Yahoo Answers is shutting down on May 4th, 2021 (Eastern Time) and beginning April 20th, 2021 (Eastern Time) the Yahoo Answers website will be in read-only mode. There will be no changes to other Yahoo properties or services, or your Yahoo account. You can find more information about the Yahoo Answers shutdown and how to download your data on this help page.
Trending News
Why is the standard classroom experiment demonstrating the GHG effect of CO2 wrong ?
Two identical, open top, plastic containers are lit by identical heat lamps. Probes shielded from direct radiation are placed close to the absorbing surfaces at the bottom of the conainers and monitor the temperature. When steady state is reached (27.5 C in the experiment reported) CO2 is added into one of the containers. Temperature immediately starts to rise and reaches 34.5 C in about 700 seconds and then progressively drops. This has been used as classroom demonstration of the GHG effect of CO2, however, it has no connection with it. Why ?
The experiment which is criticized is described here: S.B. Lueddecke et al, "Greenhouse effect in the
classroom: A project and laboratory-based curriculum"
J. Geosci. Educ. 49, 274-279 (2001)
They don't seem to find much difference between closed and open containers. Ben is clearly right to stress the effect of convection/absence thereof. And this must the main effect (as it is for real greenhouses) in the experiment described as shown by another experiment using a heavy gas (Ar) for control without IR absorption bands.
However:
a/ simple calculation show that the increase in temp due to CO2 IR absorption can't account for more than a 5% increase w.r.t. the normal temperature difference (with air) between the container and the environment.
b/ even with sealed containers, convective effects/absence thereof at the interface between the CO2 layer and air can't be ruled out. The important point appears to be the existence of an interface here and not the thickness of the air layer.
@antarcticice: this is not a post about denial or faith or betrayal or religion but about physics experiments and their interpretation.
@antarticice my (over?)reacting was not meant at your posting links to other experiments but to your comment about what deniers recognize or not. I never intended to say that CO2 is not a GHG. It is, and that's happy for us otherwise we would freeze. But I hate false proofs pretending to rely on 'solid science' as in this experiment and in the article to which you can find a link in FGR's comment.
@FGR isn't there some contradiction between your points 2 and 5 ? Either CO2 does not settle at the bottom, or there is some stratification but not both.
Actually, this is really what happens. CO2 settles at the bottom and prevents convective cooling. The important point is the existence of a CO2-air interface where convection stops so that the differential warming has nothing to do with radiative GH effect but with the suppression of convection as pointed out by Ben.
This can be shown by replacing CO2 by Argon which has about the same molar mass but has no infrared absorption bands.
In a (cleaner) remake of the experiment with argon, it is found that Ar and CO2 yield the same temperature curves and clearly there can't be any GH effect with Ar.
Monoatomic gases do not have the vibrational ddf which are excited by IR radiation in CO2. A simple model shows that large temperature rise found in the CO2 container cannot be explained by the IR CO2 absorption bands but can very well be explained by the suppression of convection.
Interestingly, if you keep monitoring the temperature long enough (without introducing more heavy gas) you find that after rising and reaching an almost steady state value, it suddenly drops with a discontinuous slope, corresponding to the moment when the level of the heavy gas falls below the temperature sensor, showing that it is really the existence of an interface which is at the root of the absence of convection. There is a real greenhouse effect here, but it has nothing to do with the radiative GH effect supposed to cause global warming.
This is precisely what I deeply dislike in this article. The authors rightfully write that the difference of temperature is due to CO2 but without asking any question, they attribute it to the absorption features of this gas, and therefore to what is the cause of global warming..! Afterwards they boast of basing their claims on solid science, contrasting it with the 'half truths' and 'approximations' found elsewhere. It is a real shame and it is the duty of real scientists to say so loud and clear. There are, actually, many points in their experimental protocol which show it is unreliable, to start with them not lighting the lamps and letting a steady state be reached before introducing carbon dioxide. This could allow to check that both containers reach the same T when they are in same conditions. Also to avoid the problem posed by the endothermic CO2 producing reaction, which is used to justify not heating before, why don't they use small pipes to introduce the gas from outside ?
@antarticice sorry but I will not by any experiment 'documented' by a YouTube clip with which you can prove almost anything and its exact opposite. That's the paradise of candidate Uri Geller imitators. If you have some reference to some detailed written protocol, I would be interested.
Sorry, I should have given a reference to the remake experiment and the model calculations. This is described in Am.J.Phys. 78,5, 536-540 (May 2010)
@FGR I agree with most of what you say but you first answer gave to somewhat incompatible possibilities without deciding - but maybe you were forming your ideas while writing, as I often do here.
Also, I don't see why you said or implied that CO2 would not stay at the bottom because it is warmer. It would have to be a lot warmer to compensate for the density difference (44/29)=(273.15+t)/(273.14+27)==>t=182 °C !
What is difficult is really to understand this convection mechanism and why it is inhibited by the interface and how in the end it overtakes. We should try to estimate the diffusion rate. I'll do that if I have time.
More basically my question was: given that this experiment does not appear credible, do you know a table-top experiment demonstrating the GHG properties of CO2 (apart from measuring its absorption lines directly ?)
I tried to extend the question duration but for some reason I couldn't. So I shall re post it in a different form.
For BA, I give it to Ben in spite of the numerous interesting points made by FGR, because he was clearly the first to spell the fundamental reason for the described experiment to be unrelated to radiative GHG effect, namely that the effect here is entirely due to (lack of) convective heat transport.
9 Answers
- Ben OLv 61 decade agoFavorite Answer
Global warming from the enhanced greenhouse effect is not something that can be demonstrated using a physical experiment or first principles of physics. It's a theorised explanation for why the Earth has warmed in the second half of the twentieth century at a faster rate than predicted by some models.
If closed top containers were used in the experiment, the result would be completely different. The experiment demonstrates differences in convective heat loss - not radiation absorption.
As CO2 is a heavier than air gas, it can be made to stay inside an open top container if it's poured in gently. Because if it's density it can become heated without becoming buoyant in the surrounding air. By comparison, warm air will not remain in an open top container because it is buoyant.
- 1 decade ago
Interesting. Here is a cite to your article: http://dcdc.asu.edu/K-12Education/greenhouselab.pd...
I would be really hesitant about relying on that experiment to prove anything.
Potential areas of trouble are:
(1) The substrate -- which is absorbing high frequency IR and storing it. It will lose heat by conduction, convection, and radiation to the air inside the tank. It becomes difficult to differentiate between the various mechanism of heat loss. You should get a pretty good heat engine rolling.
(2) The open top will allow the warm air to convex out taking with it CO2 spiked air. No, the CO2 will not appreciably settle at the bottom of the tank under these conditions unless it is also colder--however, once you have established an enriched gas, it will tend to stay enriched. Unless there is some convection, the only way it can mix is by diffusion.
(3) the mean free path for an IR photon before it meets a CO2 molecule would be fairly large compared with the size of the container.
(4) the CO2 will help cool air molecules that have picked up kinetic energy from the substrate. The only way air can cool is bouncing off a solid, or bouncing off a GHG. So the CO2 could have a negative feedback under these circumstances.
Lastly, (5) the CO2 being heavier than normal air will reduce the convective transfer of gas out of the terrarium. (stratification). This decrease in convection could easily account for the increase in temperature compared with the non CO2 enriched gas.
The paper, itself has a diagram showing a decrease in temperature everytime CO2 is generated from baking soda and vinegar. Is this what you are referring to? Or is there something more.
==> I'll make a guess that you know a little more than is contained in the paper. If so, the most probable mechanism is, the increase in CO2 dampens the convection out of the terrarium, however, with time, the loss of CO2 out the top will return the terrarium to its initial equilibrium.
Given the above, I would be very hesitant about using this particular experiment to prove anything. I'd have do multiple trials with changed conditions to be confident in it
**********
JDLL, I don't think we are in serious disagreement. I was just postulating potential problems.
While CO2 will mix with air and stay mixed, if you get stratification -- which could be a CO2/gas mixture -- you will have very little mixing. Gas only moves quickly when you have group motion such as with convection. Diffusion is dirt slow because of the short mean free path.
I'm pretty sure based on your statements that a layer of CO2 forms at the bottom and because of its density, even though it is warmer, it doesn't mix. On top of that convection cells form taking heat away from the boundary layer. In addition, there is mixing at the boundary layer and CO2 becomes entrained thus gradually eating away at the bottom CO2 layer until the thermometer is above the CO2 layer.
Anyhow, a very interesting question..
- Anonymous5 years ago
Trevor, A few questions. 1.) Your graph has the CO2 and Methane together. 2.) It looks like your methods may tend to add more H2O into the CO2 bottle. How did you account for this difference. 3.) It looks like your methods would tend to make the Methane and CO2 bottle pressurized, or at least more pressurized than your normal bottle. Since it is known that denser air tends to hold more heat, did you do anything to account for this. Trevor, The temps would have to be the same prior to placing the cap on to ensure the same overall pressure. Clearly if one is colder, it will have a higher pressure than the others after the temps are equal. Your processes would likely change the temps. The indigestion tablets in water would most likely change the H2O content. Trevor, Clearly not, and I am not suggesting that your experiment is bad for a lab top. I would simply change how I did it slightly. I would keep the set-up the same, but place a small chunk of dry ice in one. Place the cap on so that it is closed, but not tight enough that no air escapes. Then, when the temps meet, I would tighten the caps. Same temp, same pressure, more CO2 in one. Otherwise I would keep the experiment the same.
- How do you think about the answers? You can sign in to vote the answer.
- MTRstudentLv 61 decade ago
Open top containers.
The greenhouse effect is what inhibits radiation heat loss. Open top containers allow the air inside the containers to quickly conduct and convect the heat out - you can't convect heat from the atmosphere into outer space, it has to be done by radiation!
Even with closed topped containers you'd get some conduction, but not much since plastics aren't great at conducting heat.
- 6 years ago
This is a very good discussion so far. I would just like to bring it up to date by linking the following article: http://www.researchgate.net/profile/Michele_Berto/... Climate_change_in_a_shoebox _a_critical_review/links/5450d6de0cf249aa53dbd4cb.pdf
The authors address the issues of radiative vs. convective effects, and show several direct measurements to support their analysis. They also propose a modified version of the experiment, using CO2 and N2O (which have similar convective, but different radiative effects), to isolate the radiative effects in the experiment.
- antarcticiceLv 71 decade ago
This experiment was done by the Mythbusters they used closed containers. I have also helped set up the same experiment many times for visiting school groups and it has always been with closed containers.
http://www.youtube.com/watch?v=pPRd5GT0v0I
The below link uses a different method using a candle and and infrared camera.
http://www.youtube.com/watch?v=SeYfl45X1wo
Even a good portion of deniers no longer claim Co2 is not a GH gas.
"@antarcticice: this is not a post about denial or faith or betrayal or religion but about physics experiments and their interpretation."
Which is why my comment was about the experiment with two links to back it up one to the closed experiment and another to an experiment using a thermal camera if you see faith or betrayal or religion in that then I feel sorry for you.
- A GuyLv 71 decade ago
Because it is not standard. I do not know of any school system where this is done!
