A simple argument is put forth against the idea that the radiative properties of an atmosphere somehow serve as the CAUSE of elevated steady-state planetary surface temps. Continue reading
Be sure to read Part 1 first, now …
DEFINING THE rGHE THROUGH THE ERL.
How is the rGHE defined in the most basic way? If you have a planet with a massive atmosphere, the strength of its “greenhouse effect” is defined as the difference between its apparent planetary temperature in space and the physical mean global temperature of its actual, solid surface. The planet’s apparent temperature in space is derived simply from its average radiant flux to space, not from any real measured temperature. It is assumed that the planet is in relative radiative equilibrium with its sun, so is – over a certain cycle – radiating out the same total amount of energy as it absorbs.
If we apply this definition to Venus, we find that the strength of its rGHE is [737-232=] 505 K. Earth’s is [288-255=] 33 K.
The averaged planetary flux to space is conceptually seen as originating from a hypothetical blackbody “surface” or ‘radiating level’ somewhere inside the planetary system, tied specifically to a calculated emission temperature. This level can be viewed as the ‘average depth of upward radiation’ or the ‘apparent emitting surface’ of the planet as seen from space. Normally it is termed the ERL (‘effective radiating level’) or EEH (‘effective emission height’).
The idea behind the ERL is pretty straightforward, but does it accord with reality? The apparent planetary temperature of Venus in space is 231-232K, based on its average radiant flux, 163 W/m2. Likewise, Earth’s apparent planetary temperature in space is 255K, from its mean flux of 239 W/m2. In both of these cases, the planetary output is assumed to match its input (from the Sun), so one ‘simple’ method one could use to derive the apparent temperature of a planet is by taking the TSI (“solar constant”) at the planet’s (or moon’s) particular distance from the Sun, and multiply it with 1 – α, its estimated global (Bond) albedo, a number that’s always <1, finally dividing by 4 to cover the whole spherical surface. Determining the average global albedo is clearly the main challenge when going by this method. The most common value provided for Venus is 0.75, for Earth 0.296.
But does the resulting value really say anything about the actual planetary temperature? If the planet absorbs a mean radiant flux (net SW) below its ToA, then how this flux affects the overall system temperature very much depends on the system’s total bulk heat capacity. If it is large, the flux will have little effect, if it’s small, the flux will have a bigger effect.
And so finally we have reached the stage where we will explain why the atmospheric insulating effect is inherently a ‘massive’ one and not a ‘radiative’ one. The answer is quite intriguing, maybe even a bit surprising to some, the solution rather subtle in many respects. I have settled for two posts, but could probably have written several, considering the bewildering amount of different aspects in some way or other pertaining to this whole issue.
I hope you can bear with me on what might seem like a rather repetitive style of writing in this first post. I have only done so in a humble attempt to punch through the basic idea presented, which might at first come off as a novel or unfamiliar one to most people.
The second post is more lengthy, gradually winding its way towards the final resolution. When reading it, always bear this first one in mind.
I will most likely at some point publish a (strongly) condensed version of these posts. However, their content and interconnected nature might take time to digest.
OK. Let’s begin …
TO NECESSITATE > TO ENABLE > TO CAUSE
In his ‘Physics Today’ feature article of January 2011, “Infrared radiation and planetary temperature”, Raymond T. Pierrehumbert stated the following about the proposed rGHE surface warming mechanism:
“An atmospheric greenhouse gas enables a planet to radiate at a temperature lower than the ground’s, if there is cold air aloft. It therefore causes the surface temperature in balance with a given amount of absorbed solar radiation to be higher than would be the case if the atmosphere were transparent to IR. Adding more greenhouse gas to the atmosphere makes higher, more tenuous, formerly transparent portions of the atmosphere opaque to IR and thus increases the difference between the ground temperature and the radiating temperature. The result, once the system comes into equilibrium, is surface warming.”
This is a most interesting quote, one that reveals a central misconception lying at the heart of the rGHE and AGW hypotheses. In order to get his message across, Pierrehumbert employs two quite specific terms – “enable” and “cause” – as if they were almost interchangeable. They are not. Read the two highlighted sentences once more. “An atmospheric ‘GHG’ enables a planet to radiate at a temperature lower than the ground’s, if there is cold air aloft. It therefore causes the surface temperature to be higher than would be the case if the atmosphere were transparent to IR.”
How did he get from “enables” to “therefore causes”?
He seems to forget that there’s crucially a third term that needs to be included before this chain is complete and one is able to see the whole picture, and that term is “necessitate”.
Something necessitates an effect, but cannot cause the effect before it is enabled to do so.
I will explain … Continue reading
If there were no atmosphere on top of our solar-heated terrestrial surface, then Earth’s mean global surface temperature would likely be about 80 degrees lower than what it actually is (209 rather than 289K). And this would be in spite of the fact that in this case the solar heat input to the global surface would be almost 80% larger on average (296 rather than 165 W/m2).
Much of this cooling of the mean would simply come as a result of greatly amplified temperature swings between day and night and between the seasons. The larger the planetary surface temperature amplitudes in space and time, the lower the mean global planetary surface temperature needs to be to maintain dynamic radiative equilibrium with the Sun. This is why the Moon is so cold.
So we need to get this straight: The Earth’s surface would be a much colder place without an atmosphere on top of it. Even with much more solar heat absorbed. There is no escaping this. The lunar surface is about 90K colder than ours, on average.
SO WHAT DOES OUR ATMOSPHERE DO?
The short answer: It insulates the solar-heated surface.
Well, so how does it do this?
Mainly in four ways, three of which concern suppressing the effectiveness of convective cooling of the surface at a certain temperature.
Why is this important? Why convective cooling?
Consider a hypothetical single-room house. Continue reading
Lately there’s been a bit of back-and-forth discussion going on on the so-called ‘Gravito-Thermal Effect’ (GTE) at a few notable climate blogs, like The Hockey Schtick, Tallbloke’s Talkshop, Clive Best and even Judith Curry’s Climate Etc. (in fact, this is where the lengthiest discussion thread on the subject is to be found).
To me the whole thing appears to arise from a fundamental misunderstanding of the adiabatic process (see the end of the post).
Something called the ‘Loschmidt Effect’, after a proposal in the 1870s by the Austrian scientist Josef Loschmidt, seems to lie at the heart of the GTE argument. Tallbloke brought it out from relative obscurity in a post in early 2012. A quote from a textbook describes the proposed effect as follows: Continue reading