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]

22 comments:

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?

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!

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.

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.

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).

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.

Lumo said...
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Lumo said...
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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?

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...

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.

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.

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 ..?

EliRabett said...

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

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."

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.

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.

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.

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.

WeatherMan@OnlineWeather.org.uk said...

Atmospheric physics crosses the boundaries to chemistry. There are several factors at play here. The complex line spectra of gases in the atmosphere mean that at some wavelengths the atmosphere is opaque to radiation (mostly in the infra-red), and almost entirely transparent in the visible part of the spectrum.

Greenhouse gases resist the flow of thermal (longwave) energy leaving the atmosphere below about 12km to space, but the infra-red absorbing gases responsible in this process tend to decrease approximately in line with air density. In the troposphere longwave absorption and emission are the dominating factors in the energy budget. The troposphere is mostly transparent to shortwave radiation.

The energy received from the Sun is absorbed by the Earth's atmosphere, surface and oceans. On a global and long term average, the energy received roughly equals the energy emitted back to space. By increasing the greenhouse gas composition, the atmosphere tends to warm in the troposphere and therefore must cool in the stratosphere so that the net energy (in - out) is in balance at the top of the atmosphere.

In the stratosphere the term "greenhouse effect" (referring to thermal heat exchanges) is negligible, an order of magnitude smaller than the shortwave effects. Ozone is only produced efficiently by photolysis by intense sunlight (high energy photons). The altitude where infra-red absorption is overwhelmed by the ozone emission is marked by a cross-over region or inversion known as the tropopause (the location of a temperature inversion). The density of the atmosphere in the lower stratosphere above this transition level is rather low, so that absorption of small quantities of energy can raise its temperature by much greater magnitudes than the troposphere.

Ozone in the stratosphere absorbs some thermal radiation from the troposphere below, but the dominating effect is governed by shortwave absorption from above.

Since the troposphere and stratophere are in a radiative-convective equilibrium (see the IPCC 2007 documentation), a perturbation in the tropospheric temperature from increases in greenhouse gases results in an adjustment to the stratospheric temperature profile. The tropopause boundary between the two layers (troposphere and stratosphere) increases slightly in altitude, and its temperature increases when greenhouse gases are higher in the atmosphere.

If you consider the greenhouse world where thermal radiation is now emerging from lower altitudes in the atmosphere closer to the tropopause.

The next important factor is that: "The surface and the troposphere do not respond instantaneously to the greenhouse gas perturbations in the atmosphere". This means that the stratosphere must exhibit a negative forcing, if the surface and lower troposphere experiences a net positive forcing (warming). As the top-of-atmosphere budget must be always in a state of balance*. This is a counteracting effect, that acts to balance out any net imbalance at the surface due to a lag-time in global warming. The surface can not heat instantaneously particularly over the oceans, due to the high thermal inertia of the system.

*Note that on a local or regional scale well-mixed quantities such as cloud and water vapour cause the net top-of-atmosphere budget to become depressed slightly frequently.

crandles said...

A few points and queries since this seems to be being referred to.

My OU S199 reference (so probably not the best sort of reference) seems to indicate the tropopause varies from about 11km at poles to 20km at equator.

>"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."

Hmm. My reference indicates that the Stratosphere temperature increases with height from -57C to 0C (annual average at mid latitudes) at the stratopause (approx 50km high) then temperature decreases with height to about -90C in the mesosphere up to the mesopause (approx 80km high). Then temperature increases with height in the thermosphere.

My ref indicates "there is virtually no convection" and "The warming in the higher layers of the stratosphere is caused by the absorbtion of UV radiation by the ozone layer. Most of the ozone is concentrated between 20 and 30Km altitude."

Are those sentences as contradictory as they at first appear? The high energy from UV may need to disperse a bit before you get CO2 radiation. The explanation seems much more in accord with how you say the temperatures change.

Avito Hong Daa Yee said...

"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."

These are vague and false. The LW forcing from sun and LW radiation from CO2 is an equilibrium. However, upon hitting masses, SW from sun is converted into LW. Thus the forcing balance is shifted to the earth and the earth heats up.