Module · Earth Systems · Prequel

The Green Sahara

The world before the dust. When the desert was a garden and the largest lake on earth was full of life.

0M km²

Green Sahara at peak

0K km²

Lake Mega-Chad

Green cycles in 8M years

~200 yrs

Speed of the collapse

Eleven thousand years ago, the Sahara is green. Not metaphorically green. Actually, physically, overwhelmingly green. Rivers run through it — wide, permanent rivers full of crocodiles and Nile perch. Lakes cover the interior, the largest of them bigger than all the Great Lakes combined. Hippos wade in water that is now sand. Elephants browse on acacia trees that will not exist for another six millennia.

This is the African Humid Period. It happens because the earth wobbles. A slow, 21,000-year cycle in the planet's axial tilt shifts how much solar energy hits the Northern Hemisphere in summer. When the tilt is right, the African monsoon strengthens. Rain falls on the Sahara. Grass grows. Then shrubs. Then trees. Then animals. Then people.

It has happened over 230 times in the last 8 million years. The Sahara breathes — green, desert, green, desert — on the rhythm of the earth's orbit. The last green period ended roughly 5,000 years ago. In geological terms, it ended this morning.

The Landscape

Lakes, rivers, and the world's largest art museum

Lake Mega-Chad~400,000 km²

Now: Bodélé Depression (dust) + remnant Lake Chad (~1,350 km²)

1,000 km north-south. 600 km east-west. Larger than all the Great Lakes combined. Possibly the world's largest lake at the time. Fed by rivers from the Tibesti, Hoggar, and Ennedi mountains. Skeletons of elephants, hippos, and hominins found on its ancient shorelines. When it dried, it left behind a bed of diatomite — the fossilised shells of billions of diatoms. That diatomite is now the dust that feeds the Amazon.

Lake Mega-Fezzan~120,000 km²

Now: dry sabkha, southern Libya

A massive lake in what is now the Libyan Sahara. Fed by rivers from the Tibesti and Tassili mountains. Crocodiles and hippos lived in it.

Lake Darfur~30,000 km²

Now: dry lake beds, western Sudan

Part of a chain of lakes across the central Sahara. Connected to the Nile drainage during the wettest phases.

Lake Ptolemy (Jebel Arkenu)~5,000 km²

Now: dry, southeastern Libya

Named by Ptolemy who mapped it from Alexandrian reports. A lake in the deep Sahara that no longer exists.

Gobero (Lake)Small lakeside site km²

Now: Ténéré Desert, Niger

The "Stone Age Graveyard." Two distinct populations buried here over 5,000 years. Kiffian people (tall, powerful, fishing culture, 10,000–8,000 BP) and Tenerian people (shorter, more gracile, 7,000–5,000 BP). Harpoon heads. The lake that sustained them vanished.

Ancient Nile overflowExpanded flood system km²

Now: standard Nile flow

During the AHP the Nile flooded much more than today. Enhanced runoff poured into the eastern Mediterranean, creating anoxic conditions and depositing organic-rich sapropel layers on the seafloor — still measurable today.

Scale

How big was Mega-Chad?

Green Sahara9,000,000 km²

Sahara today9,200,000 km²

Lake Mega-Chad400,000 km²

All Great Lakes combined244,106 km²

Lake Chad today1,350 km²

Lake Mega-Chad was 400,000 km². All five Great Lakes together are 244,000 km². Today's Lake Chad — what remains — is 1,350 km². That is a 99.66% reduction.

The Animals

What lived in the Green Sahara

The fossil record and rock art together paint a picture of a landscape teeming with life. Herds of animals that we now associate only with East African safari — elephants, giraffes, hippos, wildebeest — once covered the Sahara from the Atlantic to the Nile.

Hippopotamus

aquatic

Bones in lake sediments. Rock art depictions. Found across the entire Sahara during the AHP.

Nile crocodile

aquatic

Propagated through connected river/lake systems. Still survives in Saharan guelta pools in Mauritania and Chad — relict populations from the Green Sahara.

Elephant

megafauna

Rock art across Tassili, Acacus, Hoggar. Skeletons found at Angamma delta of Lake Mega-Chad.

Giraffe

megafauna

Rock art. The famous Dabous Giraffes in Niger — two life-sized giraffes carved into rock, 6,000 years old, 6 metres tall.

Rhinoceros

megafauna

Rock art depictions in Tassili n'Ajjer and Acacus Mountains.

Aurochs (wild cattle)

savannah

Rock art shows large herds. The ancestor of domesticated cattle. Pastoralism began in the Green Sahara.

African buffalo

savannah

Fossils in Egyptian Sahara sites. Rock art.

Warthog

savannah

Fossils in Egyptian sites.

Hartebeest

savannah

Bone deposits at lake sites across the central Sahara.

Nile perch

aquatic

Fish bones in lake sediments. A freshwater giant — up to 2 metres.

Tilapia

aquatic

Found in multiple ancient lake sites. A staple protein source.

Catfish

aquatic

Bone deposits at Gobero and other lakeside sites.

Pelican

aquatic

Fossil evidence in multiple lake deposits.

Spotted hyena

predator

Fossils in Egyptian Sahara.

Wildebeest

savannah

Fossils in Egyptian Sahara sites.

Zebra

savannah

Fossils in Egyptian sites. Savannah species requiring grasslands and water.

The Drying

Then the earth wobbled the other way

The same orbital cycle that created the Green Sahara destroyed it. As the axial tilt shifted, less solar energy hit the Northern Hemisphere in summer. The monsoon weakened. The rain retreated south. And a landscape that had been green for 6,000 years became a desert in roughly 200.

~14,600 BP

wet

The greening begins

Earth's axial tilt shifts. More solar energy hits the Northern Hemisphere in summer. The African monsoon strengthens. Rain begins to fall on what had been desert since the Last Glacial Maximum. The Sahara starts to grow grass.

~12,700 BP

transition

Younger Dryas interruption

A sudden cold snap. 1,200 years of drying. The monsoon weakens. The greening pauses. Then the cold ends — and the rain returns stronger than before.

~11,500 BP

wet

Full Green Sahara

9 million km² transforms. Rivers fill. Lakes form. Mega-Chad grows to 400,000 km². Grasslands, acacia woodland, and in places, actual forest. Elephants, hippos, crocodiles everywhere. Humans settle.

~9,000 BP

wet

Peak humidity

Maximum rainfall. The Sahara is at its greenest. Lake levels at their highest. Rock art flourishes across Tassili, Acacus, Hoggar, Aïr. Pastoralism begins — people domesticate cattle in the Green Sahara.

~8,200 BP

transition

The 8.2 kiloyear event

A sharp cold snap. The Green Sahara "pauses." Neolithic humans temporarily abandon occupation sites. Lakes shrink. Then conditions recover — but not fully.

~6,000 BP

dry

The drying begins

Earth's axial wobble shifts again. The monsoon weakens. Rainfall decreases. Vegetation retreats southward. Lakes begin to shrink. The process is slow at first. Then it accelerates.

~5,500 BP

dry

Abrupt collapse

In some areas the transition from green to desert happens within one to two centuries. Dust flux increases dramatically in ocean sediment cores. The Sahara turns off like a switch.

~5,000 BP

dry

The desert returns

Mega-Chad shrinks. Rivers stop flowing. Animals retreat south to the Sahel or die. Humans migrate — to the Nile Valley, to the coast, to the south. The civilisations that will become Pharaonic Egypt are born in this migration.

~3,000 BP

dry

Full desert

The Sahara looks like it does today. 9 million km² of sand, rock, and dust. The rivers are gone. The lakes are gone. Only the rock art remembers.

Present

consequence

The Bodélé Depression

What was the northern shore of Lake Mega-Chad is now the dustiest place on earth. The diatomite — fossilised shells of billions of diatoms that once lived in the lake — is picked up by wind and blown across the Atlantic. 182 million tons per year. 27.7 million tons land on the Amazon. The dead lake feeds the living forest.

The Connection

The dead lake feeds the living forest

When Lake Mega-Chad dried, it left behind a bed of diatomite — the fossilised silica shells of billions of diatoms that had lived in its water. That diatomite now sits in the Bodélé Depression, the lowest point in central Africa, pinched between the Tibesti and Ennedi mountains.

Wind funnels through the gap between the mountains and rakes across the dry lake bed. On an average winter day, 700,000 tons of dust are lifted into the air. The dust — made of dead diatoms, rich in phosphorus, iron, and silica — is carried west on the trade winds, across the Atlantic, 5,000 kilometres, to the Amazon rainforest.

182 million tons per year. 27.7 million tons land on the Amazon. The phosphorus in that dust replaces exactly what the rainforest loses to rain each year. The largest living forest on earth is fertilised by the largest dead lake on earth.

This is a prequel. The next chapter is The Dust That Feeds. The sequel is The Phosphate Equation.

Sources

Tierney, J.E. et al. (2017). Rainfall regimes of the Green Sahara. Science Advances, 3(1).

deMenocal, P.B. et al. (2000). Abrupt onset and termination of the African Humid Period. Quaternary Science Reviews.

Drake, N. & Bristow, C. (2006). Shorelines in the Sahara: geomorphological evidence from Palaeolake Megachad. The Holocene.

Schuster, M. et al. (2005). Holocene Lake Mega-Chad palaeoshorelines from space. Quaternary Science Reviews.

Koren, I. et al. (2006). The Bodélé Depression: a single spot in the Sahara that provides most of the mineral dust to the Amazon. Environmental Research Letters.

Yu, H. et al. (2015). The fertilizing role of African dust in the Amazon. Geophysical Research Letters.

Washington, R. et al. (2009). Dust as a tipping element: The Bodélé Depression, Chad. PNAS.

Sereno, P. et al. (2008). Lakeside Cemeteries in the Sahara. PLOS ONE. (Gobero site)

NASA Earth Observatory. Bodélé Depression imagery. MODIS/CALIPSO.

Nature Scitable. Green Sahara: African Humid Periods Paced by Earth's Orbital Changes.

Sources: NASA Earth Observatory, Nature Scitable, CALIPSO, Tierney et al., deMenocal et al., Schuster et al.