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CO2 saturation point?

  1. Apr 26, 2008 #1
    can any one tell me what the saturation point of CO2 is in the atmosphere in relationship to it's effect as a green house gas? At what point does extra CO2 have no more effect?
  2. jcsd
  3. Apr 26, 2008 #2
    There's no real saturation point. The effect is approximately proportional to the logarithm of the concentration, so each extra ppm will do less.
  4. Apr 27, 2008 #3
    let me rephrase that then. What is the effective saturation poit? I realize that there is a diminishing return, but at what point?
  5. Apr 27, 2008 #4
    Let's rephrase the correct answer then,


    on this graph we used the calculation model MODTRAN to compute the theoretical radiative balance with increasing concentrations of CO2 and CH4 assuming no feedbacks. For each order of magnitude increase of concentration 1- 10 -100 etc, the radiative balance is restored at about a fixed increase in temperature according to the model. The dimishining return starts at the beginning.

    http://geosci.uchicago.edu/~archer/cgimodels/radiation_form.html [Broken]

    But then again, it's only a model

    Happy to explain how it works, if it interests somebody.
    Last edited by a moderator: May 3, 2017
  6. Apr 27, 2008 #5
    please this is exactly what I was looking for.
  7. Apr 28, 2008 #6
    Okay, I did it here before, but it's always relevant and topical. So rather than linking, I propose another round, also since I would like to discuss some relevant issues.

    MODTRAN here is a basic mathematical tool to calculate radiation profiles in certain kinds of atmosphere, using the HITRAN database. The USAF originally develloped it, decades ago for analsying IR windows in the atmosphere for develloping IR missile seeker heads.

    So let's open MODTRAN, leave every entry to default and hit "submit the calculation".

    So this situation that emerges in the null hypothesis which assumes radiative balance. Radiation energy in is radiation energy out. On the right top we see that this should be average

    while the graphs show a nice absorbtion spectrum with dips for CO2 around 700 (?units?), O3 around 1000 and CH4 as of 1200

    Anyway lets double the CO2 (750) and calculate again to find:

    Assuming no change in the solar input, we now have radiation unbalance, less energy, about 3.2 W/m2 getting out. Hence the surplus energy, not reradiated back to space is heating the Earth until the balance is back to the original energy value (287.844 W/m2). Therefore we increase (trial and error) Earths temperature (Ground T offset, C) with 0.89 degrees and, lo and behold, we are back to the original radiation balance value. Hence doubling CO2 in a tropical atmosphere would seem to lead to an increase of 0.89 degrees C. That is if radiation balance is necesary and you wait long enough for that to happen.

    To be continued
  8. Apr 29, 2008 #7
    Ok, so I played with MODTRAN last night for quite a while. I also called up a friend of mine who uses MODTRAN in his daily job to discuss things.

    It seems to me that the effective saturation point for CO2 is about 30ppm.

    Does this seem about right to you?
  9. Apr 29, 2008 #8
    As said, there is no real saturation. How would you define an "effective saturation point"?
  10. Jun 6, 2008 #9
    If the partial pressure equals the vapour pressure, the saturated is acquired.
  11. Jun 6, 2008 #10
    That's a different approach, I think. The idea was, at which point would adding more CO2 to the atmosphere not lead to more greenhouse effect and according to the common ideas there is no saturation point as the assumed relationship is more or less logarithmic. Hence the increase of greenhouse effect from an concentration change from one to two part per million volume (ppmv) equals that of 280-560 ppmv or 1000-2000 ppmv.

    However this relationship is now being challenged by Miscolzki.
  12. Jun 26, 2008 #11
    I've read before that there is a 'CO2 saturation point' where the atmosphere will hold no more CO2 and the excess is lost. Is this incorrect? or maybe just meaningless if the emissions rate maintains or increases CO2 levels despite what ever will eventually be lost?
  13. Jun 27, 2008 #12
    That should be correct. Of course we are looking at a complex carbon cycle.

    You could compare the carbon (carbon dioxide + methane etc) in the atmosphere with a bucket of water, filled by several water taps but drained by holes in the bottom. Within certain limits, if the draining equals the filling, both being constant, then the water level is constant as well, increase the rate of filling and the water level will increase, which increases the draining rate due to the increased water pressure. As soon as the draining rate matches the filling rate again, the water level stabilizes again but at a higher level. Dynamic stability. If that's what you would call, saturation point, then sure, with constant rates of filling and draining the CO2 level should reach a dynamic stability point/saturation point.

    The carbon filling and draining of the atmosphere is supposed to look like http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/earth_system/carbon_cycle_NASA.jpg [Broken].

    It's more complex than the bucket, looks more like many buckets, where the rate of exchange between fast and slow cycles is important. The upper ocean - atmosphere exchange is fast but http://www.geographypages.co.uk/weathering.htm [Broken] as well as organic carbon burial are much slower cycles. It would require for that part to match the fossil fuel burning rate before dynamic stability can be reached. The fossil fuels would probably deplete first, which also would limit the maximum CO2 concentration.
    Last edited by a moderator: May 3, 2017
  14. Jun 27, 2008 #13
    Perhaps 'saturation point' is the wrong term for what I am talking about. The article I read(which I unfortunately can't find any more) stated that our atmosphere was only capable of 'trapping' or 'holding on to' a certain level of CO2. Something akin to say... carbonating soda water. The water will only 'hold' so much carbonation and eventually the extra gas will just be lost. So according to this article the excess CO2 apparently would leave the atmosphere. I'm unsure now of this idea that it would just drift off into space, whether or not it is possible (though I imagine it would be).
    At any rate I doubt it's going to go straight up and out of the atmosphere. It'll be there anyway until it's capable of escaping (assuming that it can).
    What I'm wondering then is (if this is true and if we've exceeded the "saturation point") how quickly should CO2 levels drop when emissions are reduced? Is it effectively trapped for a significant amount of time regardless?
    Last edited by a moderator: May 3, 2017
  15. Jun 27, 2008 #14
    Apparently edits must be approved.
    Sorry Andre. Rereading your post I realize that I misundertood. Your analogy is certainly better for illustrating the effect. I'm just tired.
  16. Jun 30, 2008 #15
    In the "Gore warming"(my name) years, which show very convincing evidence that man is the primary cause of global warming, has the very significant increase in CO2 in that period been accompanied by a similar and corresponding measurable increase in the biomass of photosynthetic organisms that perform the main CO2 reduction function on earth?

    I would surmise that a rapid and manyfold increase in the latter would be the natural response to a similar increase in the former until a saturation point is reached. Has that saturation(tipping) point been reached? Tough questions. Anyone? And, links to research data if so? Thanks.
  17. Jun 30, 2008 #16
    Have you seen the (mandatory in the UK) guidelines?


    Doesn't seem so, as the rate of increase of CO2 concentration is not levelling off at all.
  18. Jul 1, 2008 #17

    I think a statement in this story is in error about the additional amount of water the Greenland ice cap would add to the oceans and how it would be added.

    It is my understandingof Gore's words that the world's oceans would immediately rise at least 20 feet if the Greenland cap slid off its rock base into the sea at one time, not by gradually melting for many years. Isn't that what Gore said? Could be wrong.

    Also, I read a report after Gore's film that stated in the unlikely event that the Antarctia ice sheet also slid into the sea at the same time as Greenland's, almost 1/3 of the world population would immediately drown. I haven't done any math calculations on that re: ice volume/ocean volume/population centers, but think offhand that it is a gross exaggeration. And, nature rarely does its work so dramatically and instantly, and even massive killing floods, volcanos, earthquakes, and tsunamis are rare if you graph them over time.

    Anyone know more on this?
  19. Jul 5, 2008 #18
    You ask about a CO2 saturation point for the atmosphere, implying that above a certain value, carbon dioxide would have no further effecton infrared absopption. You received responses that indicated no saturation value exists. But you are trying to get at something important. You have focused on the Earth for CO2 saturation but you may want to add Venus to your model. It has an atmosphere that is 96.5% CO2 more abundant than that of Earth. Its surface pressure is 9.63 mPa, compared to Earth's 101.3 kPa, a ninetyfold higher value. The incredibly high carbon dioxide density at Venus’ surface has attracted greenhouse believers into explaining its high surface temperature, 462-480 oC., using the greenhouse effect. But the massive nature of its atmosphere and its adiabatic behavior are more important in surface temperature production. Venus radiation balance is based on its sulfuric acid cloud, an opaquely thick aerosol that increases its albedo to 65% of solar light and has a temperature at its outer surface of 260 oK, http://www.datasync.com/~rsf1/vel/1918vpt.htm As with Earth, solar radiation breaks some molecules down. For Earth it is molecular oxygen to atomic oxygen and then ozone. For Venus it is carbon dioxide to carbon monoxide and atomic oxygen. This time the atomic oxygen converts atmospheric sulfur dioxide (150 ppm) to sulfur trioxide that adds water to become sulfuric acid. The effect is to add energy to the layer where chemical change is happening, raising its temperature. In the case of Earth, this makes the top of the stratosphere warmer than its bottom. In the case of Venus, it flattens the rate of fall above the sulfuric acid cloud. The Venus atmosphere above the cloud needs only to reflect 28.5 W/m2 back into the cloud to balance the post-reflection solar radiation input of 230.6 W/m2, a value lower than the 32 W/m2 attributed to CO2 by Kiehl and Trenberth for Earth’s radiation balance http://www.atmo.arizona.edu/students/courselinks/spring04/atmo451b/pdf/RadiationBudget.pdf [Broken]. You could see this as a saturation behavior, but I believe that there is a different explanation and will post it on this forum shortly. Please look for it and comment. I have also noted the discussion of the Miskolczi paper by Andre and will try to compare my explanation with his in the future.
    Last edited by a moderator: May 3, 2017
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