2005-03-02

Why does the stratosphere cool under GW?

One of the strongest predictions of global warming is that the stratosphere will *cool* - unlike the troposphere, which will warm, of course. See the IPCC here for example. This turns out to be not as useful for *detecting* climate change as it might be, because ozone decreases also lower the stratospheric temperature. However...

The interesting question is, *why* does the stratosphere cool? From asking colleagues, its quite clear that very few people have thought about this, and of those few who do think about it few get the right answer. Indeed, I'm not absolutely sure that what I've written below *is* the right answer, but I think it is. For a long (and possibly doomed) attempt to explain it, see this at RealClimate.

[Clarification: 2005/03/05: I fear I may not have been quite as explicit as I might have been: this post is about why the stratosphere cools if all you do is change the GHG's, e.g. CO2. It is not about what happens if you decrease the ozone - that, trivially, cools the stratosphere. Consequently, I am not talking about the observed decrease in temperature in the strat - which is caused by a mixture of ozone depletion and GHG increase - but about what *would* happen in a though experiment if GHG's are increased but ozone is held fixed.]

Anyway: my explanation (thanks HKR) is:

in a uniformly grey non-convecting atmosphere (ie, if the atmosphere were equally transparent at all wavelengths, and uniformly through its depth) heated from below (ie, solar radiation warming the surface; assuming of course that we've relaxed the grey assumption to let the solar through), then increasing the greenhouse gases (GHG's) *doesn't* lead to a cooling at the top: instead, the whole atmosphere warms, though not uniformly. You can see some calcs and pictures and code here;

of course, the real atmos does convect; isn't totally transparent to solar; etc; but the real difference is:

the reason that the real atmosphere has a stratosphere is because of ozone absorbing UV, thereby warming that portion of the upper atmosphere;

hence the stratosphere is considerably warmer than it would be under just longwave (LW, or IR) forcing; and CO2 is only effective in LW frequencies;

hence, increasing CO2 *increases* the stratospheres ability to radiate in the LW, but doesn't substantially increase its ability to gain heat, because most of that comes from the SW;

hence it cools.

In the troposphere (ignoring convection etc etc; the real atmos is complex...) increasing CO2 increases both the ability to gain and lose heat, and this first-order argument doesn't tell you what will happen; as it turns out, it warms.

Note: of course the fact that many people couldn't explain this makes no difference at all to the fact that climate models produce the correct answer: they just integrate the equations, and don't care about *why* things happen.

[Update in response to comment: the troposhere is the lowest bit of the atmosphere - up to about 8km. Temperature generally decreases with height at about 7 oC/km. The stratosphere comes next, temperatures *increase* with height (the temp min defines the interface, called the tropopause) until the mid-strat, then declines again to - I think - the stratopause. See IPCC glossary for more.

CO2 is only radiatively active in the LW - ie the infrared portion of the spectrum. Its just about transparent to visible (SW) light]

19 Comments:

Anonymous Anonymous said...

This reader finds the jargon barrier a bit high. For instance: the stratosphere and the tropospere - which is which? What does 'CO2 is only effective in LW frequencies' mean?

6:53 pm  
Anonymous Thomas Palm said...

Not only does the stratosphere cool, but also several layers above it, and since a cooler gas occupies less volume for a given pressure this means that it shrinks. The troposphere does expand as it is warming, but it occupies much smaller volume so in total the atmosphere of the Earth is shrinking as a consequence of ozone depletion and increased greenhouse effect.

Amusingly enough that means that when contrarians deridingly talk about chicken littles and 'the sky is falling' they are actually right: the sky is falling!

8:26 am  
Blogger Belette said...

Thank you Thomas. I like that.

Meanwhile, I have discovered that I and the radiative people mean slightly different things by "stratosphere". To me, it meant the bit above the tropopause where the temperature starts going up again. To them, it meant the "stratified" bit of the atmos (hence the name), ie the bit where convection has ceased. These are the same things in the real atmos, but possibly different in simplified models, or on other planets.

8:50 pm  
Blogger Lumo said...

Are you joking, William? Do you call yourself a climate expert?

Stratosphere cools because it contains ozone which is itself a greenhouse gas. Ozone is what normally keeps stratosphere warm, and if it decreases, stratosphere obviously cools.

Even tropospheric ozone is a powerful greenhouse gas that has caused about 1/3 of the warming since the beginning of the industrial revolution.

It's amazing that the same people who claim that they have understood everything - like you - don't even know why stratosphere cools.

You have already brainwashed yourself so much with the CO2 that you can't actually see the reality. CO2 is not too important. The primary overall greenhouse gas is water, and O3 is primary for the stratosphere.

4:48 am  
Anonymous Thomas Palm said...

LuboŇ°, given your uncivil tone I'm not sure you deserve any answer, but you'll get one anyway.

It is true that ozone depletion is part of the answer to stratosphereic cooling, but not for any reason you state. The reason there is a temperature inversion in the stratosphere is that ozone absorbs UV from the sun, getting heated from it. When you reduce ozone levels you reduce this absorbtion, which cools the stratosphere, as do the presence of the CFC:s being greenhouse gasses.

That ozone is a greenhouse gas means that reduced concentration in the stratosphere will cause warming of it (for the same reason more CO2 cause cooling).

10:46 am  
Blogger Belette said...

Thank you Thomas. Lubos has (as usual) got the wrong end of the stick by failing to read carefully. In my post, I'm *not* talking about the *observed* decline in stratospheric T (which is partly ozone depletion and partly CO2): I'm talking about the *predicted* decline, under a scenario in which only GHG's change. In that scenario, as I say, stratospheric temperatures are predicted to decline, even with ozone remaining constant.

3:23 pm  
Blogger Lumo said...

This comment has been removed by a blog administrator.

3:33 pm  
Blogger Lumo said...

This comment has been removed by a blog administrator.

3:38 pm  
Blogger Peter Hearnden said...

At the risk of feeding Lubos, I have to say I don't understand what Thomas said. To me his comments seems to say reducing ozone causes both warming and cooling. Typo?

11:50 am  
Blogger Belette said...

I think its TP's last para you're querying. I think what Thomas is saying in that is that, O3 being a GHG like CO2, decreasing it in the strat will have a slight tendency to cause warming (for the same reason that adding CO2 to the strat will cause cooling). In other words, that the LW and SW effects go in different directions. Errmm...

8:06 pm  
Anonymous Thomas Palm said...

The net effect of removing ozone has to be either warming or cooling of the stratoshpere (cooling as it happens), but as ozone has several effects some of them may work towards a warmer stratosphere while others towards a cooler. As belette said, the shortwave and longwave effects are in opposite direction, but in this case the shortwave effect dominates.

You will find the same thing with clouds. They cool Earth when they reflect sunlight into space but they warm it when they reflect IR from the surface back to the surface. Which effect is stronger depends on a lot of different factors, such as time of day and the altitude and thickness of the cloud.

5:50 pm  
Anonymous Thomas Palm said...

The net effect of removing ozone has to be either warming or cooling of the stratoshpere (cooling as it happens), but as ozone has several effects some of them may work towards a warmer stratosphere while others towards a cooler. As belette said, the shortwave and longwave effects are in opposite direction, but in this case the shortwave effect dominates.

You will find the same thing with clouds. They cool Earth when they reflect sunlight into space but they warm it when they reflect IR from the surface back to the surface. Which effect is stronger depends on a lot of different factors, such as time of day and the altitude and thickness of the cloud.

5:50 pm  
Blogger Pat N said...

--- The global climate was very warm 55 myrs ago. Global climate was also very warm 70-230 mya. What was the ozone layer like back then?
--- Holding all but but CO2 and water vapor constant, will the protection from uv radiation by the ozone layer disappear as CO2 doubles, triples ..?

9:48 pm  
Blogger EliRabett said...

Try this http://rabett.blogspot.com/2006/02/why-does-stratosphere-cool-while.html

4:18 am  
Blogger Belette said...

Replied there. In summary: I don't believe you! In fact I shall paste in my wise reply here, so I don't lose it:

"Hi Eli. I think your explanation fails because its not just simpler its *different*. I believe that my explanation is correct, so yours must be wrong. As far as I'm able to understand it, the crucial point is that the O3 in the strat makes it warmer up there than it would be if the atmos were grey.

So, lets try a thought exp: imagine an atmosphere with no ozone layer (either because it has no oxygen, or perhaps because solar radiation stops at the visible). There will be no real strat: temperatures will decline above the tropopause (which won't exist, but temps there and in the trop should be unaltered).

My theory says that GW will, in this case, waarm the "strat". Yours, I think, still predicts it will warm. Yes?

Also: you say: "let us say that most of the currently observed stratospheric cooling is due to ozone depletion." I'm not sure how correct this is. "Most", perhaps: but not 90%. Perhaps 60-40, or maybe 75-25? I'm not sure its even known."

12:11 pm  
Blogger Tom Brogle said...

1)Gavin wrote a peper last year attributing Antarrctic cooling to the raising of the temperature of the stratosphere because of the depletion of Ozone and increase in CO2.

2:52 pm  
Blogger CapitalistImperialistPig said...

Belette - If your explanation is correct, and I believe it is, it should only be the extra CO2 in the stratosphere that contributes to stratospheric cooling, right? How long does it take for CO2 increases in the troposphere to make it up into the stratosphere, especially the high stratosphere where most of the cooling happens?

Also, 8 km is the stratosphere at the poles. It starts rather higher in temperate regions and especially in the tropics.

2:56 am  
Anonymous Anonymous said...

Its not CO2 doing it.

Whats happening is the sun warms up... then that small effect is amplified as water vapour begins to accumulate in the air.

Now because the water vapour concentration, in the air, gets less the higher up we go...... the result is that there is very little water vapour in the air in the Stratosphere.

The water vapour blocks a lot of the long wave radiation coming off the earth but it does so most powerfully at around 6.5 microns.

Now it just so happens also that Ozone has been reducing in the Stratosphere. And Ozone blocks Long Wave radiation at about 10 microns.

It is at these two wavelengths that we see the reduction in Long-Wave radiation reaching the stratosphere. So thats why the stratosphere has been cooling.

If it was substantially due to CO2 then we would see this. But its not due to CO2 and the effect of CO2 is not detected either here or in the troposphere. So negative or positive, CO2's effects have to be assumed to be negligible.

8:55 am  
Anonymous Anonymous said...

How about this:

the lowest layers of CO2 are insulating the heat in the low atmosphere turning their reflective side to the IR coming from surface. But in the stratosphere there is no IR reradiation left and the CO2 molecule reflecting side will always point to the IR side, that is reflect incoming sunlight out to space. That is why it is cooling down!

SO if we increase the CO2 in the stratosphere alot we might be able to lower surface temps as the CO2 at surface is enough to trap all IR - good idea... Practically, more airplanes are needed, not less.

Problem is that the extra radius at this altitude makes it hard to use this concept.

8:34 am  

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