Radiative Heat Transfer by CO2 or “what’s the quality of your radiation?”
Note: This is a contentious subject, and I have often shied away from it because it often erupts in food fights. However, Mr. Gill is making a good-faith effort here, and asks some relevant questions that I consider worth discussing. His original essay was sans graphics, and I’ve added two relevant graphics to aid in the discussion. – Anthony
Do Wien’s Law and Quantum Physics 101 prove CO2 can’t warm anything?
Guest essay by Rod Gill
WUWT has happily demonstrated many ways CO2 fails to produce measurable warming. I’ve thought of another way. It’s so simple I must have missed something, but I simply can’t work out what. It goes like this…
Experts suggest there is a net down welling 2W/m2 of long wave infra-red radiation (LWIR) that is causing global warming. I suggest the quality of that 2W of radiation is crucial to determining whether or not it causes any atmospheric warming at all. First a few key points which I think are facts and not open to dispute.
My understanding of Thermodynamics and Radiation from CO2 is as follows:
- In Thermodynamics, Temperature is the average kinetic energy of the particles in a body (solid or gas).
- The temperature of a volume of air has nothing to do with the amount of radiation (sometimes mislabelled as heat by scientists) passing through it. Unless that radiation is at a frequency that can be absorbed by the air, its temperature is completely unaffected by the radiation (ignoring any convectional heating).For example at the top of Mount Everest, there is a lot of solar energy (long and short wave radiation) there when the sun is out but the temperature is still cold.
- Different gases have different emission spectrums. For example Oxygen and Nitrogen do not absorb or emit Long Wave Infrared Radiation (LWIR) at all, so are not considered to be “Greenhouse” gases.
- The temperature of a body (gas, liquid or solid) directly affects the wavelength of the radiation it emits and absorbs.
- Wien’s Law defines the temperature – wave length relationship. The formula is Temperature (in degrees Kelvin) = 2898 / peak wave length in µm (micro metres). So for the average temperature of the Earth, lets call it 15C (=289 Kelvin), the wave length is 2898 / (15+274) = 2898 ÷ 289 = 10um.
The wavelength of the peak of the blackbody radiation curve decreases in a linear fashion as the temperature is increased (Wien’s displacement law). This linear variation is not evident in this kind of plot since the intensity increases with the fourth power of the temperature (Stefan- Boltzmann law). The nature of the peak wavelength change is made more evident by plotting the fourth root of the intensity. Source: http://hyperphysics.phy-astr.gsu.edu/hbase/wien.html
Carbon Dioxide’s absorption spectrum shows it absorbs LWIR at three different narrow wave lengths, sometimes called finger frequencies. Two of those wave lengths happen at temperatures too hot to exist in the atmosphere, the remaining wave length is 15um.
15um equates to 2898 ÷ 15 = 193K = -80C or -122F. In the atmosphere this temperature only occurs about 90-100Km high in the atmosphere.
Carbon Dioxide only emits and absorbs radiation at -80C from a narrow layer of atmosphere 90Km above the Earth’s surface.
So now we need to examine the quality of that 15um radiation and its ability to heat the lower atmosphere. To do this we need to understand basic Quantum Physics as taught in 101 classes to Physics and Engineering students at University. Confession: I’m an Engineer, but trained before Quantum Physics was introduced to University courses so I’m self-taught, hence my need for a sanity check. Which, dear reader, is where you come in.
The key points in basic Quantum Physics, regarding radiative heat transfer, are:
- Molecules have one or more electrons circling them. Their orbital height is not variable, But fixed. The electrons only orbit at set altitudes, the closer to the molecule the lower the kinetic energy of the molecule and so the lower the molecule’s temperature.
- For a molecule to “warm up” (have more kinetic energy) it needs its electrons to move to a higher, more energetic orbit. This can happen in one of two ways, get energy from a more energetic molecule via collision or receive energy via radiation.
- For an electron to move to a higher orbit from radiation it must receive a photon with sufficient energy for an electron to reach that higher orbit.
- Photons with too much energy raise the electron to the higher orbit then the molecule immediately re-radiates surplus energy.
- Photons with not enough energy to raise the orbit of any of the electrons are either scattered or immediately re-radiated (effectively reflecting or scattering them) with no change to the molecule’s kinetic energy, or temperature.
- The Photon must have a frequency that resonates with the molecule, otherwise the Photon is just scattered or reflected immediately with no temperature change to the molecule.
Carbon dioxide can only absorb Long Wave Infrared Radiation (LWIR) energy and radiate it at 15 micro metres, a fraction of the LWR spectrum.
- Electrons orbiting molecules of a liquid or solid need more energy to boost an electron’s orbit than electrons in a gas, so require more energetic photons again to warm them.
Therefore it is my understanding that it is impossible for the LWIR emitted by a cold low energy CO2 molecule to have the energy required to warm any molecule in the atmosphere warmer than -80C and certainly no molecule in a liquid (EG water) or a solid body, as their electrons require even more energy.
- LWIR from CO2 simply bounces around the atmosphere until it escapes into space and it causes no warming of the lower atmosphere at all. The energy level of that 2W of LWIR is too poor to have any affect. It needs to be closer to 10um to be energetic enough to warm anything.
So the idea of CO2 trapping heat in the atmosphere is all wrong. Yes LWIR from CO2 is retained in the atmosphere longer, but it simply bounces around until it escapes into space without causing any warming.
So am I right? I deliberately have not included any references because I want you to confirm or deny my understanding independently. If I gave you my references, which knowing the web may or may not be accurate, you might erroneously come to the same conclusions I have. However I have tried to limit my research to University papers and lecture notes hoping they are more reliable.
If I’ve got this right, CO2 caused global warming isn’t possible. If I haven’t got this right, then exactly how does LWIR radiated from CO2 warm anything?
Many thanks and please limit comments to specifics mentioned above. And if you disagree with the science above, please explain which sentences you disagree with and exactly how, at the Quantum Physics level, photons from a CO2 molecule at -80C can warm anything.