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Could the Sahara ever be green again?

The Sahara Desert is seen here, in Northern Africa.
The Sahara Desert is seen here, in Northern Africa.
(Image: © Shutterstock)

Sometime between 11,000 and 5,000 years ago, after the last ice age ended, the Sahara Desert transformed. Green vegetation grew atop the sandy dunes and increased rainfall turned arid caverns into lakes. About 3.5 million square miles (9 million square kilometers) of Northern Africa turned green, drawing in animals such as hippos, antelopes, elephants and aurochs (wild ancestors of domesticated cattle), who feasted on its thriving grasses and shrubs. This lush paradise is long gone, but could it ever return? 

In short, the answer is yes. The Green Sahara, also known as the African Humid Period, was caused by the Earth's constantly changing orbital rotation around its axis, a pattern that repeats itself every 23,000 years, according to Kathleen Johnson, an associate professor of Earth systems at the University of California Irvine. 

However, because of a wildcard — human-caused greenhouse gas emissions that have led to runaway climate change — it's unclear when the Sahara, currently the world's largest hot desert, will turn a new green leaf. 

Related: Has the Earth ever been this hot before?

The Sahara's green shift happened because Earth's tilt changed. About 8,000 years ago, the tilt began moving from about 24.1 degrees to the current day 23.5 degrees, Space.com, a Live Science sister site, previously reported. That tilt variation made a big difference; right now, the Northern Hemisphere is closest to the sun during the winter months. (This may sound counterintuitive, but because of the current tilt, the Northern Hemisphere is tilted away from the sun during the winter season.) During the Green Sahara, however, the Northern Hemisphere was closest to the sun during the summer. 

Notice how the Northern Hemisphere is closer to the sun during the winter months (right).

Notice how the Northern Hemisphere is closer to the sun during the winter months (right) than it is during the summer months. (Image credit: Shutterstock)

This led to an increase in solar radiation (in other words, heat) in Earth's Northern Hemisphere during the summer months. The rise in solar radiation amplified the African monsoon, a seasonal wind shift over the region caused by temperature differences between the land and ocean. The increased heat over the Sahara created a low pressure system that ushered moisture from the Atlantic Ocean into the barren desert. (Usually, the wind blows from dry land toward the Atlantic, spreading dust that fertilizes the Amazon rainforest and builds beaches in the Caribbean, Live Science previously reported.)

This increased moisture transformed the formerly sandy Sahara into a grass and shrub-covered steppe, according to the National Oceanic and Atmospheric Administration (NOAA). As animals there prospered, humans did too, eventually domesticating buffalo and goats and even creating an early system of symbolic art in the region, NOAA reported. 

Image 1 of 6

The Aorounga impact structure is located in northern Chad, in the Sahara Desert.

The Aorounga impact structure is located in northern Chad, in the Sahara Desert. (Image credit: ISS Crew Earth Observations, Image Science & Analysis Laboratory; Johnson Space Center; NASA)
Image 2 of 6

This 2014 photo from the International Space Station shows cloud bands over southern Mauritania in the Sahara Desert.

This 2014 photo from the International Space Station shows cloud bands over southern Mauritania in the Sahara Desert. (Image credit: ISS Crew Earth Observations; Image Science & Analysis Laboratory, Johnson Space Center; NASA)
Image 3 of 6

These dried-up river channels in Algeria, which is mostly covered by the Sahara Desert, are known as wadis.

These dried-up river channels in Algeria, a country that is mostly covered by the Sahara Desert, are known as wadis. (Image credit: Image by Robert Simmon; NASA GSFC, based on Landsat Thematic Mapper data)
Image 4 of 6

About 134 miles (215 kilometers) of the Saharan coastline were captured in this 2014 image taken from the International Space Station.

About 134 miles (215 kilometers) of the Saharan coastline were captured in this 2014 image taken from the International Space Station. (Image credit: SS Crew Earth Observations Facility; Earth Science and Remote Sensing Unit, Johnson Space Center; NASA)
Image 5 of 6

More dust is blown out of the Sahara than any other desert in the world, according to NASA's Earth Observatory. The International Space Station was over Libya when this photo was taken in 2014.

More dust is blown out of the Sahara than any other desert in the world, according to NASA's Earth Observatory. The International Space Station was over Libya when this photo was taken in 2014. (Image credit: ISS Crew Earth Observations Facility; Earth Science and Remote Sensing Unit, Johnson Space Center; NASA)
Image 6 of 6

In March 2018, Saharan dust plastered parts of Europe, coating ski slopes and Mediterranean cities in orange particles, according to NASA's Earth Observatory.

In March 2018, Saharan dust plastered parts of Europe, coating ski slopes and Mediterranean cities in orange particles, according to NASA's Earth Observatory. (Image credit: NASA image by Jeff Schmaltz, LANCE/EOSDIS Rapid Response)

Wobbling Earth

But why did Earth's tilt change in the first place? To understand this monumental change, scientists have looked to Earth's neighbors in the solar system.

"The Earth's axial rotation is perturbed by gravitational interactions with the moon and the more massive planets that together induce periodic changes in the Earth's orbit," Peter de Menocal, the director at the Center for Climate and Life at Lamont-Doherty Earth Observatory  at Columbia University in New York, wrote in Nature. One such change is a "wobble" in the Earth's axis, he wrote. 

That wobble is what positions the Northern Hemisphere closer to the sun in the summer — what researchers call a Northern Hemisphere summer insolation maximum — every 23,000 years. Based on research first published in the journal Science in 1981, scholars estimate that the Northern Hemisphere had a 7% increase in solar radiation during the Green Sahara compared with now. This increase could have escalated African monsoonal rainfall by 17% to 50%, according to a 1997 study published in the journal Science

Related: Why does rain smell good?

What's interesting to climate scientists about the Green Sahara is how abruptly it appeared and vanished. The termination of the Green Sahara took only 200 years, Johnson said. The change in solar radiation was gradual, but the landscape changed suddenly. "It's an example of abrupt climate change on a scale humans would notice," she said.

"Records from ocean sediment show [that the Green Sahara] happens repeatedly," Johnson told Live Science. The next Northern Hemisphere summer insolation maximum — when the Green Sahara could reappear — is projected to happen again about 10,000 years from now in A.D. 12000 or A.D. 13000. But what scientists can't predict is how greenhouse gases will affect this natural climate cycle. 

Paleoclimate research "provides unequivocal evidence to what [humans] are doing is pretty unprecedented," Johnson said. Even if humans stop emitting greenhouse gases today, these gases would still be elevated by the year 12000. "Climate change will be superimposed onto the Earth's natural climate cycles," she said. 

That said, there's geologic evidence from ocean sediments that these orbitally-paced Green Sahara events occur as far back as the Miocene epoch (23 million to 5 million years ago), including during periods when atmospheric carbon dioxide was similar to, and possibly higher, than today's levels. So, a future Green Sahara event is still highly likely in the distant future. Today's rising greenhouse gases could even have their own greening effect on the Sahara, though not to the degree of the orbital-forced changes, according to a March review published in the journal One Earth. But this idea is far from certain, due to climate model limitations.  

Meanwhile, there is another way to turn parts of the Sahara into a green landscape; if massive solar and wind farms were installed there, rainfall could increase in the Sahara and its southern neighbor, the semiarid Sahel, according to a 2018 study published in the journal Science

Wind and solar farms can increase heat and humidity in the areas around them, Live Science previously reported. An increase in precipitation, in turn, could lead vegetation growth, creating a positive feedback loop, the researchers of that study said. However, this huge undertaking has yet to be tested in the Sahara Desert, so until such a project gets funding, humans might have to wait until the year 12000 or longer to see whether the Sahara will turn green again. 

Originally published on Live Science.

  • FB36
    IMHO any desert wasteland could be turned to forests etc, only if humanity invents fusion power tech someday!

    Some people may think "Why humanity needs fusion power? Why not just use solar & wind power etc?"

    IMHO, humanity definitely/absolutely needs fusion power, because it could really take humanity to a whole new level (which cannot ever be done using solar & wind power etc)!

    Imagine, we could build a global permanent water pipeline network (& do seawater desalinization) & provide plenty water to everywhere on Earth (for agriculture & forests etc)! (& so any desert wastelands (like Sahara Desert) could be turned to forests and/or agricultural fields!)

    Imagine, very tall poles w/ very powerful infrared heater lamps (etc) on top!
    Imagine, using many of them in cities/towns to turn winters to springs/summers!
    And/or, using many of them in agricultural fields to grow any hot climate (even tropical) crops/trees, even in coldest places on Earth!

    For example, titanium is an extremely durable & strong & light material & Earth has plenty of it, but AFAIK, it requires so much electricity to mine/process!
    Imagine, if we had so much (clean) electricity, we could build all kinds of land/air/sea vehicles, buildings, roads, even whole cities from titanium!

    & no doubt, many new techs would become reality, once we have enough electricity power for them! (For example, consider how computers & internet are used in many ways today, which were pretty much unimaginable to their inventors!)
    Reply
  • Mario Sanchez
    IMHO, qualquer terreno baldio do deserto poderia ser transformado em florestas, etc, apenas se a humanidade inventar a tecnologia de energia de fusão algum dia!

    Algumas pessoas podem pensar "Por que a humanidade precisa de energia de fusão? Por que não usar apenas energia solar e eólica etc?"

    IMHO, a humanidade definitivamente / absolutamente precisa de energia de fusão, porque ela poderia realmente levar a humanidade a um nível totalmente novo (o que nunca pode ser feito usando energia solar e eólica, etc)!

    Imagine, poderíamos construir uma rede global de encanamentos de água permanente (e fazer a dessalinização da água do mar) e fornecer bastante água para todos os lugares da Terra (para agricultura e florestas, etc)! (e assim, qualquer deserto (como o deserto do Saara) pode ser transformado em florestas e / ou campos agrícolas!)

    Imagine, postes muito altos com lâmpadas infravermelhas muito potentes (etc) em cima!
    Imagine usar muitos deles em cidades / vilas para transformar invernos em primaveras / verões!
    E / ou, usando muitos deles em campos agrícolas para cultivar qualquer cultivo / árvores de clima quente (mesmo tropical), mesmo nos lugares mais frios da Terra!

    Por exemplo, o titânio é um material extremamente durável e forte e leve e a Terra tem muito disso, mas AFAIK, ele requer muita eletricidade para minerar / processar!
    Imagine, se tivéssemos tanta eletricidade (limpa), poderíamos construir todos os tipos de veículos terrestres / aéreos / marítimos, edifícios, estradas e até cidades inteiras de titânio!

    E sem dúvida, muitos novos técnicos se tornariam realidade, assim que tivermos energia elétrica suficiente para eles! (Por exemplo, considere como os computadores e a Internet são usados de muitas maneiras hoje, que eram praticamente inimagináveis para seus inventores!)D fato é uma questão de levarem a sério e fornecerem algum fundo para que eu faça um experimento com uso da força que posso usar convertendo o efeito espiral em energia sem consumo de nada e com esse processo fazemos espaços de micro-clima onde a vegetação pode ser implantada. É bem mais barato do que Israel faz cultivando seus desejos. Eu posso fazer um bolsão de abelhas e em breve, com o mel e o turismo que isso vai atrair, alavancaremos o reflorestamento progressivo de todos os desertos, com a experiência que teremos.
    Reply
  • Minamii
    FB36 said:
    IMHO any desert wasteland could be turned to forests etc, only if humanity invents fusion power tech someday!

    Some people may think "Why humanity needs fusion power? Why not just use solar & wind power etc?"

    IMHO, humanity definitely/absolutely needs fusion power, because it could really take humanity to a whole new level (which cannot ever be done using solar & wind power etc)!

    We already have fusion power. It's manufactured every second by the nearest star to us and it's called "the sun".
    Reply
  • Travelmait
    The climate of North Africa could be altered by flooding the Qattara Depression in
    Egypt. It would increase the overall humidity in the westward flowing trade winds
    increasing rain. An added bonus would be a small decrease in sea level. A channel
    or tunnel from the Med would accomplish this and is feasible with current tech.
    Reply
  • hellopunyhumans
    Travelmait said:
    The climate of North Africa could be altered by flooding the Qattara Depression in
    Egypt. It would increase the overall humidity in the westward flowing trade winds
    increasing rain. An added bonus would be a small decrease in sea level. A channel
    or tunnel from the Med would accomplish this and is feasible with current tech.
    I feel like I read something about that. Isn't the issue something about ww2 land mines?
    Reply
  • Travelmait
    There have been a number of proposals to flood the depression as it is
    well below sea level - 147 meters at it's lowest. There is one oasis
    settlement of about 300 that would need to be resettled. Earliest
    proposal was 1912. Egypt is looking at it as a hydro electric project.
    Reply
  • katesisco
    Travelmait said:
    The climate of North Africa could be altered by flooding the Qattara Depression in
    Egypt. It would increase the overall humidity in the westward flowing trade winds
    increasing rain. An added bonus would be a small decrease in sea level. A channel
    or tunnel from the Med would accomplish this and is feasible with current tech.
    Quite possibly something like that happened when the active volcano on the floor of the Gulf of Mexico blew and destroyed the protected areas within the Yucatan, Cuba, and island chain.
    Perhaps the theory of DeGrzie/Milton regarding the formation of the inner planets thru magnetic connection of Sol and the minor sun, Jupiter is correct. As the magnetic connection fades. Mars would transform from blue to green to red and dead. IF so, Earth may be transforming to red and Venus, which was the original for the inner planets, may become the last habitat planet before it too becomes red and dead.
    Reply
  • Edward
    FB36 said:
    IMHO any desert wasteland could be turned to forests etc, only if humanity invents fusion power tech someday!

    Some people may think "Why humanity needs fusion power? Why not just use solar & wind power etc?"

    IMHO, humanity definitely/absolutely needs fusion power, because it could really take humanity to a whole new level (which cannot ever be done using solar & wind power etc)!

    Imagine, we could build a global permanent water pipeline network (& do seawater desalinization) & provide plenty water to everywhere on Earth (for agriculture & forests etc)! (& so any desert wastelands (like Sahara Desert) could be turned to forests and/or agricultural fields!)

    Imagine, very tall poles w/ very powerful infrared heater lamps (etc) on top!
    Imagine, using many of them in cities/towns to turn winters to springs/summers!
    And/or, using many of them in agricultural fields to grow any hot climate (even tropical) crops/trees, even in coldest places on Earth!

    For example, titanium is an extremely durable & strong & light material & Earth has plenty of it, but AFAIK, it requires so much electricity to mine/process!
    Imagine, if we had so much (clean) electricity, we could build all kinds of land/air/sea vehicles, buildings, roads, even whole cities from titanium!

    & no doubt, many new techs would become reality, once we have enough electricity power for them! (For example, consider how computers & internet are used in many ways today, which were pretty much unimaginable to their inventors!)
    Why should solar and wind farms increase temperature and humidity? The sun energy falling on that piece of ground is the same t no matter what you do with it.

    Re. Titanium, what' s the big deal here? Aluminum can do almost everything titanium can do except that titanium is more temperature resistant and more corrosion resistant in chloride systems (e.g. salt). The abundance of aluminum is huge whereas titanium, not so much.

    The trouble with hydrogen fusion is that it does not work, so don't make any big plans. If it could be made to work, I doubt it will ever be economical since the plant would be so complex that operating costs would be prohibitive. The fuel is cheap enough, as it is in uranium plants but the infrastructure is outrageous, especially magnified by all the political interference.
    hellopunyhumans said:
    I feel like I read something about that. Isn't the issue something about ww2 land mines?
    What would land mines have to do with flooding the area? They would be less dangerous underwater than above.
    Reply
  • hellopunyhumans
    Edward said:
    Why should solar and wind farms increase temperature and humidity? The sun energy falling on that piece of ground is the same t no matter what you do with it.

    Re. Titanium, what' s the big deal here? Aluminum can do almost everything titanium can do except that titanium is more temperature resistant and more corrosion resistant in chloride systems (e.g. salt). The abundance of aluminum is huge whereas titanium, not so much.

    The trouble with hydrogen fusion is that it does not work, so don't make any big plans. If it could be made to work, I doubt it will ever be economical since the plant would be so complex that operating costs would be prohibitive. The fuel is cheap enough, as it is in uranium plants but the infrastructure is outrageous, especially magnified by all the political interference.

    What would land mines have to do with flooding the area? They would be less dangerous underwater than above.
    No, the area where the cannal would be dug is a ww2 battleground.
    Reply
  • Uncle Al
    "The Sahara was once a grass and shrub-covered steppe. Could this ever happen again? "

    If Arabia had spent its money on geoforming rather than palaces and Rolls Royces, sure. Israel made its deserts furiously bloom by investing intelligence creating technology. Natural or engineered, Arabia is philosophically crippled. The UAE + israel might be effective.
    Reply