[MUSIC] So in the green house model that we did so far, we have the pane of glass and we had it absorb all of the infrared light because it made it easier to understand. But then we figured out that gases don't really do that. Gases are very selective about what kinds of light they absorb. So the situation kinda looks like this, where you have different frequencies of light that are shining up from the ground as part of the ground's black body spectrum. And some of the light is in the frequency range around 700 wave numbers. Where the CO2 bending vibration absorbs light. And so the light, it sort of has to fight its way out of the atmosphere. It gets absorbed by CO2 very quickly and then some more gets reemitted by that CO2 going up and down and on and on and on. And then finally the light of that frequency that escapes to space has to come from someplace fairly high up in the atmosphere, where it's fairly cold. Whereas another frequency range around 900 wavenumbers, we call that the Atmospheric Window, because if there are no clouds in the sky or airplanes or birds, or any other sort of condensed matter up there just the gases. There's no gases that really absorb light in the atmospheric window. So the light in the atmospheric window around 900 centimeter, wave numbers. 900 cycles per centimeter goes right from the ground out to space. So if we think about how this looks from space. We can visualize this within the context of two black body curves, one that's fairly cold, like the upper atmosphere, as if you had a black body that was cold at that temperature. And then another as if you had a black body that's fairly warm like the temperature of the ground. And because it's warmer that means that curve is larger because that's epsilon sigma T to the fourth is larger number because temperature is higher figure black body curve. So if you're looking down from space and the range around 700 wave numbers, what you will see when you look down is cold, a cold CO2 molecule up in the upper atmosphere. And so if this CO2 molecule was a black body, and could shine at all frequencies, what you would see is a cold black body curve. But it's only really effective if CO2, in the CO2 bending vibration range. And so in this part of this spectrum what you see is following the cold black body curve. Because the CO2 is emitting in that range and it's cold. And then if you look in the atmosphere of window range. What you see is all the way to the ground. You see light is coming from the warm ground. And so that light following the black body curve is at this intensity. And so you see a higher intensity there for the atmospheric window region. So this is how the selectivity of the CO of the greenhouse gases to the light kinda manifest itself in the intensity of the light that's leaving the planet. It works in the frequency ranges that it can cover. And the other frequency ranges, it just leaves the light alone. [SOUND] So, let us look at the online model of infrared light leaving the atmosphere. And I'm going to show you an effect called the Band Saturation effect. So if we start out and we have no carbon dioxide in the atmosphere at all, what we see is in that 700 wave number range, the intensity follows the black body curve of the warm object because we can see light all the way from the ground. Now if we put just a little bit of CO2 into the atmosphere, I'm going to put one part per million in, and what you see is the peak Is very prominent. Just putting a little bit of CO2 goes a long way. The peak comes down quite noticeably from the warm black body curve. And then putting in more CO2, I'm gonna do ten parts per million, and you can see now that the CO, the absorption peak, the bite that CO2 takes out of the outgoing light from the planet, has gotten much deeper. And so it's changed quite a bit the amount of energy that's leaving the planet. But as we add more CO2, so now I'm gonna go up to 100 parts per million. What you see is that the peak has gotten sort of broader and a little bit deeper but as the CO2 concentration rises even further. The peak can't get any deeper. It's absorbing everything that's coming up from the ground, and all you see is the cold. The cold black body curve. And then going up still more to a thousand parts per million, you'll see the peak is getting even fatter still but it's not getting any deeper. This is called the Band Saturation Effect. The impact of that is that I've plotted the data that I just took with the model here. As we put just a little bit of CO2 in the atmosphere, we see that the intensity of the energy leaving the planet has decreased quite a bit from 318 watts per square meter down to 313, just per one part per million CO2. So that's basically the difference between this point and this point. This axis here goes all the way up to 1,000 PPMs. So one PPM is almost the same as zero on this plot. It's almost nothing. And then as you go up higher and higher in CO2 concentrations, it decreases the rate at which energy is being lost to space, but you get less bang for your buck, the more CO2 you have in the atmosphere. It never becomes insensitive to CO2 but the sensitivity goes down the more of the gas you have in the atmosphere. So it turns out that the impact of CO2 as a greenhouse gas or most other greenhouse gases tends to follow kind of a logarithmic relationship. Where any doubling has about the same impact on the energy balance and on a climate of the Earth as any other. So going from 10 ppm to 20 ppm would give you just as much global warming amazingly as going from 1000 ppm to 2000 ppm. So one thing that controls how this shape looks is the details of these absorption peaks. So I said earlier that a gas can absorb the light if the frequency of the vibration of the gas is sort of the same as the frequency of the light that's coming in. That should be sort of, it seems like it should be sort of an absolute thing, it has to be exactly the same frequency to really resonate and absorb. But it turns out that there is some slop in that, that makes this absorption peak, if you'd imagine sort of a very, very sharp edged absorption peak like it will absorb all of this light, but none of that light is how it might be ideally, there's some sort of slop in that absorption peak. And that slop is very important to determining how much adding more CO2 changes the climate if you've already got a lot there, because it allows that bite out of the spectrum that you saw to get fatter as you add more of the greenhouse gas. So the reasons why there are slop in this, why it's not just on or off the way you might think it would be, one is due to what they call Pressure Broadening. So, remember we said that if the carbon dioxide was frozen into dry ice and so it was condensed. It would be a black body. It could absorb and emit pretty much everything, probably, or at least that's the way most condensed things are. And that's because you can sort of vibrate at some pre-characteristic frequency, but if a light photon comes in and it's a little bit off, maybe you can absorb it and kind of shovel off some of the excess energy to your neighbor, who's just there anyway. Right? And it turns out that making a gas more compressed makes it more like condensed matter. And so it makes it, spreads out how much of the light it can absorb. The other thing that, this is most important in the lower atmosphere, the other thing that's important is Doppler Shifting. So if the gas is moving and the light is going up past it and the frequency might not be exactly right if the gas molecule were steady, but if it's moving, that changes a little bit the frequency that it sees. It's kind of like when a train goes past it makes [SOUND] this kind of a sound. And say you had, some sound absorption thing, and it couldn't absorb the sound of the train whistle if it was sitting there stationary. But maybe it's just exactly right for the train whistle as it's going away. So it would absorb some of that train whistle, because of the velocity of the train, relative to you. But it turns out that even if the gas absorbed everything, adding more of the greenhouse gas would still affect the climate. And we can see that by thinking back to our layer model, we had this layer model where having no panes of glass left the atmosphere cold. And then putting a couple panes of glass made it warmer. And then more panes of glass make it warmer and warmer and warmer and warmer and you saw that Venus you couldn't even with a single pane of glass explain all the warming, the greenhouse effect of Venus. And so the more of these layers you add up, the more warming you get at the ground. And these panes of glass are completely saturated, they absorb everything. That's how we defined it to make it simple. So the band saturation effect is a very important effect. For understanding how the climate of the Earth responds to greenhouse gases like CO2 which is very abundant, or methane which has a very low concentration, it's kinda up in this part of it's curve here. I mean the curve isn't exactly the same but it looks sort of like this, and there's not much methane so methane is very sensitive, very strong greenhouse forcing. And then freons, the chlorofluorocarbon compounds that we use in refrigerators and air conditioners and things like that. They actually absorb in the atmospheric window region, and there were no freons on Earth before we started making them. So they basically start from zero and as a result of this band saturation effect it turns out that one molecule of freon is worth something like 10,000 molecules of CO2 in its climate impact. However, the fact that CO2 is so abundant and is therefore band saturated does not mean that the Earth is insensitive to adding more CO2. It's a good thing we didn't have any CO2, it's a good thing we already had CO2 in the atmosphere before the industrial revolution because if there wasn't and we started putting CO2 into a virgin atmosphere it would've just totally melted down the climate. [MUSIC]
