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Percent water vapor - greenhouse gas

  1. Mar 9, 2011 #1
    I'm have armchair interest in greenhouse gases.
    One of the interesting points is the amount of greenhouse effect that CO2 has versus water vapor.
    I go to this page
    and find that the relative percentage of atmospheric humidity is 45%. I suppose that is the best guess average of measurements taken throughout the atmosphere of planet earth. Does that mean 45% of the atmosphere is made of water vapor?
    Then on the other hand we have CO2 which makes up .0390 percent of the earths atmosphere.
    When we measure the greenhouse effect of each I know we are not comparing apples to oranges. I have read a thread on this forum talking about saturation effect of CO2. Meaning the more you have the less of a greenhouse effect it creates. Can the same thing be said of water vapor?
    How much of the greenhouse effect is from water vapor and how much is from CO2 at current levels?
  2. jcsd
  3. Mar 9, 2011 #2
    Lots of questions. Start with the easy one. Relative humidity at a given is the percentage of water vapor relative to the point at which condensation occurs. The actual humidity can be much higher at higher temperatures, thus the old saw about it being "Too cold to snow."

    In general, water vapor swamps CO2 as a greenhouse gas. The reason most climate modelers will tell you that atmosphere models are worthless junk, is partly the non-equilibrium behavior of water vapor. (Take two masses of air, at the same temperature and pressure, with the same water content. One will have clouds, the other won't.) The other problem is that it is just too computationally expensive to run large scale simulations with data at every thousand feet--and even those won't catch some of water's trickier behavior.

    Finally, just to drive you up the wall--clouds can remove CO2 from the air, and of course, release some of it when it rains. Since the level (feet or meters from the surface) of CO2 determines whether it traps or releases heat, good models try to deal with this as well.

    When these large scale models can come close to the results observed with weather balloons, then they may be useful in predicting future global warming.
  4. Mar 9, 2011 #3
    on this page
    it says
    "Another gas, water vapor, also exists in small amounts. It varies in concentration from being almost non-existent over desert regions to about 4% over the oceans."
    and either there or somewhere it says it's averages about 2-3% of concentration. Does that seem right?

    Also, I know that as CO2 increases, it has less of a greenhouse effect, it gets saturated, does the same thing happen to water vapor?

    >The reason most climate modelers will tell you that atmosphere models are worthless junk

    you must not see/read the news too much!!!<g>
  5. Mar 9, 2011 #4
    you must not see/read the news too much!!!<g>

    LOL! There are two groups of climate scientists, those who believe the atmospheric models, and those who create them.

    Personally I think that there are very good reasons to limit CO2 levels that have nothing to do with global warming. Dinosaurs may have evolved to handle high levels of atmospheric CO2, humans didn't.

    Having gotten that out of the way, I can tell you I got involved with such models back in the '70s when there was no way that a model was realistic. You may remember the "nuclear winter" of the TTAPS model. The TTAPS model was one dimensional (height, no length or width) and as a result it held smoke at high altitudes for months--there was no convection of any kind going on.

    Today's models are much, much better. But next time there is a hurricane in your area--or a winter storm--pay attention to how the weather forecast changes from day to day as the storm approaches. The weather forecasters have several supercomputers at hand, and real-time feedback several times a day. Even so, they don't trust their models beyond 3-5 days.

    Now that hurricane significantly changed the vertical distribution of CO2 (as well as the geographical distribution) but the NOAA model doesn't include that as a modeled parameter.

    So the belief in global warming, is a belief that, yes, CO2 acts the way simple models predict. Disbelief in AGW comes from the real data--that still doesn't match our models.
  6. Mar 9, 2011 #5
    >I can tell you I got involved with such models back in the '70s
    >that still doesn't match our models.

    considering the information you are giving me, you sound more like a climate scientist "hobbyist", because a $real$ climate (rush limbaugh cough cough) $scientist$ can't possibly think that way and get paid by anyone, not in this day and $age$. must be retired.

    water vapor 2%, CO2 .039%, I mean who needs that kind of info these days? what good would it do anyone?

    This kind of information would only lead to doubt and questions. besides, just look out your window. you can see what's happening.
  7. Mar 9, 2011 #6
    sorry to be a pain.
    but this page, an "environmental science" exercise for student says on page 2 that CO2 has about the same concentration in the atmosphere as water vapor, about .04%.
    does that seem remotely possible?

    I read and linked in a previous post the page that stated that water vapor is around 3% of the atmosphere, give or take a percent. someone is wrong big somewhere. but the page linked here has the word "environment" in it, so it must be right, huh?

    Here's another page, it talks about the natural greenhouse gas effect, caused by "certain naturally occurring greenhouse gases", they have a side bar about water vapor, down below, but they certainly forgot to mention what percentage of the greenhouse effect it has.
  8. Mar 9, 2011 #7
    I did retire in January, but not from climate science. Even though I have a Masters in Operations Research and Statistics, I spent most of my career trying to get computers to do more than they could. It seems crazy that computers are now over a billion times faster than when I got interested in computer modeling, but they still are not fast enough.

    It works like this. Faster computers (or better algorithms, usually my part of the picture) means that you can do more modeling at a lower price. As the price drops demand rises. However, people like me (with my statistic hat on) keep finding "interesting anomalies or features" in the data. So run the model a few more times, probably at higher resolution.

    When modeling a car crash or bending an aircraft wing, someone has the job of telling me to put my concerns in a memo, and consider the job finished. (I've worn that hat too, just don't try to wear both at the same time--you get ulcers.) For well behaved models, that process ends pretty quickly. It is very unusual to have issues that can't be nailed down by tweaking the model and running it again.

    As for climate modeling, and atmospheric modeling in general, the big problem can be stated succinctly. You want to model four dimensions, latitude, longitude, height above sea level, and time. It is possible to tile a full Earth model so points don't get close together at the poles and far apart at the equator. But that saves you less than a factor of two. Let's start with modeling every fifteen minutes of latitude and longitude, and twenty points between the surface and the stratosphere. About as rough as you can expect any results at all from. So 360x4x180x4x20, just over 20 million points per iteration. How about 5 minutes for the time step? You might be able to do it today on a desktop computer--OpenCL and DirectCompute allow you to use your high-end video card to crunch the numbers. (But you will need help from me or some other expert to flow the data through the computer in the best order to keep it crunching data not moving it around.)

    In any case, you might be able to model 5 minutes of real time in say two minutes. NOAA has worked with models like that where to predict three days forward from today, you started the model running the day before yesterday.

    Go to a model about five times finer in every dimension, and you should get some really good climate data. (Assuming the math of your model is right--that is the easy part.) But we just made the model run 3125 times slower, and it would be nice to model a decade or three to get useful data. Maybe in another ten years, but not happening today.

    So why are the current models not good enough? Unfortunately the big 64 dollar question is how do clouds and CO2 interact? Heat transport from the surface to the stratosphere occurs through radiation and convection. If you bias the models in a non-realistic manner, say no radiative transport, convection increases to take up most of the slack. But the models also do strange things like moving the snow cover line way south. So when you try to relate the models back to reality, you need "fudge" factors to reflect what you really learned. You also try to put brackets on the model results rather than looking at one "perfect" run.

    As a result anyone who argues that the net effect of CO2 is to trap heat either fell asleep during the discussion, or has a bridge they are trying to sell you. In any decent model I have seen it traps heat on Tuesday but not on Wednesday...
  9. Mar 10, 2011 #8


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