托福听力_考前小试牛刀（二）

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.