Need Help: Can You Model CO2 as a Greenhouse Gas (Or is This Just Wishful Thinking?)

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  • #176


It's hard for deserts to lose much heat at the ground because the evaporation term in the surface energy budget is small compared to the moist tropics. This has little to do with water vapor feedback. If you could make the Sahara moister, the surface would cool even if you increase CO2 a bit.

It also doesn't follow at all from Clausius-Clapeyron that global cloudiness (or low clouds in particular which control the albedo more than any other kind) will increase in a warmer world. I don't understand andre's objections at all to sylas...they're repetitive and rather ill-posed.
 
  • #177
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It's hard for deserts to lose much heat at the ground because the evaporation term in the surface energy budget is small compared to the moist tropics. This has little to do with water vapor feedback. If you could make the Sahara moister, the surface would cool even if you increase CO2 a bit.
So latent heat is more important than CO2. But it's not about that, it's about why the sahara is warmer than the tropics. And it's not about feedback to variation in CO2 It's about feedback in the direct heating processes. Let's try again then,

If we assume the constructions presented by Pierrehumbert, which already incorporates convection in the Planck 'response', then the temperature reponse is not affected by convection, if I understand it right.

So for the Sahara compared to the tropics:

1: Latent heat evaporation/ condensation negative (cloud) feedback is virtually non existent
2: Absolute Atmospheric moisture is much lower hence the positive water vapor feedback is much lower in the desert
3: albedo is unclear but the low forest albedo of the tropics is neutralized by the high albedo of the more abundant clouds

So if the current greenhouse feedback ideas attribute the strongest effect to water vapor compared to the latent heat/convection then the tropics should be much warmer than the desert, which aint so.

Also, once more, the results of Lindzen et al suggest that there is something wrong with the estimation of the total feedbacks being positive.

Now why is the Sahara several degrees warmer than the tropics? Two possible reasons.

A: the negative cloud feedback (caused by moist convection) is stronger than the positive water vapor feedback.

B: The subsiding/ decending air above the desert heat up dry adiabatically while it had been cooled previously in the tropics by moist adiabatic convection. This results in Foehn effect, the dry air heating up much more than the original moist air cooled. Wouldn't this put some question marks by the assumption that convection (positive or negative) merely corrects the lapse rate?

It also doesn't follow at all from Clausius-Clapeyron that global cloudiness (or low clouds in particular which control the albedo more than any other kind) will increase in a warmer world.
No, http://www.science.uwaterloo.ca/~cchieh/cact/c123/clausius.html assume an exponential relationship between temperature and evaporation. Now the key is in "exponential". So if the lapse rate temperatures reacts lineair to an higher surface temperature, the difference in maximum water vapor content is exponential and when in the convection cooling starts from a higher temperature the difference in absolute humidity is larger with the same adiabatic cooling rate and more water will condensate.

Another question would be the source of the additional energy required to evaporate all that water with higher temperature. Would the assumed 3-4 W/m2 for double CO2 also be enough to evaporate enough water to attain that positive water vapour feedback?
 
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  • #178
sylas
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I'm struggling with this thread also. It's becoming a real mess; and every time I think there's some progress, it just mutates into a new set of confusions.

In my opinion, one difficulty is that we keep shifting from the topic.

The thread topic was modeling CO2 as a greenhouse gas. This ought to be a quite straightforward, in the sense that of all the aspects of climate science it stands out as one of the simplest. The physical thermodynamics is comparatively straightforward, and mostly goes back to the nineteenth century; though with quantum mechanics having since given a better understanding of matter/radiation interactions.

On the way, we've covered some useful issues. A number of quite basic misconceptions have come up along the way, which have been addressed pretty clearly; though not, unfortunately, with a clear recognition from all participants, so I am still uncertain about how much real progress has been made in terms of getting the underlying physics sorted.

One of my own personal aims is to acknowledge explicitly and with thanks anyone who can find errors in my own work. This happens from time to time. In this thread, Andre noted correctly that I had implied convection gives a transport of heat both up and down, whereas of course it really only works to transport heat upwards. I acknowledged this promptly in [post=2299540]msg #125[/post]. It was a genuine error that I should not have made, but I'd messed up way the atmospheric equilibrium is maintained. I actively seek out useful corrections like this, and as a result the whole matter of radiative-convective equilibrium is on a firmer footing; for me in particular.

There have been a number of other similarly fundamental points which have just been left hanging. I don't know how much of what I consider the underlying physical basis for talking about feedbacks and sensitivity and radiation and convection and so on I can rely on as common ground for looking at different hypotheses about their magnitudes or signs.

Here are a couple of examples:

Magnitude of the greenhouse effect on Earth -- 33 degrees

This is an example where the discussion suddenly stopped with no recognition of the arguments I presented.

It is a fairly basic result for the nature of the Earth's surface temperature that for the amount of solar energy absorbed, surface temperatures are about 33 degrees warmer than it would be without longwave interactions in the atmosphere. This follows directly from basic thermodynamics; and it is specifically a consequence of the greenhouse effect -- primarily from CO2 and H2O.

Andre disputed this conclusion for a while, on the basis that the model was "too simple". I gave what amounts to the formal proof of why the 33 degrees is a strong lower bound on the magnitude of the effect in [post=2296677]msg #96[/post]; complexity can only make this number larger. Andre never commented on this bound further; so I don't actually know if he understands or accepts the argument for 33 degrees. I'm not just trying to rub noses in errors here; I do really find it disconcerting when someone proposes a focus ("Now let's concentrate on that." -- Andre, [post=2296495]msg #95[/post]) and then never makes any further comment when I give a detailed response.

If Andre recognizes the validity of the argument there's no shame in that. We can take it as progress and move on to consider other matters where we differ. If he doesn't -- then what was wrong with it? What about that suggestion we concentrate on this?

So just for the sake of seeing where we stand on what is recognized as common ground and what is disputed... Andre, did you understand the mathematical argument for 33 degrees being a lower bound on the magnitude of the greenhouse effect for Earth? It's a widely used number, with a clear physical basis. Can we now take this as common ground? If not, do you have any comment on the demonstration of the lower bound using Holder's inequality?

Planck response

I have asked many times for Andre to spell out what HE understands by the term "Planck response". (Messages 156, 161, 166, 171). I can appreciate one might not answer every question that shows up, but I've singled this one out as basic, many times. I STILL don't know whether Andre is on the same page as I am, or the same page even as his own sources that use the term!

This is a basic question, because it goes to the heart of how you identify what is and is not a feedback. Andre has spoken of certain processes being "assumed" as non-feedback; or about being "selective", words that suggest he considers this all rather arbitrary. But it is not arbitrary at all, and it is not a way of ignoring anything or leaving it out. It is simply a way of structuring the analysis of a complex system, and what is or is not feedback is not a matter of assumption, but a consequence of what the base response actually means. I've given the definition as I understand it many times now, and in fact this is not something that ought to be a matter of "debate" at all, in my opinion. It's really a matter of base level comprehension of the technical terminology of atmospheric physics.

Andre wants to talk more about feedback. Fine... but do we know what this actually means yet? What is the nature of the base relation to which feedback applies? If we can at least have that as common ground, it’s a basis for making some kind of substantive engagement on other points where we may differ on what evidence implies for the magnitudes and signs of feedback.

Thread focus

Both the net magnitude of the greenhouse effect, and the radiative-convective equilibrium for a given atmospheric composition which is the basis of no feedback response, go directly to modeling of the greenhouse effect, which is the main thread focus as I understand it.

CO2 is modeled as a greenhouse gas on the basis of its interactions with infrared radiation. Calculation of the base no-feedback equilibrium response shows up clearly how temperatures relate to the way energy flows between space and the surface. Any feedback process occurs when temperature feeds back into changes on some variable that appears in calculation of Planck response. Albedo alters solar input. CO2 alters longwave absorption. Cloud alters both albedo and longwave absorption. Humidity alters lapse rate as well as longwave absorption. And so on. The base response everything works upon is called the Planck response, or the net flux of energy for a given atmosphere and surface and solar input.

Now, unfortunately in my opinion, the whole matter of cloud feedbacks and humidity feedbacks -- which is really a distinct topic -- seems to be raised as a new focus.

And yet, this has come up while we are still left hanging with basic thermodynamic fundamentals unresolved from earlier in the thread.

-----

On the tropical feedbacks

OK. This seems to be a new direction Andre would like to take the discussion.

I have no problem with considering the matter of humidity feedbacks. There's been a fair bit of work on this recently, both theoretical and empirical, and nearly all of this work indicates a strong positive feedback effect. There are still a small number of papers proposing that the feedback is very small or even negative; and looking at this can be useful. Andre has provided a couple of useful references for this. But it is a new focus, and frankly, given the issues with basic underlying thermodynamics seen in this thread, I have little hope that it will make any real progress.

When I do post on this subject, I'll do it as a new post entirely, where I don't worry about other subjects or previous threads of discussion.

I just want to go on record for now concerning what I see as a whole pile of loose ends and missed opportunities in this thread.

The main issues ought to be clear and we should have been able to come to a clear mutual statement of what is common ground with respect to basic physics relating to the thread topic, of how CO2 works to give a greenhouse effect and to impact Earth's surface temperature. We could have done that without needing to resolve the question of "warming", or the magnitude of changes to CO2 concentrations, or the magnitude of Earth's sensitivity in general to small changes in forcings. The topic is more fundamental than that, and had good potential for clearing up a lot of deep misunderstandings of the relevant physics that sometimes degrades popular discussion.

Cheers -- sylas

PS. For Andre... I have referred to you above in the third person, from time to time, and I don't mean any offense by that. Sometimes people have taken offense at this in the past, so I'm adding this postscript to disclaim any attempt to belittle you. The grammar reflects nothing more than whether I am writing as a summary intended for readers in general, where I refer to you as an important thread participant; or whether I am intending to write specifically to you as one person in a two-way dialogue... that's all. Best wishes as always -- sylas.
 
  • #179
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Greetings all. Brand new here, I came across this site Googling for the 270/385/1500ppm warming experiment under controlled conditions. So far, it's the closest I've come to an answer, which is I'll see essentially no difference in the temperature between the two containers. This makes sense to me when dealing with a trace gas, and it's what I expected.

I've also been on other threads, pointed to "whole atmosphere" experiments and direct measurements, where there are no doubt infinite variables. Basic scientific procedure, at least to me, would be to eliminate your variables and isolate what you want to test. Without 2 controlled containers of atmosphere, there will be a difficulty in convincing some of CO2's warming ability.

Those few controlled experiments use 100% CO2 to get a 5 C difference in temperature. Hopefully they are using a barometer to make sure they aren't over-pressuring the CO2 box. While Venus is a hot box full of CO2, I am also aware that its atmospheric pressure is 90 times that of Earth.

That being said, I've correlated two common data sets and I'd like your thoughts.

The first is the yearly ppm increase in CO2 at Mauna Loa:

http://www.esrl.noaa.gov/gmd/ccgg/trends/

The second is the temperature anonomly in degrees:

http://data.giss.nasa.gov/gistemp/2005/2005cal_fig1.gif [Broken]

I had to blow the diagram up to get the yearly detail. What I found was quite interesting.

- The year to year CO2 ppm increase at Mauna Loa differed by as much as 600%, 1992 vs 1998.

- I was able to predict which years were cool, versus which years were warm, simply by looking at the ppm increase for that year. Lower ppm increases were associated with lower temperature for that year.

The cool years:

1960

1964 (significant)

1982

1992 (significant)


And these years will be on the warm side:


1965 (definitely warmer than 1964)


1972 (defintely warmer than 71)


1977 (warmer than 76)


1987 (warmer than 86)

1993, though not warm, will be warmer than 92.

Of course, 1998 and 2005 were heavy with CO2, and their yearly temperature reflects it.

This correlation, which is just about perfect, occurs in an atmosphere of ever-increasing CO2. What does this mean?
 
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  • #180
sylas
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Welcome aboard Rickeroo.

Greetings all. Brand new here, I came across this site Googling for the 270/385/1500ppm warming experiment under controlled conditions. So far, it's the closest I've come to an answer, which is I'll see essentially no difference in the temperature between the two containers. This makes sense to me when dealing with a trace gas, and it's what I expected.
The atmospheric greenhouse effect arises in ten kilometers of atmosphere, with a substantial temperature gradient (lapse rate). It's not just the absorption that matters; but the fact that the portions of the atmosphere for which thermal radiation can escape into space are much colder, because they are at a high altitude. Without this temperature gradient, there would be no greenhouse effect.

The 270/385/1500 ppm CO2 corresponds to about 4/6/23 kilograms of CO2 per square meter, in about 10 kilometers of atmosphere. The forcing from this is very well known, and follows from well measured properties of CO2 and radiation, along with some basic thermodynamics.

The experiments that give the physical basis for inferring CO2 "forcings" do not work by simply testing a ten kilometer gas cell with atmospheric compositions. What experiments give you are the emissivity/absorbtivity of the gas at different frequencies, along with all the basic physics of thermodynamics that gives you the way heat flows through different materials of all kinds.

You can calculate from well established thermodynamics what is called the "Planck response", which gives you a temperature required for Earth to radiate back out again the solar input, but only for given fixed conditions. That's enough to define the "greenhouse effect" by itself, which is actually one of the most straightforward aspects of climate there is, and not in any credible scientific doubt at all.

This is a long LONG way from a full understanding of climate! The real problem is that conditions are NOT fixed. The Earth is a complex system, and all the genuine uncertainties of climate and temperature are not with the basics of the greenhouse effect itself, but with the whole sensitivity of the Earth climate system. You can't just use Planck response by itself, because as temperature changes, you also get changes in surface cover, humidity, cloud, carbon cycle, and so on. These are called "feedbacks", because they are changes driven by temperature that impact the variables that in turn establish the basic temperature response.

I've also been on other threads, pointed to "whole atmosphere" experiments and direct measurements, where there are no doubt infinite variables. Basic scientific procedure, at least to me, would be to eliminate your variables and isolate what you want to test. Without 2 controlled containers of atmosphere, there will be a difficulty in convincing some of CO2's warming ability.
Frankly, it is pretty much impossible to convince some people, and I am personally fairly relaxed about that. My main interest here is to try and help give a better general understanding of the underlying physics.

Experiments are nice, but they work best in the context of a theory to be tested by the experiment. For example, if you set up a long 10 kilometer tunnel, with a big lamp at one end to represent the sun, and used that to try and infer impact of different densities of CO2, you'd get effects quite different from the atmosphere, because you don't have a gradient of pressure with an adiabatic lapse rate driven by convection. And how would you know that this is important? Mainly, by knowing the theory of the greenhouse effect that you are supposedly testing.

The real issue for most people, I think, is not the lack of experiment. There are heaps of experiments and measurements that demonstrate the simple fact of a powerful greenhouse effect on Earth, but to see their relevance, you have to first understand the physics that they are testing. One of the clearest direct measurements of our greenhouse effect, in my opinion, is simply the direct measurement of the huge flux of atmospheric infrared backradiation coming down to the surface from the sky, with the spectrum matching our major greenhouse gases.

Atmospheric thermodynamics are quite complicated, but it is well within the capacity of a decent physics student to learn the basics of how the greenhouse effect works. The physics behind it is truly not in any credible doubt at all.

That being said, I've correlated two common data sets and I'd like your thoughts.
My first thought, on this sentence in isolation, is that correlation is a weak basis for confidence. It can be very useful as a test of predictions from theory, but in my opinion you don't really understand a physical situation until you have a theory; which means a proposed explanation of how something occurs. Finding correlations can be suggestive in looking for theories, but until you have the theory that is consistent with the observed correlations, the correlation alone can only be suggestive.

- The year to year CO2 ppm increase at Mauna Loa differed by as much as 600%, 1992 vs 1998.

- I was able to predict which years were cool, versus which years were warm, simply by looking at the ppm increase for that year. Lower ppm increases were associated with lower temperature for that year.
This isn't because of a greenhouse effect.

You are looking at a rate of increase of CO2, and comparing that with temperature. You can't explain this correlation (if it holds up) by proposing that CO2 is driving temperature. If CO2 was all that mattered, then you would expect temperature to rise all the time as CO2 is rising, but when CO2 rises more gradually, temperature would rise more gradually. But you are looking at temperature that goes up and down, which means there's something more than CO2 going on here for temperature.

And of course, there is. There's a heck of a lot going on with climate, all the time, which gives all kinds of natural variation on short terms. The major factor the big temperature increase in 1998, for example, was a very strong El Nino in that year. 1992 was cool, mainly because of the big Pinatubo volcano eruption. And so on. These are not merely correlation based arguments; there are good physical theories which indicate why you get hotter years with El Nino, and cooler ones with a big volcanic eruption.

The changes in the rate of increase of CO2 from year to year do not lead to the big temperature swings that you are looking at. We know the forcing involved, and it's a fairly strong steady increase, but not something that has huge short term forcings to make temperature swing wildly between different years.

This correlation, which is just about perfect, occurs in an atmosphere of ever-increasing CO2. What does this mean?
I don't think the correlation is all that good. I measured it for myself with a spreadsheet just now, using the annual mean grown rate for CO2 from the Mauna Loa site, and the GISS data underlying the graph you linked, from the Global Land-Ocean Temperature Index. The correlation I got was 0.735.

I think the most likely reason for any such correlation, if it is a real effect, is an impact of temperature on the carbon cycle. There are enormous fluxes of carbon in and out of the atmosphere from vegetation, and temperature is likely to have an impact on that, rather than the other way around.

Note that I am guessing at a theory for the correlation. This is the first step in a genuinely scientific project. The next would be to try and test the theory, with an experiment that has the potential to falsify it. I have no idea what that might turn up. The point is... merely noting a correlation is not a sufficient basis for a good scientific theory.

Cheers -- sylas
 
  • #181
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Thanks Sylas.

With the correlation being .735, would that be enough to say that the comparision is relevant, or at least something to be looked at further?

There are enormous fluxes of carbon in and out of the atmosphere from vegetation, and temperature is likely to have an impact on that, rather than the other way around.
Would that suggest that temperature leads carbon, at least to some degree?
 
  • #182
sylas
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Thanks Sylas.

With the correlation being .735, would that be enough to say that the comparision is relevant, or at least something to be looked at further?
It's enough to be suggestive; worth looking at. I would not be at all surprised to see some kind of temperature related effect, but as I said, it's a bit of a guess. I suspect that if we hunted through the literature on the carbon cycle we'd find some consideration of such effects. I don't know off the top of my head and I haven't take time to look.

Would that suggest that temperature leads carbon, at least to some degree?
Sure. If you look at the Mauna Loa data, you should see that the total atmospheric CO2 looks like a tilted sine wave. There is a very strong annual flux of CO2 in and out of the atmosphere every year, and then a steady continuous increase going on all the time as well.

In a way, the Earth "breathes". The effect is seasonal, and it arises mostly (I think) from changes in the way vegetation takes in and emits CO2 between winter and summer. The total flux of CO2 involved is huge.

However, it is not cummulative. The carbon taken into vegetation is released again later. There's a continual cycle of carbon between atmosphere, vegetation, soil and ocean, going on all the time. The human input is a bit different, because we are basically acting as a way for carbon from geological reserves (fossil fuels) to make its way into the carbon cycle, and this leads to a gradual increase in carbon in all parts of the carbon cycle: the atmosphere and ocean especially.

Basically, the carbon cycle consists of several "reservoirs" of carbon, each with a different total capacity, and with carbon fluxes between them. Here's a diagram, from an online textbook: http://www.uwsp.edu/geO/faculty/ritter/geog101/textbook/earth_system/biogeochemical_cycles.html [Broken].) The numbers are the capacities of the reservoirs, and the total amount of carbon moving between them annually.
carbon_cycle_NASA.jpg


What human emissions do is add 5.5 GigaTonnes per year into the atmosphere. Over a century, this has resulted in a large increase in total carbon in the atmosphere, ocean and terrestrial reservoirs. But at the same time, there is about 90 Gigatonnes per year going each way between ocean and atmosphere, and about 120 Gigatonnes per year each way between atmosphere and vegetation/soil on land. Temperature effects can shift the balance of the reservoirs a bit, enough to make the net atmospheric increase rise or fall a bit, and I would guess this is the main reason for the correlation you have observed.

Cheers -- sylas
 
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  • #183
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Thanks Sylas. Yes, the sine wave at Manua Loa makes perfect sense with the seasonal vegetation level, as does the net addition of CO2.

It also makes sense that temperature would have an effect of CO2 transfer or absorption, something to be looked at anyway.

My next task will be to correlate the temperature with the rise in CO2, and to correlate the melting ice with the rise in sea level.
 
  • #184
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silas - thank you for your comments and advice way back in post #155.
First I must admit I am not the originator of that thought experiment. I read it on a blog first, but sadly did not record the url. Later I attempted to work out the mass of the atmosphere on 1sq/m at standard pressure and found my answer was about right. This chuffed me no end, so I attempted to calculate the mass of global CO2 and got it wrong.

Minor correction here: CO2 is about 0.04% by volume; so you have to scale by 44/29 (the molecular weight of CO2 and the average molecular weight of air) to get pretty close to 6 kilograms.
Your correction improved my calculation but not enough yet. I was attempting to calulate the global mass of CO2. There are 10^6 square meters in a square kilometer so we have 6 x 10^6 kg/km^2.
From this link: http://www.net-comber.com/worldarea.html I selected 510,072,200 km^2 as the total global surface area and arrive at 3.06 x 10^15 kg. (umm.. still something wrong)

Towards the end of your post you mention dealing with the atmosphere in slabs and integrating the temperature changes, well, its 55 years since anyone last attempted to teach me calculus, so I am a lost cause there.

A point for clarification; in our column of well mixed gasses, as we progress upwards with a constant lapse rate, we not only have less temperature, we also have less density so the total mass of CO2 per "slab" will also be less. Therefore I feel we must take into account reduced mass as well as temperature.

I would like to get hold of the following book but it is not available in my local library. Might be in the Uni library. I will have to wait until Amazon offers used copies at much reduced prices. A short critique at:

http://climatesci.org/2006/05/05/co2h2o/

Relative Roles of CO2 and Water Vapor in Radiative Forcing
Filed under: Climate Change Forcings & Feedbacks, Climate Change Metrics — Roger Pielke Sr. @ 6:09 am
In the second edition of our book

” Cotton, W.R. and R.A. Pielke, 2006: Human impacts on weather and climate, 2nd Edition, Cambridge University Press, New York, in press ”
 
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  • #185


before I make my views on this, how would current rate of change in global temperature affect the rate of which major greenhouse gases such as water vapour from the ocean, methane in garbage dumps, and CO2 trapped in soils, are released?
 
  • #186
sylas
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Your correction improved my calculation but not enough yet. I was attempting to calulate the global mass of CO2. There are 10^6 square meters in a square kilometer so we have 6 x 10^6 kg/km^2.
From this link: http://www.net-comber.com/worldarea.html I selected 510,072,200 km^2 as the total global surface area and arrive at 3.06 x 10^15 kg. (umm.. still something wrong)
That sounds pretty much exactly correct. That's what is given in wikipedia's Carbon Dioxide article. (Wikipedia is an unsafe source for this forum, but it's okay as a confirmation of your calculation.)

Note that people often speak of the mass of carbon in the atmosphere, which would be 12/44 times the mass of carbon dioxide. This corresponds to about 8.2 * 1014 kg, or 820 Gigatonnes. The carbon cycle diagram I've just posted gives 750; but that may just be an older value, corresponding to about 355ppm CO2 in about 1990.

A point for clarification; in our column of well mixed gasses, as we progress upwards with a constant lapse rate, we not only have less temperature, we also have less density so the total mass of CO2 per "slab" will also be less. Therefore I feel we must take into account reduced mass as well as temperature.
Quite so. However, we often use pressure as the altitude co-ordinate, which doesn't have that problem. This makes all sorts of calculations more straightforward.

I would like to get hold of the following book but it is not available in my local library. Might be in the Uni library. I will have to wait until Amazon offers used copies at much reduced prices. A short critique at:

http://climatesci.org/2006/05/05/co2h2o/
I can confirm for you right away the main conclusion. H2O is easily the most important gas in our atmosphere for giving the greenhouse effect. I've noted this a couple of times in the thread. An increase in humidity has a very strong effect; much more than a similar increase in carbon dioxide.

This is, in fact, the reason why "water vapour feedback" is such an important part of the more complex question of climate sensitivity. The amount of water in the atmosphere is mostly a function of temperature.

Industry emits huge amounts of water vapour into the atmosphere. Ironically, many pictures trying to show a picture of pollution are actually showing discharges of water vapour. A picture of CO2 emissions is much more boring, because it is invisible.

But the effect of human H2O emissions is almost nil on atmospheric water vapour. Anything extra we add comes out again almost immediately, because the water cycle is so rapid. So you really can't hope to increase humidity just by adding water. The best way to increase the water content of the atmosphere is simply to heat things up somehow. That's why carbon dioxide, despite being a smaller part of the total greenhouse effect, is what is forcing the changes. The warming effect of carbon is amplified by the effects of additional water from this feedback. See our previous discussion on "Planck response" and feedback. There are a lot of other effects to consider as well. Water vapour will reduce the lapse rate, which is a negative feedback; and changes to cloud can reflect sunlight (negative feedback) and also absorb infrared even more strongly than gaseous vapour (positive feedback). It looks like we may be pulling apart some of the scientific literature on this question as the thread progresses.

Cheers -- sylas
 
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  • #187
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Water vapour will reduce the lapse rate, which is a negative feedback; and changes to cloud can reflect sunlight (negative feedback) and also absorb infrared even more strongly than gaseous vapour (positive feedback). It looks like we may be pulling apart some of the scientific literature on this question as the thread progresses.
When water vapour reaches dew point and starts to condense on whatever CCN's are available, there is a drop in air pressure. I believe this can result in fierce updraughts within large cumulus clouds. You can see this effect here in Pembrokeshire. The "Finger of God" extending upwards from the cloud tops. Very impressive and a warning to any aircraft to keep clear. I would assume a lot of energy would be transported upwards even while the cloud is accumulating energy from the sunlight above and longwave radiation from below. At night I assume the "feedbacks" will change due to lack of solar input.

I feel I must aquire more understanding of the "greenhouse effect" of water vapour and liquid water (clouds, fog) and ice crystals (cirrus clouds) in the atmosphere and the effect on positive/negative feedback. Possibly, then, an understanding of the "feedback" due to increasing CO2 will be more clear to me.

So back to my imaginary 1m^2 column of air and a dry adiabatic lapse rate of 3C per 1000 feet and assuming the air temparature has stabilised from about 2 meters above surface level I expect the air temperature at 10,000 feet (plus 6 feet or so) to be some 30C cooler.

Now consider the air in 1000 foot slabs/layers, each layer 3C cooler than the layer below and that a net transfer of heat will only flow from hotter to cooler. We must also bear in mind that each layer has less mass than the layer below. The flow of energy is upwards. It appears only the bottom layer of a 1000 feet or so seems to have any feedback to the surface even as the net flow is upwards. It has been established that increasing the water vapour content does not effect the dry adiabatic lapse rate therefore any increase in CO2 also has no effect in dry air.

Sea surface temperatures appear to range from a minimum of -2C to a maximum of about 33C. A much smaller variation than on land and also less inclined to change sharply over short time periods. Seeing that slightly more than 70% of the Earth's surface is water I thought this might be a good place to start. In my attempts to gain some knowledge about water I have been looking at the Water Absorption Spectrum page on Martin Chaplin's site.

I must confess I find this site very heavy going, but extremely interesting. I never knew water could take on so many different molecular configurations which seem to be responsive to different temperature regimes. Every change seems to have its own spectral response. Quite awesome.

On the above page is a graph titled The visible and UV spectra of liquid water

http://www1.lsbu.ac.uk/water/images/watopt.gif [Broken]

You can see clearly how light and some UV can penetrate quite deeply into clear water. (I read somewhere that you can get sunburn under water and thought Huh!) The area of the graph I am trying to get to grips with is the IR region. From about 3µm to 100µm. Here penetration seems limited. If I read that correctly I fail to see how downwelling IR from any source can possibly provide any significant heating into water. From other literature (haven't found it on Chaplin's site) I read that IR reacts with surface molecules of water to increase the rate of production of water vapour. How this may be quantified I haven't clue.

So to satisfy my curiosity I will suspend a shielded IR source over a measured quantity of water and try to record any temperature change. The IR source, still to be obtained, will be a circular slab of steel or cast iron of about 2kg mass and the shield will be a small drum such that airflow past the source is minimal but heat radiated downward will have a clear path to the water surface. Should be interesting. I will post the result in due course.
 
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