> The MODTRAN web user interface allows you to see radiative fluxes
> given a temperature profile. The temperatures are an input, not an
> output.
>
> No one denies that the atmosphere becomes opaque at absorption bands
> as the absorbing constituent increases. In the tool exposed on the
> web, though, and in MODTRAN standing alone, the feedback to
> temperature is absent. So it doesn't answer the question.
It certainly looked like you were making that denial.
From one of your responses "It seems to me your argument (which
peculiarly seems perfectly happy with a 7 C increase in mean
temperature, albeit on an unspecified time scale) remains based on the
idea that the greenhouse effect saturates."
Looked rather like a denial to me.
And this one: "You may wish to work through it, and play with the
parameters to see
if you can get it to saturate." Looked rather like a denial to me.
Now, I know that modtran doesn't output to temperature, it isn't a
climate model, it's a culumn atmospheric gaseous spectrum analyzer.
however, since the whole basis for global warming is that these are
radiatively active gasses, the radiative responses of rhe relevent
gasses over the relevent concentrationsDO signify. Since the modtran
results show VERY minimal difference in the spectrum response from
concentration 600 ppm to 2000 ppm, however, a fairly dramatic
difference between 280 ppm and 600 ppm, regardless of the latitude or
temperature input, I'd have to say that's fairly clear on the point
that band opacity is a fairly important factor in the analysis of
global warming.
> Once again, Bill, you are (at best) assuming you understand things you don't.
Once again michael, you are at best, assuming I said something I
didn't.
> about the project:
> I can take no credit for this project. (Remarkably, David's young son
> Jeremy did the web interface, I think at the age of 12, building on
> some work of David's to simplify setting up the runs for his undergrad
> classes.)
Impressive!
> MODTRAN itself is a product of the USAF, the result of many years of
> professional effort, though. For some reason some patents apply, which
> I find not especially satisfactpry as a taxpayer, but taht's a bit
> beside the point.
> logarithmic behavior of CO2 in Earth's atmosphere only applies over a
> limited (but rather extensive) range of concentrations. At very low
> concentrations (say, around 1 ppm) bands are unsaturated and OLR
> becomes more sensitive to CO2 than in the logarithmic range. At
> sufficiently high concentrations (say, when you start to get around
> 10% or 20% of CO2 in the atmosphere) the absorption starts to be
> dominated by weak bands that have a different probability distribution
> than the bands that dominate in the present climate; this again starts
> to lead to an increase in sensitivity. Radiative transfer is
> complicated because of the complex line structure of greenhouse gases,
> but for a long time I have been looking for a simple, accessible
> explanation of the typical logarithmic behavior. I'm writing that
> section of my climate book now (check Chapter 4 of The Climate Book
> in a few months). As far as I can tell, the simplest way to put it is
> like this: CO2 opacity for the present Earth is dominated by the 15
> micron band group. The envelope of the absorption strength in this
> group tails off roughly exponentially from the center of the group,
> once the lines are broad enough to overlap significantly within each
> sub-band of the interval, and the resulting probability distribution
> of absorption can be shown to give rise to the logarithmic behavior.
> However, the exponential envelope is only approximate, and only
> extends a certain distance out from 15 microns, so once you put in
> enough CO2 you get out of the logarithmic range. Hence the answer to
> your question, roughly, is that it depends both on
> pressure/temperature broadening and on CO2 concentration. To get a
> logarithmic behavior, you need enough pressure or temperature to make
> the lines broad enough to start overlapping, but if you put in too
> much CO2, you make the overall width of the principal absorption
> region (that's not the line width!) wide enough that you get out into
> a different shape of envelope, and lose the logarithmic behavior.
It can be plainly seen in the modtran interface. Between
approximately 100 and 50000 ppm, the response to doubling co2 is
relatively small and diminishes, thus each doubling produces a smaller
effect, and concerns only the response in the 600-800 band. Below
that, the 6-800 band isn't saturated and the response is fairly
dramatic. Above that, the graph becomes more "ragged", with absorption
bands appearing in spectra that were unaffected at the lower
concentrations, and the response to doubling co2 becomes very
significant again.
What else becomes pretty obvious is that from 550 to 1100 ppm co2
is an almost undetectable difference significantly smaller than the
difference between 280 and 550. Now, since that's a LOT of co2
release, and the 550 ppm stabilization is a moot point, since it has
poorer odds of happening than an extinction level meteor impact, next
question, and one that hasn't been much looked into, is what's involved
in an 1100 ppm stabilization?
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