Lecture 4 – greenhouse effect
Very broad assumptions about the BB.
o Earth is warmed by solar radiation coming in
o Solar radiation is largely invisible light
o Only a fraction of it is being absorbed
o Shadow are = the area of the sunlight reaching the earth at the earth’s orbit, the portion
of the area in which the radiation is absorbed by the earth.
o Assume that the earth is a BB., and if it emits like a BB then we can use the Stephan
o The emission area is the whole of the earth, because the earth’s temperature is
constant all through out.
o This temperature is referred to es T = Effective temperature = 255 K.
1. There is a problem with this solution because if that is the basic science, then we are predicting
that the average temperature of the earth would be -18 degrees. But it is not so.
2. The model of the energy in and out 3. What happens to the IR light which gets emitted by the earth?
4. We have gases in the atmosphere which are IR active, they absorb the IR radiations
5. Then they themselves start to act as a BB source.
6. The atmosphere we have is a little thin slice of gas. It’s the atmospheric layer.
7. It only absorbs IR radiation, and not visible light.
8. It is a BB, so the amount it absorbs, it is going to emit.
9. When it is emitting, there is no reason it shouldn’t emit it up or down. So the amount of energy
going up, is also coming down.
10. The radiations are also IR. The atmosphere may be cooler than the end of the ground, but not
that cooler. It still emits IR
11. Molecules absoribind the radiation and dumping it into the atmosphere and the atmosphere
absorbs it and reemits it again
12. If the alien from the mars or sun looks at earth, they see Ir radiation coming from the
atmosphere, not from the ground. If you look up from the ground, you can see as if IR is coming
from the atmosphere.
13. This is the Greenhouse effect
14. Greenhouse effect is the emission from the atmosphere of radiation back down to the earth.
15. The IR energy goes up, and comes down, and gets absorbed by the ground again.
16. So the Greenhouse effect is to warm the surface of the earth.
17. In addition to the solar warming, you get an additional warming. And this increases the
Radiative Equilibrium - II
1. E in = E out ( for the planet) still has to be true
2. If we did this for the planet, we would be writing the same thing. But what would be changed in
this scenario is: we would be solving for the temperature of the part of the atmosphere which emits energy back out of the place. So we would be solving for the temperature of the
3. T atmosphere = 255K we should get the same solution.
4. 2) E in = E out ( for the atmosphere)
5. The sun doesn’t know what the temperature of the earths is. They only see photons coming out
of the atmospheric layer. Assuming the atmosphere is a BB, then the temperature of that Bb is
what someone would observe.
6. E =in out– for the atmosphere
7. How much energy is the atmosphere absorbing?
a. It’s absorbing all the energy that is coming from the earth. because it is a BB
b. E have the same amount of energy going up as well as down. So we do it twice.
8. We get a very simple equation. We don’t need the radius and everything, because it gets
deleted. (lol )
9. The temperature of the surface = 2 x T of the atmosphere = 2 x 255 K = 303 K
10. There is an atmosphere which gets warmed because of the radiation of the earth.
11. Then it absorbs it and reemits it back into the earth. 12. How do we simplify this? What is not realistic about this?
a. The atmosphere is not a perfect BB. For something to be a perfect BB, it should absorb
every wavelength of light which hits is with equal efficiency.
b. But gases don’t do that. They absorb IR light in specific frequencies. Others don’t
13. Slide 2
a. The BB curve is at 220, 225, 250, 275 K
b. The jagged line is what a satellite would measure if it was above the earth
c. It looks something like BB curve, has the general shape of it.
d. There are different gases in the atmosphere absorbing at different levels.
14. Slide 3
a. When the no is around 100, the atmosphere is largely IR, when it is 0 it is mostly
b. There are huge chunks in the areas where there are H20, co2, o3, CH3. these are
c. They absorb the IR, and then make the absorption non zero.
d. The chunk at 10microns is ozone. The other chunk next to this is Co2
e. What is the temperature of the earth when this spectrum was made?
i. Around 270 K
ii. The reason? The region between 800- 1200 wavelengths. Forget about the little
chunk in the middle.
iii. If you draw a BB curve the line would have lined near 270K
iv. Atmospheric window has very little absorbing material
v. It is called the atmospheric window = the part of the spectrum where if a
photon was emitted at the ground, it would whizz carefree to the outer space