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

旁白：听下面一段环境科学课堂上的讲解。

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 effect of clouds 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.

教授：基本上，云的作用是降低或升高地球表面的温度。地球天气系统不断地试图在云的降温和变暖效应中寻求一个平衡。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。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，我们称之为地球辐射收支。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。

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... rain forests also have low albedos.Well, by contrast, deserts and areas covered by ice or snow, these places have high albedos.And clouds-in general-clouds also have high albedos.That means that a large percentage of the solar energy clouds receive is reflected back into space.

表面反照率是指地表反射回太空的能量与所有入射太阳的能量的比率。海洋的反照率很低，因为反射的能量很小，大部分到达海洋的太阳能量都被吸收并使海水增温。雨林也具有低反照率。相反的，沙漠地区或被冰雪覆盖的地区具有高反照率。一般来说，云也具有高反照率。这就是说，云接受的大部分太阳能量都被发射回太空了。

OK... now when we say that clouds have a high albedo, we're 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.

好的。当我们说云具有高反照率，我们说的是所有的云的平均反照率。但是不同种类的云有不同的反射性，也就有不同的反照率。

So which type of clouds cool Earth, and which type heat it?

学生：那么什么类型的云为地球降温？什么类型的又使地球升温呢？

Well, high, thin clouds contribute to heating, while low, thick clouds cool Earth.High, thin clouds are very transparent to solar radiation, like, eh, clear air.So, they mostly transmit incoming solar energy down to Earth; there's not much reflection going on 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.

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。太阳辐射可以轻易地通过高空薄云层，就像通过空气一样。所以大部分的太阳能量都会传输到地面，薄云层没有多少反射作用。同时，这些云层还会留住一些地球的热量。由于这些截留的能量，这类云总体上有升温作用。

Oh, OK, so since low, thick clouds are not transparent to radiation...

学生：哦，这样。由于辐射无法透过低空厚云层。

...exactly, they block much of the solar energy so it never reaches Earth's surface- they reflect much of it back out in to space.

教授：没错，他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，大部分的太阳能量被发射到太空中。

So that's how clouds contribute to cooling.

学生：这就是为什么他们使地球降温？

Yep. And as I said earlier, this cooling effect predominates.Now, what if there was a process that could control the types of clouds that form?

教授：是的，之前我说过，这种降温作用占主导地位。现在想想，要是有一种方法可以控制这种云的形成？

Are you talking about controlling the weather?

学生：您是说控制天气？

Well, I'm 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- 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.Well, if 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.

我不确定能够办得到。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。是低空厚云层的增加，可以将大部分太阳能量反射回太空给地球降温的云层。造成云盖增加的原因，是大量微小的的海洋植物。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，创造了我们观察到的低空厚云层形成的条件。如果这个假设是正确的，它有着巨大的意义。所以我们很有可能控制天气。如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。