I’ve come across evidence that could be interpreted as showing that there is some kind of mechanism in the atmosphere that compensates for changes in greenhouse gas composition so that changes in CO2 levels would be insignificant. This is in no sense finished work, but is instead a very brief outline to make others aware of this potential.In a paper by Ned Nikolov & Karl Zeller (“New Insights on the Physical Nature of the Atmospheric Greenhouse Effect Deduced from an Empirical Planetary Temperature Model“) they show a close relationship between planetary pressure (and a few other things) and greenhouse temperature as shown below:
This appears to show a very close relationship, and this is not surprising because greenhouse temperature is effectively the temperature from the “average emission height” x the lapse rate. And the average emission height is itself largely dependant on where the “top of the atmosphere is” (a very crude model of this is shown below but for a more detailed explanation see The Greenhouse effect)
This means, there is no surprise that the greenhouse temperature for other planets is related to pressure. What however is surprising is Nikolov & Zeller’s contention that there is no apparent effect from greenhouse gas concentration.
For obvious reasons I’ve questioned this, not least because the scale of the Greenhouse effect is so small relative to pressure effects that it could easily get lost. To explain, it’s well known the greenhouse temperature is the log of greenhouse concentration. But when you then plot this log on a log graph, you have a log of a log which is then a very small quantity. To put it in perspective I very quickly estimated the change from 1pp to 1,000,000ppm (i.e. 100%) CO2 and drew this as below:
So, even if the earth’s atmosphere were 100% CO2, by a slight redrawing of the line, the earth could be said to be “fitting” to the curve. Thus, I felt it was safe to argue that they were not seeing any affect of greenhouse gas composition, because it was “lost in the noise”. This still meant that changing greenhouse gas composition would effectively lower or raise the average height of emissions and change temperature, but compared to the overall greenhouse effect (determined by pressure) this change would be small.
This argument however was based on my assumption that the temperature profile through the atmosphere was constant. And that a change in height would mean moving up or down the temperature profile to change the average temperature of emitted IR, and therefore average heat lost. (Lower temperatures mean IR comes from cooler atoms so less heat is emitted).
However… I then came across this paper:
“Common 0.1 bar tropopause in thick atmospheres set by pressure-dependent infrared transparency” (Robinson & Catling 2014 ). And this graph say it all:
This shows that the temperature profile of atmospheres as diverse as Earth and Jupiter seem to be determined in the lowest parts by the lapse rate (which fit with Nikolov & Zeller) and then there is an effective “top of the atmosphere” at 0.1bar – a top which seems to be present in almost all atmospheres including some with very different greenhouse gas compositions.
This appears to show that rather than greenhouse gases acting on a static temperature profile as I was using as a model, that the temperature profile may adapt so that the behaviour is largely independent of the exact composition (As Nikolov & Zeller) have argued. If this is right it’s a very important observation and could completely change our understanding of how greenhouse gases may affect the earth’s atmosphere.
In layman’s terms this could “blow a hole in the greenhouse gas theory” – at least in terms of how atmosphere respond to changes in gas composition. Because if the temperature profile changes so that we have in effect (largely) the same atmosphere irrespective of the exact composition, then changes in CO2 may have almost no impact on the climate (except to grow big fruit etc.).
The big caveat here is that again the above graph is a log with respect to pressure (i.e. height); and the atmospheres are far from being an exact fit with each other and so you could hide an awful lot of greenhouse gas change in there. However, this 0.1bar is close or at the boundary between the over-turning troposphere and the static stratosphere. And it is not impossible that the dynamics of the lower atmosphere in some way compensate and tend to reduce the impact of any change in the actual greenhouse gas composition. In othe words, the overturning may in some way actively compensate for changes in greenhouse composition so that we end up with largely the same greenhouse temperature.
“Greenhouse gases are important in planetary atmospherics because of their affect on planetary atmospheric temperature by the emission of infrared. As such any increase in greenhouse gas temperature within a fixed temperature profile would tend to cause warming within regions of the atmosphere where temperatures lower with height because the average radiative temperature would reduce. However … according to work by Robinson and Catling (2014) the temperature profile of planetary atmospheres is dependant on pressure such that there is an effective “top of the atmosphere” at around 0.1bar, corresponding to the effective top of bulk infra-red heat emissions, which on earth corresponds to the top of our troposphere or convective region. Likewise the lower atmosphere on other planet(oids?) show similar convective regions(I think that is right?).
This indicates that rather than greenhouse gas composition acting in isolation, the planetary atmosphere appears to adjust such that the dominant factor affecting surface temperature is not the gas composition, but instead the height of this 0.1bar “top of atmosphere”. The greenhouse gas temperature is then largely determined by the height of the 0.1bar pressure and the lapse rate (which is determined by gravity & specific heat capacity of gases).