How many of us actually understand how the greenhouse effect works? The simple physics explanations you find on the Internet are usually quite wrong. With this in mind, I've decided to attempt a very simple explanation of the physics behind Earth's energy balance. Bear in mind that this is a simple radiative model, there are actually many more parts to the greenhouse effect than I present here, they're just unnecessary for such a basic explanation.


I'm curious to know if any of you have any objections to this explanation, since this understanding of Earth's energy balance is fundamental to anthropogenic global warming theory. 


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First, an important idea is that hot objects tend to cool off relatively quickly, while cool objects cool off more slowly, that is, the rate that energy flows from an object is dependent on the object's temperature. This means that a hot object in the presence of a constant energy source will tend to cool down until the amount of energy it emits equals the amount of energy it receives, while a cool object will warm up until the amount of outgoing energy equals the amount of incoming energy. 


Get it? Got it? Good. Moving right along.


A planet is in thermal equilibrium when the amount of energy radiated from the planet to space equals the amount of radiation the planet receives from the sun. Due to its position relative to the sun, the Earth receives about 240 Watts per square meter (W/m^2) of solar energy, which means that in equilibrium, the planet must radiate 240 W/m^2 back out into space.


Some rough calculations would show that if Earth was absorbing heat from the sun and then radiating it directly out into space, the average temperature of the planet would be about 0ºF—well below freezing.


Happily for us, this isn't the case. There are certain trace gases in the atmosphere that are transparent to sunlight but opaque to radiation at wavelengths emitted from Earth's surface. These gases are called the greenhouse gases (although they have nothing to do with greenhouses). The heat radiated from the Earth's surface must pass through these greenhouse gases before it can radiate into space. 


The atmosphere is free to radiate heat in all directions (but for our simple model, we can think of it as radiating either "up" or "down"). So while the atmosphere radiates 240 W/m^2 into space, it also radiates 240 W/m^2 back toward the ground. So Earth's surface receives both 240 W/m^2 from the sun and 240 W/m^2 from the atmosphere for a grand total of 480 W/m^2*.


 However, unlike the atmosphere, the ground can only radiate heat in one direction: up. So the surface of the planet radiates radiates a total of 480 W/m^2 up into the atmosphere, and this heat is absorbed by the atmosphere, rather than escaping straight into space. And viola! We have a greenhouse effect that heats up the planet's surface! In fact, the greenhouse effect I just described keeps Earth's surface at a cozy average of about 15ºC (59ºF).


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All righty then, now that we understand how Earth's energy balance works, and how the greenhouse effect warms the Earth's surface (I hope you didn't have too much trouble hacking through my garbled verbiage), doesn't it make sense that changing it would have an effect on Earth's temperature? CO2 makes up 9—36% of the overall greenhouse effect, and we humans have increased its concentration in the atmosphere by about 35% over the past century. Given this understanding of the physical processes driving the Earth's climate, doesn't AGW theory seem perfectly reasonable?


I look forward to your thoughts, comments, and, most importantly, criticisms. 


—EP


* If you're having trouble imagining this part, try thinking in terms of this simple progression. The planet gets 240 W/m^2 from the sun, and radiates the same back out. The heat is absorbed in the atmosphere, where 120 W/m^2 is radiated out to space, and another 120 W/m^2 is radiated back toward the ground. Now the planet has absorbed 240 W/m^2 from the sun and 120 W/m^2 from the atmosphere, which it reradiates. Now the atmosphere absorbs this 360 W/m^2 and radiates half toward the Earth, and half out to space. Now the Earth receives 240 W/m^2 from the sun, and 180 W/m^2 from the atmosphere. You can continue this progression until the atmosphere is absorbing a total of 480 W/m^2, at which point the energy flux "balances out". (Whew!)

I don't believe in global warming, the Sun controls our temperature but I do understand that with China and India entering the 21 st century the price of oil will keep going up and the nasty fact that we are funding the most despicable regimes in the world.

So I'm all for alternative energy but am Totally against mandatory cuts in greenhouse gases as the US has the most to lose under those circumstances.  We already have the technolgy for hydrogen fueled cars but the build out for the new infrastructure is going to be costly, instead of throwing money down the drain on carbon credits the government could add a tax to gasoline to finance the new infrastructure needed to switch to hydrogen. IT won't happen overnight and it doesn't have to, the price of oil will plunge once the plan is underway and we will have cleaner air and the IslamoNazis and Hugo Chavez can start feeding their people oil soup!

Hi Enraged,

As you know from our past discussions on the forum, I am a global warming skeptic.  I do not, however, consider myself anti-environment.

I would be interested in your response to these questions:

You state that "the earth's surface is kept at a cozy average of 15 C (59 F)".  How do you know that the earth's surface would not be even more 'cozy' were the average temperature one degree higher?

You state "given this understanding of the physical processes driving the earth's climate, doesn't the AGM theory seem reasonable?"  I would say possible, but not necessarily reasonable.   I'd be interested to know your answer to these questions:  

What percentage of all the "physical processes driving the earth's climate" does science fully understand?  
Is the greenhouse effect the only 'physical process' controlling the earth's temperature, or are there other factors?  
Is it reasonable to think that science is capable of predicting with great precision the earth's temperature over the next 100 years?  (How accurate was science in predicting the amount of hurricanes this season?)

In the 70s, the scientists were proclaiming "It's the O Zone, stupid!" and we were headed into a major Ice Age.  They wanted to spread ash all over the polar icecaps to absorb heat.  Now it is Global Warming and the Greenhouse effect caused by humans throwing polluting gases into the atmosphere, and they want to throw iron into the oceans to perk up the plankton so they absorb the CO2 and then die with it in them and go to the bottom of the ocean thereby taking alot of CO2 out of the atmosphere (long sentence   ).

Here's the problem.  The increase in CO2 occurred after the heat up of the sun and the planet, not before. Therefore, while the levels may be impacting now, they are not the cause of the global warming.  The sun is.

I have no problem saying the Earth is having climate change, but I will say it again, it is cyclic.  Greenland and Iceland used to be exactly as their names describe.    I have a problem with declaring Global Warming to be human caused and therefore, socialism is the answer to saving the planet.

dittohead wrote:Hi Enraged,

As you know from our past discussions on the forum, I am a global warming skeptic. I do not, however, consider myself anti-environment.

I would be interested in your response to these questions:

Hello, Ditto. Happy to oblige.

You state that "the earth's surface is kept at a cozy average of 15 C (59 F)". How do you know that the earth's surface would not be even more 'cozy' were the average temperature one degree higher?

Quite frankly, I don't. It could be that a warmer planet would be much nicer than it is today. But then again, it might not be. And it'd take a powerfully good argument to convince me that it's worth taking the risk to find out.

What percentage of all the "physical processes driving the earth's climate" does science fully understand?

I don't think I can quantify that. I would say that science currently has a solid understanding of the physical processes behind the climate.

Is the greenhouse effect the only 'physical process' controlling the earth's temperature, or are there other factors?

No, there are a great many factors that determine Earth's temperature. Changes in solar output, in cloud cover, and in surface albedo all affect Earth's radiative balance.

However, changes in Earth's greenhouse effect are by far the most important factors in the current warming.

Is it reasonable to think that science is capable of predicting with great precision the earth's temperature over the next 100 years? (How accurate was science in predicting the amount of hurricanes this season?)

That depends on what you mean by, "great precision." Climate projections obviously can't give us an exact picture of what the climate will be like a century from now. But they're perfectly capable of giving us a solid idea.

SoldiersMum wrote:In the 70s, the scientists were proclaiming "It's the O Zone, stupid!" and we were headed into a major Ice Age.  They wanted to spread ash all over the polar icecaps to absorb heat.  Now it is Global Warming and the Greenhouse effect caused by humans throwing polluting gases into the atmosphere, and they want to throw iron into the oceans to perk up the plankton so they absorb the CO2 and then die with it in them and go to the bottom of the ocean thereby taking alot of CO2 out of the atmosphere (long sentence   ).

Here's the problem.  The increase in CO2 occurred after the heat up of the sun and the planet, not before. Therefore, while the levels may be impacting now, they are not the cause of the global warming.  The sun is.

I have no problem saying the Earth is having climate change, but I will say it again, it is cyclic.  Greenland and Iceland used to be exactly as their names describe.    I have a problem with declaring Global Warming to be human caused and therefore, socialism is the answer to saving the planet.

Hi, Soldier'sMum.


First, I must admit that I remain entirely unconvinced that any scientists were seriously predicting an imminent ice age in the 70's. The idea just isn't reflected in the scientific literature of the time. But never mind, on to more important matters.

You're absolutely right that the sun was, in fact, driving the early period of the current warming trend. In fact, most scientists believe that changes in solar output were responsible for the bulk of the warming up to 1940, and quite a few even believe that humans weren't fully driving the current warming period until about 1975. It's a common misconception that human activity has been driving this change from the very get-go. So for once, we agree on something (yay!).

For the Global cooling data, all you have to do is to go to the library and research the 70s issues of Newsweek and Time Magazines.  It's all there, I didn't make it up, Mr. Parrot, and Hi back to you.  There are still scientists who believe the Earth is headed towards another Ice Age.

As for the Earth, it's remarkable that we have any point of agreement but I am not really sure of your politics so we might have others.  What I think you are doing is merely talking the science of global warming but the science that you speak of does say humans created the problem, if it is really a problem, which I am truly not convinced that it is.  The Earth has warmed less than one degree.  While some ice sheets at the northern pole are melting, others are thickening and the southern pole is thickening, not melting.  

Are you also a green person?  Might I ask you a question?  Well..here it is without your permission..haha..  One of the things that the Global Warming people are saying is that we should conserve all energy.  In so doing, they advocate the use of the compact florescent lightbulbs.  This is of great concern to me as they are sold everywhere now and they are loaded with mercury.  They are being disposed of in people's regular trashcans and not by a special procedure because in actuality, they are hazardous waste.  What will eventually happen is our ground water tables will become polluted.  These lightbulbs are primarily made in China and marketed by anyone and everyone including the normal lightbulb people like GE.  So, what is your opinion of the use of these compacts and do you think the Global Warming people have stock in their manufacture?? haha  How much stock do you think Algore owns???

To All,
   I'm not going to discuss Global Warming anymore.

To Independent,
    For many years I was a huge proponent of hydrogen fuel cells specifically for cars, but also for power plants.
There is one little problem that I have searched for the answer to.  

The problem/question is this: where do we get the hydrogen from?

Fuel cells (large ones anyway) have a theoretical efficiency of nearly 90%.  Smaller fuel cells that could be used in cars have a theoretical efficiency around 50% (I'm pulling that from memory, don't quote me on it); as opposed to the maximum efficiency of an internal combustion engine of around 30% to 33%.  So, it would make sense to use hydrogen fuel cells as one of the most efficient forms of energy conversion available.  However, even assuming we reach an economically viable fuel cell, there's still one little problem with hydrogen fuel cells  --  i.e., where do we get the hydrogen?

There are efforts underway to use bacteria to separate hydrogen gas from organic compounds.  I don't know enough about this technology to say if it will be the answer to our energy woes -- I hope that it will.

Currently, however, there are only two reliable means of obtaining hydrogen:
   1. Extraction from Methane (natural gas)
   2. Electrolysis of water

The problem with the first method is that natural gas is almost always found only along with oil fields.  This means that (although nat. gas is cheap & plentiful now) eventually our dependence on foreign oil fields (from which the methane comes) will not be reduced.

The problem with the second method is a little thing I like to call the First Law of Thermodynamics.  You cannot put "X" amount of energy (electricity) into separating hydrogen from water, and then expect to get more energy from the use of that Hydrogen.  This would be the equivalent of a perpetual motion machine.  It is physically impossible.  In short,  more energy is required to generate the Hydrogen than you can ever get from the Hydrogen.

If I were King of America, I would abolish all current energy subsidies (and all other subsidies to boot), and would subsidize two untapped sources of energy: 1. Oil Shales and Oil Sands; and, 2. Methane Hydrates.

Unless there's some technological breakthough in the generation of hydrogen (not to mention safe & efficient storage & distribution), I'm afraid that my initial support for a hydrogen fuel cell economy was misplaced.

I'd be more than happy to hear about alternative methods of accumulating hydrogen that don't require reliance on current oil fields, and that don't defy the laws of physics  -- anyone with better info on the subject, I'd be encouraged to hear of it.

SoldiersMum wrote:For the Global cooling data, all you have to do is to go to the library and research the 70s issues of Newsweek and Time Magazines.  It's all there, I didn't make it up, Mr. Parrot, and Hi back to you.  There are still scientists who believe the Earth is headed towards another Ice Age.
  

Are you also a green person?  Might I ask you a question?  Well..here it is without your permission..haha..  One of the things that the Global Warming people are saying is that we should conserve all energy.  In so doing, they advocate the use of the compact florescent lightbulbs.  This is of great concern to me as they are sold everywhere now and they are loaded with mercury.  They are being disposed of in people's regular trashcans and not by a special procedure because in actuality, they are hazardous waste.  What will eventually happen is our ground water tables will become polluted.  These lightbulbs are primarily made in China and marketed by anyone and everyone including the normal lightbulb people like GE.  So, what is your opinion of the use of these compacts and do you think the Global Warming people have stock in their manufacture?? haha  How much stock do you think Algore owns???

Oh, I don't doubt that some people (mostly in the mainstream media) thought scientists were predicting an imminent ice age in the 70's, I just don't think any actually were. I have yet to find a single peer reviewed article saying anything of the kind. And since the scientific literature provides a clear picture of mainstream scientific opinion, if many scientists thought we were headed toward an ice age, it would be reflected there. 


To answer your question, no, I doubt that many climate scientists have stock in Compact Fluorescent Light-bulbs, although one or two might have stock in GE, or some other company that manufactures them. Also, since CF bulbs contain much, much less mercury than standard fluorescent bulbs, which are broken and thrown in the dump all the time, I can't see how they're going to pollute our ground water any more than it all ready is. Of course, if they are disposed of properly, there won't be any problems with them at all. So I think they're marvelous.

Why am I not surprised that you really don't care about mercury poisoning?  Your one issue..your global warming model..will keep you busy enough in the days ahead.  If we could wave a magic wand and do away with the less than one degree of warming and make everything all right with the world, the mercury in those lightbulbs will still eventually find their way into our water but it isn't any concern of your's..you think they are fine.  Oh well..keep marching on with your model and your theory.  We are all going to eventually die anyway..right?