MAN: 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 just painted what they saw around them.

And instead of what you just imagined the scenery that 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 that they were depicting.

For the artist 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 is now the desert.

And paleo climatologists 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 artist 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 the 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 metal sediment that was blown off the African continent and into the ocean.

When there was more dust coming off the Sahara, geologists know that 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 the 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 the 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 studies 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, kind of like geological bathtub rings, and these rings showed that at least one lake was truly massive, up to 160 meters deep and covering more than 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 up of those ancient shorelines.

So the artists basically created another high watermark 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 Tenere 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 the lakes, there's 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 human 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 9000 and 5,000 years ago.

And on top of their 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 savanna to desert.

And paleo climatologists 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 monsoon south again closer to where they are now.

And sure enough, the archaeological record shows that people who the Sahara abandoned their northern sites first and then the 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 it 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 eight million years.

And since solar radiation is always changing due to natural orbital cycles, it will 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.

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