The 2 Good Greenhouse Gas III (Looking Up)

We have been looking down so far. What happens when you stand on the ground and point a spectrometer up? Interesting things.

downwelling ground up


In the CO2 bands around wave number 667 a spectrometer pointed up from the ground sees radiation at very near surface temperature. In the cases above that near surface temperature was about 245  K in Barrow and 300 K at Nauru. The spectra are very different in form as a result of different humidities in the atmospheric window and ozone bands to the right. In the cold, dry air at Barrow the instrument reads near zero except for the ozone bands. Those photons are running away straight to space and there is nothing to see. In the warm, moist air of Nauru the window is very “dirty” and noisy due to scatter and radiation by water.

Looking Up Barrow and Nauru vs Looking down Sahara

We can add our trusty Sahara looking down for comparison. The hot dry air is a good inverse for Barrow.

All Up

Finally we add an interesting spectrum looking up from Cerro Toco at 5 km altitude (but still on the ground) in the Atacama Desert in Chile. Notice how in the dry desert air the atmospheric window very closely matches Barrow with the photons fleeing at the speed of light and the instrument seeing next to nothing, while the higher near surface temperature shows clearly in the CO2 bands.

Note the difference between Barrow and Cerro Toco. The dip in the Barrow (Grey) CO2 bands results from a strong surface inversion. Close to the instrument, the radiance reads at a lower temperature. You essentially get an inverted altitude profile of CO2  absorption. Below is the 1 meter path at 400 ppm.


infrared_spectrum surface looking up and down Arctic Ice

In an amazing experiment shown above, the same instrument, one pointed up from the surface of Arctic ice, and another looking down from some sort of platform at 20km took measurements of the same place at essentially the same time. In the atmospheric window the U-2 looking down sees a very chilly near surface at~267 K. The one on the ice looking up sees a much chillier 160 K. The 20 km looking down sees a very clean signal of high intensity, the ice looking up sees a noisier signal of very low intensity.

They both see a clean blackbody signal in the CO2 bands with the 15 micron/667.4 wave number “spike” pointed up towards higher intensity and temperature. No surface inversion here.

In this series we first explored the lack of correlation between temperature and CO2 at long time scales, the dependence of CO2 on temperature at Neogene scale, and the dependence of CO2 variation around the trend on temperature, but not the trend itself as the trend becomes distorted by human emission in the satellite era.

We followed up with a look at radiance on the IR earth bands and found that satellites are seeing CO2 radiance taking place between ten and twenty kilometers in the upper troposphere and lower stratosphere. We then found that the same instruments pointed up from the surface see CO2 radiating very near the surface.

The problem is that the 2 good greenhouse gas  gobbles up all the surface IR within a hundred meters of the surface. This is clear from transmission.


Even at 280 ppm preindustrial levels of CO2 there is ZERO transmission of  surface IR to the tropopause in the fundamental bending bending bands. Furthermore, the 667.4 band and its homologous rotational bands DEFINE the zero transmission gap.

The fundamental bend commands 89% of the Boltzmann absorptive and radiative potential of CO2. This is why it gobbles up the surface IR so fast. This is why CO2 does not drive temperature at any time scale. It has already eaten all the earth photons in its strike zone at preindustrial levels. Adding more CO2 has no effect in these bands.

It is just too good.

This entry was posted in Climate, climate sensitivity, Greenhouse Spectra, Optical Material Properties, Spectra and tagged . Bookmark the permalink.

11 Responses to The 2 Good Greenhouse Gas III (Looking Up)

  1. Pingback: Modtran, Up and Down | geosciencebigpicture

  2. Pingback: MODTRAN Up and Down VII | geosciencebigpicture

  3. tom0mason says:

    Interesting. What effect has the differing heights of the atmospheric layers between Barrow and Nauru on these results? That is to say the top of the troposphere/tropopause/stratosphere are all lower nearer the poles than at the equator.

    • gymnosperm says:

      Assuming for the moment that the WordPress truncated version of your much appreciated comment is all I get, the answer is clear. The effective radiative altitude is given by the Planck temperature.

      Looking up, radiometers see basically 245K at Barrow, and 300K at Nauru.

      This 55K difference is the pressure modulated delta between the tropics and the poles.

      The instruments are seeing the first few meters of surface temperature in both cases.

      The downward inflection below 245K in the CO2 fundamental bend and associated rotation bands results from a strong surface inversion.

  4. SebastianH says:

    Have you considered that a shorter absorption path with higher CO2 levels results in a different starting temperature for the temperature-gradient/lapse-rate? More absorption nearer to the ground would equal higher temperatures at the ground and therefor increased blackbody radiation in the CO2 bands, wouldn’t it?

    • gymnosperm says:

      Sure, more absorption closer to the surface increases the lapse rate (more lapse) by increasing near surface air temperature. Increasing temperature, with more kinetic interaction, does increase the odds of photon emission.

      You can look down in Modtran in one meter increments for a CO2 only GHG atmosphere. This gets very tiresome because you do this about 900 times before you see any difference from the blackbody curve in the CO2 bands.

      The above is looking down from a kilometer. The red curve is looking down, and the blue is looking up from the same altitude according to Modtran.

      The above is down and up from 100 meters. The results are the same from 1 meter to about 900 meters. Not until this altitude do you see any deviation from the blackbody curve in the CO2 bands. Both up and down CO2 bands radiate at the Planck temperature according to the lapse rate and altitude.

      Even after CO2 bands begin to separate from the Planck curve at 900 meters, up and down continue to radiate at the same temperature/altitude.

      Same thing at 17 km in the lower stratosphere.

      Whether at the blackbody temperature or not, CO2 radiates at a very narrow range of altitude and temperature. This is because it is an exceptional absorber and a poor emitter. It much prefers to vibrate rather than spit photons.

      • SebastianH says:

        Thank you for the quick reply. Does the downwelling IR Heat Flux change at a distance of 100 m (or 1 meter) from the ground when you increase CO2 concentration? This shouldn’t be the case if CO2 is already “too good”.

        You mention that the CO2 effect would not get any stronger, because it is already maxed out and yet you agree that surface air temperature would increase? I’m confused.

      • gymnosperm says:

        If you go to Modtran CO2 only at 100 meters looking down and bump CO2 from 400 to 600ppm, it will tell you that “flux” has increased by .31 w/m2.

        Above is a table of the CO2 transitions and their relative intensities based on the Boltzmann populations at each state. Not all of these transitions are “saturated”. 544.3, 594.7, and 1063.7 are nowhere near saturated, but their intensities are orders of magnitude weaker than 667.4; which is saturated.

        Co2 is not “good enough” to cause no warming at all, it just doesn’t cause very much.

        Surface temperatures as measured at the height of our thermometers increases because radiant energy formerly released higher–but never escaping to space–is now released closer to our thermometers.

      • SebastianH says:

        Surface temperatures as measured at the height of our thermometers increases because radiant energy formerly released higher–but never escaping to space–is now released closer to our thermometers.

        So you are saying increased CO2 levels increase temperatures at the height we are measuring them. But isn’t that what an increased greenhouse effect is all about? Trapping heat closer to the surface?

        On the other hand you say it doesn’t cause much warming. How much is not much? The figure 4 W/m² is often mentioned. That’s a lot of additional Joules per year that the surface must get rid of by increasing its temperature, isn’t it?

      • gymnosperm says:

        The greenhouse effect is ultimately about thermalizing radiation the formerly escaped to space.

        Modtran CO2 only from 70km, 280 vs 400ppm gives a difference of 2.2 w/m2. Even this may be too high, as radiative transfer models do not account for the low emissivity of CO2 in the Schwarzchild equation. They plug in the CO2 absorption coefficient as the “source function”(emissivity). This would be valid if CO2 were a good blackbody with am emissivity approaching 1. CO2 is actually a lousy blackbody with measured and calculated total column emissivities ranging from .002 to .2.

        Only zealots “know” how much warming CO2 causes. The rest of us puzzle and poke at the complexity of the interactions that determine this.

        Does moving energy that was formerly dissipated at 10 meters down to 2 meters contribute to ocean surface warming? It might.

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