rev 1/24/22,9/15/22,1/19/23,6/30/23,8/15/23,3/7/24,4/23/24,4/26/24,5/24/24,10/12/24

Theory of Redirected Energy

Top-of-atmosphere (TOA) graphs of radiation flux from earth show ‘notches’ (‘ditches’?) centered at the nominal wavenumbers of long wavelength infrared (LWIR) active gas molecules, mostly CO₂, which do not condense in the atmosphere. Figure 1 appears to be a typical mid-latitude TOA graph. Radiation flux intensity is plotted vs. wavenumber at any one geometric location. Wavenumber is simply the number of radiation wavelengths per centimeter. Wavenumber, cm^-1 and wavelength in microns are easily converted: Divide 10,000 by either to get the other.

Figure 1: Typical mid-latitude TOA emission of radiation from earth. (Graph is from NASA [1])

On average, radiation emitted from the liquid and solid surfaces of the earth is very closely described by the Planck spectrum for a black body, at 288 K (15° C, 59° F) (The colors you see are reflected sunlight). Actual emissivity for earth’s surface is closer to 0.99. The red trace on Figure 1 shows the Planck spectrum for a black body (perfect emitter/absorber, emissivity = 1) at 294 K (69.5 F).

Part of the energy radiated from the surface goes directly to space through the ‘atmospheric window’ in the approximate wavenumber range 770-1230 (13-8.13 microns) where no gas molecules (except ozone, O₃) significantly absorb radiation. Over the rest of the total range of significant terrestrial radiation (wavelength 5-2000 microns, wavenumber 2000-5 cm^-1) the radiation is absorbed by Long-Wave-InfraRed (LWIR) active gases (which, including water vapor, are misleadingly called greenhouse gases (ghg)).

Height of the black line trace is the flux (energy rate) as a function of wave number at the TOA at a single line-of-sight. This line accounts for the energy that radiates from the surface plus that from latent and convective heat from the surface and solar radiation absorbed by clouds and atmosphere. The vertical distance on the graph from a curve representing radiation from the earth’s surface to the black line indicates the amount at each wave number that the radiation at TOA (energy rate) is less than the energy rate radiating from the surface. Energy is conserved so the rate is what changes, i.e. the flow of energy is slowed down as it passes through the atmosphere. The slowing is proportional to the logarithm of the number of molecules of the responsible ghg.

 At the scale of atoms, the atmosphere can be visualized as molecules bouncing elastically (no energy loss) off each other in empty space. At sea level conditions, the time between collisions for the air molecules (molecule diameter about 4 Angstroms; 4E-10 m) is extremely short, less than 0.0002 microseconds [4]. Among other properties, these collisions are the basis for thermal conduction in the gas. Therefore, electromagnetic radiation (EMR) energy absorbed by ghg molecules is immediately (within a few microseconds) shared with surrounding molecules both ghg and non-ghg. The sharing is thermal conduction in the gas. The process of absorbing radiation energy and sharing it with surrounding molecules is called thermalization.

 Logic mandates that the elapsed time between when a molecule absorbs a photon and when it emits one must be more than zero or there would be no indication the photon had been absorbed. This elapsed time, called decay time, has been calculated for CO2 and averages up to about 1.1 second [2, 3]. Decay time for WV is expected to be up to about 0.4 sec. The probabilistic accumulation of the elapsed time in outward directed radiation flux, especially from water vapor (WV) molecules, accounts for the so-called Greenhouse Effect (GHE) which is the difference between observed average global temperature and what it would be if there were no IR active gases.

 A common observation which shows that water vapor increase contributes to global warming is that cloudless nights cool faster and farther when absolute water vapor content of the atmosphere is lower. Clear nights cool faster and farther in the desert than where it is humid. 

Below the tropopause (below about 8 to 16 km depending mostly on latitude; more at the equator) WV decreases from an average of about 8,000 ppmv (0.8%, 4% near the equator) at ground level to, because of the low temperature (about negative 50 °C), at most, 203 ppmv at the tropopause (assume about 12 km). Saturation vapor pressure of ice at -50 C/total pressure at 12 km = 3.94 Pa/19400 Pa = 0.000203 = 203 ppmv (at saturation).

In addition to the decline in ppmv due to temperature decline, is the decline resulting from pressure decline with altitude of 19.4/101.1 = 0.191. The total WV molecule population gradient from surface to tropopause is thus about 8000/203 * 101.1/19.4 = 206 to 1.

Radiation from WV molecules below about wavenumber 600/cm can only be absorbed by other WV molecules. This steep WV molecule population gradient means that much of the outward directed radiation from WV molecules will make it all the way to space. This is demonstrated by the ‘hash’ in flux intensity observed in this wavenumber range measured at the top-of-atmosphere as shown on Figure 1. At about 2 km and higher, outward directed photons from WV molecules can make it all the way to space. Thermalization causes some of the energy absorbed by CO₂ molecules to be redirected to WV molecules. The Schwarzschild equation [12] accounts for this by assuming that “as radiation passes through an isothermal layer, its monochromatic intensity exponentially approaches that of blackbody radiation corresponding to the temperature of the layer.” Much of the radiant energy along a line-of-sight is ‘redirected’ with respect to wave number so it becomes transparent to the slowing of CO2 molecules.

The redirection mechanism is summarized as follows: Much of the radiation energy that is absorbed by CO₂ below the tropopause is shared with all molecules (thermalization) and emitted to space by WV molecules. This is discussed further in Sect 10 of Ref [10].

Above the tropopause the remaining energy is conducted from non-ghg molecules to all ghg molecules for eventual radiation towards space. For lack of a better term, call the conduction of energy from non-ghg to ghg molecules and radiated from them reverse-thermalization.

Ghg in the warmed air can significantly emit photons only at a limited number of wavelengths (or wavenumbers) characteristic for each molecule species. Furthermore, all theoretically possible wavenumbers are not equally likely.

The Water Vapor Factor

Water vapor is a transparent gas that, molecule for molecule, is more effective at absorb/emit of earth-temperature infrared radiation (IR) than carbon dioxide (at low altitude, absorb lines for CO₂ overlap). From Jan 1988 thru Dec 2023 NASA/RSS accurately measured and reported monthly the global average water vapor as Total Precipitable Water (TPW). The anomaly data are reported at [11]. The nominal value is about 29 kg/m^2 so the trend from Jan 1988 thru Dec 2023 is about 1.43 % per decade. Given that at ground level average global water vapor is about 0.8% or 8,000 ppmv (parts per million by volume), the increase in water vapor molecules in 3.6 decades is about 0.0143 * 8000 * 3.6 = 412 ppmv. From Mauna Loa data at [9] the CO₂ increase in that time period is 423 - 349 = 74 ppm. Per ideal gas laws, ppm = ppmv. With that, at ground level, water vapor molecules have been increasing 412/74 = 5.5 times faster than CO₂ molecules. Thus, regardless of the initial source of warming, at ground level, water vapor molecules have been increasing about 5.5 times faster than CO₂ molecules. The idea that CO₂ starts the increase is ludicrous.

Radiation from water vapor molecules can be in any direction but, because of the steep decline with altitude of the population gradient of water vapor molecules, the distance traveled by a photon before it encounters another water vapor molecule is greater towards space than towards earth so the prevailing direction of IR flux is towards space. This is shown on a Top of Atmosphere (TOA) graph of radiation flux vs wavenumber (wavenumber is the number of wavelengths in a centimeter) by the jagged line below about wavenumber 600. Because of the characteristic absorb/emit signature of every gas, no other gas can significantly absorb or emit radiation in the wavenumber range occupied by water vapor. The line is jagged because radiation that reaches TOA/space is from water vapor molecules at different temperatures/altitudes.

Fig 1.5 is a TOA graph with overlaid constant temperature curves for black body radiation at noted temperatures. For any specified atmosphere, these are also constant elevation curves. The temperature and associated altitude for standard atmosphere are shown in the upper right corner of Fig 1.5.

Figure 1.5: Typical TOA radiant emission. (U Chicago version of MODTRAN [8]

At about 2 km and higher, the outward directed radiation from water vapor can make it all the way to space. Below the tropopause, much of the energy absorbed by CO₂ and other IR active molecules is redirected with respect to wave number via thermalization to water vapor molecules. This mitigates any warming from increased CO₂ in the troposphere (or any other IR active gas that does not condense at earth temperatures).

At the tropopause (about 8 to 16 km altitude depending mostly on latitude; higher at the equator) and above, water vapor molecules are greatly diminished because of low temperature so radiation to space is mostly from CO₂ and other IR active molecules that do not condense in the atmosphere. Increased CO₂ in the extremely thin air there actually counters warming.

The end result is that CO₂ does not cause significant climate change, sequestering it is an expensive mistake and the Green New Deal would have no significant effect on climate.

Radiance calculated by MODTRAN6

MODTRAN6 [5] is a computer program developed for the Airforce Research Laboratory which (besides other things) can calculate the radiation flux at selected elevations in the atmosphere for specified constituents and conditions. It contains default values for several environments including the tropics and the 1976 Standard Atmosphere. Values for water vapor change rate and atmospheric temperature vary with altitude for different latitudes and seasonal conditions as shown in MODTRAN6 documentation [7].

 Most of the photons emitted by the water vapor molecules are at wavelengths different from the comparatively narrow band that CO₂ molecules can absorb so they are absorbed by other WV molecules. Effectively, below the tropopause, much of the terrestrial thermal radiation energy and other energy absorbed by CO₂ (and other non-condensing ghg) is thermalized, redirected to, and radiated to space from water vapor.

 At very high altitudes, temperature, molecule spacing and time between collisions increases to where reverse-thermalization to CO₂ (and O₃) molecules becomes significant as does radiation from them to space. At low altitude the tiny amount of energy absorbed by CO₂ and much greater amount absorbed by water vapor are thermalized; contributing to warming the low altitude atmosphere.

 The water vapor content of the atmosphere diminishes rapidly as the temperature decreases with increasing altitude. Above the tropopause it has declined to a level where emission from water vapor ceases to dominate and emission from CO₂ molecules becomes significant. The result is most of the residual energy not emitted to space by water vapor at low altitude is, at high altitude, redirected back to the wavenumber range 600-740 cm^-1 and emitted to space by CO₂. The ’redirection’ is not geometric because all wavenumbers refer to photons at essentially the same line-of-sight.

 Energy redirection helps explain why average global temperature tracks average global water vapor and not CO₂ [6].

 

References:

1. NASA/GISS TOA graph https://www.giss.nasa.gov/research/briefs/2010_schmidt_05/

2. Average elapsed time to emit a photon https://sealevel.info/Happer_UNC_2014-09-08/Another_question.html

3. Average elapsed time to emit a photon http://rabett.blogspot.com/2013/04/this-is-where-eli-came-in.html

4. Time between gas molecule collisions http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/frecol.html

5. MODTRAN6 calculator http://modtran.spectral.com/modtran_home#plot

6. Climate change drivers http://globalclimatedrivers2.blogspot.com

7. MODTRAN6 defaults http://modtran.spectral.com/modtran_faq

8. MODTRAN calculator http://climatemodels.uchicago.edu/modtran/

9. Mauna Loa data for CO2:  https://www.co2.earth/monthly-co2

10. Water Vapor vs CO2 for Planet Warming: https://watervaporandwarming.blogspot.com

11. NASA/RSS measurements thru Dec 2023 of Total Precipitable Water anomalies  https://data.remss.com/vapor/monthly_1deg/old_files/tpw_v07r02_198801_202312.time_series.txt

12. Wallace and Hobbs: http://www.gnss-x.ac.cn/docs/Atmospheric%20Science%20An%20Introductory%20Survey%20(John%20M.%20Wallace,%20Peter%20V.%20Hobbs)%20(z-lib.org).pdf