نطق دوم

Lecture 2 - Earth Radiation Budget

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.

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.