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旁白：请听一段环境科学的课堂讲解。

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

教授：基本上，云的作用是降低或升高地球表面的温度。

Professor: Basically, a cloud either contributes to the cooling of Earth's surface or to its heating.

地球天气系统一直试图在云的降温和保温作用中寻求一个平衡。

Earth's climate system is constantly trying to strike a balance between the cooling and warming effects of clouds.

这十分相近，但是总体上来说，云的累积作用是降低地球的温度而不是加热。

It's very close, but overall the cumulative effects of cloud is to cool Earth rather than heat it.

这个平衡是指太阳的辐射量，也就是被地球吸收太阳能量与反射回宇宙中的能量之间的平衡。

And this balance between the amount of solar radiation, energy from the Sun, that's absorbed by Earth, and the amount that's reflected back into space.

我们观察辐射收支的方法之一就是通过观测地球不同表面的反照率。

And one way we keep tracking the radiation budget is by looking at the albedo of the different surfaces on the planet.

表面反照率是指接受的太阳能量、阳光与反射回空间中的能量的比率。

A surface's albedo is the percentage of incoming solar energy, sunlight, that's reflected off that surface back into space.

海洋的反照率很低，因为反射的能量很小。

Oceans have a low albedo, because they reflect very little energy.

大部分到达海洋的太阳能量都被吸收并使海水增温。

Most of the solar energy that reaches the ocean gets absorbed and heats the water.

相反的，沙漠地区或被冰雪覆盖的地区具有高反照率。

Well, by contrast, deserts and areas covered by ice and snow, these places have high albedos.

And clouds, in general, cloud also have high albedos.

这就是说，云接受的大部分太阳能量都被发射回太空了。

That means that a large percentage of the solar energy clouds receive is reflected into space.

好的。当我们说云具有高反照率。

OK. Now, when we say that clouds have a high albedo.

我们说的是所有类型的云的平均反照率。

We are talking about the effect of all the clouds on earth averaged together.

但是不同种类的云有不同的反射性，也就有不同的反照率。

But different types of clouds have different reflective properties, they have different albedos.

学生：那么什么类型的云为地球降温？

Student: So which type of clouds cool Earth?

教授：高空的薄云层使地球升温，而低空的厚云层为地球降温。

Professor: Well, high thin clouds contribute to heating while low thick clouds cool Earth.

太阳辐射可以轻易地通过高空薄云层，就像通过空气一样。

High thin clouds are very transparent to solar radiation, like, uh, clear air.

所以大部分的太阳能量都会传输到地面。

So they mostly transmit incoming solar energy down to Earth.

There's not much reflection going on at all.

同时，这些云层还会留住一些地球的热量。

At the same time, these clouds trap in some of Earth's heat.

由于这些截留的能量，这类云总体上有升温作用。

Because of the trapped heat, these clouds have an overall heating effect.

学生：哦，这样。由于辐射无法透过低空厚云层……

Student: Oh. OK. So, since low thick clouds are not transparent to radiation...

教授：没错。它们阻挡了大部分的太阳能量，所以太阳能量无法到达地球表面。

Professor: Exactly. They block much of the solar energy so it never reaches Earth's surface.

大部分的太阳能量被发射到太空中。

They reflect much of it back out into space.

学生：这就是为什么它们使地球降温？

Student: So that's how clouds contribute to cooling?

教授：是的，之前我说过，这种降温作用占主导地位。

Professor: Yep. And as I said earlier, this cooling effect predominates.

现在想想，要是有一种方法可以控制这种云的形成？

Now, what if there was a process that could control the type of clouds that form?

Student: Are you talking about controlling the weather?

Professor: Well, I am not sure I would go that far.

但是，近期我们注意到在南极洲附近的海水上空的云层覆盖量有所增加。

But we recently noticed an increase in cloud cover over an area of the ocean waters around Antarctica.

是低空厚云层的增加，可以将大部分太阳能量反射回太空的云层。

An increased area of low thick clouds, the type that reflects a large portion of solar energy back to space and cools the Earth.

造成云盖增加的原因，是大量的显微镜可见的海洋植物。

Well, the reason for this increased cloud cover, it turns out, is the exceptionally large amount of microscopic marine plants.

近期的一个假设是，这种微生物产生一种化学物质，即硫化物，它与空气中的氧相互作用，创造了一个适合我们所观察到的低空后云层产生的环境。

Well, the current hypothesis is that these microorganisms produce a chemical, dimethyl sulfide, that interacts with the oxygen in the air, creating conditions that lead to the formation of the low thick clouds we've observed.

So, maybe we are talking about controlling the weather.

若果南极洲的微生物真的是引起云层增加的原因，或许我们能够加快这个进程。

Perhaps, if the microorganisms near Antarctica really are responsible, perhaps we can accelerate the process somehow.

According to the professor, what is Earth's radiation budget?

What can be inferred from the lecture about the effects of different types of clouds on Earth's climate?

What does the professor say about microscopic plants in the ocean near Antarctica?

Listen again to part of the Lecture. Then answer the question. Why does the professor say this:

Narrator: Listen to part of a lecture in an environmental science class. Professor: Basically, a cloud either contributes to the cooling of Earth's surface or to its heating. Earth's climate system is constantly trying to strike a balance between the cooling and warming effects of clouds. It's very close, but overall the cumulative effects of cloud is to cool Earth rather than heat it. And this balance between the amount of solar radiation, energy from the Sun, that's absorbed by Earth, and the amount that's reflected back into space. We call this Earth's radiation budget. And one way we keep track of the radiation budget is by looking at the albedo of the different surfaces on the planet. A surface's albedo is the percentage of incoming solar energy, sunlight, that's reflected off that surface back into space. Oceans have a low albedo, because they reflect very little energy. Most of the solar energy that reaches the ocean gets absorbed and heats the water. Um... rainforests also have low albedos. Well, by contrast, deserts and areas covered by ice and snow, these places have high albedos. And clouds, in general, cloud also have high albedos. That means that a large percentage of the solar energy clouds receive is reflected into space. Well, by contrast, deserts and areas covered by ice and snow, these places have high albedos. And clouds, in general, cloud also have high albedos. That means that a large percentage of the solar energy clouds receive is reflected into space. OK. Now, when we say that clouds have a high albedo. We are talking about the effect of all the clouds on earth averaged together. But different types of clouds have different reflective properties, they have different albedos. Student: So which type of clouds cools Earth? And which type heat it? Professor: Well, high thin clouds contribute to heating while low thick clouds cool Earth. High thin clouds are very transparent to solar radiation, like, uh, clear air. So they mostly transmit incoming solar energy down to Earth. There's not much reflection going at all. At the same time, these clouds trap in some of Earth's heat. Because of the trapped heat, these clouds have an overall heating effect. Student: Oh. OK. Since low thick clouds are not transparent to radiation... Professor: Exactly. They block most of the solar energy so it never reaches Earth's surface. They reflect much of it back out into space. Student: So that's how they contribute to cooling? Professor: Yep. And as I said earlier, this cooling effect predominates. Now, what if there was a process that could control the type of clouds that form? Student: Are you talking about controlling the weather? Professor: Well, I am not sure I would go that far. But we recently noticed an increase in cloud cover over an area of the ocean waters around Antarctica. An increased area of low thick clouds, the type that reflects a large portion of solar energy back to space and cools the Earth. Well, the reason for this increased cloud cover, it turns out, is the exceptionally large amount of microscopic marine plants. Well, the current hypothesis is that these microorganisms produce a chemical, dimetho sulfide that interacts with the oxygen in the air, creating conditions that lead to the formation of the low thick clouds we observed. Well, that's true. It could have huge implications. So, maybe we are talking about controlling the weather. Perhaps, if the microorganisms near Antarctica really are responsible, perhaps we can accelerate the process somehow. Student: Are you talking about controlling the weather? Professor: Well, I am not sure I would go that far. But we recently noticed an increase in cloud cover over an area of the ocean waters around Antarctica. An increased area of low thick clouds, the type that reflects a large portion of solar energy back to space and cools the Earth. Well, the reason for this increased cloud cover, it turns out, is the exceptionally large amount of microscopic marine plants. Well, the current hypothesis is that these microorganisms produce a chemical, dimetho sulfide that interacts with the oxygen in the air, creating conditions that lead to the formation of the low thick clouds we observed. Well, that's true. It could have huge implications. So, maybe we are talking about controlling the weather. Perhaps, if the microorganisms near Antarctica really are responsible, perhaps we can accelerate the process somehow.