Getting Rudy's Back
The next latest issue of C&E News, house organ of the American Chemical Society continues the back and forth about Rudy Baum's forthright statement on climate change on which Eli has been making a reporting from the scene. There has been an intelligent response to the harumphing chorus. We start with something nice
I am writing to thank you for your editorial on climate change. I understand that you're being bombarded by the forces of ignorance, but as a climate scientist, I can assure you that your views of the science are spot on. Keep up the good work, and don't be intimidated by the screaming from those for whom politics matters more than science.something old
Andrew Dessler
Readers who may be confused about the causes of climate change might want to read the Nongovernmental International Panel on Climate Change (NIPCC) summary report, "Nature, Not Human Activity, Rules the Climate" (www.sepp.org/publications/NIPCC_final.pdf). The title tells the story. . .something useful
S. Fred Singer
Many chemists are skeptical about the science of global warming because it doesn't fit well with the frame of their chemistry mind-set. An example is the letter by Thomas E. D'Ambra in which he asks, "How many kilocalories of infrared energy can a ton of carbon dioxide absorb?" (C&EN, July 27, page 6). The question implies that the amount of carbon dioxide in the atmosphere may be insufficient to cause a change in the trapping (the greenhouse effect) of the outgoing energy by Earth's thermoradiation.This time there are 16 letters supporting Baum's editorial and 4 opposed. Many of the supportive letters are quite blunt
I encountered questions from professional chemists similar to this while giving presentations on global warming, and I was initially unable to come up with a satisfying answer. The basis of the question is legitimate: CO2 absorption in the infrared region of the spectrum is weak on a per-molecule basis, and CO2 is a minor component of the atmosphere, with a current concentration of 380 ppm (only 380 molecules out of 1 million molecules in air are CO2).
Any person, particularly a skeptical chemist, would expect that, with the nonstop emission of thermoradiation from Earth's surface, all CO2 molecules would soon be in the excited vibrational and rotational levels of their molecular energy states, and none would be left to absorb more outgoing energy. Hence, the greenhouse effect would be very limited.
However, CO2 molecules do not exist alone in the atmosphere. The excited molecules can and do transfer their excess energy to other molecules and return to ground states and are therefore ready to absorb thermoradiation again. The transfer of the initially absorbed energy to other nonabsorbing molecules, called "quenching" in photochemistry, enables a relatively small amount of greenhouse gases such as CO2 to continuously absorb the thermoradiative energy, which otherwise would escape into space, and to convert the radiation back to thermal energy that stays on Earth.
Therefore, the answer to D'Ambra's question is that an unlimited amount of infrared radiative energy can be absorbed and returned back to Earth by small quantities of atmospheric CO2 and other greenhouse gases. The greenhouse effect is continuous along with Earth's thermoradiation.
Jihong Cole-Dai
After reading your editorials of June 22 (page 3) and July 27 (page 5) and the letters to you about the June 22 editorial, I am amazed. The level of illogical thinking and ignoring of obvious evidence in many of the letters is appalling to me. I expect such stuff in my local newspaper, the Seattle Times, but not from such a highly educated readership as yours.
It makes me ashamed of my profession (chemistry). But then again, it just drives home to me that even educated people who are talking about something that is not in their area of training and/or expertise sometimes act in strange ways. I'm not sure why, but I suspect it has to do with their religious outlook, their political leanings, or their paranoid ideas regarding conspiracies.
Your editorials were right on the mark. They contain some of the most concise and pointed discussions of climate change I have seen. . . .
Harvey F. Carrolland how can Eli do anything but close with
Ouch! I think you got a taste of what it has been like teaching environmental chemistry in Utah.
Stephen E Bialkowski
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9 comments:
lol Utah
Wait...why am I laughing,? I live in Tennessee.
That quenching stuff was interesting. Can people who know what they're talking about (IOW, not me) verify that it's right?
In dynamics you call it energy transfer, in kinetics quenching (e.g. you quench the energy in the vibrational mode). It's basically the same explanation Eli has been posting for years, but well put.
To restate it which hopefully means I understand it, the quenching effect is a way that CO2 returns to ground state that's different from simply re-radiating IR. Quenching is a physical contact between excited CO2 and other molecules that transfers energy.
I guess if I've been exposed to this idea before, then it just took some repetition to sink into my skull. I always thought the saturation argument was wrong because there were many frequencies where CO2 could absorb IR.
Actually I don't think the quenching argument is completely germane - pretty much all the energy a CO2 molecule exchanges with its neighbors will eventually comes back to excite it again, according to whatever the local temperature is. But the end of that answer is perfectly correct - the issue is the reradiation back down to Earth, which a small quantity of CO2 can of course do indefinitely without limit on the total energy exchanged.
So here is one version of what Eli said from Real Climate comments:
Having set myself up to fail let me point out that as in many of the issues discussed here it is a problem of time scales. Briefly put, the process can be defined as a CO2 molecule absorbing a ~650 cm-1 photon (equivalent to a thermal energy of about 900 K), and losing that energy to the surrounding bath of atmospheric gases. In turn other CO2 molecules can be excited by collisions with atmospheric molecules. Some of them (and a very small percentage of the originally excited CO2 molecules can re-emit).
The processes (absorption of light, collisional energy transfer and emission) can be separated because the average time that an isolated CO2 molecule takes before it emits a photon is much longer that the time for collisional de-excitation (~tens of microseconds at atmospheric pressure, less, higher in the atmosphere). Therefore you can model the situation by separating absorption and emission.
The absorption rate is readily calculated from Beer’s law I/Io=exp(-N sigma L) (the proportion of light absorbed in a layer of thickness L depends exponentially on the Number density of the absorbers, the absorption cross-section of each molecule and the length of the layer L. Both I and sigma are functions of the light frequency/wavelength).
Because collisional energy transfer to and from the excited molecules is rapid, the chunk of energy (650 cm-1) rapidly degrades into the heat bath of the atmosphere. The temperature of the heat bath can be determined from simple thermodynamics by figuring out the rate of heat flow into and out of the system by radiation, convection and conduction. A nuclear engineer should be able to do that part. Thermodynamics assures you that the CO2 is always at the temperature of the heat bath as long as the collisional thermalization step is rapid and it is. To claim that it is at a higher temperature is an amusing form of Zeno’s paradox. It is the same as claiming that if I put hot and cold chunks of metal together the originally hotter one will always be microscopically hotter.
At 300K (27 C) ground state CO2 molecules are continually colliding with oxygen, nitrogen and other molecules. The average collision has an energy equivalent to kT where k is Boltzmann’s constant. In units usual to the field this is ~200 cm-1 (multiply by the speed of light in cm/sec and Planck’s constant to get Joules) but some of them have much more energy. A few of the energetic collisions can vibrationally excite the CO2 to the same 650 cm-1 excited vibrational level. Because collisional processes are fast wrt radiation, the number of vibrationally excited CO2 molecules can be characterized by a Boltzman distribution. At 300 K about 6% of the CO2 molecules in the atmosphere are vibrationally excited and can radiate.
This, and the radiative emission rate allows you to calculate the radiative heat loss from a packet of atmosphere. It is important to note that the radiative loss from greenhouse gases is ONLY at frequencies that can be absorbed by near-by other near by greenhouse gases* and in all directions. Thus, the process becomes a giant game of pass it on until, by chance the packet of energy reaches the atmospheric door to space. Think of it as a crowded room full of blindfolded people passing packets to each other and a small door through which the packet (eventually) is shoved.
*Because of Doppler and pressure broadening the absorption and emission profiles from CO2 are slightly narrower for colder, less dense gases. Thus some of the radiation from a hotter, denser gas lower in the atmosphere, will pass transparently through the colder, diffuser atmosphere higher up.
Arthur,
Kinetically speaking the N2 and O2 act as an energy reservoir for the energy absorbed by the CO2/H2O. without the reservoir you have the classic two state Rabi system in which case the absorption rate would be limited.
The absorption *rate* would be limited, right, but not total energy absorbed, because whatever gets absorbed gets re-emitted. You'd get almost the same effect with a half-silvered mirror or a diffuser that randomizes photon directions until you hit that absorption rate limit. But I didn't think we were anywhere close to those limits for CO2 in the atmosphere? I guess there's different ways of thinking about these things...
The absorption rate WOULD be limited without collisional energy redistribution because for practical purposes there would be many fewer CO2 molecules in the lower state. With the heat bath, it is constant as you point out.
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