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旁白：听一段讲地球科学的课堂讲座。

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

教授：我们刚刚了解了激烈的气候变化发生的有多快。

Professor: We're really just now beginning to understand how quickly drastic climate change can take place.

我们看到过去几百年前发生的气候变化事件。

We can see past occurrences of climate change that took place over just a few hundred years.

比如说，北非的撒哈拉大沙漠。

Take uh...the Sahara Desert in Northern Africa.

6000年前的撒哈拉大沙漠是很不一样的。

The Sahara was really different 6,000 years ago.

我是说，你不会叫它热带天堂或什么的。嗯，如果你想想在撒哈拉的有些地方今天是什么样子你也可能会这么叫，撒哈拉的有些地方一个世纪才下一场雨。

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.

不过，基本上，你还是有绿色植物和水的。

Um... but basically, you had greenery and you had water.

而且我觉得特别有意思和不可思议的是一些画在岩石上的东西，史前艺术，石头上画了河马，这些东西显示了撒哈拉在几千年前是多么的不像沙漠。

And what I find particularly interesting... amazing really, what really indicates how undesert like the Sahara was thousands of years ago, was something painted on the rock, prehistoric art, hippopotamuses.

因为你们知道，河马是需要大量的水才能生存的，并且因此？

As you know hippos need a lot of water and hence?

学生：它们需要终年生活在一个巨大的水源附近。

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

学生：但是那怎么证明撒哈拉过去潮湿的多？

Student: But how is that proof that the Sahara used to be a lot wetter?

我是说那些画河马的人，嗯，难道就不可能是他们在旅途中见到的吗？

I mean the people who painted those hippos, well, couldn't they have seen them on their travels?

教授：好的，他们是有这种可能，卡尔。

Professor: Okay, in principle they could, Karl.

但是岩石绘画不是唯一的证据。

But the rock paintings aren't the only evidence.

在撒哈拉下面有大大的蓄水层，一片淡水的海洋，那可能是经过岩石层过滤的，有一百万年历史了。

Beneath the Sahara are huge aquifers, basically a sea of fresh water, that's perhaps a million years old filtered through rock layers.

并且，嗯，有一些石化的花粉，来自于曾经生长在撒哈拉的小灌木丛和草丛。

And... er, and then there is fossilized pollen, from low shrubs and grasses that once grew in the Sahara.

实际上，这些植物现在仍然在生长，嗯，但是离这儿几百英里远了，生长在一些有更多植物生长的区域。

In fact these plants still grow, er, but hundreds of miles away, in more vegetated areas.

不管怎么说，就是这些石化花粉和蓄水层，还有岩石上的画作，这三类东西都是证明撒哈拉曾经比现在有更多植物的证据，可能不仅有河马，还有大象和长颈鹿等等。

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.

学生：那么发生什么事儿了呢？

现在，我们很习惯听到听到说人类活动是如何影响气候的。

Well, Now, we're so used to hearing about how human activities are affecting the climate, right?

但是这却将将注意力从地球气候的自然变化转移开了，比如说冰河世纪，对吧？

But that takes the focus away from the natural variations in the earth climate, like the Ice Age, right?

这个星球在几千年前都是被冰雪覆盖的。

The planet was practically covered in ice just a few thousand years ago.

现在就撒哈拉沙漠而言，近期有些文学作品指向了那个地区季候风的迁移。

Now as far as the Sahara goes, there is some recent literature that points to the migration of the monsoon in that area

是这样，季候风是指季节性的刮风，这种风能带来大量的雨水。

Okay, a monsoon is a seasonal wind that can bring in a large amount of rainfall.

现在如果季候风迁移了，也就意味着雨水也跟着去了另一个地区，对吧？

Now if the monsoon migrates, well, that means that the rains move to another area, right?

那么是什么使得季候风迁移呢？

嗯，答案就是，地球运动的动力学。这也是造成冰河世纪原因。

Well, the answer is: the dynamics of earth's motions, the same thing that caused the Ice Age by the way.

地球不总是和太阳离得一样远的，也不总是以同样的角度对着太阳的。

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 parameters.

他们变化都很缓慢，但是他们的影响却有可能很突然。

They're gradual variations but their effects can be pretty abrupt.

可以引起气候在短短几百年间发生变化。

And can cause the climate to change in just a few hundred years.

教授：嗯，你们想想啊，其他的气候变化可是花了几千年啊，这个已经是很突然了。

Professor: Well, yeah, considering that other climate shifts take thousands of years, this one is pretty abrupt.

所以这些星球运动中的变化，他们称之为“气候变化”。

So these changes in the planet's motions, they caused the climate to change.

撒哈拉沙漠所经历的变化，嗯，是一种逃离干燥效果。

What the Sahara experienced was uh...a sort of...runaway drying effect...

就像我说的季候风本身会迁移，所以撒哈拉沙漠的雨水变少。

As I said the monsoon migrated itself, so there was less rain in the Sahara.

土地开始变干燥，于是造成了植物的大量减少，因为植物在干燥的土壤里无法很好的生长。

the land started to get drier, which in turn caused a huge decrease in the amount of vegetation, because vegetation doesn't grow as well in dry soil, right?

后来，植物太少也导致了土壤无法保留住水分，土壤也失去了在下雨时保持住水分的能力。

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.

所以，后来湿度不够也难以形成云层，因为没有可以蒸发的能形成云层的东西。

So then you have less moisture to help clouds form, nothing to evaporate for cloud formation.

然后形成了这样的循环，雨水少，土地变干燥，植物变少，云层减少，下雨就更少，如此循环下去……

And then the cycle continues, less rain, drier soil, less vegetation, fewer clouds, less rain etc. etc.

学生：但是，那些在岩石上画那些画儿的人呢？

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

教授：这个问题问得好。没有人真正知道。

Professor: Good question. No one really knows.

但是也许与古埃及有点联系，与撒哈拉变成沙漠在同一时期。

But there might be some connections to ancient Egypt.

教授：五千年前，埃及在尼罗河山谷真正开始兴旺繁荣。

At about the same time that the Sahara was becoming a desert... hmm ...5,000 years ago, Egypt really began to flourish out in the Nile River valley.

当很多人意识到这不仅仅是暂时性的干旱的时候，他们就迁移到了尼罗河山谷，这种假设也符合逻辑。

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.

有些人对这个做了进一步的研究。

And some people take this a step further.

科学上假设这次的迁移实际上在古埃及的发展中提供了一个重要的动力。

And that's okay, that’s science and they hypothesize that this migration actually provided an important impetus in the development of ancient Egypt.

Not long ago, the Sahara had a different climate. What evidence does the professor mention to support this?

In the lecture, what do the Ice Age and the creation of the Sahara Desert both illustrate about past climate changes?

What started the runway effect that led to the Sahara area of north Africa becoming a desert?

The professor mentions a theory that people migrating from the Sahara were important to the development of the Egyptian civilization. Which sentence best describes the professor's attitude toward this theory?

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 Saharait…it only rains about once a century. Um… but basically, you had greenery and you had water. 
And what I find particularly interesting... amazing really, what really indicates how undesert-like the Sahara was thousands of years ago, was something painted on the rock, prehistoric art, hippopotamuses, as you know hippos need a lot of water and hence? Hence what? 
Student 
They need to live near a large source of water year around. 
Professor 
That’s right. 
Student 
But how is that proof 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 principle 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 hippo sand probably elephants and giraffes and so on. 
Student 
So what happened? 
Professor 
How did it happen? Well, 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 parameters. They’re gradual variations but their effects can be pretty abrupt. And can cause the climate to change in just a few hundred years. 
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 parameters. 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 caused the climate to change. 
But it was also compounded. What the Sahara experienced was uh…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 a 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…hmm ...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. 
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 Saharait…it only rains about once a century. Um… but basically, you had greenery and you had water.