Now tell me, which came first, the chicken or the egg? Discussions of Forcing and Feedback can be about like that. When some talk about the greenhouse effect they mention that the "Earth is 33C warmer than it would be if there were no Greenhouse Gases". I guess they think the egg came first?
With no atmosphere, the Earth would have a "surface temperature" of about 4 C degrees based on TSI/4 or ~1361 Wm-2/2 = 340 Wm-2 which if you assume a very perfect black body, the Stefan-Boltzman Law would give you an effective temperature of about 4 C degrees. If the Earth were a less than perfect but close black body, the "subsurface temperature" with no atmosphere would be about TSI/Pi=433 Wm-2 which by the S-B law would be a temperature of about 22.5 C degrees.
The subsurface temperature would be due to one and only one radiant surface, the assumed to be ideal surface, that would produce an average of ~4 C degrees. Instead of an atmosphere insulating the surface, a few meters of assumed uniform soil of some sort provides insulation. There is a page on Lunarpedia related to lunar temperature you can check if you like. If the actual absolute "surface" temperature of the Earth is 15C, then Earth's "surface" temperature would fall right in the range of an "ideal" black body surface and subsurface. If our atmosphere thickens or becomes a better insulator, the temperature would increase to closer to the subsurface 22.5 C and if it thins, closer to the 4 C surface temperature.
No where have I mentioned clouds or CO2 or anything forcing/feedback related. This is what I consider the egg. This is why I continually ask for a better definition of the "surface" folks are talking about in reference to Global Warming or the Greenhouse Effect.
The actual Earth doesn't have a uniform "ideal" surface/subsurface it has oceans. The "average" temperature of the ocean subsurface is about 4 C degrees. That is likely due to a variety of "lucky breaks". The first lucky break is water has a weird maximum density at 4 C degrees. Even though that is for pure water, the salty oceans still are influence by the 4 C density phenomenon and the ~0 to -2C freezing point of the current blend of fresh to salty water. The second lucky break is that water which is close to an ideal black body provided it isn't frozen or evaporated, would tend toward the ideal black body temperature related to TSI/Pi. That's right sports fans, if there were no clouds the oceans average temperature would be close to 22.5 C degrees. The third lucky break is that as the briny oceans saturate with salt, the freezing point can drop to about -18 C degrees.
If you have ever looked into solar ponds, a heavy brine subsurface with a stratified fresh water lens makes one hell of a solar energy collector. That makes it very difficult for Earth to remain a "snowball" for any length of time as long as there is the sun and salt limits in the equatorial oceans. It also means that there will be considerable water vapor if the oceans are not frozen over.
With warm equatorial oceans there will be water vapor in the atmosphere. Water vapor is also a greenhouse gas, but it is limited to a more narrow temperature range of roughly -43 C in a "normal" atmosphere but in a very clean atmosphere with very few cloud condensation nuclei (CCN), the limit could be as low as -100 C degrees depending on the overall concentration. How much impact water vapor all alone would have on "surface" temperature I have no clue. The atmosphere of the Earth also contains oxygen which in strong solar irradiation reacts to form ozone mainly near the equator. I have seen estimates that tropical ozone and water vapor advected towards the poles increase the temperature of the highest latitudes by about 50 degrees in a very critical region. So we have two greenhouse gases, ozone and water vapor that exist due to solar irradiance, atmospheric composition and oceans that are responses to the circumstances, "lucky breaks", that are not easily classified as forcing or feedback. Changes in water vapor and ozone can be feedback or forcing or more realistically responses to general system conditions. You would have to define "normal" in order to pigeonhole the changes.
With warm oceans and some CCN, there would be clouds which tend to reflect solar irradiance. That in my opinion is a response which can become a forcing or feedback depending our your perspective i.e. frame of reference. Since the subsurface depends on the solar insulation at the surface, clouds a few kilometers above the surface would tend to reduce the solar impact. Remember that if the actual albedo of the ocean area is 0.23, then the subsurface temperature would be 4C using the (1-a)*TSI/Pi approximation. Try that on your own just for kicks.
This is were climate science in its "Global Warming" mode went wrong in my opinion, they assumed a normal instead of considering a range of normal that would be related to the surfaces selected as references.
As it is, additional CO2 would increase the atmospheric insulation potential and the impact would always depend on the current conditions. The largest of the current conditions would be the average temperature of the oceans which have that tendency toward a 4 C average as a relatively clear liquid subsurface.
So now go figure out what "discrepancy" there might be in "surface" temperature.
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