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Elevator Pitches - Chapter 02 - Radiative Gases
Posted on 24 March 2017 by Rob Honeycutt
This is another excerpt from my book 28 Climate Change Elevator Pitches. I'll be publishing one chapter here on SkS each month.
Chapter 02
Radiative Gases
A Musical Basis for Scattering Heat
The scientific basis for understanding climate goes back to the 1820’s when brilliant French mathematician Joseph Fourier first proposed the idea that our planet’s atmosphere had heat-trapping properties. Fourier was trying to calculate what should be the temperature of a planet at our distance from the sun. He derived a figure about 33°C (59°F) colder than the actual average temperature of the Earth. For his figures to be correct, he thought gases in our atmosphere must have “radiative properties” with the capacity to absorb and re-emit heat energy. When visible sunlight passes through our atmosphere it warms the surface of the Earth. The heat that is emitted upward we refer to as infrared radiation, or IR. Infrared radiation is just another wavelength of energy which is invisible to the human eye, but we can feel that energy as heat. It’s this heat energy that is scattered by radiative gases in the atmosphere.
In the 1850’s a British scientist, John Tyndall, devised an apparatus enabling him to measure the heat absorbing properties of various gases. Earth’s atmosphere is composed primarily of nitrogen (78%) and oxygen (21%). The remaining 1 percent of gases are known as “trace gases.” Tyndall discovered that the radiative properties of nitrogen and oxygen are insignificant and transparent to infrared radiation (heat). But, he further discovered that some trace gases do efficiently block heat.
But, how does this work? Why would one gas be transparent to heat and another gas block it?
The most common radiative gases in our atmosphere are water (H2O), carbon dioxide (CO2), and to a lesser extent, methane (CH4), so let’s look at how these molecules are constructed. The first two have a single core atom with two other atoms attached to it. With H2O, there is a central oxygen atom with two hydrogen atoms attached. With CO2, there is a central carbon atom and two oxygen atoms attached. You can picture these being something like soap bubbles joined together, but imagine if you can, that these soap bubbles have an electromagnetic field incorporated into them. This electromagnetic field gently locks the molecule into a specific configuration. That magnetic field also allows the atoms to wobble around a bit as the molecule is floating about in the atmosphere. Methane is somewhat similarly constructed as CO2, but with a central carbon atom surrounded on four sides by hydrogen atoms making it a far more potent radiative gas than the others.
Infrared radiation is a wavelength of light. In a way, it’s analogous to sound waves traveling through the air. If you tap an A note tuning fork on your knee and then hold it against the soundboard of a guitar the A-string of the guitar will vibrate sympathetically. Infrared radiation also has a frequency range, so when visible sunlight (higher frequency energy) comes in and hits the surface of the planet, that energy warms the surface. The surface then emits lower frequency energy as heat (IR) back up through the atmosphere.
The capacity of these molecules to vibrate (the “wobbling”) is “tuned” like the guitar string and when infrared radiation in the right frequency interacts with these gases, the molecule vibrates sympathetically. What they’re doing is absorbing and re-emitting that IR heat energy. The difference with the dominant molecules, like oxygen (O2) and nitrogen (N2), is they can’t vibrate in this same manner nor at the same frequency ranges, thus they are invisible to IR.
That is the fundamental physics of climate change: the vibrational modes of greenhouse gases acting to absorb and scatter heat energy in the atmosphere. This was a cutting-edge discovery of the mid-19th century but now an indisputable fact of science. Scientists have empirically measured, modeled, and applied these facts in numerous ways for well over a century.
Confusion in the 3rd sentence.33 degrees C is not equivalent to 59 degrees F.
The sentence is right. Average temp of the earth is about 15C (59F).
33C cooler than that is -18C. That's -0.04F, which, with rounding is 59F cooler.
Interesting that Ozone is also a greenhouse gas, although a weak one. It's apparently that three atom structure again.
Confusion in the third sentence is due to two interpretations of "33 degrees Celcius". A better phrasing, now rarely used, is "33 Celcius degrees".
33 degrees Celcius is a temperature, equal to about 91 degrees Fahrenheit.
33 Celcius degrees is a difference in temperature, about equal to 59 Fahrenheit degrees.
Thank you, Rob. For clarification, and do I have it right?:
Heat is thermal energy, that is, whole molecules BOUNCING around, hitting each other. When a molecule absorbs IR, the energy goes into atoms WOBBLING, or electrons jumping, within the molecule, which is NOT thermal energy.
So the CO2 does NOT get hotter when it absorbs IR.
Please comment on the accuracy of my (mis)understanding of the quantum electrodynamics. Thanks
More, misunderstanding: How, quantum electrodynamically, do we explain classical black body radiation, where the emitted frequency is a function of its thermal temperature? I am missing something, photons reacting with the thermal energy.
sailingfree, thermal energy, by which I assume you mean heat, in a gas includes the energy of motion as each molecule moves ("energy of translation"), but it also includes the energy of vibration and/or rotation within the molecule. It does not include energy involved in changes energy states of electrons in ordinary atmospheric conditions. In contrast, only energy of translation contributes to the temperature of a gas.
Tom Curtis@7,
That is accurate only for monoatomic noble gases like helium that have the possibility of kinetic energy only from translational kinetic energy of point masses, that their single atomic molecules represent. For any other materials. The so called "kinetic temperature" you're refering to, defines temperature in terms of the average translational kinetic energy of the independent point masses only.
Molecules of all other materials can have other forms of kinetic energy. That includes rotational kinetic energy of vibrating milti-atomic molecules of most other gasses. The zero-th law of thermodynamics (which defines the termperature as the state of energy equilibrium between any two parts of a single "body") includes all kinds of kinetic energy in the material molecules. With such definition of material's "overall temperature", the vibrational energy must also be included. Such definition translates to the everyday understanding that the3 temperature of a gasuous material we measure, includes both translational and vibrational kinetic energy of the material's molecules. If it did not include vibrational kinetic energy, the subsequent laws of thermodynamics would be less intuitive to understand and the energy conservation laws would not be implied therein.
sailingfree@5,
Not exactly. By increasing the vibrational energy, the CO2 molecules become "super-hot" in terms of thermal energy definition, or overall "kinetic energy" definition I refered to in my previous comment to Tom Curtis. Further, by interacting eith other moelcules in the air (by bouncing around), CO2 molecules transfer that energy to the rest of air, resulting in air temperature increase.
I don't understand you question. You need to be more specific.
The fact that shorten wavelngth are radiated by hotter objects follows from Wien's law. Later, Plank described the spectral distribution of black body radiation. Intuitively, it means that various molecules of a body are able to emit various quants of electromagnetion radiation, depending on their energy state at that instant. The collective amount of radiation (all quants combined, as emitted from 6.02*10E23 molecules per mole of material) form the continuous Plank spectrum or radiation.
What is unlear here? What do you mean by seeking "quantum electrodynamically" explanation?
My post @10 above is the response to sailingfree@6. Sorry for the omission.
What's missing here is the fact that below the effective radiating height relaxation through collision with another gas molecule (mainly O2 or N2) is far more likely than relaxation through spontneous emission of a new photon, thereby converting the vibrational energy of absorption to thermal energy within the atmosphere.
I am new here so bear with me. I liked the article. I'm wondering though about one thing. I'm not sure of the exact data, but I've heard that atmospheric concentration of CO2 has increased in the last century or so from around 300 ppm to 400ppm. If the change has only been one additional molecule of CO2 per 10,000 (namely 4 molecules of CO2 out of every ten thousand gaseous molecules as opposed to there having been only 3/10,000 a hundred years ago) how does this account for the rise of (correct me if I'm wrong) 1.5F? The molecular percentage change is miniscule. I don't see how it correlates to a temperature change of one and a half degrees globally. I realize the N2 and O2 are not participating in the IR equation, but it just doesn't seem to make sense mathematically. Also, are the molecules of CO2 and Methane refracting or reflecting the IR? I like the name of your website, by the way. A scientist's job is to try and poke holes in any hypothesis. Copernicus challenged the Ptolemaic model which was the scientific consensus of his day. We aren't called to have faith in other's experiments. We're called to look for the weak links. The main thing that concerns me however about the so-called climate skeptics are their bedfellows.
Welcome, Sancho! See if this post helps: "How substances in trace amounts can cause large effects." Then read "How Do We Know More CO2 Is Causing Warming?" Read the Basic tabbed pane first, and if you want more read the Intermediate and then Advanced tabbed panes.
"reflect or refract" isnt a great description of the interaction. A better understanding can be found here. As to effect of raising by 100ppm, you can always try the "shut up and calculate" approach. The interactions are described by radiative transfer equations. You can solve for the atmosphere and compare direct measurements at earths surface or outgoing IR from satellite with the results of the calculation. This thread has some of the results. This paper for an even better direct measurement of the effect of raising CO2.
Sancho, the key to understanding the power of adding only one molecule per 10,000 (from 3 to 4) is to understand that the other 9,997 N2 and O2 molecules do not absorb IR energy, only the 3 CO2 molecules do, and thus only the 3 are responsible for the greenhouse effect. The other 9,997 molecules might as well not even be there. Thus increasing the CO2 number from 3 to 4 has to increase the power of its greenhouse effect.
Think of it this way: if the effective dosage of a drug is 3 in 10,000 when disolved in distiled water and you increase the dosage to 4 in 10,000 you have increased the dosage by 33%, which could be fatal in some cases.
Or another analogy: think about what would happen if you added 3 liters of carbon black to a swimming pool containing 9997 liters of water. You might still see the bottom of the pool at the shallow end, but as you moved toward the deep end at some point you would no longer be able to see the bottom. Now remove a liter of water and add 1 more liter of carbon black. What would happen to the point where you could no longer see bottom?
As for the increase of 1.5F so far when we haven’t even doubled CO2 yet, remember that a warmer atmosphere will hold more water vapor, and that water vapor is also a greenhouse gas. So, by warming the atmosphere by adding CO2 we have also indirectly added water vapor to the atmosphere, thus increasing the H2O greenhous effect as well.
Jim Eager @16, very well explained. I do have to quibble, however, that CO, CH4, NO2, O3 (all four of which have natural and anthropogenic sources), various long chain carbon compounds of anthropogenic origin, and H2O all also contribute to the greenhouse effect. So also do clouds independently of the contribution of water vapour
Of course, Tom, I was just trying to simplify by keeping it to CO2 since that is the greenhouse gas that sancho asked about, but there is always a risk of oversimplifying, just as there is a risk of over complicating a concept by mentioning all the caveats. Still, "contribute to" would have been a much better choice than "responsible for."
Now for a question: How does CO, being a diatomic molecule, act as a greenhouse gas?
Jim Eager @18:
In order to be a greenhouse gas, the molecule must exhibit a dipole, ie, a spatial difference in electrical charge resulting from one element within the compound attracting electrons more strongly than the other. In carbon dioxid, and carbon monoxide, the oxygen atom pulls the electrons more closely to itself. That is neutralized in carbon dioxide because the molecule is linear, and the oxygen atoms at either end of the molecule end with the same electric charge. Certain vibrations, however, result in a relative change in charge as shown in this image:
In the assymetric stretch, as the carbon atom approaches closer to one of the oxygen atoms, that side gains a slight positive charge relative the other creating a dipole. Likewise when the carbon atom moves out of the direct line between the oxygen atoms when bending, that also creates a dipole.
Carbon monoxide is simpler because it always has a dipole because of the different electrical strengths of the oxygen and carbon atoms.
Tom, I understand the CO2 bending and asymmetric stretching modes, but I'm still not quite grasping how CO works. It's not a stretching or bending vibration, it's simply becuse of the electrical charge imbalance?
Jim Eager @20, the CO2 oscillations that emit or absorb IR photons do so because they generate an oscillating dipole. If they did not, they would not be able to do so (as is conveniently shown by the symmetric stretching oscillation which generates no dipole, and emits and absorbs no IR photons). In CO, I suspect that there is only a single oscillation, ie, a stretching oscilation. Because CO always has a dipole, that stretching oscillation will also generate an oscillating dipole, and hence allow the emission and absorption of IR radiation at a precise frequency.
OK Tom, now it makes sense to me. Thanks.
Also Jim, Tom, just speculating,
Since CO has a permanent dipole like H2O, it might also capable of being IR active through rotational absorption.
Glenn Tamblyn @23, evidently so (scroll down to figure 4.14).
Tom@24, Glenn@23,
Rotational energy quants at the molecular level are smaller than the vibrational. The result is that molecular rotation produce spectral lines in the radio part (some 100MHz or less), therefore not playing a mejor part in greenhouse effect (radiation in radio frequency constitutes a miniscule part of an outgoing body radiation at Earth temperature). That's probably why most GHE textbooks talks about virbational energy quants as the main phenomenon of interest.
But I suspect, in combination with vibrational changes, rotational changes may result in broadening of virbational spectral lines since the comination of two quants of a magnitude difference (if allowed) would produce a quant slightly shifted. However that's only my intuition as I'm not an expert in this field.
Interesting find Tom.
And thus maybe rotational isn't a factor in the IR bands for CO. Interesting however that it is a factor for H2O - another feature to add to waters status as a really 'interesting' molecule.
Anything that increases the translational, rotational, or vibrational energy of a molecule will increase its temperature. Energy added to one storage mode (e.g., vibration) is redistribted to the other storage modes until they are all in equilibrium. This happens within a time scale so small that for most considerations it is instantaneous.