Last week in WattsUpwithThat.com I published the article “Did Trump Appoint Jack the Ripper as Guardian of a Girls’ School?” It lampooned, and rebutted, attacks on Princeton physicist Dr. Will Happer and President Donald Trump’s intent for him to head a committee on climate security at the National Security Council.

That generated lots of comments, mostly favorable, and the unfavorable ones were, mostly, handily refuted by others.

One favorable one, though, was was so brilliant I just have to share it here.

The main issue of debate is whether Happer is qualified, as a physicist, to advise on climate science. My article pointed out that Happer’s expertise,
Atomic, Molecular and Optical physics, is precisely the expertise most relevant to the most important question facing scientists trying to figure out how much impact CO2 and other greenhouse gas emissions are likely to have on global average temperature. Without an answer to that question, we have no basis from which to launch into other questions about the impacts of the GHG-driven warming, and hence no basis on which to answer whether this or that response to it makes sense.

Roger Taguchi commented (and though it’s technical, it’s also comprehensible—just “put on your thinking cap”):

William Happer’s qualifications to comment on climate change: As an atomic spectroscopist, he knows that very high-energy collisions between ground state sodium (Na) atoms and inert gas molecules like argon (Ar) can occasionally knock the outer (valence) electron of Na into an outer orbital. This electronically excited Na* atom can then drop back down to the ground state by emission of a photon (a particle of light); the drop from a 3p orbital to the ground state 3s orbital corresponds to emission of yellow light (because of electron spin, the yellow line is a doublet, at 5890 and 5896 Angstroms; see details at http://www.ifsc.usp.br/~lavfis/images/BDApostilas/ApEspectrRedeDif/Sodio.pdf ). OTOH, the excited sodium atom Na* can be quenched during non-radiative collisions with Ar molecules. The energy of the Na* atom does not disappear, but ends up as increased translational energy of the departing atoms.

The mechanism of the greenhouse effect is analogous. At 288 K, collisions between air (mainly N2, O2 and Ar) and CO2 molecules knock about 3.6% of the CO2 molecules into the v=1 first vibrationally excited state in the bond-bending mode. The solid and liquid surfaces of the Earth emit infrared (IR) photons in a Planck black-body spectrum. As the surfaces warm up during the daytime, the black-body radiation increases in intensity (the total outgoing flux is given by the Stefan-Boltzmann law, with emissivity 0.98 ). At 667 cm^-1, ground state (v=0) CO2 molecules will absorb some of the IR photons and be boosted to the v=1 first excited state, creating an excess. These excited state molecules can re-emit 667 cm^-1 photons, in which case there is no net absorption. However, these excess excited state CO2 molecules can also be quenched during non-radiative collisions with the air (mainly N2, O2 and Ar) molecules; this time, the excitation energy ends up in extra translational and rotational energy of the departing N2, O2 and CO2 molecules, and translational of Ar. After a few more molecular collisions, the energy ends up evenly distributed among the air molecules, and the air will have warmed up.

Because the heat capacity at constant pressure of linear molecules like N2, O2 and CO2 is the same, at 7k/2 per molecule, where k is the Boltzmann constant (see https://en.wikipedia.org/wiki/Heat_capacity ) and because at 400 ppmv CO2 the air molecules outnumber CO2 by 1,000,000:400 = 2,500:1, almost all of the absorbed energy ends up increasing the translational and rotational energies of N2 , O2 and Ar which cannot and do not re-emit any significant amount of IR (dipole emission requires a changing electric dipole moment, and N2 and O2 are non-polar molecules with no changing electric dipole moment during vibration). So the troposphere warms up.

Since net outgoing IR energy is absorbed by the troposphere, there would be an energy flow imbalance. The same incoming Solar visible radiation during the daytime would then warm up the solid and liquid surface until the increased Stefan-Boltzmann black-body emission (minus net absorption by the troposphere) to outer space once again balances the net incoming Solar energy flux (after considering reflection to outer space and absorption in the stratosphere by ozone). This is the mechanism of the greenhouse effect which William Happer would understand as a direct analog of his deep knowledge of atomic spectroscopy.

I’ve simplified the argument, of course. During the nighttime (and especially during Polar winters), there is no incoming Solar radiation, so the Earth’s solid and liquid surface would cool by loss of IR photons directly to outer space, especially in the frequency “windows” where there is little absorption by CO2 or water vapour molecules in the troposphere. The considerable heat content (enthalpy) stored in the troposphere during the daytime can moderate the rate of heat loss, via back-radiation: as vibrationally excited CO2 molecules emit IR photons, half of them directed back at the Earth, the number of v=1 excited state molecules is replenished via collisions of ground state v=0 molecules with fast-moving N2, O2 and Ar molecules, which cool down. This replenishment is not total, however (LeChatelier’s Principle says that if a stress is applied to a system at equilibrium, the equilibrium shifts in such a direction as to PARTIALLY relieve that stress). So the troposphere and hard deck surface of the Earth both cool down at nighttime.
In the Polar regions, back-radiation from excited state CO2 molecules may not be intense enough to maintain a warmer surface temperature, and you can get temperature inversions in the first few hundred metres from the surface. In this case, there can be net heat flow via back-radiation from a warmer troposphere to a colder surface which does not violate the Second Law of Thermodynamics.

Under normal circumstances, however, when there is a continually decreasing temperature with increasing altitude, back-radiation from the troposphere to the surface does exist (as detected by spectrometers looking upward from the surface), but cannot be used to explain the greenhouse effect. For example, if the Earth’s surface emits X W/m^2 upward which is totally absorbed within metres at 667 cm^-1 by CO2 which at the same temperature emits X W/m^2 in the forward direction, and X W/m^2 in the backward direction, conservation of energy appears to be violated (how can X W/m^2 end up powering a total emission of 2X W/m^2?). The answer is that the X W/m^2 of back-radiation just balances ANOTHER X W/m^2 emitted by the hard deck surface, since the two close emitters are at the same temperature, in thermal equilibrium (or very close to it). The X W/m^2 escaping outward from the first opaque layer of the troposphere is powered by the net X W/m^2 emitted by the hard deck (solid & liquid) surface of the Earth. Energy balance diagrams showing back-radiation warming the Earth are simply wrong.

The MODTRAN spectra available at https://en.wikipedia.org/wiki/Radiative_forcing show that they are almost identical for 300 and 600 ppmv CO2. The slight difference in area (net absorption) of the blue and green curves occurs at 618 and 721 cm^-1, which are the band origins for absorptions from the v=1 first excited state, and not from the v=0 ground state. (For the energy level diagram, see Diagram 3 in the section “Spectral transitions” athttp://www.barrettbellamyclimate.com .) Because v=1 state molecules are only 3.6% of v=0 molecules at 288 K (and only 1.3% at 220 K), lines in the band wings of these overtone absorptions are far from being saturated, so doubling CO2 will have a measurable effect in the enhanced greenhouse effect.

By contrast, the 667 cm^-1 absorption from the v=0 to the v=1 bond-bending state is so strong that almost all the lines in the band are saturated to altitudes of 20 to 40 km in the stratosphere. In this case, complete absorption is followed by complete emission (Kirchhoff’s Law that a good absorber is a good emitter), and as temperatures decrease with increasing altitudes in the troposphere, the emission goes down. However, because the temperature actually increases from 20 to 50 km due to absorption of incoming Solar UV and visible radiation by ozone in the stratosphere, the emission of the 667 cm^-1 band actually increases when CO2 is doubled, so for energy balance the Earth’s hard deck surface need not be as high (i.e. doubling CO2 means global cooling, not warming, when only the main 667 cm^-1 band is considered). To see the effect when MODTRAN calculations are carried to 70 km altitude, and not truncated at 20 km, see the section “The hard bit” at http://www.barrettbellamyclimate.com .

Finally, the MODTRAN calculated spectra necessarily involve net absorption in a cloud-free 10 km of the troposphere. When the 62% of the Earth’s surface that is covered by clouds is considered, the climate sensitivity is decreased from 1 degree to about 0.7 degrees before feedbacks, and the 50% positive feedback due to increased water vapour absorption is mostly cancelled by any reasonable estimate of the expected slight increase in cloud cover. These considerations would be obvious to someone with Will Happer’s background.

E.J. Zuiderwijk also commented, saying “A physicist who is not trained as a climate scientist is much, much, to be preferred over a climate scientist who is not trained as a physicist.”

Case closed.