That'll resonate with Mason Williams.

"I have been away a while, so this might seem to be a bit old-hat for the more regular readers, but I felt I needed to get it off my chest, so to speak"

That's OK. Going through it again often brings out new nuances and approaches. Science is about constantly challenging even well-established results. And if you're honestly interested in learning, it's a joy to help.

"It has been witnessed in laboratory experiments, where the contents of a closed container enriched with CO2 rises in temperature faster than a control closed container of ordinary air when both have been subjected to infra-red radiation"

If you're talking about those "CO2 in a jar" experiments, they're bogus. There's not nearly enough depth of CO2 to make a detectable difference, and the thermal contrast between the gas and the general surroundings is too small. The reason for the different temperature is that gases have different physical properties for convection efficiency - density, viscosity, heat capacity, etc. It's just that CO2 is less effective at convecting heat away than air.

If you want to do a 'greenhouse in a jar' experiment, then the best example I know of is a solar pond. Liquid water is about 20,000 times stronger a 'greenhouse' agent than the atmosphere, and if you prevent convection by using a salt-water gradient, you can get the bottom of the pond up to about 90 C in the sunlight. Some people in hot countries use them to generate power.

The experiments to show CO2 is a greenhouse gas consist of using a tube several metres long full of gas and shining infrared light down it - detectors at the other end can see whether it gets through. The total atmospheric air column contains the equivalent of about 4 metres depth of pure CO2, so you need at least this much to get a comparable effect.

"CO2 exists as a minute proportion of the atmosphere"

The atmosphere is also very thick. It's equivalent to about 10 km thickness at its full surface pressure.

To make the idea more intuitively reasonable, I usually use an analogy with a glass of milk. Skimmed milk is about 0.3% fat suspended in water, or about 3000 ppm. If you dilute it ten-fold, you're down to around 300 ppm comparable with atmospheric CO2. Now imagine a 10 km deep lake of 10-fold diluted semi-skimmed milk. Would you expect to be able to see through it?

The physics of scattering in milk is different, but it illustrates that even small concentrations can be very visible.

To explain how, consider a 1x1x1 metre box containing centimetre cubes. There are a million cubes in the box, so if we imagine 400 of them are opaque and the rest perfectly transparent, this is about what we would expect from CO2 in air. Now looking in from the top, we have 100x100 columns, or 10,000, of which around 400 are likely to be blocked. (Some columns may have more than one opaque cube, but the odds are against it.) That's about 400/10,000 = 4% of the light being blocked, by every metre of depth. Air molecules are a lot smaller than centimetre cubes, so to contain 400 CO2 molecules in a box, the box would have to be pretty tiny, and you'd be losing 4% of the transmitted light through every layer this thick.

Actually, 400 ppm of real CO2 is far more transparent than you would expect on this basis - it only intercepts light at specific narrow frequencies, and only with a fairly low probability. Nevertheless, 400 ppm isn't a small number in molecular optical terms.

"There are other “greenhouse gasses” extant within the atmosphere, [...] why are none of these considered to have an impact as significant as that claimed for CO2? "

The impact of water vapour is claimed to be about twice as big as that of CO2. 1.2 C/2xCO2 gets trippled to 3.5 C/2xCO2 by (mainly) water vapour feedback in the models. The difference is that changes in humidity are thought to be driven by the changes in CO2. So they just talk about CO2, and regard water vapour and all the other stuff as a multiplier.

It's like somebody saying that their car is driven faster by their foot on the accelerator. They're ignoring stuff like the engine and wheels and so on because they all happen automatically. It's the foot on the accelerator that's the dynamic *cause* of the change.

"Bearing in mind the “environmental sensitivity” being xK per doubling of CO2, what would be the calculated temperature if Earth’s atmosphere was 99% CO2?"

That's tricky to work out, in detail. I'll answer a slightly different question, which is what would the surface temperature be if the atmosphere was perfectly opaque to infra red?

The convective greenhouse equation is T_surface = (T_eff + LapseRate * altitude) where T_eff is the effective radiative temperature - the temperature of a black body that would radiate all the energy being absorbed from the sun - and altitude here is the average altitude of emission to space. At the moment, T_eff is around -20 C, the lapse rate is about 6.5 C/km, and the altitude of emission to space is around 5 km. If the atmosphere was opaque right up to the 10 km tropopause, the surface temperature would rise 6.5 * 5 = 33 C. If you go any higher, things get complicated because the lapse rate is no longer constant, but since it's a thought experiment we can ignore that and just plug the numbers into the formula. If the top of the atmosphere was at 80 km (like the top of the cloud layer on Venus, say) then the surface would be several hundred degrees hotter.

As noted by someone else, the logarithmic relationship is only an approximation. To see this, try working out what would happen if the concentration of CO2 approached zero!

"What was the average global temperature when CO2 concentrations were about 4,000ppm, ten times present concentrations, or somewhat under 7 doublings (i.e. 6.65)? Is there evidence that it was more than 6xK the present average?"

There's a very nice chart of Earth's temperature/CO2 history here:
http://www.geocraft.com/WVFossils/Carboniferous_climate.html

Read the references to understand the evidence.

"Why were the Minoan, Roman and Mediaeval Warming Periods as warm as or warmer than today, when CO2 levels are generally agreed to have been lower?"

Climate scientists dispute that they were, and so don't have to explain it.

But the simple answer is that there's more than one control knob.

"is about 66°C, which, it has been calculated, is what the Earth’s atmosphere would be, were its orbit the same distance from the Sun as Venus’s."

Any such calculation is strictly "back of an envelope" stuff. Don't consider them to be anything like that accurate or precise. Atmospheres are hugely complicated.

"Possible explanations for a warm atmosphere?"

The most likely, I'd say, was something to do with clouds (Roy Spencer or Henrik Svensmark have plausible theories). A reasonable alternative is Bob Tisdale's ENSO hypothesis.

"The reason the sky is blue has been shown to be because of particles in the air absorbing some of the visible solar radiation (a.k.a. light); that it is blue does suggest that it is the red end of the spectrum that is being absorbed."

Not quite. The reason the sky is blue is that blue light is *scattered* more, so the red goes straight on overhead while the blue spreads out to the side. The converse is that sunsets are red, because the blue end has been diverted off to the sides.

Scattering is not the same thing as absorption.

"Another good example that supports this guess is a clear, cloudless night, especially if there is no air movement."

This is slightly tricky. Yes, the radiation effects do make a difference - particularly in explaining how you can get ground frosts when the air temperature is still above zero.

But clear skies are a sign of high pressure systems when cold dry air is descending from above, while clouds are an indication of low pressure systems when warm moist air is rising and condensing. Thus, the noticeable change in temperature when it clouds over at night might be putting the causal arrow partly the other way round. It's the warm temperatures that cause cloud, not entirely the cloud causing the warm temperatures.

"My understanding is that if something is a poor absorber of radiation, it is also a poor emitter of that radiation"

At the same wavelength, yes. But sunlight is at much shorter wavelengths than thermal radiation from the ground. They don't have to be the same.

Hope that helps.

Let me approach it from a different angle (one I've used here before but you may not have read the thread)

If you pump up a football, you increase the pressure, and the temperature goes up. Pressure and temperature are linked in this closed system, so that if you increase the pressure, you increase the temperature. This is because work is done on the system - you are forcing air into the football. The heat energy which causes the temperature doesn't come from the pressure itself, but because you have to force the air molecules in there against entropy. The increase in temperature is equivalent to the extra energy you use to get the air in there. Pressure is simply an expression of the increased number of air molecules bouncing off the walls of the football.

If you leave that football in a room overnight, then that raise in temperature leaks away through radiation until the football is at the same temperature as it was before you pumped it up. The pressure is still much the same (give or take - pressure is a measure of both the number and the energy of collisions, so will have gone down as the air molecules lose heat) - no air has leaked out - the football is still inflated. But the temperature has fallen. Under static conditions, a static pressure cannot maintain a static temperature.

A good example of why air pressure alone is not enough to maintain the temperatures we enjoy today is that of Jupiter.
At the top of its atmosphere, it is about -150 Celcius. So what level of air pressure would you have to descend to get reach say room temperature or 20 degrees or so? The answer is 10 atmospheres. The earth does not have enough of a gravitational gradient to produce the sorts of potential energy heating that you get in the centre of stars.

The take-away fact from this is that compression (i.e. changing pressure) and temperature rises are directly linked, but static pressure can't maintain a static temperature above ambient.

Ferenc Miskolczi resurfaces with a new paper:

The Greenhouse Effect and the Infrared Radiative Structure of the Earth's Atmosphere

http://www.seipub.org/des/paperInfo.aspx?ID=21810

After the pounding he took after his terrible 2007 paper, I hope he's thought this one through.

I'm not sure why TBYJ finds the assertion that all gas molecules contribute the greenhouse effect incorrect. That conclusion follows from mere consideration of the gas laws and what's normally taken to be a basic course in Properties of Matter. Even if we were surrounded by an atmosphere of completely inert argon, that atmosphere would be heated by radiation from the earth (the gas must have a thermal capacity) and that would lift the energy balance point between incoming and escaping energy away from the earth's surface. To contradict that, you'd have to take the argon atmosphere to be at absolute zero (i.e PV=0!)

What is added on with molecules (such as CO2, CH4, etc.) is a trapping phenomena that behaves, superficially, nearly the same as the energy exchange exhibited by ordinary gas molecule collisions. What is different is that the energy steps become very well defined (equal to the depth of the traps) and during the trapping event, the molecule exhibit no increase in vibrational energy. Having thought about this I can see that consideration of trap capture cross-sections, residence times, etc. might be able to arrive at equations for the impact of a particluar trapping gas's impact on the greenhouse effect. This will be fiercely complicated: after just one trapping event the radiation from that level in the earth's atmosphere will, strictly speaking, no longer follow the Stefan-Boltzman equation since it's been modified by an effect outside classical physics! At small CO2 concentrations that's probably not an issue. It might be possible to measure this through some study of specific heats. It's not an area I've any experience of.

To answer MJ, in my view: 'it's both'.

TBYJ
Your football illustration merely explores adiabatic and isothermal compression and expansion. Your cooled down football still has pressure because the gas inside it is still at room temperature. PV=RT. It won't be at the same presuure as it was when first inflated because it's cooled slightly. And if you cooled the football to absolute zero, the gas inside would exert no pressure at all, because the gas molecules would be motionless.

And if you heated your football with and without gas inside it to measure it's specific heat you'd get different reusults.

I'm not sure why TBYJ finds the assertion that all gas molecules contribute the greenhouse effect incorrect.

That would be a ludicrous assertion, luckily I didn't make it.

Your football illustration merely explores adiabatic and isothermal compression and expansion. Your cooled down football still has pressure because the gas inside it is still at room temperature. PV=RT. It won't be at the same presuure as it was when first inflated because it's cooled slightly. And if you cooled the football to absolute zero, the gas inside would exert no pressure at all, because the gas molecules would be motionless.

You say "merely" in front of my description as if it doesn't completely demolish your previous assertion that a static pressure ALONE can maintain an atmospheric temperature above ambient.

Yes, the ball loses a little pressure as it cools - I said as much in the description - but it does not lose ALL of it - but you have lost ALL the temperature. The increase in temperature caused by compression has ALL GONE, whereas some of the compression remains. So you have the following two snapshots:

1. Ball at low pressure, low temperature.
Pump it up
2. Ball at high pressure, high temperature
Let it cool
3. Ball at medium pressure, low temperature.

So comparing 1 and 3, if pressure alone was enough to raise a temperature then the temperatures should be different between situations 1 and 3. but it's not, The temperature is the same, despite the differing pressures.

A static pressure cannot maintain a static temperature above ambient.

Your final paragraph is correct.

You're again exploring adiabatic compression and then isothermal cooling.

A football at ambient temperature with an equal concentration of gas molecules to the atmosphere outside will have be at the same pressure as the atmosphere outside (1 Bar absolute pressure).

Cool that football to absolute zero and the pressure inside will fall to zero bar absolute. Take the football to double atmodpshere temperature and you'll double the pressure (provided the volume stays constant). So what. we're just stating the gas laws - in this cases Charle's Law.

And at a molecular level R in the equation PV=RT which embraces both Boyle and Charles can be broken down into various constants that describe the constituent gas molecules, such as molecular weight, and Avogadro's and Boltzman constants. Interestingly R is also equal to the difference between the gas specific heats measured with constant pressure and constant volume.

And for a gas which is a mixture of several types of molecules or atoms, the gas law for each gas follows its own version of PV=RT with the relevant R value for each gas.

Implicit with all this is the fact that gases can be seen as molecules undergoing thermal agitation, and displaying, as a result of that agitation, both a pressure and a temperature. They have thermal mass. And as a result, all gases must act as greenhouse gases when they envelope the earth.

Now throw in quantum mechanical trapping AS WELL as the above . . .

Capell,

I'm afraid your last post completely fails to mention temperature, so explains nothing, merely restates what I said.

Cool that football to absolute zero and the pressure inside will fall to zero bar absolute. Take the football to double atmodpshere temperature and you'll double the pressure (provided the volume stays constant). So what. we're just stating the gas laws - in this cases Charle's Law.

Let's forget absolute zero for now, apart from it being impossible, it's an edge case with peculiar characteristics. You're wringing the handle from the wrong direction - yes, if you heat or cool a gas in a closed system, then it gains or loses pressure in direct proportion. That is not in dispute. Unfortunately, that's not what you are claiming. You are claiming the inverse - that pressure drives temperature. As so it does - but temperature in an open system can change on other ways. Whilst temperature is a direct knob on pressure in a closed system, in an open system pressure is not a direct knob on temperature, because temperature can change by other means.

You are claiming that a pressure gradient in the air caused by gravity can completely explain the temperature near the surface. I am saying that compression in a closed system can indeed raise the temperature, but it will leak away radiatively in an open system. You keep mentioning the gas laws, which are specifically talking about closed systems, not open atmospheres.

Gravity can indeed squash together molecules of air at the surface, but this would only heat during the compression stage, while the molecules lose potential energy. Unless the atmosphere is continually shrinking (such as happens in large gas giants such as Jupiter), then no extra heat can be produced once the air is at maximum compression. Any that was produced will leak away radiatively. This is no perpetual motion machine!

I am not claiming that pressure drives temperature. I am simply stating the fact that all gases follow the gas laws experssed by PV=RT.

I am not claiming that a pressure gradient in the air caused by gravity can completely (or for that matter is even an issue) explain the temperature near the surface. I am simply stating the fact that all gases follow the gas laws experssed by PV=RT.

The gas laws do not specifically talk about closed systems.

Absolute zero is not an edge case as far as the gas laws are concerned, it's merely a point at which T=0.

I am claiming that the atmosphere follows the gas laws, behaves like a classical gas and as such it has thermal mass. In a gas (and in a solid, for that matter) that thermal capacity is understood to be expressed in the energy of vibration of the gas molecules. This thermal agitation also describes the pressure of the gas, and the interaction between pressure, volume and temperarure as described fully by the gas law PV=RT.

Because the atmosphere has thermal capacity and is also a heat conductor there's a temperature gradient in the atmosphere with temperature falling as altitude increases. If you now determine the energy balance point at which incoming, black-body radiation from the sun exactly balances the black-body radiation from the earth, that that point will no longer be on the earth's surface but somewhat higher. The temperature of the surface is thus raised by the presence of the atmosphere. This 'model' would be exactly the same if I replaced the atmosphere with a layer of asbestos.

Now within that gas that forms the atmosphere we have some molecules that have quantum mechaniocal energy traps. We're now adding another aspect of physics whilst remembering that CO2 is also capable of behaving like a classical gas. This quantuim mechanical trapping of CO2/H2O/CH4 will change the heat transfer patterns within the atmosphere - but by how much?

I give up. The thing about slayers is there is no point.

TBYJ
I'm going to assume that we understand that all gas molecules (even the inert gases) contribute, classically, to the greenouse effect. But that's not the whole story, of course, because we've got quantum trapping in certain gases which manifestly affects the black body radiation from the earth's surface. I say manifestly because the evidence is clear to see in the various IR absorbtion peaks.

But what's happening in these dips? The IR wavelengths of these absorption peaks tell us the depth of the traps involved and they're not particularly deep. When in the trapped state the CO2 will appear unchanged as far as its thermal agitation is concerned; the trapped energy appears in the valence bonds between the carbon and oxygen atoms making up the CO2 moilecule. Thermal agitation of the gas molecules will have sufficient energy to release the trapped energy from these molecules to allow them to revert to their normal state. Trap residency times will be low because the trap depths are low. When release happens, the trap energy is released in the form of a new IR photon. And these photons must be, then, at exactly the same wavelength of the absorption peak. They could be retrapped if they collide with another CO2 molecule, or just continue in normal thermal exchange (i.e. momentum exchange) with other gas molecules. So some of the escaping trap energy must reappear in the IR spectrum at longer wavelengths.

What impact this has on the behaviour of the CO2 within the GHE must be calculated by modelling this process; how this is verified by experiment, I don't know. That's the part of the description I'd like to see.

You've worn me down, Capell. That doesn't mean you've won the argument. It just means life is so very short.

I hadn't realised it was a competition; I thought it was a discussion.

Capell
Perhaps you can answer the question regarding CO2 and its disportionately large contribution as described in previous posts in the discussion. According to NASA data Mars has roughly 10x as much atmospheric CO2 as Earth. The inverse square law givens energy input from the Sun at about 40% of Earth if my mental arithmetic is correct. This should mean significantly higher back radiation on Mars than Earth. Does what happens on Mars actually match the theory, Mars having no active volcanoes, little or no water vapour and clouds and not much atmospheric dust. Therefore measurements there should be better at confirming theory.

Simple answers will suffice as I am curious about the differences.

sandy, you answered it yourself. Mars has no water, so it immediately loses 70-80% of the GHE. CO2's effects decrease logarithmically, so 10x may only produce twice the effect. So in total I'd expect Mars to have somewhat less of a GH warming than earth, a guesstimate of about 40%.

But it is a very good question, and if I can find some data on average Martian surface temperature, surface atmospheric pressure, and solar insolation later on once my hangover clears, I may attempt the calculation. Or if you have a clear head, you can find me these figures, I'll take some painkillers and get back to you.

It is not possible for me to build a greenhouse in my garden using gas. I could do so using glass. Could it be that the greenhouse effect is more to do with reducing convection of heat out of the system than radiation? Could it be that the term "Greenhouse Effect" in relation to climate change due to gasses is in fact a misnomer?

I simply don't know. I read that the daily temperature variation (day/night) can be 170 F suggesting that any GHE is rather weak.

John Hume, it's generally agreed across the board that 'greenhouse' is a terrible description for the mechanism. Unfortunately that is what it is called, so we continue to use it as a lable only.