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A ‘Green’ Sahara Was Far Less Dusty Than Today

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As recently as 5,000 years ago, the Sahara — today a vast desert in northern Africa, spanning more than 3.5 million square miles — was a verdant landscape, with sprawling vegetation and numerous lakes. Ancient cave paintings in the region depict hippos in watering holes, and roving herds of elephants and giraffes — a vibrant contrast with today’s barren, inhospitable terrain.

The Sahara’s “green” era, known as the African Humid Period, likely lasted from 11,000 to 5,000 years ago, and is thought to have ended abruptly, with the region drying back into desert within a span of one to two centuries.

Now researchers at MIT, Columbia University and elsewhere have found that this abrupt climate change occurred nearly simultaneously across North Africa.

The team traced the region’s wet and dry periods over the past 30,000 years by analyzing sediment samples off the coast of Africa. Such sediments are composed, in part, of dust blown from the continent over thousands of years: The more dust that accumulated in a given period, the drier the continent may have been.

From their measurements, the researchers found that the Sahara emitted five times less dust during the African Humid Period than the region does today. Their results, which suggest a far greater change in Africa’s climate than previously estimated, will be published in Earth and Planetary Science Letters.

David McGee, an assistant professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences, says the quantitative results of the study will help scientists determine the influence of dust emissions on both past and present climate change.

“Our results point to surprisingly large changes in how much dust is coming out of Africa,” says McGee, who did much of the work as a postdoc at Columbia. “This gives us a baseline for looking further back in time, to interpret how big past climate swings were. This [period] was the most recent climate swing in Africa. What was it like before?”

Getting to the core of dust

To trace Africa’s dust emissions through time, McGee analyzed sediment samples collected in 2007 by researchers from Columbia and Woods Hole Oceanographic Institution. Researchers sampled from sites off the northwest coast of Africa, spanning a distance of more than 550 miles.

At each site, they collected a core sample — a 10-foot long cylinder topped by a weight — which scientists submerged, collecting a column of sediment.

McGee says a 10-foot column represents approximately 30,000 years of sediments deposited, layer by layer, in the ocean — sediments like windblown dust from the continent, marine deposits brought in by ocean currents, and leftover bits of organisms that sank to the seafloor. A centimeter of sediment corresponds to about 100 years of deposition, providing what McGee calls a “high-resolution” record of dust changes through time.

To trace how much windblown dust accumulated over the past 30,000 years, McGee used a combination of techniques to first determine how fast sediments accumulated over time, then subtracted out the accumulation of marine sediments and biological remnants.

Layer by layer

Using a technique called thorium-230 normalization, McGee and his colleagues calculated accumulation rates for sediment layers every two to three centimeters along the column. The technique is based on the decay of uranium in seawater: Over time, uranium decays to thorium-230, an insoluble chemical that sticks to any falling sediment as it sinks to the seafloor. The amount of uranium — and by extension, the production rate of thorium-230 — in the world’s oceans is relatively constant. McGee measured the concentration of thorium-230 in each core sample to determine the accumulation rates of sediments through time.

In periods when sediments accumulated quickly, there was a smaller concentration of thorium-230. In slower-accumulating periods, McGee measured a greater thorium-230 concentration.

Once the team calculated rates of sediment accumulation over the past 30,000 years, it went about determining how much of that sediment was dust from neighboring Africa. The researchers subtracted biological sediment from the samples by measuring calcium carbonate, opal and organic carbon, the primary remnants of living organisms. After subtracting this measurement from each sample layer, the researchers tackled the task of separating the remaining sediment into windblown dust and marine sediments — particles that circulate through the ocean, deposited on the seafloor by currents.

McGee employed a second technique called grain-size endmember modeling, charting a distribution of grain sizes ranging from coarse grains of dust to fine grains of marine soil.

“We define these endmembers: A pure dust signal would look like this, and a pure marine sediment would look like this,” McGee says. “And then we see, OK, what combination of those extremes would give us this mixture that we see here?”

This study, McGee says, is the first in which researchers have combined the two techniques — endmember modeling and thorium-230 normalization — a pairing that produced very precise measurements of dust emissions through tens of thousands of years.

In the end, the team found that during some dry periods North Africa emitted more than twice the dust generated today. Through their samples, the researchers found the African Humid Period began and ended very abruptly, consistent with previous findings. However, they found that 6,000 years ago, toward the end of this period, dust emissions were one-fifth today’s levels, and far less dusty than previous estimates.

McGee says these new measurements may give scientists a better understanding of how dust fluxes relate to climate by providing inputs for climate models.

Natalie Mahowald, a professor of earth and atmospheric sciences at Cornell University, says the group’s combination of techniques yielded more robust estimates of dust than previous studies.

“Dust is one of the most important aerosols for climate and biogeochemistry,” Mahowald says. “This study suggests very large fluctuations due to climate over the last 10,000 years, which has enormous implications for human-derived climate change.”

As a next step, McGee is working with collaborators to test whether these new measurements may help to resolve a longstanding problem: the inability of climate models to reproduce the magnitude of wet conditions in North Africa 6,000 years ago. By using these new results to estimate the climate impacts of dust emissions on regional climate, models may finally be able to replicate the North Africa of 6,000 years ago — a region of grasslands that were host to a variety of roaming wildlife.

“This is a period that captures people’s imaginations,” McGee says. “It’s important to understand whether and how much dust has had an impact on past climate.”

This research was funded by the National Science Foundation and by a National Oceanic and Atmospheric Administration postdoctoral fellowship to McGee.

None of the green tax brigade like to hear rational discussion about the reality of climate change.

One Vesuvius, Pinatubu, Eyjafjallajökull, or Mount St. Helens event can spew out more 'dust' and gasses in a day than years, perhaps decades, of modern human existence can.

Why are we still arguing over man made climate change, when in fact the we play very little role in anything this world.

It's like arguing the effect of a pound to the greater national economy.

Perhaps we can pay a dust tax now.

Al Gore, suck my balls.

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Certainly, I have become more sceptic about the doom-mongering!

The earlier articles put forward an interesting hypothesis, but once the politicians took it up, after throwing out their old ideologies, well I became more sceptical! And there is cash in it! :huh:

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None of the green tax brigade like to hear rational discussion about the reality of climate change.

The "green Sahara" or Neolithic Subpluvial is not news nor does it derail theories on climate change. This is just more research on a known topic.

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Why are we still arguing over man made climate change, when in fact the we play very little role in anything this world.

The carbon sequestered in fossil fuels and carbonate rocks by biology over millenia was once in the atmosphere.

We are now returning organic and inorganic fossil carbon to the atmosphere faster than it is removed by biology and faster than it has been added to the atmosphere at any time over the last 800,000 years, and this has given levels of atmospheric carbon today that are higher than any recorded during that time.

Up to you whether you choose to argue about climate change, but it is difficult to argue with the above and so argue that we play very little role in the Earth's carbon cycle.

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I love these pro and anti global warming debates - they are great to sit back and watch.

I applaud your stance! Actually you are probably right! It's an angels on pinheads argument!

A bit like whether to put he clocks back and forward twice a year! Generates a lot of debate, but actually will not affect the tilt of the Earth!

And Scottish farmers do not care, becaues cows cannot read a clock!

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None of the green tax brigade like to hear rational discussion about the reality of climate change.

One Vesuvius, Pinatubu, Eyjafjallajökull, or Mount St. Helens event can spew out more 'dust' and gasses in a day than years, perhaps decades, of modern human existence can.

Why are we still arguing over man made climate change, when in fact the we play very little role in anything this world.

It's like arguing the effect of a pound to the greater national economy.

Perhaps we can pay a dust tax now.

Al Gore, suck my balls.

It's pretty much established science that human GHG emissions will significantly affect the Earth's climate. Read virtually any scientific publication, and you'll see that this is the case. It's almost impossible to explain how past variations in the Earth's climate could have occurred without invoking greenhouse effects, and from that it's obvious that human greenhouse emissions will be important. The scientific arguments concern details of the speed, extent and geographical variations of climate change resulting from greenhouse emissions, not the basic theory.

The only reason that there is any argument about the fundamental theory of AGW is because of the attempts of those with a vested interest in continuing to burn fossil fuels to mislead the public, largely through the complicity of the mainstream media.

Edit: Regarding emissions of greenhouse gases by volcanoes, you are simply wrong. Human emissions of CO2, for example, dwarf volcanic emissions by a factor of about 150. Your antipathy towards those concerned about the effects of human greenhouse emissions is obviously founded on ignorance. Perhaps if you were to read up a little on the subject, you might change your stance on the issue.

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Perhaps if you were to read up a little on the subject, you might change your stance on the issue.

You really are hard work.

I'm taking the US Geological Study and others on their word as to volumes of gasses, not you; and you can't even seem take the boys at MIT at their word! MIT! Massachusetts Institute of Technology! Which is considered the best of the best in academia and minds... in the world. Who are you?

Greenhouse gasses I don't deny, nor their effect on a closed system. However, most optimistic forecasts see traditional hydrocarbon global reserves on the wain now, and to be minimal in 40 years, almost non-existent in 100. That's coming from SPE's and other academic and industry studies.

So what's the worry? Tax for CO2 is just plain dumb, and so are you for buying into a fabricated crisis.

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You really are hard work.

I'm taking the US Geological Study and others on their word as to volumes of gasses, not you; and you can't even seem take the boys at MIT at their word! MIT! Massachusetts Institute of Technology! Which is considered the best of the best in academia and minds... in the world. Who are you?

Greenhouse gasses I don't deny, nor their effect on a closed system. However, most optimistic forecasts see traditional hydrocarbon global reserves on the wain now, and to be minimal in 40 years, almost non-existent in 100. That's coming from SPE's and other academic and industry studies.

So what's the worry? Tax for CO2 is just plain dumb, and so are you for buying into a fabricated crisis.

You're talking out of your **** again. I refer you to the US Geological Survey's web page on the topic, which states that:

Human activities, responsible for a projected 35 billion metric tons (gigatons) of CO2 emissions in 2010 (Friedlingstein et al., 2010), release an amount of CO2 that dwarfs the annual CO2 emissions of all the world’s degassing subaerial and submarine volcanoes (Gerlach, 2011).

Also, nothing I have said contradicts the MIT paper you quoted which, while suggesting how climate models might be improved, in no way claims that AGW is not a real and serious problem. I suggest you brush up your reading and comprehension skills before you make an even bigger fool of yourself.

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The only reason that there is any argument about the fundamental theory of AGW is because of the attempts of those with a vested interest in continuing to burn fossil fuels to mislead the public, largely through the complicity of the mainstream media.

I don't doubt that, but there's another side to it: the attempts of the green bureaucrats to manipulate data and secure their pensions.

As far as I can tell energy constraints will get us first.

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I don't doubt that, but there's another side to it: the attempts of the green bureaucrats to manipulate data and secure their pensions.

As far as I can tell energy constraints will get us first.

I think you are right Mr Cuckoo! There will be nothing left to burn and we will all turn to canibalism! Where is Ken, and his tasty Oriental road-kill recipes?

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bigger fool

I wrote about release of dust and gasses not specifically CO2. Go back and read.

CO2 c-o-shmo. If required, we humans can easily geo-engineer this world into any environmental state we desire. I'm an engineer and that's what we do. We create the world. Will it happen? No, because nobody will agree what is a good balance.

Gas releases through the chemical reactions of hydrocarbon for our modern day heat engines, which we will be taxed on, play a very insignificant part of the bigger picture. Take that page you linked to in your 2 seconds of googling for example.

Yes, the earth is subject to changes by what we do, small changed, and putting up an article that suggest that also nature does a much better job of it gets you knickers in a knot?

AGW is an area of study; Studying phenomena. Folk like you that want to support taxation of consumption, based on very large assumptions and impure science... that will do nothing other than enrich those supplying the consumption are dangerous. Very dangerous indeed.

Traditional hydrocarbon consumption will decline and be all but a thing of the past in a lifetime. You ignored that bit. why?

What is your worry?

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You really are hard work.

I'm taking the US Geological Study and others on their word as to volumes of gasses, not you; and you can't even seem take the boys at MIT at their word! MIT! Massachusetts Institute of Technology! Which is considered the best of the best in academia and minds... in the world. Who are you?

Greenhouse gasses I don't deny, nor their effect on a closed system. However, most optimistic forecasts see traditional hydrocarbon global reserves on the wain now, and to be minimal in 40 years, almost non-existent in 100. That's coming from SPE's and other academic and industry studies.

So what's the worry? Tax for CO2 is just plain dumb, and so are you for buying into a fabricated crisis.

Because it's already too late.

Read some Lovelock

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It's pretty much established science that human GHG emissions will significantly affect the Earth's climate. Read virtually any scientific publication, and you'll see that this is the case. It's almost impossible to explain how past variations in the Earth's climate could have occurred without invoking greenhouse effects, and from that it's obvious that human greenhouse emissions will be important. The scientific arguments concern details of the speed, extent and geographical variations of climate change resulting from greenhouse emissions, not the basic theory.

The only reason that there is any argument about the fundamental theory of AGW is because of the attempts of those with a vested interest in continuing to burn fossil fuels to mislead the public, largely through the complicity of the mainstream media.

Edit: Regarding emissions of greenhouse gases by volcanoes, you are simply wrong. Human emissions of CO2, for example, dwarf volcanic emissions by a factor of about 150. Your antipathy towards those concerned about the effects of human greenhouse emissions is obviously founded on ignorance. Perhaps if you were to read up a little on the subject, you might change your stance on the issue.

It's pretty much established....

I once said in a maths class 'It's reasonable to assume'...

I only said it once.

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I don't doubt that, but there's another side to it: the attempts of the green bureaucrats to manipulate data and secure their pensions.

As far as I can tell energy constraints will get us first.

In fact what will probably happen is that uninformed Joe Bloggs will get really pissed off ate being poorer, colder and hungrier and we will burn LOTs of strip mined coal while new nukes are built. With any luck a few warmie bureaucrats will be lynched along the way.

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I wrote about release of dust and gasses not specifically CO2. Go back and read.

You mentioned gases. Which gases were you taking about then if not CO2?

CO2 c-o-shmo. If required, we humans can easily geo-engineer this world into any environmental state we desire. I'm an engineer and that's what we do. We create the world. Will it happen? No, because nobody will agree what is a good balance.

Total ********. If you were a real engineer, you'd know that there are hard constraints on what is achievable in any particular circumstance. You'd also realise that good decisions are taken on the basis of cost-benefit analyses, not by simply winging it and hoping for the best.

Gas releases through the chemical reactions of hydrocarbon for our modern day heat engines, which we will be taxed on, play a very insignificant part of the bigger picture. Take that page you linked to in your 2 seconds of googling for example.

The page that I linked from the US Geologic Survey clearly states that effects of human greenhouse emissions dwarf those of natural emissions. I suggest you read it again a bit more carefully!

Yes, the earth is subject to changes by what we do, small changed, and putting up an article that suggest that also nature does a much better job of it gets you knickers in a knot?

I am simply pointing out that the article you posted does not support your position. In fact, in emphasising the importance of dust in determining climate, it actually strengthens the validity of climate change models that are otherwise unable to explain past climate changes in the Sahara. The fact that the authors themselves do not claim that their findings invalidate AGW should be enough to tell you that you've got the wrong end of the stick!

AGW is an area of study; Studying phenomena. Folk like you that want to support taxation of consumption, based on very large assumptions and impure science... that will do nothing other than enrich those supplying the consumption are dangerous. Very dangerous indeed.

That makes no sense at all. Why would taxing fossil fuels enrich suppliers of fossil fuels?

Traditional hydrocarbon consumption will decline and be all but a thing of the past in a lifetime. You ignored that bit. why?

What is your worry?

I have no worry; I simply ran out of time after disproving your earlier statements. I have other stuff to do. It's very easy to make vague and unsupported claims like yours, but takes more work to find and understand reliable sources of data. Since you are making the claim, the onus is on you to give some evidence to support your claim that fossil fuels will run out before CO2 emissions can have a significant effect on the climate. Let's see what you've got.

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It's pretty much established....

I once said in a maths class 'It's reasonable to assume'...

I only said it once.

You're just playing word games, presumably because you have nothing of substance to contribute to the debate. Why not have a shot at actually understanding the issues rather than allowing yourself to be spoon fed with crap?

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You're just playing word games, presumably because you have nothing of substance to contribute to the debate. Why not have a shot at actually understanding the issues rather than allowing yourself to be spoon fed with crap?

What I'm saying is that 'It's pretty well established' means as much as 'It;s reasonable to assume' when trying to prove hard facts,

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That makes no sense at all. Why would taxing fossil fuels enrich suppliers of fossil fuels?

'Carbon taxes' hit coal hard and increase demand for gas and oil. Why do you think Evil Big Oil donates money to Global Warmers?

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  • 239 Brexit, House prices and Summer 2020

    1. 1. Including the effects Brexit, where do you think average UK house prices will be relative to now in June 2020?


      • down 5% +
      • down 2.5%
      • Even
      • up 2.5%
      • up 5%



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