Did Thomas Allmendinger show that Nitrogen and Oxygen absorb infrared radiation?

In 2016, Thomas Allmendinger, a retired chemist, published a paper, “The thermal behaviour of gases under the influence of infrared-radiation,“ which purported to offer experimental evidence of thermal radiation being absorbed by monatomic (1-atom) and diatomic (2-atom) gasses which are conventionally believed to have negligible capacity to absorb such radiation.

This sort of conclusion is beloved of climate skeptics, since it suggests that nitrogen (N2) and oxygen (O2) could absorb radiation in a manner similar to what is done by water vapor (H2O) and carbon dioxide (CO2) in association with the atmospheric “Greenhouse effect.” If Allmendinger’s conclusions were valid, that would mean that “Greenhouse gasses” aren’t in any way special. (It would also mean that much of modern physics is wrong.)

However, Allmendinger’s claims are readily seen to be self-contradictory, and his experimental setup was fatally flawed.

Some of Allmendinger’s experiments involved illuminating a tube of air 1 meter long with sunlight. The air warmed by about about 7℃ in 5 minutes (per his Figure 20). Allmendinger apparently assumed the warming was due to the sunlight being absorbed by air. Given that air has a heat capacity of about 1.2 kJ/m3/℃, this explanation would involved air absorbing sunlight at a rate of (1.2 kJ/m3/℃)×(7℃)/(300 s) ≈ 28 W/m3. 

However, the atmosphere has a scale height of around 8500 meters. So, if 1 cubic meter of air at the surface absorbs 28 W of sunlight, the atmosphere as a whole would be expected to have absorbed over 8500 × 28 ≈ 240 kW/m² of Sunlight. Since the intensity of sunlight at the top of the atmosphere is at most about 1.3 kW/m², this is clearly impossible. Air could not possibly be that good at absorbing energy from sunlight. And, if air was that good at absorbing sunlight, then the air above would have absorbed all the relevant energy in the sunlight; by the time sunlight reaches the ground, there would be nothing left in the ground-level sunlight to be absorbed by the air in Allmendinger’s experiments.

So, something is clearly very wrong with Allmendinger’s assumption that the warming he observed was due the gasses he was testing absorbing radiation.

What else could have caused the measured warming?

Given the design of Allmendinger’s experiments, there is every reason to believe that the observed warming was due to the inner walls of the gas container absorbing radiation, and then heating the contained gasses via conduction and convection.

So, contrary to what was claimed, no, the experiment did NOT demonstrate infrared radiation being absorbed by air and noble gasses.

For those interested in details, here’s some additional analysis…

Allmendinger tried to prevent his apparatus from absorbing radiation by lining the gas container with aluminum foil. However, he apparently simply assumed that using aluminum foil would address the problem, without validating that assumption.

Aluminum has a reflection spectrum as shown below.

So, for visible light, aluminum absorbs around 7% of radiation and there is an absorption peak of around 14% absorption at a wavelength of around 0.85 microns. (The source website explains that, in practice, uncoated aluminum oxidizes, increasing its absorption beyond that shown in the graph.)

Allmendinger used two light sources for his experiment: sunlight and a 150 W “basking lamp” for reptiles. By an unfortunate coincidence, the basking lamp used in the experiment was claimed by its manufacturer to have a peak output wavelength of 0.85 microns—the same wavelength at which aluminum is most absorptive!

How much heating was observed to occur in the “basking lamp” experiments? The observed rate of temperature increase was roughly 20℃ in 5 minutes (Allmendinger Fig. 22). The volume of gas was apparently (19 cm)2 × (100 cm) = 0.036 m3 in each of two adjacent tubes of gas. For air (used in some experiments), the heat capacity is around 1.2 kJ/m3/°C. So the energy required to raise the temperature of the air in one tube would be about 43 J/°C. To warm by 20℃ in 5 minutes would require a heating rate of (43 J/°C)(20℃)/(5×60 s) ≈ 3 W. (Given that the heat capacity of different gasses differ, this is only a rough calculation.) For a 150 W lamp, with light being directed into two tubes, this amounts of about 4% absorption of the incident radiation.

The light from a lamp is not well collimated, so it is to be expected that much of the light traveling through the tube would bounce off the walls of the tube multiple times before exiting the far end of the tube. On each bounce, the aluminum foil lining the tube would have been expected to absorb from 3-14% of the incident radiation.

So, it’s easy to believe that potentially ALL the observed heating was due to radiation being absorbed by aluminum foil, which would have then heated the contained gas via conduction and convection.

So, no, the experiment did NOT disprove the known thermal physics of gasses!

As to why the different gasses were observed to exhibit different maximum temperatures… 

The maximum temperature would occur when the rate of heat absorption was balanced by the rate of heat loss. What determined the rate of heat loss? Given the insulating properties of the styrofoam tubes that were being used, likely most heat loss occurred through the ends of the tubes. The ends of the tubes were thin plastic membranes (“preferably 0.01 mm thick Saran-wrap”). I would expect that the primary heat loss mechanism was:

  • Convective heat transport in the gas under test
  • Conduction across the plastic membranes at the ends of the tube
  • Convective heat transport in the ambient air surrounding the apparatus

One could do some calculations to verify it, but, given the large temperature differences (e.g. 40℃) relative to the ambient air, this mechanism could easily account for around 3 Watts of heat loss to the environment.

Given that that was the heat loss mechanism, the differences in what was observed for different gasses would presumably be explained by differences in how effective each gas is at convective heat transport.

Did radiation absorption/emission by gasses play any role at all in this experiment?

The only gas tested that would be expected to interact with radiation was CO2. Note, however, that the Greenhouse effect due to CO2 is associated with its ability to absorb and emit radiation with a wavelength of 14-16 microns. Sunlight contains essentially no significant amount of radiation at those wavelengths. The “basking lamp” that Allmendinger used would have also emitted very little radiation at those wavelengths. So, the experiment in no way tested the “Greenhouse” properties of CO2.

It does happen to be the case that CO2 can also absorb and emit radiation at shorter wavelengths (around 2, 2.7 and 4.3 microns). Those absorption bands don’t play a role in the atmospheric Greenhouse effect, but could have played a role in the experiment. In particular, CO2 would have absorbed some radiation at these wavelengths, and also emitted some radiation in the 14-16 micron band. It’s possible that those effects might have roughly balanced out one another. However, the calculations that would need to be done to check that are a bit more work than seems worth doing for the sake of understanding an experiment that was so poorly controlled.

Note that some people think, “If CO2 is a Greenhouse gas, then using CO2 should make things warmer.” That idea reflects a failure to understand how the Greenhouse effect functions. The Greenhouse effect functions ONLY when there is a large temperature drop from one side of the gas to the other (just as there is a large temperature drop from Earth’s surface to high in the troposphere). If the gas under test in an experiment is roughly the same temperature, one SHOULDN’T observe any Greenhouse effect, not from CO2, and not from any other Greenhouse gas. The Greenhouse effect happens only when the gas is warm where heating is happening (e.g., from sunlight being absorbed), and cold where heat is leaving (e.g., in the upper atmosphere).

Allmendinger’s experiment was poorly controlled and failed to differentiate between radiation being absorbed by gasses and radiation being absorbed the liner of the gas container.