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Post by socold on Nov 11, 2010 19:49:15 GMT
Mars has 12 times more co2 than Earth, but it doesn't have a water vapor feedback. There's also the pressure difference which I think makes a difference to absorption. What the models show for these question is covered here: www.giss.nasa.gov/research/briefs/schmidt_05/
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Post by af4ex on Nov 11, 2010 20:37:39 GMT
socold: > Mars has 12 times more co2 than Earth, Yes, globally, but Earth is much bigger. The important effect is locally, where Mars has x 30 the concentration of CO2. > but it doesn't have a water vapor feedback. But we're not discussing feedback. Let's do this scientifically and tackle the issues one by one. The issue is the "GH warming" effect of CO2 caused by IR blockage. So, how much of the Terran 33C "comfort blanket" is due to that effect? Latest results in from our Mars Greenhouse Lab are suggesting the effect may be much smaller than Gavin Schmidt would like to admit. > There's also the pressure difference which I think makes a > difference to absorption. Steve and I have been discussing that. Even if there is a spectral shift caused by pressure, how does that effect the IR blockage? The "shifted" molecules don't have any extra capacity. And the main absorption band (14k nm) is right in the middle of the 210K black body curve. So please explain how small shifts in the molecules resonant frequency causes the negligible warmth to happen? I.e. show me the formula, not a bunch of hand waving. Attachments:
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Post by socold on Nov 11, 2010 21:13:49 GMT
The paper contains: In a broader perspective, CO2 greenhouses also operate on Mars and Venus, because both planets possess atmospheres with substantial amounts of CO2. The atmospheric greenhouse effect requires that a substantial fraction of the incident solar radiation must be absorbed at the ground in order to make the indirect greenhouse heating of the ground surface possible. Greenhouse parameters and relative surface pressure (PS) for Mars, Earth, and Venus are summarized in Table 1. Earth is unique among terrestrial planets in having a greenhouse effect in which water vapor provides strong amplification of the heat-trapping action of the CO2 greenhouse. Also, N2 and O2, although possessing no substantial absorption bands of their own, are actually important contributors to the total greenhouse effect because of pressure-broadening of CO2 absorption lines, as well as by providing the physical structure within which the absorbing gases can interact with the radiation field.pubs.giss.nasa.gov/docs/2010/2010_Lacis_etal.pdfTable 1 contains: Surface Temperature Mars 215K Earth 288K Emission Temperature Mars 210K Earth 255K
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Post by af4ex on Nov 11, 2010 22:55:39 GMT
> pubs.giss.nasa.gov/docs/2010/2010_Lacis_etal.pdfI've heard about this paper but haven't read it. Thanks. > Table 1 contains: > Surface Temperature > Mars 215K > Earth 288K I've said that I expected the Martian atmosphere to have some warming effect, so I was a little surprised that NASA shows the mean temp as "~210K". I would like to know how this 215K 'surface temperature' was derived. Perhaps cited in the paper. [But looks like there is a spread of opinions on how this should be done: hypertextbook.com/facts/2001/AlbertEydelman.shtml]In any case, I believe the case for "CO2 GH Warming" has been greatly overstated.
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Post by socold on Nov 11, 2010 23:23:39 GMT
I suspect it's calculated backward, ie calculating what the greenhouse effect should be on Mars, and adding that to the emission temperature, so not based on observational data.
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Post by nautonnier on Nov 12, 2010 2:26:30 GMT
I suspect it's calculated backward, ie calculating what the greenhouse effect should be on Mars, and adding that to the emission temperature, so not based on observational data. " not based on observational data."There's a surprise!
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Post by af4ex on Nov 12, 2010 13:10:01 GMT
FYI, WUWT has posted an article by Roger Pielke Sr. on the Lacis paper (which I just started to read). Interesting discussion, with some comments provided by Lacis himself. wattsupwiththat.com/2010/11/11/27720/#more-27720There was an earlier article on WUWT when the paper first came out. Lacis made a comment there too and was heckled considerably. I don't like that kind of attitude, on either side of the Skeptic/AGW fence. I may disagree with Lacis' findings, but I respect his work as a scientist. A lot of these 'hit and run' barbs are hurled by "anonymous cowards" who are acting rather childishly, IMHO. :-|
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Post by jurinko on Nov 12, 2010 13:22:50 GMT
Without water vapor, the earths temp would be its blackbody temp. If 400 ppm CO2 makes 5-10% of the total "greenhouse effect" on Earth, Mars atmosphere containing 6000 ppm of CO2 creates equal "greenhouse" lid as Earth does. And - nothing. The difference is the missing bulk atmosphere, composed of nitrogen and oxygen. No atmosphere - no contained heat. All those watts per square meters are nothing. Whether the heat is really contained by four molecules of CO2 and hundred molecules of water vapor per 10000 other molecules looks as a matter of belief. Fact is, deserts with very dry air (thus much less "greenhouse effect") are not colder in average as other areas. But in this case, absence of evaporative surface cooling and clouds heats up day and cools nights there. One more thing; from each 10000 molecules in the atmosphere, all 7000 molecules of nitrogen, 2100 molecules of oxygen, 1000 molecules of Argon, 100 molecules of water vapor and four molecules of CO2 radiate heat, since they all have temperature above zero K. The only difference is, that some of them can get warmed only via conduction/convection and some can also absorb weak IR emitted by warm surface. Whether the increase of CO2 molecules from 3 to 4 is causing catastrophic warming is a million dollar question.
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Post by steve on Nov 12, 2010 13:48:47 GMT
> The Compton effect relates to the scattering of > photons by (usually) electrons ... Yes, I know. It's an analogy, trying to understand what might be happening. The end result is the same: spectrum shift. Like I said, I'm not denying this could be cause of your "broadening" effect, but I'm skeptical (of course). I don't see how the broadening could be an instrument effect if the experiment is properly designed - ie the detector is not in contact with the emitting material, and the detector has sufficient resolution. It is possible to detect to a much finer resolution than the width of the lines we are talking about. The way I see it, the CO2 spectrum is a result of the different modes of bending, rotating and vibrating of the molecule and are dependent on things like the moment of inertia of the molecule (a function of the size of the molecule). In the presence of another electric field (such as from another molecule) its size and shape will change, but only by a bit, and this would affect its quantum states. Compton scattering is just an elastic scattering and the outcome is a continuum spectrum. For a given angle of scatter the frequency is constant for a stationary electron, but is dependent on the velocity of a moving electron. A higher temperature material will broaden the spectrum of photons received by a detector at a particular angle to the point of incidence. You'd need a very hot plasma to make muvh difference. This is perhaps analogous to doppler broadening where emissions are red or blue shifted.
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Post by steve on Nov 12, 2010 14:22:45 GMT
There is an online Modtran calculator for earth geoflop.uchicago.edu/forecast/docs/Projects/modtran.orig.htmlThis is a very rough calculation, but if you set CO2 to 980,000ppm (98% of the atmosphere) altitude of sensor to 35km (approx level where pressure is the same as earth) sensor looking up and subarctic winter then you get a spectrum that peaks at just above 220K - so roughly the same temperature as Mars. This represents the back radiation (as the sensor is looking up) which is 18W/m^2. If you plug an extra 18W/m^2 to the BB formula you get a greenhouse warming of about 8K. ie. kT^4=energy emission For a surface at 210K 5.67e-8*210^4 = 110 Watts/m^2 If there is back radiation of an extra 18 Watts the surface will warm to approx: 128 Watts/m^2 = (128/5.67e-8)^0.25 = 218 Kelvin Obvious qualifications are that the calculator might be imprecise as it is running out of normal scope, the back radiation is dependent on the lapse rate of the Martian atmosphere which will be different to what is in the calculator and the diurnal variation on Mars has a big impact given that radiation is proportional to the 4th power of the temperature.
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Post by steve on Nov 12, 2010 14:35:57 GMT
This page may be of interest. www-k12.atmos.washington.edu/k12/resources/mars_data-information/temperature_overview.htmlOne thing it says is that the temperature sensor sees variations of up to 20C over periods of a few minutes. This suggests to me that the data may not be sufficient to detect an 8C greenhouse effect. In our discussions we are also neglecting the fact that atmospheric dust is far more important on Mars than it is on earth. The following paper assumes the average radiation from the atmosphere is 2% of the noon insolation (so about 12W/m^2 compared with my calculated 18W/m^2). But it also says that the dust load can vary this by a factor of 2-5.
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Post by af4ex on Nov 12, 2010 17:06:25 GMT
jurinko> Whether the increase of CO2 molecules from 3 to 4 is causing catastrophic warming is a million dollar question. Excellent way to pose the crucial CAGW question! Although I might quibble that it's a trillion dollar question, if you factor in the expense of Kyoto etc. :-] steve> line broadening, dust etc. You've made some good points too, on which I need to cogitate some more. I'll comment on these later today or tomorrow. Tnx
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Post by af4ex on Nov 12, 2010 17:56:01 GMT
socold> I suspect it's calculated backward, ie calculating what the > greenhouse effect should be on Mars, and adding that > to the emission temperature, so not based on observational > data. Yes, that's a perfectly legitimate part of any modeling: to try to predict or explain real observations. I'll have to brush up on Arrhenius (is that still considered a valid model?) and see what the range of reasonable expected temperatures might be. (or maybe some of you folks have already done that. Please post). It's not easy to calculate, or even define, what 'average temperature' means with respect to an entire planet. There's so much variation. Between day and night, equator and poles, valleys and mountains. Like trying to predict the 'average speed of an automobile' for the entire world, including parking lots. If temps are normally distributed, then mean and variance are 'complete' statistics. But I doubt that is the case. I think it would make more sense to pick a spot and calculate it's BB temp (maybe adjusted for albedo etc) and compare to actual observations.
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Post by steve on Nov 13, 2010 13:55:01 GMT
I'll summarise briefly because my input yesterday was a bit incoherent.
1. Someone seems to have calculated the back radiation from the Martian atmosphere (roughly a measure of its greenhouse effect). They reckon it is 2% of noon-time solar incidence - about 12W/m^2 which would cause warming of roughly 5 Kelvin.
The equivalent figure on earth (according to the classic Trenberth energy budget diagram) is 333W/m^2 (about 25% of noon time solar incidence at top of earth's atmosphere). According to the Modtran simulator (with water vapour and other gases set to zero) about a third of this is from CO2.
2. But they say dust content in the atmosphere can change the Martian back radiation by a factor of up to 5. We must also assume that the dust content will simultaneously reduce the solar radiation reaching Mars' surface.
3. The sensors on the Martian surface are measuring rapid changes in temperature suggesting an average temperature is hard to measure. There are only a few sensors on the surface and it is not clear whether they are standardised to measure absolute temperatures accurately.
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Post by hunterson on Nov 13, 2010 16:45:21 GMT
The question is poorly posed. Without CO2 there would be catastrophic and disastrous cooling. The amount of CO2 we have keeps us comfortable. Doubling the amount of CO2 will cause significant damage to our economy because it will change weather patterns. The absorption lines of gases including CO2 are affected by both temperature and pressure. The higher the temperature and pressure the broader the lines. The temperature on Mars is 80C cooler than earth. The pressure is 0.7% of earth's atmospheric pressure. This means that the CO2 absorption lines are much narrower than those on earth. There is also very little water vapour. This allows most of the radiation from Mars' surface to escape to space without being absobed by the CO2. Without CO2 the planet would probably cool so much that the amount of water vapour will be significantly reduced also. So quite a lot of the 33C is attributable to the warming effects of CO2 plus the water vapour that can be held by the warmer atmosphere. Close to its BB temperature. The greenhouse effect of the small amount of CO2 would, like on Mars now, be further reduced by the narrower spectral lines resulting from the reduced atmospheric pressure and temperature. You have no proof that doubling CO2 will cause destructive changes to our weather patterns.
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