Donald Rapp makes things clear and concise in his 2024 paper How Increased CO2 Warms the Earth-Two Contexts for the Greenhouse Gas Effect. Excerpts in italics with my bolds, exhibits and some added images.
Physicist Donald Rapp retired from the Jet Propulsion Laboratory and has authored many books including Ice Ages and Interglacials: Measurements, Interpretation and Models; Assessing Climate Change: Temperatures, Solar Radiation and Heat Balance; and Use of Extraterrestrial Resources for Human Space Missions to Moon or Mars (Astronautical Engineering). Most recently he published Revisiting 2,000 Years of Climate Change (Bad Science and the “Hockey Stick”)
Abstract
The widespread explanations of the greenhouse effect taught to millions of schoolchildren are misleading. The objective of this work is to clarify how increasing CO2 produces warming in current times. It is found that there are two contexts for the greenhouse gas effect. In one context, the fundamental greenhouse gas effect, one imagines a dry Earth starting with no water or CO2 and adding water and CO2 . This leads to the familiar “thermal blanket” that strongly inhibits IR transmission from the Earth to the atmosphere. The Earth is much warmer with H2 O and CO2 . In the other context, the current greenhouse gas effect, CO2 is added to the current atmosphere. The thermal blanket on IR radiation hardly changes. But the surface loses energy primarily by evaporation and thermals. Increased CO2 in the upper atmosphere carries IR radiation to higher altitudes. The Earth radiates to space at higher altitudes where it is cooler, and the Earth is less able to shed energy. The Earth warms to restore the energy balance. The “thermal blanket” is mainly irrelevant to the current greenhouse gas effect. It is concluded that almost all discussions of the greenhouse effect are based on the fundamental greenhouse gas effect, which is a hypothetical construct, while the current greenhouse gas effect is what is happening now in the real world.
Adding CO2 does not add much to a “thermal blanket” but instead,
drives emission from the Earth to higher, cooler altitudes.
Background
Were it not for the Sun, the Earth would be a frozen hulk in space. The Sun sends a spectrum of irradiance to the Earth, the Earth warms, and the Earth radiates energy out to space. This process continues until the Earth warms enough to radiate about as much energy to space as it receives from the Sun, reaching an approximate steady state. If for some reason, the Earth is unable to radiate all the energy received from the Sun, the Earth will warm until it can radiate all the energy received. It is widely accepted that rising CO2 concentration reduces the ability of the Earth to radiate energy to space. In a dynamic situation where the CO2 concentration is continually increasing with time, the Earth will continuously warm as it tries to “catch up” to the effect of increasing CO2 and reestablish a steady state. It is a conundrum that while it is widely accepted that rising CO2 concentration produces global warming, the exact mechanism by which warming is induced in the current atmosphere by rising CO2 is not widely understood. The concept of a “thermal blanket” imposed by greenhouse gases to warm the Earth has merit in some contexts but is mainly irrelevant to the question of how adding CO2 to the current atmosphere produces warming.
Before attempting to deal with the question of how rising CO2 concentration affects the current Earth’s climate, it is appropriate to first discuss the Earth’s energy budget. The exact values for each energy flow are not important, but the relative values are important to show which processes dominate.
Finally, we provide an explanation of how adding CO2 to the current atmosphere produces global warming in the current atmosphere. The mechanism is not widely known and is likely to be surprising to some. Warming does not occur by increasing the thickness of the thermal blanket but instead occurs by raising the altitude at which the Earth radiates to space.
IR radiation
A fundamental law of physics states that all bodies emit a spectrum of radiant power proportional to the fourth power of their absolute temperature. A body at absolute temperature T (K) emits power per unit area: P = σ T 4 = 5.67 x 10 -8 T 4 (W/m 2 ) For example, a body at T = 280 K is said to emit 348 W/m 2 . However, this law of physics is academic and not directly applicable to real-world experience. In the real world, we never have a single isolated body emitting radiation, instead, we deal with pairs of bodies where the warmer one radiates a net flux to the cooler one. (If you stand next to a body at 280 K, you don’t feel an incoming heat flux of 348 W/m 2 ). For example, if there is one body at 280 K and a second body at 275 K, the warmer body will radiate through a vacuum to the cooler body at a net of 24 W/m 2 . That is a real-world parameter that can be measured. But the academic model involves calculating the emission of the warm body as 348 W/m 2 and the emission of the cooler body as 324 W/m 2 , and subtracting, the net transfer from the warm body to the cool body is 24 W/m 2 . But the calculated values are academic and cannot be measured in the real world with 348 W/m 2 in one direction and 324 W/m 2 in the opposite direction. Those values are only of academic use to infer the measurable net of about 24 W/m 2 . See the simple model in Figure 1 presented here for illustration.
Figure 1: Radiant heat transfer between warm and cool bodies
The two contexts of the greenhouse effect
We are all aware of the widely discussed greenhouse effect that warms the Earth as the concentration of greenhouse gases increases. But just how does it work? Here, we define two contexts for greenhouse gas effects:
1) The fundamental greenhouse gas effect can be described by a “gedanken experiment” in which one imagines a dry Earth starting with no water or CO 2 and begins adding water and CO 2 . The original atmosphere, lacking water and CO 2 , will transmit IR radiation completely. As a result, the Earth will be quite cool. As H 2 O and CO 2 are added to the atmosphere, the transmission of IR radiation from the Earth’s surface is increasingly inhibited, and the Earth warms. As the Earth warms, evaporation and thermals transmit more energy from the Earth to the atmosphere. By the time H 2 O and CO 2 levels reach current levels, the atmosphere is almost opaque to IR radiation, and a “thermal blanket” greatly reduces IR transmission from the Earth to the atmosphere. The Earth cools primarily by evaporation and thermals, and it is much warmer than if CO 2 and water were absent. The notion of a “thermal blanket” of IR absorbing gases warming the Earth has validity in this context starting with a transmitting atmosphere and adding greenhouse gases. However, once the thermal blanket is established with ~ 400 ppm CO 2 , adding more CO 2 has only a small effect on reducing IR radiation from the surface.
2) The current greenhouse gas effect deals with the question: How does the addition of CO 2 to the atmosphere affect the global average temperature in 2024 and beyond, with CO 2 around 400+ ppm? It was shown previously that starting with no water or CO 2 , adding H 2 O and CO 2 to the atmosphere generates a “thermal blanket” for radiation. But once that “thermal blanket” is well established and the lower atmosphere is very opaque to IR radiation, what is the effect of adding even more CO 2 ? Dufresne, et al. provide a detailed technical analysis to show how the current greenhouse effect works [7]. However, this reference is complex and written for expert specialists in IR transmission through the atmosphere. In the sections that follow, a simpler, qualitative interpretation will be presented.
Figure 3: Energy flows in the Earth’s system. (Based on LTWS references).
Energy budget of the earth
Energy transfer in the Earth system can take place by thermal transfers (“thermals”) where winds carry warm air up to colder regions, evaporation from the surface (removes heat), and condensation in the atmosphere (deposits heat) and radiation (further discussion follows).
After analyzing the data in the LTWS references (see Section 1.2), a rough estimate of key energy flows per unit time in the Earth system is given as follows. The exact numbers are not critical; only their relative values are important for this discussion.
These results can be visualized in Figure 3 which is based on the references LTWS. As shown in Figure 3, incoming solar irradiance (341 W/ m 2 ) is partly reflected by the lower atmosphere back out to space (79 W/m 2 ), partly reflected by the Earth’s surface back out to space (23 W/m 2 ), partly absorbed by the lower atmosphere (76 W/m 2 ), and finally about 163 W/m 2 is absorbed by the surface.
Radiation from the Earth’s surface to the lower atmosphere requires further discussion. The LTWS references show high up and down radiation flows. For example, Trenberth, et al. did not show radiation transfer between the Earth’s surface as a simple 25 W/m 2 net radiative transfer from the surface to the lower atmosphere. Instead, they showed 356 W/m 2 radiated upward from the surface and 333 W/m 2 of “back radiation” from the atmosphere to the surface [2]. The figure 356 W/m 2 radiated upward from the surface corresponds to the theoretical radiation from a blackbody at 281.5 K. The claimed downward figure is difficult to explain. But both of these figures are academic. What is happening is that the warm Earth is radiating upward through an optically thick gas of H 2 O and CO 2 absorbers, and the radiant transfer through that thick gas is estimated to be only a mere ~25 W/m 2 . This is the “thermal blanket” so often referred to in discussions of global warming. The thermal blanket is real. But the problem with so many discussions of the greenhouse effect is that there is a preoccupation with radiant energy transfer between the Earth and the atmosphere (which is “blanketed”) while neglecting the more important transfers of energy to the atmosphere by processes other than radiation.
Figure 4: Pressure, temperature, and relative humidity vs. altitude [8].
Energy transfer in the lower atmosphere takes place by conduction,
convection, and radiation. Energy transfer in the upper atmosphere
takes place primarily by radiation.
The greenhouse effect
The greenhouse effect can only be fully understood by comprehensive modeling of upward energy flows in the Earth system. Excellent studies by Dufresne, et al. and Pierrehumbert provide detailed physics [7,9]. Here, we interpret these results qualitatively.
Within the Earth system of land, ocean, atmosphere, and clouds, energy transfer is taking place continuously. There is a net energy flow upward toward higher altitudes. From the surface of the Earth, much of the upward flow of energy in the lower atmosphere is through evaporation and convection. The lower atmosphere is almost opaque to IR radiation due to water vapor and CO 2.
Figure 5: Qualitative sketch to show radiation is dominant at the highest altitude. By adding CO2 to the atmosphere, radiative energy transport is carried to a higher altitude where it is colder, reducing the radiant power emitted by the upper atmosphere.
Radiation energy transfer will persist out toward a high altitude until the CO 2 concentration diminishes. Each CO 2 molecule that absorbs an IR photon can reradiate in all directions, but in a thin atmosphere, some upward IR radiation will be lost, and on a net basis, this allows the Earth to radiate out to space. The presence of an IR transmitting/absorbing gas (CO 2 ) will allow energy transport to higher altitudes. The highest altitude where there is enough thin gas to maintain radiation is the region of the atmosphere that mainly radiates energy outward to space. This is illustrated on the left side of Figure 5. Figure 5 was created here to illustrate how the predominant energy transfer mechanisms gradually change to IR radiation at higher altitudes, and the presence of CO 2 carries the IR radiation to higher altitudes.
Conclusion
There are two different contexts for discussion of the effect of greenhouse gases on the Earth’s climate.
In one context, one can imagine an Earth with no water vapor or CO 2 in the atmosphere. This Earth can radiate effectively to space and is relatively cold. As water vapor and CO 2 are added to the atmosphere, the IR-opacity of the atmosphere increases and the Earth system warms. The greenhouse gases act as a “thermal blanket” to warm the Earth by impeding upward IR radiation. This is labeled the fundamental greenhouse gas effect. However, once the thermal blanket is established, adding more CO 2 has only a minimal effect on the thermal blanket, and reduced upward IR radiation from the surface does not produce significant warming. This is referred to by Dufresne, et al. [7] as the “saturation paradox”.
In the other context, we are concerned with the effect of adding more CO 2 to the current atmosphere where the CO 2 concentration is already 400+ ppm, and the thermal blanket is already in place, restricting upward IR-radiation. This is labeled the current greenhouse gas effect, and it is quite different from the fundamental greenhouse gas effect. In the current atmosphere, energy transfer from the Earth to the atmosphere is primarily by evaporation and thermals, and IR-radiant energy transfer is significantly impeded by an almost opaque lower atmosphere. The “thermal blanket” is in place, but it doesn’t change much as CO 2 is added to the atmosphere. Adding CO 2 to the current atmosphere slightly increases the opacity of the lower atmosphere but this is of little consequence.
In the upper atmosphere, CO 2 is the major means of energy transport by IR radiation. The greatest effect of adding CO 2 to the current atmosphere is to extend the upward range of IR-radiant transmission to higher altitudes. The main region where the Earth radiates to space is thereby extended to higher altitudes where it is colder, and the Earth cannot radiate as effectively as it could with less CO 2 in the atmosphere. The Earth warms until the region in the upper atmosphere where the Earth radiates to space is warm enough to balance incoming solar energy.
My Comment:
The explanation above is clear and understandable in qualititative terms. It does not reference empirical evidence regarding a GHG effect from a raised effective radiating level (ERL). Studies investigating this theory find that the effect is too small to appear in the data.
Refresher: GHG Theory and the Tests It Fails
Postscript on Raised Effective Radiating Level
The following diagram by Andy May shows the pattern of emissions by GHGs, mainly H2O and CO2.
Helpfully, it shows the altitudes where the emissions occur. As stated in the text above, the upper and lower tropopsphere shift occurs about 12km high, with variations lower at poles and higher in tropics. Note the large CO2 notch appears at 85km, which puts it into the thermosphere, where temperatures increase with altitude. Raising the ERL there means greater cooling, not less. The Ozone notch at 33km is in the stratosphere, where temperatures also rise with altitude. Otherwise almost all of the IR effect is from H2O.
The best context for understanding decadal temperature changes comes from the world’s sea surface temperatures (SST), for several reasons:
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