The greenhouse effect | Physics | Khan Academy

Our Earth's surface temperature is somewhere close to 15° C—nice, cozy, and warm for us living beings. But what keeps us so warm? Well, my instinctive answer is that it's the sun, right? But it actually gets more interesting. Our atmosphere has these gases called greenhouse gases. There's nothing green about them, okay? The name is just stuck. They have something called the greenhouse effect on our planet.

And it turns out that if we didn't have these gases, then even with the sun shining as much as it does, the temperature on our Earth would have been way, way below 0° C. So along with the sun, we have to thank the greenhouse gases because they are the ones that warm up our planet. But how do they do that? What exactly is the greenhouse effect? Well, let's find out.

Let's start by thinking about what would be our temperature if there was no atmosphere and there was no sun. Then the Earth would have the coldest temperature we can think of: very close to absolute 0, which is -273° C. Okay, but now if you put the sun, the light shining on the Earth gets absorbed by the Earth. The energy from the light gets converted into thermal energy, and now the Earth will start warming up.

But does that mean its temperature will now keep rising indefinitely because there's continuous light that's shining on the Earth? No, because every object that has any temperature will start radiating light. We call this thermal radiation, and that way it starts losing energy. But at very low temperatures, the thermal radiation is not much, so the Earth is not losing a lot of energy per second. It's still gaining a lot more energy per second, which means its temperature will keep rising for some time.

But as its temperature rises, it will start losing even more energy to thermal radiation. So you can see where you're going with this. At some particular temperature, the Earth's thermal radiation will be so high that the energy lost will be exactly equal to the energy gained, and we'll now have thermal equilibrium. The temperature will no longer rise. And by the way, this would be the average temperature because the side that's facing the sun would be much hotter, and the side that's away from the sun would be much colder. The Earth is spinning, so it's a dynamic system, but on average that temperature would be very, very low without an atmosphere.

And by the way, don't think that Earth is only radiating on this side; Earth is radiating all around. It's just a way—it's just a depiction. Okay, but now that brings us to the question: What happens when we add an atmosphere along with the greenhouse gases? What do they do? Well, they have a very interesting effect on both the incoming and the outgoing light. But before we get into the nitty-gritty, let's oversimplify a little bit so that we can get the big picture idea of what's really going on. Then we'll dive into the details.

The big thing to understand is that the greenhouse gases are mostly sensitive to infrared radiation. All right? So the incoming light from the sun is pretty much visible light; therefore, the greenhouse gases have pretty much no effect on it. So nothing changes over here. However, the light that is being radiated by the Earth is infrared radiation. Why is that? Well, that's because thermal radiation purely depends on the temperature. At very low temperatures, like what we have over here, the radiation would be pretty much all infrared.

Now, the greenhouse gases will absorb these thermal radiations. And what do they do when they absorb? Well, just like how an atom absorbs radiation, it gets excited, and then it will re-emit it. Just like that, these greenhouse gases will re-emit these radiations, but in all directions. That's the idea. And so, some of the radiation will go back into space like before, but some of the radiation will come back to the Earth. As a result, look at what has happened because of this: the outgoing radiation now has reduced. As a result, it's no longer in equilibrium.

Now the Earth and the atmosphere—the whole system—is gaining more energy than it is losing, and that's how the temperature of the planet will start rising. But again, will it keep rising indefinitely? No, just like before, as the temperature rises, the Earth will start radiating even more infrared radiation. Eventually, at some particular temperature again, the outgoing radiation will equal the incoming radiation per second, and we'll have a new equilibrium. But that new equilibrium will be at a much higher temperature, and that temperature happens to be where we are.

And that's how the greenhouse gases warm the globe by trapping the infrared radiation. So that's how they cause global warming, which is a good thing because we want our Earth to be warm. But now the big question is: What happens if we add even more greenhouse gases due to human activity? What will that do? Does it significantly affect the climate anymore? Well, to answer that question, we now have to dig into the details.

So let's do that. A couple of details that we want to talk about is, first of all, what really happens to the incoming radiation? How does it interact with the atmosphere? Why do the greenhouse gases absorb infrared radiation in the first place? And then finally, we'll try to understand what would happen if we were to increase the greenhouse gases like what we've been doing due to human activity.

Our first oversimplification was assuming that all the light that reaches the Earth warms up the Earth, but that's not true at all. Because a lot of light actually interacts with the atmosphere. And just to help us see that, here is the spectrum of the sunlight at the top of our atmosphere. You can see that it nicely follows—it's very close to the black body radiation spectrum that we talked about in one of our previous videos.

Okay, so you can see that a lot of light is, you know, is in the visible region. Some of it is the UV, and some of it is in the infrared region as well. But now let's look at the spectrum of the light that eventually reaches the ground. You ready? Here goes. Look at that! It's not the same spectrum; there's a lot of light that has been absorbed.

So let's look at some of them. First of all, let's look at the light that is absorbed in the visible region. Who's absorbing it? Where is it going? Well, most of the light is actually being scattered by our atmosphere. And because of scattering, a lot of light goes back into space, which doesn't reach the ground.

Some of that light does reach the ground, which is why we see the sky to be blue. We see the clouds to be white; they also scatter light. So a lot of light is actually lost to scattering. The second part we can see is there's a lot of light that's being absorbed in the UV region. Which one is that? You can probably guess it: that's our ozone. Ozone absorbs a lot of high-frequency, high-energy UV light and ensures it doesn't reach the ground, which is great; otherwise, that would be bad for us.

And then in the infrared region, also you can see a lot of sharp absorptions happening. Who's causing that? Well, these are the greenhouse gases. We said that they are very sensitive and they absorb infrared radiation. Well, incoming light also has some infrared radiation, and so they absorb them as well.

And so, look, a lot of light has already been absorbed, which does not reach the ground. But even there, not all of that light will warm up the Earth because some of the light will be reflected back into space by the clouds themselves. And that's why you can see the cloud from space itself.

Then, of the rest of the light that reaches the ground, again, some of that light will be reflected by the Earth's surface. The surfaces covered with ice or snow are highly reflective; we say they are high-albedo surfaces. They will not absorb a lot of light; they will just reflect a lot of light back into space. Okay, what about the remaining light that falls on the low-albedo surfaces, which are less reflective? There, it gets absorbed.

What happens there? Well, even there not all of it is used up in warming up the Earth. Some of it that falls on the water, for example, is used up in evaporating the water, and so the energy is carried away by the evaporating water. Some of it which falls on the greeneries is photosynthesized and gets converted into chemical energy.

Finally, the rest of the light that reaches the ground which is not absorbed, not reflected, not scattered, which is not photosynthesized, which is not used in evaporation—it's finally that portion of light that, you know, heats up the Earth. This detail, as we will see, will be important for us in understanding how climate may change.

But anyways, because of Earth's temperature, it gives out thermal radiation. And at this temperature, the thermal radiation is primarily in the infrared region. Now, if there were no greenhouse gases, this would have escaped to Earth—sorry, escaped to space, and the Earth would have cooled down pretty quickly. But because we have greenhouse gases, they absorb a lot of these radiations.

Not all of them, remember; it's all wavelength dependent. Some of the wavelengths do escape back into space, but a lot of these will be absorbed by the greenhouse gases. Then, when they re-emit, a lot of that actually gets redirected back to Earth. As we discussed earlier, um, that's basically how the greenhouse gases heat up the Earth.

But now comes the big question: Why do they absorb infrared radiation? What is so special about them? Well, there's actually nothing special about them. It's just that, because they're made up of different bonds, these bonds can vibrate, and these atoms can wiggle in different ways. For example, this is a water molecule: here are some ways in which the molecule can wiggle. It can wiggle by stretching; it can wiggle by stretching this way asymmetrically. It can also wiggle when their bonds bend.

It turns out that the wiggling frequency of most of these molecules lies in the infrared region. It just happens to be that. And therefore when light of that specific frequency—the light which has frequency equal to the wiggling frequency—happens to fall on them, they will absorb it and wiggle and eventually re-release that radiation.

But wait a second! What about oxygen and nitrogen molecules? Even they should be able to wiggle, right? But why does an infrared radiation make them wiggle? Ah, here's the thing: all electromagnetic waves are made of electric and magnetic fields. If you want to interact with these fields, you need to have some kind of charges, right? But where are the charges? Anywhere? They're all neutral molecules, right? Yes!

But because the greenhouse gas molecules are made up of different atoms bonded, the electrons are shared not equally between them. As a result, they will have some charge separation. For example, when it comes to the water molecule, oxygen pulls the shared electrons more towards itself compared to the hydrogen. Therefore, the oxygen side will have a slight negative charge compared to the hydrogen.

We call such molecules dipoles. Now, in some cases—like in the water molecule—this is a permanent effect, while in some other cases—like in methane or carbon dioxide—the effect is temporary and only appears while wiggling. But the point is, because there's a charge separation, they can interact. Electric fields can interact with these molecules, and that's why they can absorb the infrared radiation.

But oxygen and nitrogen molecules are made of the same atoms, so the electrons are shared perfectly symmetrically between them. Therefore, they will not have any charge separation even when they're wiggling, and that's why they will not be able to absorb infrared radiation, and it just goes straight through. This is why 99% of our atmosphere does not cause greenhouse effect.

And so, what I find now fascinating is that this means, because some of the molecules in our atmosphere—less than 1% of the molecules—because they share the electrons asymmetrically between the atoms, is eventually what's responsible for global warming. That is unreal if you think about it. Oh my God! But that is what it is. This is it; all boils down to this.

That finally brings up the question: What would happen if we add even more greenhouse gases? This is not a hypothetical scenario at all. Since the industrialization, there is evidence that the greenhouse gases concentration has increased. So what's the effect of that? Well, at first, the answer sounds pretty obvious: if there are more greenhouse gases, there'll be more absorption. As a result, the Earth will heat up even more.

But the question is, how much? And just to give you a glimpse of how complicated the system is and how so many variables are involved, think about this: let's say the Earth heats up slightly, but as a result of that slight heating from the direct addition of more greenhouses, some of the ice can melt. That can reduce the high-albedo surface that we have, and that could now increase the amount of sunlight that we absorb, further increasing the temperature of the Earth.

That could also increase the amount of evaporation that happens, increasing the amount of water molecules, increasing the amount of greenhouse gases even more, which further will heat up the Earth. And there can be now a runaway effect. So you see what I'm talking about—the runaway effect. Not only can it dramatically increase the temperature of the planet, but it can dramatically change climates.

It can dramatically change our ecosystem, which could be catastrophic. So the big question would be: at what levels of concentration will these runaway effects start kicking in? I don't think we should try and find out the hard way. Of course, on one hand, we should try and do as much theoretical calculations as we can, but on the other hand, prevention is the best cure.

That's why we should try and prevent it as much as possible, and that's why we talk about reducing our carbon footprint. A few ways of doing that could be slowly transitioning from burning fossil fuels to maybe using solar power, wind energy, and of course using nuclear power. Other ways could be making our transportation and our other processes more efficient.

Now of course, there are a ton of challenges: economic, technological, even political. So I don't think we can solve this problem just by being extremists. Here we have to be smart about it; we need to have critical debates around it. Because, at the end of the day, we're talking about the future of our species.