Just pause for a moment and imagine the Sahara
Desert.
You’re probably picturing an arid landscape
covered in sand dunes.
There’s almost no water.
Out in the distance, there might be a few
camels walking in the hot sun.
It’s not the most hospitable place.
But ancient artists in northern Africa once
saw a very different Sahara.
Like many artists, they painted what they
saw around them.
And instead of what you just imagined, the
scenery they re-created on rocks, starting
at least 12,000 years ago, was dramatically
different.
They made pictures of hippos and giraffes,
and other savanna species that need to live
near water.
There are even images of livestock and grazing
animals, like cattle and sheep.
While you might see these mammals in southern
or central Africa today, you’d never find
them in the modern Sahara.
But this rock art is everywhere, from the
western Sahara to Saudi Arabia.
And it’s also incredibly accurate, which
means the artists were really familiar with
the animals they were depicting.
For the artists to be that knowledgeable about
hippos and giraffes, those animals had to
have lived there.
So all of this means that the climate of the
Sahara must have been completely different
thousands of years ago.
And I’m not talking about just a few years
of extra rain.
I’m talking about a climate that was so
wet for so long that animals and humans alike
made themselves at home in the middle of the
Sahara.
The many rock artists who lived there created
a record of this ecological change.
But they didn’t record why it all happened.
It would be up to scientists from lots of
different fields to figure that out, thousands
of years later.
Geologists would find the first clues, not
in the Sahara, but at the bottom of the Atlantic
Ocean.
Archaeologists would uncover seemingly unlikely
evidence of vibrant societies, in what’s
now the desert.
And paleoclimatologists would be able to trace
all of these bizarre events back to changes
in the movement of our planet.
All of these lines of evidence would
come together to tell the story of when the
Sahara was green.
In the mid-1800s, a German explorer crossing
the Sahara encountered the paintings and engravings
left behind by those early Holocene artists.
And he puzzled over the mismatch between the
scenes depicted in the rock art and the desert
around it.
Since then, modern geologists have been able
to use many lines of evidence to confirm what
the rock artists saw: Northern Africa was
once much wetter, starting somewhere between
15,000 and 11,000 years ago, and ending 5,000
years ago.
Geologists call this span of time the African
Humid Period, but it’s also known as the
Green Sahara.
And one of the best pieces of evidence for
a green Sahara comes from nearby deep-sea
sediment found off the coast of Mauritania.
Geologists have sampled cores of underwater
sand and mud to study what’s known as Saharan
dust flux, the amount of sediment that was
blown off the African continent and into the
ocean.
When there was more dust coming off the Sahara,
geologists know it was drier, with very little
vegetation -- like it is today.
And if there was less dust, that means it
was wetter.
These sediment cores show that there was much
less dust - potentially only half as much
- coming off northern Africa during this humid
period than there is today.
And ancient pollen from the area confirms
this.
The pollen trapped in those same sediment
cores showed an increase in plants like grasses
and sedges, and a decrease in desert plants
like ephedra, throughout the wet period.
So, scientists think that the area where the
Sahara is today was covered with vegetation,
and that it stretched all the way to the Arabian
Peninsula.
So what could turn the biggest hot desert
in the world green?
Well, the main driver behind this humid period
is actually planetary in scale.
At the start of the humid period, cyclical
changes to both the tilt and orbit of our
planet resulted in about 4% to 8% more solar
energy hitting the Earth than it gets today,
and this warmed up the northern hemisphere.
And when one of the hemispheres gets warmer,
powerful winds tend to move toward it, bringing
lots of rain.
That’s because air rises in the warmer area,
combining with wind to draw moisture up into
the atmosphere.
This cycle, called the African monsoon, happens
on a small scale every year, during the summer
and winter.
And as northern Africa warmed because it was
getting more solar energy, summers became
hotter and longer than usual.
So the African Summer Monsoon was able to
strengthen and move farther north over the
Sahara desert.
Then, as vegetation grew, the plants held
onto moisture better than bare sand could.
And that ended up decreasing the land’s
albedo - that is, the proportion of solar
radiation that it reflected.
And this helped keep the northern hemisphere
even warmer and wetter.
In time, the increased moisture made the Sahara
so wet that there were actual bodies of water
there.
Recent research found that the Sahara had
isolated pockets of lakes and wetlands that
formed in natural basins.
Some lakes were there long enough that they
left behind ancient shorelines, kinda like
geological bathtub rings.
And these rings show that at least one lake
was truly massive -- up to 160 meters deep
and covering more 340,000 square kilometers.
That’s bigger than all of the Great Lakes
combined!
And the location of the rock art confirms
the lake levels.
Because, the scenes created during the humid
period were only made above those ancient
shorelines.
So, the artists basically created another
high water mark that suggests the water was
very deep indeed.
This damp environment was also home to lots
of animals and plants, the kinds that today
we see in savannas, not deserts.
And they left behind fossils both big and
small, even in the driest places.
Take the Ténéré desert, which is part of
the southern Sahara.
Today it’s known as a desert within a desert.
But archaeologists have found proof that the
humid period extended there, too.
For example, there are traces of an ancient
lake there that’s full of the remains of
crocodiles, hippos, and turtles, as well as
giant Nile perch.
Now, along with lakes, there was also a whole
river system across the Sahara.
In Algeria and Libya, researchers have found
river deposits and evidence of human occupation,
like fish hooks, around ancient riverbeds.
And two modern rivers, the Nile and the Niger,
also increased their runoff dramatically.
These riverways allowed the central Sahara
to connect to the Atlantic and the Mediterranean,
which aided the migration of both humans and
non-human animals.
And archaeologists have found that ancient
cultures were able to take full advantage
of the green Sahara.
Along the waterways where there’s now just
sand, there are traces of fireplaces, grinding
stones, hunting tools, and even mounds of
fish bones.
Researchers have also managed to get radiocarbon
dates from organic-rich sediments and artifacts
like baskets, which show that human populations
throughout the Sahara peaked between 9,000
and 5,000 years ago.
And on top of the archaeological evidence,
there are even little hints of the African
humid period within some cultures of Africa
today.
For example, languages from Mali, in western
Africa, and Ethiopia, in eastern Africa, are
now very different from each other.
But they still have similar words for “hippo.”
Some linguists think this could mean that
people from these cultures once lived in the
same place among ... hippos.
Of course, the Sahara returned to a sandy
desert at some point.
The rock artists kept painting, and their
later art showed that new animals arrived
as lakes and rivers began to dry up, and the
ecosystem shifted from savannah to desert.
And paleoclimatologists have used the same
ocean sediment cores that revealed the start
of the green Sahara to figure out when it
ended.
Dust records show that there was a dramatic
decrease in moisture around 5,500 years ago,
and that the humid period only took a few
centuries to end.
When Earth’s orbit shifted once more, the
incoming solar energy decreased, and the northern
hemisphere cooled down.
This pushed the monsoons south again, closer
to where they are now.
And sure enough, the archaeological record
shows that people who lived in the Sahara
abandoned their northern sites first, and
then other sites farther south.
All in all, the African humid period ended
quickly, at least in geological terms.
And when it did, people clustered back around
the Nile and other water sources, while the
Sahara became more like the environment we
know today.
So, thanks to a collaboration between ancient
rock artists and modern climate scientists,
we’ve been able to figure out that the Sahara
was green not all that long ago.
But what does it tell us about future climate
change?
Could this happen again?
Well, what we call the African humid period
was actually just the most recent of 230 green
periods in the Sahara that have occurred in
the past 8 million years!
And since solar radiation is always changing
due to natural orbital cycles, it’ll almost
certainly happen again.
It might be thousands of years from now, and
human-induced climate change has to be factored
in, too.
Either way, we have every reason to expect
that, if and when the Sahara greens up again,
future artists will capture that transformation
in their work.
They may not do it on rock, but their message
will be the same: the world around us is always
changing.
Now that we’ve got you thinking about extreme
climates, you should check out “Antarctic
Extremes,” a new mini-series from NOVA and
PBS Digital Studios.
It’s a journey to Earth’s most remote
laboratory—Antarctica— where science and
survival meet.
Hosts Caitlin Saks and Arlo Pérez reveal
a world that is sometimes harsh, sometimes
hilarious, sometimes gross—but always thrilling.
Find Antarctic Extremes on PBS Terra, PBS
Digital Studios’ new science channel.
Check out the episode in the description below,
and tell them Eons sent you.
Also a big shout out to this month’s Eontologists:
Patrick Seifert, Jake Hart, Jon Davison Ng,
Sean Dennis, and Steve!
Pledge your support at patreon.com/eons and
become an Eonite!
And thank you for joining me in the Konstantin
Haase Studio.
Be sure to subscribe at youtube.com/eons!
Thanks fam.
