sahara was already a desert well over a million years ago. Whereas last glacial termination occurred 12-14k yrs ago. Then 5000 years ago the dust levels increased – at a point when we are talking of major loss of remaining polar ice, not a start of de-glaciation (and equatorial ice too for that matter). I think that the model in the Ellis departs substantially from observed. Also one has to bear in mind that the changes, as evident in the sediments -video at 15:10 and at 32.30–, were very abrupt. the last at ~5500yrs ago (one my ask: why evidence keeps pointing to Dodwell?).

  12. tom0mason says:

    I wonder where all that sand and dust came from.
    Is sand from sandstone (maybe the other way about) if so where was all this sandstone originally?

  13. daveburton says:

    That “we featured” link in the first sentence of the article doesn’t work. I think it should point here:
    https://tallbloke.wordpress.com/2015/10/11/albedo-regulation-of-ice-ages-with-no-co2-feedbacks/

    [mod] amended, thanks

  14. oldmanK says:

    Paging down daveburton’s link I came across this from PV: “Careful ralfellis, you may inspire military vision:
    large fleets of tiny drones sparking widespread northern hemisphere forest fires.—–Perhaps we’ll live to see some of your theories tested—“. Recent news on tv came to mind; of burning oil-fields and blackened children. Would that black soot reach the poles and test the theory?

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