In the late 19th century Svante Arrhenius proposed a theory of global warming where carbon dioxide and water vapor increases in the atmosphere would warm the Earth like a "Greenhouse". The greenhouse analogy was a simple way to explain the molecular level interactions of radiant heat energy and atmospheric gases. Like most analogies, the Greenhouse Effect was less than perfect. In order to produce a better analogy, many have provided simple descriptions of how carbon dioxide, water vapor and trace gases interact with outgoing heat radiation to produce global scale warming, then relate that processes to other analogies like blankets and tanks. For some reason, improved analogies have done little to improve understanding of what happens in the atmosphere. Even well educated physicists are still challenging the concept of global warming by molecular interaction with heat radiation. The Book "Slaying the Sky Dragon - Death to the Greenhouse Theory" by Schreuder and O'Sullivan is a recent example of the complete misunderstanding of the relatively simple process of how Earth's atmosphere regulates temperature.
Since the space age there are plenty of examples of what happens to the surface temperatures of objects with no atmosphere when exposed to periodic sunlight. They warm with the light and cool when there is no light. The unregulated temperature swings are huge by Earth standards. The moon for example, has day temperatures as high as 123 degrees C and night temperatures as low as -233 degrees C. Even in the vacuum of space, heat flows from warm objects to cool objects. The heat flows via radiation. Electromagnetic radiation, some visible like sunlight and some invisible to the human eye. How quickly the heat flows for these bodies in space is well understood thanks to the work of many 19th century scientists.
There are two types of objects in space, those that produce electromagnetic radiation (heat) and those that do not. The Sun is an example of a radiation producer and the moon an example of one that does not. The Stefan-Boltzmann equation describes the rate that non heat producing bodies, called black bodies, emit heat energy to the cold reservoir of space.
The coldest temperature there is, called absolute zero, is -273.15 degrees C or zero degrees Kelvin (K). This is the theoretical temperature where all atomic motion stops. Motion is a form of energy and all energy can produce heat. The simplest atom is a positively charged nucleus (proton) orbited by negatively charged electron. Theoretically, at absolute zero, the electron stops orbiting the proton. Since no one knows what actually happens should that orbiting electron stop moving, absolute zero is a theory. We can save that discussion for another day and just admit that the theory of absolute zero is a good one.
During the day, the lunar surface is warmed by radiant heat from the sun. This is one point no one disagrees with. It is obvious that the sun warms people and things. When the sun is not present people and things cool which is also obvious. So sticking with the obvious, how do we keep from becoming too cool? By insulating ourselves to reduce the rate of cooling, cuddling up with something warm or generating heat to warm ourselves. The moon doesn't have any of those options so its surface just cools when not in sun light. It does not cool to absolute zero though. Since the moon is pretty big, it takes time to lose all its heat and the colder it gets the slower it loses heat. So the moon makes a good laboratory for us Earthlings to study to find out how much we would cool if there was no atmosphere.
The average temperature of the moon is about -23 degrees C. By looking at the difference in the thermal mass of the Earth and the thermal mass of the moon, it can be calculated that the Earth's average temperature would be about -18 degrees C if we had no atmosphere. We will leave that calculation for another day, but despite all the uncertainties involved, that is a pretty solid estimate. The actual average temperature of the Earth with atmosphere is about 33 degrees warmer than it would be without an atmosphere. There are all sorts of uncertainties involved with this determination as well, but it is a pretty solid estimate. So unless someone really wants to pic nits, the atmosphere reduces the rate of Earth's cooling and heating to maintain a temperature roughly 33 degrees C greater than it would be if we did not have an atmosphere.
In sunlight, the Earth does not warm as much as the moon because of really two things, reflectivity and thermal mass. Reflectivity, called albedo, is fairly simple to explain. If you walk barefoot on a black top road in the sun you will notice that the road is a lot warmer than the sand that may be beside the road. Darker surfaces absorb more heat than lighter colored surfaces. Black is less reflective of light than white. The Earth has white clouds, snow and other colored surfaces than black that absorb less light energy. The moon is also not black or we would not be able to see it so easily in the night sky.
Thermal mass is a little harder to explain, but certain objects take longer to heat and/or cool than others. Solid things like land masses warm more quickly than liquid things like oceans. Gaseous things like air warm much more quickly because air has very little thermal mass. Anything that has mass, has thermal mass. Thermal mass is how much heat something can hold. So in general the more mass something has, the more heat it can hold, the more heat something can hold, the longer it takes to transfer heat to it relative to objects that have less thermal mass. There is a difference between pure mass and thermal mass. Seventy percent of the Earth's surface is water which has a very high thermal mass. Land area has thermal mass that varies considerably, but is less than water as far as the Earth's surface is concerned. So the oceans heat and cool slower than the land. This is obvious to anyone that lives near an ocean or a large lake.
In the day, reflectivity and thermal mass regulate how quickly the Earth's surface warms. At night reflectivity is replaced by emissivity. Emissivity describes how quickly heat can flow through a gas mixture. This is harder for many to understand since we can't see radiant heat and for the most part, can't see the gases interacting with the heat flow. Luckily, clouds are easy to see and are part of what changes emissivity. Most people have noticed how a blanket of clouds at night tend to keep the temperatures warmer than on a clear night. The clouds slow down the flow of heat from the surface. They don't stop the flow of heat. They don't reverse the flow of heat. They just slow down the flow of heat, just like a blanket slows down how quickly we lose heat. So the blanket analogy is pretty good for a basic understanding of how atmospheric gases "blanket" the Earth to regulate temperatures.
Even on a clear night things are happening to regulate temperature that we can't see. Think of it as a very thin blanket if you must, but this is where the neat stuff is happening on a molecular scale. Some describe it as down welling heat or back radiation, but it is not. The direction of heat flow never changes, only the rate of flow. That is why the tub, dam or sink analogies were proposed. The heat flow is like a faucet or stream flowing into a basin with a drain or overflow. If the flow in is equal to the flow out, the water level remains the same. Carbon dioxide is like hair building up in the drain or a beaver sticking a log in the overflow. If nothing else changes, the water level will rise. Molecules made of different elements temporarily can capture a tiny packet of heat energy called a photon and release it quickly. The molecules release their energy captive in random directions. The molecules don't aim the packets back at Earth. The molecules don't fly back down to Earth to release their hostage. The molecules just momentarily impede the progress of the packets on their journey to space. The more they impede the flow the higher the water gets.
Added 2/10/11: In the atmosphere the water level is like the lapse rate. The lapse rate is the rate of decrease of temperature with height. I may expand on this later.
So probably the best analogy for many people would be a highway patrol car with its lights on parked beside the interstate. People slow down to avoid getting a ticket, traffic starts to get a little congested and there is no obvious reason why until you see the cop car.
All of these analogies are lacking in some form or the other, since they only look at a small part of the picture. They neglect the other things that happen because something else is happening. These are known as feedbacks. If you see the traffic congestion and get off the road to have lunch, you just provided negative feedback to the traffic situation. In other words you did not add to the problem. If you slow down because you see some traffic congestion or you are a lookie lou, you have a positive feedback to the traffic situation. If you keep honking your horn, you are positively a pain in the butt feedback to the traffic situation. (Sorry, I could not resist.)
Water vapor is a lookie lou. Warmer air can hold more water vapor and water vapor is a packet grabbing molecule. Water vapor can also be the driver opting for lunch since just because warmer air can hold more water vapor doesn't mean the water vapor wants to be held. And water vapor is a pain in the butt because it forms various kinds of clouds that may provide positive or negative feedback.
Thermal mass is a pretty good driver that goes with the flow most of the time. Like any other driver on "your" road, thermal mass can be a pain in the butt. The deep oceans are a huge thermal reservoir that can be a little bit irritable. As long as another driver does not aggravate them, they warm gradually and cool gradually. All good drivers know that some time, some one is going to do something stupid and hack you off. In climate terms this is called internal variability. Most of the time, good drivers regain their composure rather quickly and start doing the good driver thing again. This would be short term variability that averages out quickly and has little if any impact on long term climate.
Even good drivers can only put up with so much crap, so they may start messing with the stupid drivers for a while until they get whatever measure of satisfaction they need at the time. This would be like decadal or multi-decadal internal variability. They seem to have some impact on climate, but it looks like they may average out over all.
On rare occasion, a couple of the good drivers may go postal and start taking it out on everyone. This would be climate shifts that can last for decades. These climate shifts probably have an impact on climate, but we really are not sure because we don't have enough data to know for sure.
So to conclude:
Carbon dioxide is a traffic cop with lights on slowing the heat traffic down a little.
Water vapor is most of the time a lookie lou slowing things down more because of the traffic cop.
Thermal mass, aka ocean heat content, is a good driver staying out of trouble most of the time.
Those three things really should not be a matter of debate.
The question is how many cop cars are going to show up, will the guys behind the lookie lou start laying on their horns and will that drive the good driver postal?
Like pretty much everything I have on this blog I may be back to update things :)
Added 2/10/11
If you want to get a more detailed explanation of back radiation or downward long wave radiation you can go to the blog, Science of Doom. They have a multi-part post on The Amazing Case of "Back Radiation" It is very comprehensive, as in very long and somewhat technical, though not a bad read really. I have no arguments with anything in their post other, than I feel that the terms back radiation and downward longwave radiation and down welling, needlessly complicate the situation making some believe that the second law of thermodynamics is being violated. Their description kinda goes against the zeroth law of thermodynamics which was added so that temperature could be interchanged with heat flow to simplify understanding of the laws of thermodynamics.
KISS or Keep It Simple Stupid, it the first thing most thermodynamics professors teach, which is why I find the concept of reversed, cold to hot, heat flow needlessly over complicated when the direction of flow, other than on a very small atomic level, does not change.
Should you read their posts you will learn a lot of neat stuff. Hopefully, you will pick up on how your frame of reference changes what your perception will be. So an Earth bound observer with a neat tool called a pyrometer which measures temperature, would be lead to believe that infrared radiation is flowing back at him while in actuality he is just measuring changes in rate of traffic flow :)
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