According to robbservations.blogspot, which may not be the most reliable of sources. Also, it is from 2009, so not fresh either; but someone asked about it so I thought I’d reply. Context:
The theory of anthropogenic Global Warming rests on the so-called “idealized greenhouse model”. This Wikipedia link (Idealized greenhouse model) by a “climate researcher” and global warming advocate presents the core theory, and offers excellent insight into the problems of the fundamental premise of global warming, though that is not his intent.
It might be fairer to look at the version from December 2009, but I don’t think it has changed greatly since. First off, this was aimed at me, and although I had the last edit at that point, I didn’t create it or even add most of the content; that was User:Incredio, inactive since 2013 (no-one from the outside understands wiki). But it is fair to say that AGW “rests on” the ideas in that page. If you’re interested, I have a 2014 post grinding through the details1; R. W. Wood: Note on the Theory of the Greenhouse is also worth a read. Continuing:
The principle cause of any greenhouse effect is that of a one-way thermal blanket — one which passes most wavelengths of the sun’s energy, yet insulates infrared from the other direction — energy radiated back up from the heated ground below. This is how a real greenhouse works — most energy is in the visible bands, and it passes through glass panes to heat the ground. The ground heats up and re-radiates back in the infrared wavelengths for which the glass is opaque.
Calling it a “thermal blanket” is unhelpful but commonplace. Asserting that this is how a real greenhouse works is wrong: real greenhouses work by suppressing convection; see the R. W. Wood link, for example. But although this indicates that Rob’s observations may not be entirely reliable, they aren’t yet fatal to whatever he is trying to say, because whatever heats a real greenhouse is irrelevant. Continue, Rob, but do try to focus.
The premise of global warming is that certain gases in the upper atmosphere — principally water, CO2 and Methane — are disposed to absorb infrared. In the simplified analysis, they absorb ALL the infrared coming up from the Earth. The absorbed radiation eventually is re-emitted, but isotropically — in all directions. Consequently, 50% of the absorbed infrared is radiated up toward space, and the other 50% is “trapped” — radiated back toward the Earth. This causes the Earth’s surface temperature to rise till a state of equilibrium is reached, where the rate of re-radiation into space increases till it balances the energy absorbed from the Earth below.
He’s doing pretty well with this, so on:
In essence the Greenhouse hypothesis is developed like this:
- The atmosphere is transparent to all solar energy (not true);
- The Earth absorbs all incident solar energy without reflection (not true);
- The ground and heated air above the ground re-radiate the solar spectrum with a blackbody temperature shifted downward toward the infrared spectrum (true);
- Certain atmospheric gases make the atmosphere mostly opaque to all this ground radiation (false);
- The ground radiation is absorbed in thermally broadened atomic lines of certain gases concentrated in an infinitely thin atmospheric layer (reasonable);
- The absorbed ground radiation is radiated isotropically, so half of it goes back toward Earth, raising the surface temperature (reasonable).
1 is indeed the usual approximation. It isn’t true – and indeed, you don’t even need to assume it in the idealised model – but you may as well assume it because it makes the basic principles clearer, simplifies the maths, and doesn’t lose you any precision because so many other things are imprecise anyway. Abstraction is important when trying to understand the underlying principles of things; don’t get bogged down in details until you need to. All that is actually necessary for the model to work is this regard is that the atmosphere is transparent to most or just to some of the incoming solar; and this is undoubtedly true.
2 is also a usual approximation, and just like 1 is a useful simplification. Just like 1 it isn’t necessary; all that is necessary is that some is absorbed at the surface; or even more generally that the atmosphere is heated from below. 3 we’re agreed on, although he’s garbled it slightly. 4 is again an approximation, depends on wavelength; again, see 1. 5 and 6 will do. OK, so, great: where’s the great Head-Shooting going to happen?
There’s then a couple of paragraphs worrying about exactly which bands CO2 (and methane) absorb in, and how these interact with water vapour absorption. Then:
The Wiki article calculates and asserts that if the atmosphere absorbs 78% of all radiation from the ground, it predicts the average global temperature of 288K to within 0.3 degree. Remarkable. I’m in awe.
Rob is looking for something to attack, so attacks some spurious precision, but this is unimportant. The entire model is an approximation. You can get lots of different numbers out of it, depending on exactly what constants you put into it. One of the more important is Earth’s albedo to incoming solar shortwave radiation, which obviously you can’t calculate from this model and need to specify. Then:
calculates from this an average global temperature increase [due to CO2 doubling] of 1.2K in the absence of water vapor (clouds)
Oh. Oh dear. We’re now onto looking at GW with this simple model; but we’ve failed to identify any important flaws in the model, other than that it’s an approximation, but we already knew that. This is disappointing, but let’s push on; perhaps there’s some flaw in how it handles GW that’s of interest.
some hand-waving assumptions that says higher surface temperature increases water vapor with positive feedback (because water absorbs longwave radiation up from the Earth), so the real temperature rise from a doubling of CO2 will be 2.4K — even more dire.
Even more disappointing. Positive feedback on water vapour (only a colonial would write vapor) is indeed a commonplace, but it is no part of the idealised model. If you’re interested in how the real world will respond to CO2 increases then you’ll need to think about how water vapour will change; but that’s a different matter. Let’s plough on a bit further:
higher water vapor (from evaporation of the Earth’s oceans and lakes) means more clouds, and more clouds means more sunlight is reflected back into space. This will reduce the amount of solar energy absorbed by the Earth.
Indeed. How cloud feedback affects the overall feedback is an interesting and complex research item; but it forms no part of the idealised model, because it can’t. The idealised model is too simple to contain clouds, except subtly folded into things like the overall albedo. You can, if you like, specify changes in cloud albedo into the model; but you can’t possibly deduce them from the model. So Rob has wandered well off his subject onto other matters, without as far as I can see making the slightest dent on the idealised model. Is there more? Alas there is:
To say that this is an egregious omission is being inadequate. It would be like saying ignoring the Nazi conquest of Poland…
Godwin!. Rob loses. There’s another post which (I didn’t bother read the details, I hope you understand) appears to be doing the usual there’s-a-lot-of-overlap-with-water-vapour-so-CO2-doesn’t-do-anything stuff. What happens is this: people read the simple explanations (like the wiki one) and perhaps other stuff from govt websites; and then from whatever source discover the overlap stuff, and have the arrogance to assume that this is a major hole in the theory that people haven’t thought about, just because they haven’t seen it written into the popular expositions. Weird. Anyway, see stuff like
A Saturated Gassy Argument if you want the std.reply.
1. Sigh: some of the pix from someone else’s site have rotted; never mind, they’re only the take-down.