Thursday, November 09, 2006

Stratospheric cooling rears its ugly head....

UPDATE: For a parallel posting with wit consult the Capitalist Imperialist Pig

Stratospheric cooling is a base result of global circulation models (GCMs) and various radiative transfer codes. Stoat and Rabett have tried to find simple explanations, but maybe not succeeded. One of the problems is trying to figure out how much of the cooling is due to ozone loss (less ozone, less UV absorption, less heat in the stratosphere, that one is simple:( and how much is due to increasing greenhouse gases. To the surprise of all small cute animals, the ozone contribution is small

but the GHG contribution is large.

UPDATE: What Rabett Labs originally put up for Andrew Bolt correction has metastisized a bit (S. RAHMSTORF ALERT: the last link has spread to Shaviv and Verzier territory) the bunny will hop back into the Schreibetorium and add a few details, most of which come from the on-line Stratospheric Ozone Textbook, which REALLY has all you want to know about stratospheric ozone (this is the stuff up there, not teh stuff down here which leads to ozone alerts).

We have located a simple explanation of the phenominon (The bunch of carrots with parsley decoration to Dr. Elmar Uherek) Go read the whole page, which has much useful information, but the gist is

The second effect is more complicated. Greenhouse gases (CO2, O3, CFC) absorb infra-red radiation from the surface of the Earth and trap the heat in the troposphere. If this absorption is really strong, the greenhouse gas blocks most of the outgoing infra-red radiation close to the Earth's surface. This means that only a small amount of outgoing infra-red radiation reaches carbon dioxide in the upper troposphere and the lower stratosphere. On the other hand, carbon dioxide emits heat radiation, which is lost from the stratosphere into space. In the stratosphere, this emission of heat becomes larger than the energy received from below by absorption and, as a result, there is a net energy loss from the stratosphere and a resulting cooling.
(Looks like Stoat was more right than Rabett but he can graciously disagree if he wishes or try a Lubos.)

9 comments:

CapitalistImperialistPig said...

I don't think I buy it. In the lower stratosphere, the only net long wave absorber is O3. CO2 and H2O are net radiators in the thermal.

Also, the notion of "trapping radiation" in the troposphere is a bit of a misnomer, since we have overall radiation balance. What happens is that the mean radiating level moves higher (remember, Venus absobs less sunlight and radiates *less* thermal than the Earth)

To reiterate - stratospheric thermal absorption is small, and all of the net is in the O3.

If (as is likely) I'm missing something important, I would appreciate a less cryptic answer.

EliRabett said...

About the only thing in the strat that can radiate is CO2 and O3 (there is not much H2O or CH4). Vibrationally excited states are populated by collisions with other molecules and radiate (or lose energy by collision). If you increase the concentration of the CO2, the number of molecules that can radiate increases, and the loss of energy by radiation increases.

If there were no CO2 in the troposphere the additional CO2 in the stratosphere would absorb more IR radiation from below and on net, nothing much would happen. But the high concentration of CO2 in the troposphere blocks emission from below at wavelengths that the CO2 in the stratosphere can absorb. As a result you have an increase in heat loss by radiation with no balancing increase in heating from absorption.

Figure 3 in Uhreck's article shows this beautifully.

CapitalistImperialistPig said...

Eli,

I believe you and your colleagues are misinterpreting Uherek's figure 3. You (and Uherek) seem to be interpreting this Clough and Iacono figure as representing radiative spectral brightness vs altitude. Instead, the values in the figure must be interpreted as integrated cooling - in effect an integrated contribution function as seen from space.

Another, and perhaps better way to interpret it in terms of energy sources and sinks. The light blue color regions are regions where longwave radiation is in thermal equilibrium, the other colors represent net radiation loss, and the gray represents net radiative heating by the IR. At 250 cm^(-1) for example, the region near the surface is light blue because radiation absorbed and emitted are in balance. Higher up, in the same spectral region the radiation decouples from the matter for lack of water molecules, and is lost to space. At 670, it's light blue from surface to the tropopause, because at the center of the CO2 band we are in thermal equilibrium all the way up.

I don't believe that you can claim that the atmosphere is not radiating in the center of the CO2 band at (say) 1 km - it's radiating like crazy (or more precisely, like a pretty black body), because it's the CO2 band center.

Uherik - If this absorption is really strong, the greenhouse gas blocks most of the outgoing infra-red radiation close to the Earth's surface.

That sentence is nuts. There is plenty of upwelling radiation in that spectral region of the tropopause, it's just matched by the downwelling radiation from above.

No frequency makes much of a *net* contribution until the sky above it is largely transparent, which doesn't happen for CO2 band center until you reach the stratosphere,

It's simply not true that more CO2 in troposphere causes a significant decrease in the upwelling radiation in the CO2 band received by the stratosphere. It's the additional CO2 in the stratosphere that makes it a more effective thermal radiator and hence cools it. The figure is useful, but I think that Uherek is misinterpreting it.

CIP

EliRabett said...

Tell you what. Go to the MODTRAN web page at UChicago

Run the model at 20 km looking DOWN. You see that there is comparatively little radiation in the CO2 bend region at 600 nm. That means that the radiation is being blocked (and redistributed) by the CO2 in the troposphere. Now run the model looking UP. You see that there is energy being radiated by the CO2 in the stratosphere (roughly the same amount goes up. Result cooling.

There are other fun games you can play here like changing the concentration of the CO2, but you have to look carefully because most of the change appears as a widening/narrowing of the bands, but you can say something by looking at the amount of energy that the detector sees.

Neat toy that

CapitalistImperialistPig said...

Eli,

Thank you for the cool link to the MODTRAN tool. It's my favorite Christmas and Channukah present yet. There is an adamantine truthiness to your statement that more radiation would reach the statosphere if there were no CO2 in the troposphere. Adding a few ppm of CO2 has a fairly dramatic effect. However, band center seems to saturate at about 10 ppm and the band is pretty broad by the time you get up to 300 ppm. The Uherik effect would seem to be important at low CO2 concentrations.

You mentioned a CO2 band at 600 nm – but I'm assuming that you meant the thermal band at 670 cm^(-1) [in wavelength units, about 15 cm = 15000 nm]. I have run the model both at 20 km and at the top of the troposphere – say 12 km for mid-latitude winter.

First I looked at 12 km, with default parameters. Looking down, we see a brightness temperature of approximately 219 K, while looking up the band lies at about 218 K. Clearly, net radiation transfer here is small but directed up.

Next, I try your 20 km, well above the tropopause. Looking down, at band center, we see about 215 K. Looking up, we can see some finer structure, but the band average brightness temperature is clearly much colder – say 205 K. Here we see significant thermal energy transfer from the lower stratosphere to the upper.

As mentioned above, it clearly is true that less thermal radiation reaches the stratosphere in the relevant band than would if there were *no* CO2 in the troposphere. However, even a few tens of ppm is enough to present plenty of opacity between the low troposphere and the stratosphere.

The upward flowing radiation at band center is surprisingly insensitive to the CO2 concentration. I tried running the model at concentrations from 30 ppm to 300000 ppm. The change mainly affects only the wings of the band, which broaden but stay at about the same temperature. If , for example, you reduce CO2 below 10 ppm,. you do see a lot more radiation transferred to the upper stratosphere, and Uherik’s effect becomes important. It certainly doesn’t *look* important at current CO2 levels. The small increases are all on the far wings where there is hardly any stratospheric absorbtion (less pressure broadening).

PS - I really like your blog. I hope you don't mind that I added it to my blogroll.

jim steele said...

Quote "In the stratosphere, this emission of heat becomes larger than the energy received from below by absorption and, as a result, there is a net energy loss from the stratosphere and a resulting cooling. "

My trouble with Dr. Uherek's explanation for stratospheric cooling is as follows: If the net loss of heat is the result of less LW infrared from earth reaching the stratosphere then our satellites should also observe less LW leaving the earth. However that appears not to be the case.

A paper on the strengthening of General Circulation by Chen et al, 2002, in Science reported

"Satellite observations suggest that the thermal radiation emitted by Earth to space increased by more than 5 watts per square meter, while reflected sunlight decreased by less than 2 watts per square meter, in the tropics over the period 1985-2000, with most of the increase occurring after 1990."

http://www.sciencemag.org/cgi/content/abstract/295/5556/838

I suggest better explanations all around are required.

EliRabett said...

The results of Chen et al were mostly ascribed to clouds (actually there is a comment by Trenberth that raises some issues as to whether the result would hold up).

Absorption in the stratosphere is pretty much limited to ozone and CO2. An increase in emission outside of those bands would have little effect.

jim steele said...

EliRabett said

"The results of Chen et al were mostly ascribed to clouds (actually there is a comment by Trenberth that raises some issues as to whether the result would hold up).

Absorption in the stratosphere is pretty much limited to ozone and CO2. An increase in emission outside of those bands would have little effect."

I am not sure what your point is. It doesn't matter what Chen attributes increased LW emissions to. The point is that satellite data reported there has been an increased of LW be emitted from earth around the tropics. Such a fact is totally contadictory to the CO2 explanations for cooling and global warming. The CO2 explanation requires that less LW leave the earth because it is absorbed by CO2 thus creating the radiative imbalance.

Fred Moolten said...

I don't know whether this thread is still active, but I have a question. Granted that stratospheric cooling is complex, but wouldn't it be likely that one factor is a shift in the spectral distribution of energy escaping to space, so that proportionately less escapes from CO2-absorbable wavelengths and more from non-absorbable wavelengths such as the atmospheric window? An Earth warmed by increased CO2 will radiate more non-absorbable energy, which in turn puts limits on total warming. The troposphere will still see some increase in temperatures at any given altitude, but it will be limited by the escape of heat outside of absorbable wavelengths, and therefore not commensurate with the increased CO2. The consequence would be a reduction in CO2-absorbable wavelengths emitted into the stratosphere, increasing disparities between absorption and radiation by stratospheric CO2, with a resultant cooling.

Given that radiative balance is only slightly perturbed by increasing CO2, it would seem reasonable to think that more heat escaping in non-absorbable wavelengths should be matched by less in the absorbable bands.

Fred