This post from some years ago was in response to David A. elaborating on his thinking and questions on this topic. There is much uncertain and unknown about the functioning of our climate system. I listen when a seasoned expert such as John Christy says:
“The reason there is so much contention regarding “global warming” is relatively simple to understand: In climate change science we basically cannot prove anything about how the climate will change as a result of adding extra greenhouse gases to the atmosphere. So we are left to argue about unprovable claims.”
See Christy’s Data-Based Climate Science
So everyone is theorizing and wondering if and when the best theory will win–that is, become the new conventional wisdom. According to Christy, the science is far from settled, and he has examined the datasets extensively, having built some of them himself.
I have also learned a lot from Nullius in Verba, who is one of best explaining these things to us laymen. For example, he comments:
“It would be slightly more accurate to say that the lapse rate is the vertical temperature gradient at which convection switches off and therefore stops cooling the surface.
The sun warms the surface, but the heat escapes very quickly by convection so the build-up of heat near the surface is limited. In an incompressible atmosphere, it would *all* escape, and you’d get no surface warming. But because air is compressible, and because gases warm up when they’re compressed and cool down when allowed to expand, air circulating vertically by convection will warm and cool at a certain rate due to the changing atmospheric pressure. Air cools as it rises and expands, and warms as it descends and is compressed. This warming/cooling effect means that hot air no longer rises when it would cool faster from expansion than the surrounding air. Cold air can sit on top of warm air and be stable. The adiabatic lapse rate is why the tops of mountains are colder than their bottoms.
It’s a bit like the way a pot of boiling water sticks at a temperature of 100 C. If you turn the gas up, the water boils more vigorously, carrying more energy off as steam, which balances the extra energy supplied and keeps the temperature still at exactly 100 C. The rate at which heat escapes is very non-linear – extremely fast for temperatures above the threshold, extremely slow for temperatures below it. So long as the system is driven hard enough, it will get driven up against the non-linear limit and held there. The lapse rate does the same thing, except that instead of fixing the temperature, it fixes its gradient so you get a rigid slope that can freely float up and down in level.
The temperature at the average altitude of emission to space converges on the temperature that radiates the same energy the Earth absorbs. All levels above and below it are held in a fixed relationship to it by the lapse rate. The temperature at any other level is the temperature at the emission altitude plus the lapse rate times the difference in heights. Hence, the temperature at the surface differs by the lapse rate times the average height of emissions to space.”
“It’s interesting to consider what would happen if you had a strongly absorbing greenhouse material but a zero lapse rate. You’d get lots of backradiation, but no greenhouse warming. By marvelous happenstance we do have such a physical situation in the oceans. Water absorbs all thermal radiation within about 20 microns, making it something like 20,000 times more powerful a greenhouse material than the atmosphere. It’s a (relatively) easy calculation to show that if radiation was the only way heat could be transported, as the backradiation argument assumes, the temperature a metre down would be several thousand degrees! But water is almost incompressible, having a lapse rate of around 0.1 C/km, and so convection nullifies it entirely. Fortunate, eh? . . .”
“The direction of net energy flow is determined only by the difference in temperatures, not the amount of stuff. If you have a big body at a cold temperature next to a small body at a very hot temperature, the cold body might be emitting more heat overall because of its bigger surface area, but the net flow is still from the hot body to the cold. Most of the heat emitted from the big cold body doesn’t hit the small body, because it’s so small. Only the temperature matters.
The way this is arranged varies depending on the configuration, but it always happens. People have had a lot of fun over the years trying to construct exotic arrangements of mirrors and radiators and insulators and heat engines to try to break the rule, but nobody has succeeded yet. The second law of thermodynamics is one of the most thoroughly challenged and tested of all the laws of physics. I do encourage people to try though. The prize on offer is a perpetual motion machine to the lucky winner who defeats it!”
hat tip to Homer Simpson
Nullius in Verba holds forth here:
David, I am not a fan of thought experiments about hypothetical worlds with or without CO2. I have read too many threads that go around in circles until everyone turns into wheels.
I do like what E.M. Smith (Chiefio) said sometime ago:
“It is peculiar that everyone is so taken in by the whole notion of the so-called ’radiative greenhouse effect’ being such an ingrained necessity, such a self-evident, requisite part, as it were, of our atmosphere’s inner workings. The ’truth’ and the ’reality’ of the effect is completely taken for granted, a priori. And yet, the actual effect is still only a theoretical construct.
In fact, when looking at the real Earth system, it’s quite evident that this effect is not what’s setting the surface temperature of our planet.
The whole thing can be stated in a simple, yet accurate manner.
The Earth, a rocky sphere at a distance from the Sun of ~149.6 million kilometers, where the Solar irradiance comes in at 1361.7 W/m2, with a mean global albedo, mostly from clouds, of 0.3 and with an atmosphere surrounding it containing a gaseous mass held in place by the planet’s gravity, producing a surface pressure of ~1013 mb, with an ocean of H2O covering 71% of its surface and with a rotation time around its own axis of ~24h, boasts an average global surface temperature of +15°C (288K).
Why this specific temperature? Because, with an atmosphere weighing down upon us with the particular pressure that ours exerts, this is the temperature level the surface has to reach and stay at for the global convectional engine to be able to pull enough heat away fast enough from it to be able to balance the particular averaged out energy input from the Sun that we experience.
It’s that simple.”
Update 1 May 5,2015
David, an additional point of some importance: There is empirical support for the lapse rate existing independent of IR activity.
Global warmists share an assumption that CO2 raises the effective radiating altitude, thereby warming the troposphere and the surface. Now this notion can be found in textbooks and indeed operates in all the climate models. Yet there is no empirical evidence supporting it. What data there is (radiosonde balloon readings) detects no effect from IR active gases upon the temperature profile in the atmosphere.
“It can be seen from the infra-red cooling model of Figure 19 that the greenhouse effect theory predicts a strong influence from the greenhouse gases on the barometric temperature profile. Moreover, the modeled net effect of the greenhouse gases on infra-red cooling varies substantially over the entire atmospheric profile.
However, when we analysed the barometric temperature profiles of the radiosondes in this paper, we were unable to detect any influence from greenhouse gases. Instead, the profiles were very well described by the thermodynamic properties of the main atmospheric gases, i.e., N 2 and O 2 , in a gravitational field.”
While water vapour is a greenhouse gas, the effects of water vapour on the temperature profile did not appear to be related to its radiative properties, but rather its different molecular structure and the latent heat released/gained by water in its gas/liquid/solid phase changes.
For this reason, our results suggest that the magnitude of the greenhouse effect is very small, perhaps negligible. At any rate, its magnitude appears to be too small to be detected from the archived radiosonde data.” Pg. 18 of referenced research paper
Open Peer Rev. J., 2014; 19 (Atm. Sci.), Ver. 0.1. http://oprj.net/articles/atmospheric-science/19 page 18 of 28
In summary David, it is observed and accepted by all that there is a ~33C difference between the temperature at the surface and at the effective radiating level (the tropopause, where convection stops). Warmists attribute that increase in temperature to the IR activity of CO2.
Others, including me, contend that it is the mass of the atmosphere, mostly O2 and N2 delaying the loss of heat from the surface until IR active gases are able to cool the planet effectively without obstruction. That retention of heat in the atmosphere is measurable in the lapse rate. And 90% of the IR activity is due to H2O, especially in the lower troposphere.