Q for the Scientists

[Gregory Pratt]

As you know, the atmosphere's got a hole and that's said to be caused by CO2 and that's said to be caused by the increase we send out because for so long we didn't until we really started burning fossil fuels.

 

But someone asked today,

 

Why does the atmosphere let sunlight in but not let it out?

 

You know, if the sunlight gets in but then some is retained by the Earth and some goes, but now that we've got this hole more heat is coming and staying, how come all of it doesn't just bounce right out? Does all of it come right in, easy access? How does heat and radiation coming in get in and stay in if it can be pierced in the first place?

[Balta1701]

So, the issue here is the process of conversion of sunlight to other forms of energy.

 

Overall, earth is actually roughly in balance between energy coming in and energy stored + energy going out. Roughly speaking, a fixed amount of energy comes to the earth with time from the sun. There are other inputs also (i.e. below us) but let's neglect those for now.

 

If we remove the atmosphere from the earth, make the earth nothing but a rock...then the process is fairly simple. Sunlight comes in to the planet...the planet itself then absorbs a certain percentage of this light due to the absorbance of the rocks on the surface...i.e. due to their color. We call this effect the albedo of the planet; if a planet is totally ice, it has a high albedo, because ice is colorless and reflects a ton of light, while a rocky planet has a low albedo...and consequently 2 planets the same size and same distance from the sun will have different temperatures if they have different planetary albedoes.

 

Now...the planet has taken in energy. The interesting thing is...the planet then begins to lose this energy to space through radiation. However...it does something different than the sun. The sun is so hot that it releases its energy as fairly high energy light; UV radiation and visible light. A very cold object releases its radiation as low energy infrared light. If you looked at a planet that was at 200k (73 degrees C below zero) in the infrared, you would be able to still find a wavelength where the planet was emitting light. Our eyes aren't tuned to these wavelengths though, so we can't see the energy that is being emitted by the or by other planets; we can only see the energy that is in the visible range due to sunlight being reflected around.

 

So in other words, sunlight comes in at one wavelength...some of it actually is reflected back (that's why you can see the earth from space; light is bounced back. Same reason you can see the moon). Some of the light is absorbed, and is radiated out as infrared energy. Without an atmosphere...the amount of radiation coming in and being absorbed would entirely determine the temperature of the planet; it's actually a fairly easy calculation if you know the albedo. That's how we figure out the temperature at places like Pluto; we can measure the light being reflected back, we therefore know the albedo, and can calculate what the surface temp actually is.

 

The global warming and ozone hole details of this question are the complexities. Now, we're going to take the planet and wrap an atmosphere around it. Let's just allow it to have nitrogen and water in the atmosphere for now for simplicity. These 2 molecules are important...in that they can absorb light in the infrared and turn it into heat energy; if a photon comes in at an appropriate energy, it will be absorbed by these molecules and they will begin to either rotate or vibrate, and motion of atoms is essentially what we're measuring when we measure the temperature of something.

 

Take as a given that the planet needs to stay roughly in balance between energy going out and energy coming in from the sun. With no atmosphere, the planet can radiate that energy at any wavelength it wants. But if you stick water in there...the water will prevent emission at certain wavelengths because it will absorb the light trying to leave at those wavelengths and turn it back into heat. In other words, you wrap a planet in an atmosphere containing water...it will warm up because more energy will be staying inside.

 

But, this process doesn't happen forever; the earth isn't 60 million degrees K. Water does not absorb light at every wavelength; its only can absorb light at certain wavelengths. Therefore, by just adding water into the atmosphere, you will warm the planet up. But when the planet warms, the wavelength of IR light it emits at shifts to a higher energy, as changing the energy of light changes the wavelength. Eventually, the planet will find a happy place where it can emit the incoming energy in the form of IR light at a wavelength where the water doesn't absorb the emitted photons. This is the general concept of the greenhouse effect; by having water in the atmosphere, the surface is warmed until it emits its energy at a wavelength where the water isn't in the way.

 

Where do other greenhouse gases, like CO2 and methane fit into this? They fill exactly the same role as water; when you start sticking them into the atmosphere, they absorb light that the planet is trying to release and turn it back into heat. The reason why they're important now though is that they do not absorb at the same wavelengths as water. So, if you stick both water and CO2 into an atmosphere, you wind up forcing the planet to heat up even more because it has to find a place where it can emit its energy without it being reabsorbed in the atmosphere. And since the amount of light absorbed is proportional to the concentration of light and the thickness of the layer (Beer's law), the more CO2, methane, or water you stick into the atmosphere, the more radiated energy is going to be reabsorbed, and the hotter the planet is going to need to be on average in order to dissipate the incoming energy from the sun.

 

The ozone hole is actually an unrelated issue. Ozone is to the best of my knowledge not a major greenhouse gas...but it does do one useful thing; it absorbs UV light. UV light is a bad thing...in that it damages cellular tissue. From a warming perspective, with or without that layer, the energy from the sun would still be converted into temperature, so that's not the biggest forcing. But from a biological perspective, I really care about having that layer up there, because the less UV hitting my skin, the lower my chances of getting a sunburn/cancer are.

Edited September 10, 2007 by Balta1701

[FlaSoxxJim]

Well, yeah, sure, but the real question is, How many licks does it take to get to the Tootsie Roll center of a Tootsie Pop??

[sox4lifeinPA]

Well, yeah, sure, but the real question is, How many licks does it take to get to the Tootsie Roll center of a Tootsie Pop??

 

at least until Jesus comes back.

[Texsox]

Well, yeah, sure, but the real question is, How many licks does it take to get to the Tootsie Roll center of a Tootsie Pop??

f***ing Ph.D.s

 

BTW, that little exchange is about how my History Prof is trying to teach an on-line course. Any wonder I'm in an email flame war with him?

 

[Mplssoxfan]

Well, yeah, sure, but the real question is, How many licks does it take to get to the Tootsie Roll center of a Tootsie Pop??

 

The world may never know.

 

 

 

[IlliniKrush]

Does anyone actually lick a tootsie roll pop? No.

[TheBlackSox8]

Does anyone actually lick a tootsie roll pop? No.

 

Jonathan Finn and Casey Black??

from other thread

Edited September 11, 2007 by TheBlackSox8