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How we know the greenhouse effect isn't saturated

Posted on 13 February 2014 by Glenn Tamblyn, jg

This is the Basic rebuttal to the myth 'CO2 effect is saturated'

The mistaken idea that the Greenhouse Effect is 'saturated', that adding more CO2 will have virtually no effect, is based on a simple misunderstanding of how the Greenhouse Effect works.

The myth goes something like this:

  • CO2 absorbs nearly all the Infrared (heat) radiation leaving the Earth's surface that it can absorb. True!
  • Therefore adding more CO2 won't absorb much more IR radiation at the surface. True!
  • Therefore adding more CO2 can't cause more warming. FALSE!!!

Here's why; it ignores the very simplest arithmetic.

If the air is only absorbing heat from the surface then the air should just keep getting hotter and hotter. By now the Earth should be a cinder from all that absorbed heat. But not too surprisingly, it isn't! What are we missing?

The air doesn't just absorb heat, it also loses it as well! The atmosphere isn't just absorbing IR Radiation (heat) from the surface. It is also radiating IR Radiation (heat) to Space. If these two heat flows are in balance, the atmosphere doesn't warm or cool - it stays the same.

Lets think about a simple analogy:

We have a water tank. A pump is adding water to the tank at, perhaps, 100 litres per minute. And an outlet pipe is letting water drain out of the tank at 100 litres per minute. What is happening to the water level in the tank? It is remaining steady because the flows into and out of the tank are the same. In our analogy the pump adding water is the absorption of heat by the atmosphere; the water flowing from the outlet pipe is the heat being radiated out to space. And the volume of water inside the tank is the amount of heat in the atmosphere.

What might we do to increase the water level in the tank?

We might increase the speed of the pump that is adding water to the tank. That would raise the water level. But if the pump is already running at nearly its top speed, I can't add water any faster. That would fit the 'It's Saturated' claim: the pump can't run much faster just as the atmosphere can't absorb the Sun's heat any faster

But what if we restricted the outlet, so that it was harder for water to get out of the tank? The same amount of water is flowing in but less is flowing out. So the water level in the tank will rise. We can change the water level in our tank without changing how much water is flowing in, by changing how much water is flowing out.

water tank

Similarly we can change how much heat there is in the atmosphere by restricting how much heat leaves the atmosphere rather than by increasing how much is being absorbed by the atmosphere.

This is how the Greenhouse Effect works. The Greenhouse gases such as carbon dioxide and water vapour absorb most of the heat radiation leaving the Earth's surface. Then their concentration determines how much heat escapes from the top of the atmosphere to space. It is the change in what happens at the top of the atmosphere that matters, not what happens down here near the surface.

So how does changing the concentration of a Greenhouse gas change how much heat escapes from the upper atmosphere? As we climb higher in the atmosphere the air gets thinner. There is less of all gases, including the greenhouse gases. Eventually the air becomes thin enough that any heat radiated by the air can escape all the way to Space. How much heat escapes to space from this altitude then depends on how cold the air is at that height. The colder the air, the less heat it radiates.

(OK, I'm Australian so this image appeals to me)

So if we add more greenhouse gases the air needs to be thinner before heat radiation is able to escape to space. So this can only happen higher in the atmosphere. Where it is colder. So the amount of heat escaping is reduced.

By adding greenhouse gases, we force the radiation to space to come from higher, colder air, reducing the flow of radiation to space. And there is still a lot of scope for more greenhouse gases to push 'the action' higher and higher, into colder and colder air, restricting the rate of radiation to space even further.

The Greenhouse Effect isn't even remotely Saturated. Myth Busted!

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Comments 101 to 106 out of 106:

  1. ConcernedCitizen - Are you asking about 'latitude' or 'altitude'? The latter would make sense given previous exchanges, the former not so much. 

    CO2 is indeed radiating from a higher (and hence cooler) altitude than it did before the Industrial Revolution, the spectra of emissions at the top of the atmosphere (TOA) has been observed by satellites to have changed just as expected, the tropopause has risen several hundred meters, etc etc etc. And the explanation for it is indeed part of the OP, not to mention many of the comments made in this thread and others.

    Therefore your comment above is simply nonsense, and the article continues to hold true to both theory and data. You've made a series of such comments, and have been pointed to the literature and the physics demonstrating why you are incorrect. I would suggest you take advantage of such pointers, and educate yourself. 

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  2. I am an avid defender of Climate Science. And debate the merits of the science with vigor.  Today a friend asked me the following question:  "H2O vapor is an even greater IR absorber but nobody's claiming H2O is a "pollutant" or clouds need to be eradicated. Are you aware CO2's IR absorption decreases as its concentration increases?"

    I let my friend know this was a myth.  Referencing the climate denial 101 video posted here and on Youtube here.     

    He says I am wrong and sites this website here.

    I don't have the math skills or an understanding of the physics to refute my friends information.  Can you please help me?  Thank you in advance!  Ken

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  3. Kentobin64  @  #102 , you are wasting your time looking at the clivebest blog.  ( Indeed, you are wasting your time discussing climate matters with your science-denier "friend".   Denialists are impervious to facts and impervious to rational logic  — it all derives from their determined use of Motivated Reasoning, to support their strange subconscious bias.   Ken, you should only bother to engage with them, if you have some idle time and desire that sort of sport. )

    Clive Best is/was a physicist, I gather, and it shows in his display of various formulae and graphic charts.   He has some "overly simplistic" ideas about the effects of cloud cover.   He says that H2O vapor has 5 times the greenhouse effect of CO2 . . . while elsewhere promoting the idea that (water) gives a negative feedback on global warming and so there cannot be much actual AGW from higher CO2 levels.   He uses the paradigm : Warmer oceans --> more water vapour --> more rain --> reduced water vapour --> reduced greenhouse effect --> negative feedback on warming.   Go figure these contradictions, if you can !

    Ken , check out the respected website ATTP [And Then There's Physics] where you will find at least a couple of articles about CliveBest (and commentary column discussions including posts by Best himself).   Also mentioning Best's use of 35-year-old climate models.

    ~ Possibly the most apt comment at ATTP was by "Dikranmarsupial" (a frequent poster here at SkS in earlier years) :-

    "The frequency with which climate blogs present overly simplistic analysis, without first bothering to do their homework and find out what scientists working on the problem have actually done already, is profoundly disappointing."

    In short, Ken, the good Dr Best is just one more example of those many dozens of blog writers [not climate scientific paper authors] who believe that they themselves are right and all the tens of thousands of actual climate scientists are quite wrong.   You can call them "eccentric" or "crackpot" or "crazy" . . . or whatever . . . but essentially they are deluded or are actively deluding themselves, owing to their strange psychological makeup.

    Ken , don't waste your own valuable time on them — however many formulae or graphs they wave in the air.  They talk big but turn "a willful blind eye" on the reality around them.

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  4. kentobin64 @102,

    The Clive Best post you (or your friend) linked to is mathematically not un-reasonable but when it sets out the implications of that mathematics it persents wholly unreasonable language that is, in the words of Clive Best, "completely wrong!".

    Firstly, the Clive Best post itself is addressing only CO2 forcing and itself has nothing to say on H2O.

    Secondly, using a very simple model Best obtains a value for CO2 warming-without-feedbacks which is actually 50% higher that the present scientifically-accepted value (1.5ºC per doubling rather than 1.0ºC).

    Thirdly, the post digresses and uses exceedingly poor language within its conclusions. Thus, it is well understood that CO2 forcing is constant with each doubling of concentration. This means that an extra Gt(CO2) added to a 550ppm atmosphere will provide only half the forcing of an extra Gt(C)2) added to a 275ppm atmosphere. Best describes this logarithmic CO2 effect as "Extra CO2 causes a declining radiative forcing with increasing concentration", a description which is not just exceedingly poorly phrased, it is actually plain wrong. It is not "declining radiative forcing" he describes but "declining additional radiative forcing." And it is this Clive Best error which your friend is wielding.

    Fourthly, the throw-away comments on H2O warming appear to be simply denialist hand-waving. The point with H2O warming is that while H2O does warm the planet and does provide the largest contribution to the greenhouse effect, H2O requires the climate to be 'primed' by other warming agents to achieve that H2O warming. Essentially H2O is not a long-lived GHG. Thus, half of that Gt(C) of CO2 added (as the Airborne Fraction) to the atmosphere will still be there a thousand years hence and indeed ten-thousand years hence. The continued level of atmospheric CO2 is not particularly dependent on temperature and so not dependent on the warming provided by other GHGs etc. Conversely, an extra Gt(H2O) would be back in the rivers/oceans in days and into ice bergs within decades unless some other GHG is present to keep the atmosphere warm wich allows water to evapourate up into the atmosphere and so keep it re-charged with insulating H2O.

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  5. There is currently a good discussion of the spectroscopic basis of the GH effect at Rabett Run, goes down several threads and shows what happens on the "wings." Real Climate also explored this in details a number of years ago, might be possible to find searching the site. Clive Best is run-of-the-mill disinformation.

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  6. Further to my assertions @104 concerning the requirement for CO2 warming to maintain H2O in the atmosphere, a paper looking at the impact of the zeroing of LL GHGs has featured in the evidence resulting from Judge Alsup's call for an AGW tutorial. The evidence in question is from Monckton, Soon et al (its contorted thesis of high humour content) which cited Lacis et al (2010) 'Atmospheric CO2: Principal Control Knob Governing Earth’s Temperature'. While the description @104 was bravely considering a zero-CO2 steady-state, all LL GHG are something like 75% CO2 but those other LL GHG will be much reduced in a colder world so there won't be a great difference between zeroing CO2 and zeroing all LL GHG.

    Lacis et al show by zeroing all LL GHG the world begins to cool at 4ºC/year thus becoming colder than ice-ages in 18 months. The cooling continues but at a far lower rate after the first seven years, and temperature will still be sinking beyond the 50 years of the modellng. At 50 years, average annual temperatures at the equator remain just above freezing allowing atmospheric H2O at 20% of pre-industrial values but 50% of the oceans are frozen over.

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