In the greenhouse model that we did so far, we had a pane of glass
and we had it absorb all of the infrared light,
because it made it easier to understand.
Gasses don't really do that.
Gasses are very selective about what kinds of light they absorb.
The situation kind of 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.
The light has to fight its way out of the atmosphere.
It gets absorbed by CO2 fairly quickly,
and then some more gets re-emitted 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 some place fairly high up in the atmosphere, where it's fairly cold.
Whereas, another frequency range around 900 wave numbers,
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,
there are no gases that really absorb light in the atmospheric window.
So the light in the atmospheric window (around 900 centimeters wave numbers, or
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 is 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 epsilon sigma T
of the fourth is a larger number,
because the temperature is higher, you get a bigger black body curve.
If you're looking down from space,
in 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.
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
the CO2 is only really affecting the light in the
CO2 bending vibration range.
And so in this part of the 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 atmospheric window range,
what you see is all the way to the ground.
You see light that's coming from the warm ground.
That light, following a black body curve, is at this intensity.
You see a higher intensity there for the atmospheric window region.
This is how the selectivity of greenhouse gases
to the light kind of manifests itself
in the intensity of the light that's leaving the planet.
It works in the frequency ranges that it can cover, and in other frequency ranges,
it just leaves the light alone.
So let us look at the online model of infrared light leaving the atmosphere.
I'm going to show you an effect called the band saturation effect.
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, 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 going to do ten parts per million, and you can see now that the
CO2 absorption peak, the bite that CO2 takes
out of the 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
going to go up to 100 parts per million,
what you see is that the peak has gotten
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 black body curve.
And then, going up still more to a 1000 parts per million, you
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, plotted
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 by quite a bit.
From 318 watts per square meter down to 313 just per one part per million CO2.
That's basically the difference between this point and this point.
This axis here goes all the way up to 1000 ppm.
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.
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.
It turns out that the impact of CO2 as a greenhouse gas, or most other
greenhouse gases, tends to follow a kind of a logarithmic relationship.
Where any doubling has about the same impact on the energy balance
and on the climate of the earth as any other.
So going from ten parts per million to 20 parts per million would give you just as
much global warming, amazingly, as going
from 1000 parts per million to 2000 parts per million.
One thing that controls how this
shape looks is the details of these absorption
peaks. I said earlier that a
gas can absorb 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 seems like it should be sort of an absolute thing,
that the light has to be exactly the same frequency to really resonate and absorb.
But it turns out that there is some slack. That makes this absorption peak.
Imagine a very sharp-edged absorption peak,
It'll absorb all of this light,
but none of that light, is how it might be, ideally.
But there's some, some sort of slope 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.
The reasons why there is a slope 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.
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 pretty much everything probably,
at least that's the way most condensed things are,
and that's because you can
vibrate at some characteristic frequency
but If a light photon comes in and it's a little bit off, maybe you can adsorb it
and kind of shovel off some of the
excess energy to your neighbor who's just right there anyway, right?
It turns out that making a gas
more compressed makes it more like condensed matter.
It spreads out how much of the light it can absorb.
This is most important in the lower atmosphere.
The other thing that's important is Doppler shifting.
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 the molecule sees.
It's like, when a train goes past, it makes this
[professor's impression of a train whistle]
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 from that train whistle.
Because of the velocity of the train relative to you.
It turns out that even if,
the gas absorbed everything,
adding more of the greenhouse gas would still effect the climate.
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 you saw that Venus, we couldn't, even with a single
pane of glass, explain all the warming, the greenhouse effect of Venus.
The more of these layers you add up
the more warming you get at the ground.
These panes of glass are completely saturated.
They absorb everything.
That's how we defined it, to make it simple.
The band saturation effect is a very, 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 kind of 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 thousand 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 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.