I have enjoyed reading some older explanations of "signatures" of global warming as related to Greenhouse Gas increases. "Signatures" of anthropogenic caused warming or in other words warming that could only be caused by CO2 equivalent gases would be nice. That would give the world a real test of Climate Change, as in human related climate change, so we could move on.
Stratospheric Cooling with Tropospheric Warming (SCTW) should be a signature of well mixed greenhouse gases. The well mixed CO2 equivalent gases would basically blanket the lower troposphere reducing the flow of energy through the stratosphere. Realclimate had a hilarious post on this subject some time ago. There is also the Tropical Tropsphere Hot Spot (TTHS) which is a "signature" of GHG warming but it is not a "unique" signature. If warming is caused by GHGs then there would be a TTHS, but if the warming is caused by something else then there would still be a TTHS. If there is no TTHS then that would be evidence of no warming, at least in the tropics.
The fact of the matter is neither of these are "unique" signatures. If there is any warming, since there are already GHGs in the atmosphere, the troposphere would warm and the stratosphere cool. However, you can have variations in stratospheric ozone and water vapor that can change the temperature of the stratosphere that would change the troposphere temperature. You can also have changes in ocean surface temperature which would cause changes in troposphere temperatures and moisture with water being another of those greenhouse gases. If the atmosphere were perfectly stable, then you could easily figure out what caused what, but thanks to dynamics it is not so easy.
You can get a better idea by assuming CO2 equivalent gases have a 1C per 3.7 Wm-2 impact on lower troposphere temperatures and then tease out an estimate for lower stratospheric cooling related to that change. Since any warming would also have an impact, you would need to estimate the equivalent change in energy flux due to "other" warming. The two are very similar making attribution difficult.
Iris Effect where increased SST would reduce tropical cirrus cloud impact offsetting a great deal of the tropical warming. That basically would erase most of the TTHS. I am not particularly sure that Dr. Lindzen's Iris analogy is all that great, but changes in deep convection related to increased SST would change cloud cover distribution, stratospheric water vapor, stratospheric ozone and vary the Brewer-Dobson circulation. Notice that the tropical SST is about 0.5 C warmer than "average" and about the same as in the early 1940s.
My focus has been of the Brewer-Dobson circulation changes which impact the planets real heat sinks, the poles, by changing pole ward advection of stratospheric water vapor and ozone which changes the intensity of Sudden Stratospheric Warming Events and "Arctic Winter Warming".
That is a big mouthful of not very often discussed climate mechanisms. A simple way to think of this is that variation is inefficient and stable is efficient as far as engines performance goes. We have polar winter vortexes that if stable tend to reduce heat loss and when unstable allow more heat loss. In the news the "POLAR VORTEX" is the new villain in the winters are colder that usual explanation toolbox. What they are referring to is a break down in the vortex related to large changes in the high northern jet stream. A stable jet stream/polar vortex would related to a milder winter, a milder winter allows the surface to retain more energy which is really what global warming is about, retaining more energy. If there isn't an increase in energy retention then no TTHP or SCTW.
Well if you talk to any true believer in AGW/CLIMATE CHANGE, you know that the oceans are gaining energy which is energy retention, therefore we should have the signature TTHS and SCTW. Well, that isn't a given. Right now the Southern Hemisphere is doing the heat uptake thing and the Northern Hemisphere is doing the heat rejection thing. A few years I estimated the heat rejected by a large SSW event to be of the same order of magnitude as the energy imbalance. There are now papers that tend to confirm that my estimate was pretty good.
There are several interesting things about the hemispheric imbalance. The current stage of the solar precessional cycle is a biggie. There is more solar energy available in the southern hemisphere summer along with a larger percentage of ocean surface area. With out any consideration of change in GHG concentration, there should be more ocean heat uptake in the southern hemisphere. In order for there to be an "equilibrium" the northern hemisphere heat rejection would have to increase OR yep that is a big or, more energy must be stored in the form of glacial ice. The part after the big or would mean Earth is primed for a shift into some degree of glacial period. Fortunately or not depending on your point of view, glacial mass requires land area and there aren't many people/nations willing to donate to the glacial cause. With the current state of technology it is much easier to melt snow than it is to grow glacial mass.
This low probability of glacial mass growth leads to the second interesting point. Since land area required for glacial growth isn't really symmetrical any significant growth would produce an orbital wobble. A wobble is variability which implies inefficiency for our heat engine which could trigger a more rapid rate of heat rejection should someone donate enough land for the global glacier international park.
This potential wobble influence leads to the third interesting point, where is there any "equilibrium" involved in this climate problem? We have shifting psuedo-steady states with pretty unpredictable time frames. However, if you talk with the true believers again there is no evidence of past variability or any reason to consider hemispheric imbalances. Besides that would complicate a perfectly "obvious" theory with several "signatures" that will likely pop up any decade now, provided of course we have an extremely stable atmosphere that allows us to use "equilibrium" assumptions validly.
Kind of reminds me of those Boy's Life cartoons. Instead of circular logic it is more like a circle jerk.