TPO 6. Lecture 4. Earth Science

Narrator： Listen to part of a lecture in an earth science class

Professor
We’re really just now beginning to understand how quickly drastic climate
change can take place. We can see past occurrences of climate change that
took place over just a few hundred years. Take uh⋯ the Sahara Desert in
Northern Africa. The Sahara was really different 6,000 years ago. I mean, you
wouldn’t call it a tropical paradise or anything, uh⋯or maybe you would if you
think about how today in some parts of the Sahara it⋯it only rains about once
a century. Um⋯ but basically, you had granary and you had water. And what I
find particularly interesting and amazing really, what really indicates how un
desert-like the Sahara was thousands of years ago, was something painted on
the rock, pre-historic art, hippopotamuses, ‘cos you know hippos need a lot of
water and hence? Hence what?

Student： They need to live near a large source of water year round

Professor： That’s right.

Student
But how is that proved that the Sahara used to be a lot wetter? I mean the
people who painted those hippos, well, couldn’t they have seen them on their
travels?

Professor
Okay, in principal they could, Karl. But the rock paintings aren’t the only
evidence. Beneath the Sahara are huge aquifers, basically a sea of fresh
water, that’s perhaps a million years old filtered through rock layers.

And⋯er⋯and then there is fossilized pollen, from low shrubs and grasses that
once grew in the Sahara. In fact these plants still grow, er⋯but hundreds of
miles away, in more vegetated areas. Anyway, it’s this fossilized pollen along
with the aquifers and the rock paintings, these three things are all evidence
that the Sahara was once much greener than it is today, that there were hippos
and probably elephants and giraffes and so on.

So what happened? How did it happen? Now, we’re so used to hearing about
how human activities are affecting the climate, right? But that takes the focus
away from the natural variations in the earth climate, like the Ice Age, right?
The planet was practically covered in ice just a few thousand years ago. Now
as far as the Sahara goes, there is some recent literature that points to the
migration of the monsoon in that area

Students： Huh?????

Professor
What do I mean? Okay, a monsoon is a seasonal wind that can bring in a large
amount of rainfall. Now if the monsoon migrates, well, that means that the
rains move to another area, right? So what caused the monsoon to migrate?
Well, the answer is: the dynamics of earth’s motions, the same thing that
caused the Ice Age by the way. The earth’s not always the same distance from
the sun, and it’s not always tilting toward the sun at the same angle. There are
slight variations in these two perimeters. They’re gradual variations but their
effects can be pretty abrupt. And can cause the climate to change in just a few
hundred years.

Student： That’s abrupt?

Professor
Well, yeah, considering that other climate shifts take thousands of years, this
one is pretty abrupt. So these changes in the planet’s motions, they called it
“the climate change”, but it was also compounded. What the Sahara
experienced was um⋯a sort of “runaway drying effect”. As I said the monsoon
migrated itself, so there was less rain in the Sahara. The land started to get
drier, which in turn caused huge decrease in the amount of vegetation,
because vegetation doesn’t grow as well in dry soil, right? And then, less
vegetation means the soil can’t hold water as well, the soil loses its ability to
retain water when it does rain. So then you have less moisture to help clouds
form, nothing to evaporate for cloud formation. And then the cycle continues,
less rain, drier soil, less vegetation, fewer clouds, less rain etc. etc..

Student： But, what about the people who made the rock paintings?

Professor
Good question. No one really knows. But there might be some connections to
ancient Egypt. At about the same time that the Sahara was becoming a
desert⋯

Student： Uh-huh

Professor
5,000 years ago, Egypt really began to flourish out in the Nile River valley. And
that’s not that far away. So it’s only logical to hypothesize that a lot of these
people migrated to the Nile valley when they realized that this was more than a
temporary drought. And some people take this a step further. And that’s okay,
that’s science and they hypothesize that this migration actually provided an
important impetus in the development of ancient Egypt. Well, we’ll stay tuned
on that.