Well, not my model exactly. I developed and host a web interface to the modtran model of atmospheric infrared radiation, an early example of a line-by-line code which I downloaded and use to teach and as part of a textbook. Now David C. Archibald from Summa Development Limited, Perth, WA, Australia claims that my “University of Chicago modtran facility” proves that global warming won’t happen.
Archibald begins by discovering that the IR light flux at the top of the atmosphere is more sensitive to changes in atmosphere CO2 when the concentration of CO2 is lower. This will come as no surprise to regular readers of realclimate who will know that the energy flux scales with the logarithm of CO2. The log dependence is why the climate sensitivity parameter is often posed as a temperature change for doubled CO2 concentration; to first order, a change from 10 ppm to 20 would have about as much climate impact as a change from 1000 to 2000 ppm. So Archibald is right on this score, clearly climate is more sensitive to CO2 when levels are lower. However, I think most climate models are aware that atmospheric CO2 is 380 ppm rather than 10 ppm, and they predict global warming anyway. If we were starting out from 10 ppm, the warming would be even worse.
Archibald then takes an atmospheric increase of 40 ppm which he thinks will happen by the year 2030. I’d have guessed 60 ppm by then at least, the way things are going, but whatever, we’ll see. He uses my setup of modtran to calculate that the IR flux to space would drop by 0.4 Watts / m2 as a result of this 40 ppm. Try it yourself. Run the model once with 375 ppm CO2 and another time with 415 ppm, and compare the Iout values in Watts / m2. The exact number you get depends on humidity, setting, clouds, etc. Formulas given in IPCC would say 0.5 Watts / m2; zeroing out water vapor in modtran gets the IR response up to 0.6 Watts / m2 for the default tropical atmosphere case. At any rate Archibald isn’t wildly off here either.
But then Archibald multiplies the radiative forcing by an absurdly low value of the climate sensitivity parameter. In this case he is using the parameter in units of degrees C per Watt / m2. The two forms of the climate sensitivity parameter that we have discussed here are related by a factor of about 4 Watts / m2 for a doubling of CO2. The value Archibald uses is 0.1 degree C per Watt / m2 which was “demonstrated” in a paper entitled “CO2-induced global warming: a skeptic’s view of potential climate change” by Idso, 1998. Translated, Idso’s climate sensitivity winds up to be 0.4 degrees for doubling CO2. IPCC finds it essentially impossible (yeah, I know, highly unlikely or whatever) that the climate sensitivity could be less than 1.5 degrees C for doubling CO2, and 3 degrees C is a best-guess value.
In the end, Archibald concludes that the warming from the next 40 ppm of CO2 rise (never mind the rest of it) will only be 0.04 degrees C. Archibald’s low-ball estimate of climate change comes not from the modtran model my server ran for him, but from his own low-ball value of the climate sensitivity.
Timothy,
My thanks go to you for taking the trouble of replying in such detail.
I shall try to respond in the same manner when I can. I have a fair amount on just now but hopefully this thread will stay alive long enough to offer me such an opportunity.
For now just my thanks must suffice.
Best Wishes
Alexander Harvey
#47, the water (and CO2) bands seen in the solar visible and near IR are called vibrational overtones. If molecular vibrations are harmonic, the nuclei moving like a perfect spring, only transitions between the closest vibrational levels are allowed. As real springs, molecular vibration is anharmonic. It is this anharmonic component that makes allowed vibrational transitions which take you from one vibrational level to much higher ones. These are VERY weak.
The water vapor bands that are most important to the greenhouse effect are those that result from rotational transitions (changes in the rotational quantum numbers) in the IR, and somewhat the bends. These are saturated in the CO2 sense.
A more detailed explanation would be, well much more detailed, so perhaps this is not the place.
#44, Alexander Harvey says: “There is no fundamental difference between the ways that H20 and C02 act as greenhouse gases to imply one should be a forcing and the other a feedback, it is purely a matter of convenience.”
My understanding of how feedback and forcing are used, is that forcing is external to the climate system, as for example solar insolation. If, for example, CO2 is added to the system from burning fossil carbon, it is being introduced from a source isolated from the climate system in any meaningful sense and is a forcing. If it is added by warming of the oceans, then it is being added from inside the system in response to temperature increase and is a feedback. To paraphrase, a feedback in one situation is a forcing in another, but it is either or, not both at the same time.
lgl–most incoming solar radiation is in the visible. Water would absorb what is in its absorption band, but once excited, it would tend to relax collisionally, so that energy would mostly go into heating the atmosphere anyway. Still, the outgoing flux of LWIR is much higher than that of incoming LWIR.
Re 44: I believe you can rearrange the final equation into a simpler form:
N = On(e.Se/Ns^E)
Australia is about to have a Federal Election and John Howards Liberal Party has a bunch of so called intellectuals – Archibald – who will say anything , regardless of the consequences, to get their party back in power. Politics has a lot to answer for in Australia in regards to climate change, especially if you would like to live in a world of 1000ppm CO2, as some buisiness men think would be pretty cool.
Timothy (50), a clarification please.
Does upwelling “re-emission” from water vapor really get absorbed by high altitude CO2? Why would H2O radiate, not in its natural spectra, but in CO2s spectra (other than the minor overlapped bands)? [I’m not sure if this would alter your basic premise, however.]
Isn’t there some point, eventual equilibrium of sorts, when greenhouse gases do “block” (absorb without re-emission) terrestrial radiation? Otherwise we would not see any vacant spectra in the, say, satellite imaging of terrestrial radiation, would we?
A helpful post. Thanks. You, too, A. Harvey.
Re 54#
“most incoming solar radiation is in the visible”, well – more 50/50 I would say. But yes, the energy would heat the atmosphere and a lot of that energy would radiate back to space and not reach the surface. This should give less heating of the surface and less outgoing LWIR.
In addition the H2O absorption bands in the LWIR seem to be much more saturated that those in the solar IR spectrum.
If more of the insolation is absorbed in high troposphere the increase in GHG should also make it more difficult for that energy to reach the surface.
Thirdly, increased evaporation brings more heat to “high up” (how high?). The GHG would not influence the upwelling but would reduce the downwelling IR, right?
I see from the radiation budget that the absorbed solar, evapotranspiration and outgoing thermals add up to about 170 W/m2. I don’t know how much of this is radiated back to ground but I would assume that this incoming radiation also face an increased greenhouse effect, which should then have a cooling effect.
To get to the point. I am looking for a paper (free on the web of course) where these effects are described/estimated and how they are treated in the models.
In answer to the question, “who are the organizations that continue to fund baseless attacks on climate science?” here are some of the main players:
The American Petroleum Institute: http://www.api.org/
The Edison Electric Institute: http://www.eei.org/
In the past, the now-disbanded Global Climate Coalition was the front organization for fossil fuel interests who want to prevent any reductions in fossil fuel use. Essentially, these interests know that renewables are simply less profitable and will be expensive to produce, requiring massive amounts of investment in production facilities and electrical grid infrastructure. It’s cheaper to spend hundreds of millions on slick PR campaigns designed to create doubt in the public’s mind over climate science than it is to replace fossil fuels with renewables.
Essentially, that’s the problem. An 80-90% reduction in CO2 emissions really requires an 80-90% reduction in fossil fuel sales. That energy demand must be met with real renewable sources, primarily wind and sunlight, and energy efficient technology. That requires replacing the existing trillions of dollars of fossil fuel infrastructure with trillions of dollars of renewable infrastructure, and nobody seems to want to spend the money. Vast sums will have to be spent to create a renewable-energy based infrastructure – which means a lot of economic activity, but which also means turning the global economic order upside down. No oil-selling country or corporation on the planet wants to see U.S. energy demand drop, regardless of their political viewpoint.
Across the U.S. right now we have a wave of ‘feel-good’ proposals, with local and national politicians pledging their green values left and right – but when it comes to funding real changes, it’s just not happening. Fossil fuel companies and government economists are still working on the assumption that U.S. fossil fuel demand and consumption will continue to increase over the next twenty years.
Taking action on global warming is now seen as politically popular, so we’re seeing a lot of rhetoric – but no real large-scale changes in the energy picture have been proposed by politicians to date.
lgl, OK, think about this. First, the amount of energy at wavelengths shorter than the absorption bands of the GHGs is well over 50%. Second, warming the atmosphere does heat the surface, since it means that the net outgoing radiation is less. Yes, the ghgs do absorb in their lines, but they don’t care whether radiation is outgoing or incoming–and there’s a whole lot more outgoing LWIR than incoming (look at a spectrum of Earth’s thermal radiation). And yes, water vapor does transport energy into the lower troposphere. However, it drops it there. The only way for it to escape is via outgoing IR radiation. I would strongly recommend that you get the science of the greenhouse effect under your belt before tackling the scientific literature. Check out the text by Ray Pierrehumbert http://geosci.uchicago.edu/~rtp1/ClimateBook/ClimateBook.html
Thanks Ray, at first it looked quite promissing.
CONTENT
5.10 Effects of atmospheric solar absorption . . . 198
but where is it?
Ah.. “Chapter 5, is about 3/4 complete now”
Please hurry Pierrehumbert! I’m waiting for this.
Meanwhile, just to make sure, are you really saying that none of the heat brought to the atmosphere by thermals and solar absorption is radiated back to the surface? And if it is, why do not GHGs have any impact on this radiation?
Further to #56: Guy Pearse, a former advisor to Howard, has written a book about Australian politics and climate change:
“…He reveals that the government has no plans whatsoever to reduce Australia’s emissions, and explains why this is bad for Australia’s economy. He exposes a prime minister wilfully blind to Australia’s real Interests – a man who has allowed climate change policy to be dictated by a small group of Australia’s biggest polluters and the lobbyists they fund…”
http://www.highanddry.com.au/
And a couple of articles about the book and the politics:
http://www.newmatilda.com/home/articledetail.asp?ArticleID=2363
http://newmatilda.com/home/articledetailmagazine.asp?ArticleID=2421&HomepageID=216
I think Howard, Bush and Harper all sing from the same songbook.
Ray> First, the amount of energy at wavelengths shorter than the absorption bands of the GHGs is well over 50%.
Still, looking at the solar spectrum:
http://en.wikipedia.org/wiki/Image:Solar_Spectrum.png
it appears that a lot of incoming solar energy between 900 and 2000nm is absorbed by water vapor. Does anyone know if this negative feedback from water vapor increasing with temperature is included in the climate models?
[Response: It actually isn’t much of a feedback (since it mostly intercepts energy in the troposphere that would have been absorbed by the surface), and yes, this is included in most GCMs with reasonable radiation models. – gavin]
Ups, I made a mistake in 61#
I should of course have said “to the atmosphere by thermals and evaporation” not “to the atmosphere by thermals and solar absorption” You were quite clear about the solar.
I have this notion that heat released high up in the atmosphere will not warm the planet as much as if it was released very low in the atmosphere, maybe this is where I’m taking the wrong path?
Very enlightening discussion. Many thanks. I really question Archibald’s methods and so should you all.
[[Isn’t there some point, eventual equilibrium of sorts, when greenhouse gases do “block” (absorb without re-emission) terrestrial radiation? ]]
They can’t absorb without emitting.
Re: #59 Ike, “Fossil fuel companies and government economists are still working on the assumption that U.S. fossil fuel demand and consumption will continue to increase over the next twenty years.”.
Sounds plausible, however doesn’t the reality of Peak Oil, begin to pull the rug out from under this premise? Or does oil start to become a non issue in the context of the bigger picture when taken together with natural gas and coal?
My personal hunch is that as technology progresses it becomes increasingly feasible for larger swaths of the general population, to become completely energy independent, thereby breaking the the energy companies monopoly.
I think this is the scenario that they dread the most and will surely fight to the death to maintain their lucrative status quo regardless of the consequences.
“Peak Oil” just means “invest in oil companies”. What they sell will just keep getting more valuable.
Demand hasn’t slacked much with the increases in price so I don’t have much faith in alternate energies saving our bacon.
Demand has continued to soar despite the higher prices. The demand for energy has never been higher than it is right now. The higher prices support the more costly refining that once precluded bringing certain types of fossil fuel reserves into production.
Barton (66), I understand the logic of that, but the puzzle is: a bunch of radiative power in a certain band leaves the earth’s surface, but is not much seen at the top of the atmosphere. Where did it go??
Rod, the ‘band’ refers to counting a lot of photons in arange of energies. It’s not a unique entity that persists, it’s a measurement.
Sunlight warms the Earth. The solids heat up. The molecules in the solid Earth aren’t free to bounce around, they’re pulling and tugging at each other but (short of boiling to vapor, for example) don’t break apart and float freely. All the pieces are moving, the motion of the atoms produces photons. The range of motion isn’t in discrete steps because everything’s tugging on everything else in a solid, so any range of motion possible can produce a photon of that amount of energy. From Earth in sunlight, it’s the ‘infrared band’ that’s produced.
That “band” is like a battle plan, it does not survive first contact with the enemy — the next molecule that any of those photons hits absorbs its energy. Photon’s _gone_.
On average.
Some photons from the ground, and from low clouds, and from water vapor in the atmosphere, do make it all the way out into space — you can look at the satellite photographs in the infrared bands. Compare those wavelengths to the wavelengths in which our atmosphere is less opaque to infrared.
But on average, in the dense lower atmosphere, a photon hits a molecule; the molecule’s energy increases, in some vibrational mode, while also transferring energy among those modes internally and banging into other molecules. The original photon no longer exists, it’s a bookkeeping entry for transfer of energy.
Higher in the atmosphere the gas molecules are more widely separated. While they’re not banging into one another, each one can emit photons at the specific wavelength defined by the ways its atoms can vibrate.
We talk about an average altitude (6 km and rising) where — on average — photons interact less often and more of them escape into space, removing energy from the planet.
So — there’s where your “band” goes.
Imagine a pool game in which, half the time, your white cue ball would change to another ball on impact, and if a couple or three of the other balls displaced from the rack happened to hit the right way, one of them would turn into another white cue ball.
lgl,
Once energy is absorbed by either the atmosphere the ocean or solid Earth, it is part of the system that determines climate. The only way it can escape is as radiation–and because of the temperature of Earth, mainly IR radiation. Rod, you asked where the energy goes. It goes mainly into thermal excitation of molecules in the atmosphere and also on Earth’s surface–at least until the temperature rises enough to restore equilibrium where incoming energy equals outgoing energy.
Igl (#64) wrote:
Actually this much is right.
In terms of optical thickness, longwave’s which are at a greater altitude have considerably less “distance” to travel to reach space. And the path will be one of multiple absorptions and emissions is a random walk. Therefore that which is closer to the surface is much more likely to return to the surface, and that which is closer to space (in terms of optical thickness) is more likely to make it to space.
But there are two qualifications to keep in mind. First, optical thickness is roughly distance, but with lower atmospheric density – which decreases exponentially with altitude, etc along the longwave’s path taken into account. In fact, optical thickness is actually dimensionless. Second, one can’t actually treate the energy as a single photon since the energy gets lost to collisions, spread out between different molecules in still other collisions, etc. But an early, first conceptual approximation may be thought of as a single photon or parcel of energy.
*
Anyway, if you haven’t already, I would check your question in 61 against Ray’s comment in 60. There is still a great deal of longwave being radiated by greenhouse gases going to surface. However, when he states that that which is absorbed coming up from the surface tends to be lost in collisions (see 54), this is only because the time between collisions for any given molecule tends to be much shorter than the half-life of an excited state. About a millionth of the timespan. As such, when a molecule radiates a photon, this is due to the molecule having received energy through collisions in the same way that molecules lose energy due to collisions.
Now why don’t the molecules lose this energy before they emit – given the timespans?
Most actually do lose energy that way.
But why do any radiate.?
Because we are dealing in half-lifes. The molecules have no “knowledge” of how long they have been in an excited state. They spontaneously decay. As long as a certain number are in an excited state at any given time, a certain number will undergo spontaneous decay over a given period. The coins get passed around, but whereever they are, they keep getting tossed – and some come up heads.
Hope this helps…
Dear bobn,
Re: #55
I always welcome constructive criticism.
When you have some, do please post it.
FWIW the origin of the equations is not my work and has been around in the literature for at least 20 years.
Best Wishes
Alexander Harvey
Re 64. lgl, Your intuition is mainly correct, but you need to define what you mean by “high up in the atmosphere”. Essentially that means that the optical path of the photons is longer than the residual distance to the top of the atmosphere. That depends on the density of molecules that can absorb the photon on its upward path. As we raise the ghg content of the atmosphere, we effectively raise the altitude at which IR photons are able to escape, meaning that less energy is able to rise up that high. Thus the part of the atmosphere from which radiation can escape is colder and so radiates less, meaning more energy stays in the climate system. That’s one way to look at it–essentially the same as that in “A Saturated Gassy Argument”.
Barton, Ray, Hank, Timothy, et al: I may be getting it. I’ll try a simple example, and see if it works. The 15um band radiates a bunch from the surface. Spectral analysis shows about (eyeballing it — assume for discussion) 5% transiting and leaving the atmosphere. I assume this 5% either made it out directly, or with the absorption – many collisions – secondary emission chain of events (and maybe going through that chain many times). The other 95% will get absorbed and, following roughly the same general process above, 1) re-radiate directly back to the ground (a very tiny portion of the 95%), 2) pass energy to other molecules through multiple collisions and finally end up adding to the “steady state” kinetic energy of atmospheric molecules and heat the atmosphere (bigger than #1, but still small), 3) go through the process chain above and finally break the chain(s) by emitting back to the surface (my guess, the vast preponderance of the 95% that didn’t make it out). Though I’m not sure why the downward re-emission doesn’t get absorbed by other greenhouse gases before it reaches the ground. Or… maybe it does and just keeps going through the cycle chain numerous times with the overwhelming probability that it will finally go down to the surface rather than up and out…???
Is this, while oversimplified, generally close?
I know it is off the topic, but apparently IPCC have underestimated the level of CO2e in the atmosphere…
http://www.abc.net.au/lateline/content/2007/s2054168.htm
Any comment on this from RC?
[Response: This is very confused and misleading. The AR4 WG1 report showed that the positive forcings from WM-GHGs were around 2.7 W/m2, which is 455 CO2_equivalent, but the climate basically only cares about the total forcing – which includes cooling from aerosols and the effects of soot and ozone and solar and the like. The more relevant net forcing is estimated to be around 1.6 W/m2 (which is about 380 CO2_e). Statements like Flannery’s implying that the IPCC doesn’t know what’s in it’s own reports or that suddenly there are more GHGs than we thought are just silly. He should know better. The situation is bad enough without the exaggeration. – gavin]
This question is climate related, but not related to this article:
Some skeptics point out that man-made emissions account for only a small percentage of total CO2 emissions into the atmosphere; the rest being from natural sources. I’m finding this hard to reconcile with the large CO2 jump following 1975. Are they saying that only 3-7% of the 380ppm C02 concentration in 2007 is man-made? Which would mean that a large natural CO2 increase occurred following 1975. Am I missing something here? Someone explain this to me. What is meant in these statements?
thanks Gavin, I thought is sounded strange… I’ve found he gets quite a bit wrong on energy issues so the alarm bells usually go off when he comes out with this sort of stuff. I guess it shows the poor state of the debate in our country when he is considered one of the ‘front men’.
To the rest of the world… please don’t judge us because we are Australian, most of us are ashamed enough already!
Rod, maybe David Archer can comment — someone who actually understands MODTRAN and the math, I hope, will. I think words are just approximations at best, and I don’t pretend to be able to handle the math.
Re: #78 (John Galt)
You’ve been fed a classic piece of denialist propaganda. What they omit to mention is that natural CO2 emissions to the atmosphere (from oceans, biosphere, etc.) is balanced by natural absorption (by oceans, biosphere, etc.). That’s why, for over 10,000 years, CO2 concentration was reasonably stable at about 270 ppmv.
You may even hear the false claim that more CO2 is emitted by a large volcanic eruption like Mt. Pinatubo or el Chicon, than all human activity since the beginning of time. This is just plain false. CO2 emission from large volcanos is measured in megatons, but the CO2 emissions from the U.S. alone, in one year alone, count in the gigatons. And of course, 1 giga = 1000 mega.
The truth is that all the rise in CO2 concentration since pre-industrial times is due to human activity. This is beyond doubt; the carbon in fossil-fuel CO2 has a different “isotopic signature” from that due to natural emissions, and the changing isotopic composition of atmospheric CO2 is a “smoking gun” proving that the increase is from fossil fuels.
Not only is this claim junk — it’s not even very good junk.
Oh, further for Rod, this is another diagram from the same source Timothy Chase pointed to in the parallel conversation (this thread seems more the right place for it). Your description above fits this, I think?
http://www.aer.com/images/rc/heattrap_thumbb.gif
John Galt, look up “biogeochemical cycling” to answer the point about humans adding only “a small percentage of total CO2 emissions” — what is happening that changes climate is that CO2 from fossil fuels that doesn’t all get cycled by natural processes (about half the total produced by burning fossil fuels so far is still in the atmosphere, the other half did get taken care of by nature). The “natural carbon sinks” only accept so much — the rest accumulates fast in the atmosphere and won’t get taken care of by natural cycling for some centuries. And if the natural processes get overloaded and quit working, then the excess will accumulate faster.
The ‘start here’ link at top of page will help with the basic questions like this.
thanks Gavin, I’ve written to the ABC and asked them to clarify with Mr Flannery and if appropriate, issue a pulic retractment.
Re #78 The pre-industrial level of CO2 was measured around 1900 at 280 ppm, and that value has been confirmed by the analysis of ice cores. Today the level of CO2 is at 380 ppm, an increase of almost a third.
If you go to the NASA site you can see the latest measurements of atmospheric CO2 taken on top of Manua Loa, in the centre of the Pacific. See http://www.esrl.noaa.gov/gmd/ccgg/trends/
What the skeptics are arguing is the the change during one year, as shown by the red curve,is much greater than the net change each year shown in the black curve. The seasonal change in CO2 is caused by the blooming and decay of vegetation on the continents, which are mainly in the northern hemisphere. That has a net effect of zero on the annually averaged CO2 level.
Or it would have, but with the burning of the rain forests in fact there is more CO2 released into the atmosphere than the pasture lands that replace them can absorb. So the rise in CO2 and the resulting global warming is not just a result of burning fossil fuels. Land use change is also having a not insignificant effect.
Re #78
I should have written a NOAA website not NASA, but it is no less authoritative.
Re 78:
Each year vegetation takes up ~110 gigatons (1 billion metric tons) of carbon from the atmosphere through photosynthesis. And each year, plants, animals, and microbes respire about ~110 gigatons of carbon back in to the atmosphere. Over the last several hundred millions, some amount of carbon on the order of ~5000 gigatons wasn’t respired back to the atmosphere but trapped in the lithosphere becoming what we know as fossil fuels. Our civilization is now burning fossil fuels, emitting some of that carbon back in the atmosphere on the order of several gigatons per year.
So to answer your question, yes, annually the amount of carbon released by burning fossil fuels is only 3-7% of that respired (“natural emissions”) by the biosphere. The important distinction to be made is that the carbon cycle without fossil fuel emissions would be much closer to a steady-state. That is the amount taken up by photosynthesis is very close to the amount respired.
So, each year we are adding a few gigatons of carbon to the atmosphere, even though the anthropogenic carbon emissions are small compared the natural fluxes. This can all be solidly demonstrated by measuring the changes in the isotopic composition of atmospheric carbon dioxide. I refer you to the some of the great climate science links provided by Real Climate in the panel above on the right, if you want to learn more.
“John Galt” posts:
[[Some skeptics point out that man-made emissions account for only a small percentage of total CO2 emissions into the atmosphere; the rest being from natural sources. I’m finding this hard to reconcile with the large CO2 jump following 1975. Are they saying that only 3-7% of the 380ppm C02 concentration in 2007 is man-made? Which would mean that a large natural CO2 increase occurred following 1975. Am I missing something here? Someone explain this to me. What is meant in these statements?]]
The amount emitted by human technology in one year is small compared to the total out there. But it has been emitted for many years now. About 28% of the CO2 currently in the air is manmade.
The human amount is small compared to the natural emissions as well, but the natural emissions are largely matched by natural absorptions. For example, the oceans emit about 90 gigatons of carbon every year, but absorb 92 gigatons. It’s the human emissions that are driving the net increase every year, just like a small trickle into a bathtub that is full to the top will cause an overflow. The comparison of human emissions to natural emissions is disingenuous, because natural emissions are largely balanced by sinks and human emissions are not.
Timothy,
Re: #50
I think have found a copy of “Charney J 1979 Carbon Dioxide and Climate: A Scientific Assessment” and I shall read it before making any comment.
I am aware of some of the mechanisms involved in the absorption and emission of radiation in the atmosphere. I will if I may suggest a modification to that which you wrote.
In that absorption does not necessary equal emission. It depends on the temperature. For instance in the H20 bands there is a net flux in the direction of decreasing temperature. Eventually this turns into a net outboud flux at the TOA.
You can get a feel for this from MODTRAN if you select low altitudes and select “looking up”.
In the “looking up” mode it gives you the “Path Length Radiance” e.g the component that comes from the atmosphere not directly from the surface.
At low altitudes e.g. .01km the band (wavenumber 100-400) “shines” with a temperature of ~288K i.e. surface air temperature.
At 1 km it shines with a temperature of ~280K.
At 4 km at around ~260K.
The actual temperatures that MODTRAN assigns to 1km and 4km are higher 281.7K and 262.2K.
If you do the same “looking up” you get very similar radiation temperatures. The band is close to thermal equilibrium which is I think what you were describing. But as there is a temperature gradient so there is a net flow upwards.
In these bands the “direct” radiation from the Earth’s surface is blocked. The downflow and upflow are all but identical at low altitudes. From the point of view of the ground surface that band is effectively closed.
This could lead to a paradox as at TOA this band radiates with strengths between the yellow and light blue lines ie an equivalent temperature of between 220K and 240K. This is more or less a net outbound flux.
From MODTRAN we have a net upward flux at the surface that is less than half the TOA outbound flux. I think that this is the sort of difficulty that you are aware of.
It would be much more accurate to not use the word blocked but to say that the “blocked” bands were “thermal bands” and the clear bands were “radiative bands”. As an approximation the “thermal bands” radiate at around 220K and the clear bands at around 288K. The fourth power of (288/220) is ~1/3 so a significant reduction in outbound radiation results from “blocking” a band.
A word of caution MODTRAN seems to have a rule that the temperature at the 12km altitude is fixed as is the lapse rate.
Now as more greenhouse gases enter the atmosphere due to outputs of CO2, CH4 and the effect of rising temperature on H2O vapour. The “thermal” output will become increasingly important and the direct radiative component less so. This will mean that the upper troposphere temperature will become increasingly important as a means of removing heat from the system.
I for one should like to know if the assumption of constant lapse rate and a constant 12km temperature that “seem” to be built into MODTRAN are realistic.
I do not know the sensitivity of outbound radiation to temperatures in the upper troposphere but based on MODTRAN my best guess is that a temperature rise of 1C might contribute 0.75 W/m2.
On the other hand increasing greenhouse gases will I think mean that more energy would need to be supplied to the upper atmosphere to power the thermal radiation output there. These are areas that I know little about but would be interested to know more.
I will write again regarding your other points when I can.
Best Wishes
Alexander Harvey
Re Myself #79
I have always used this http://geosci.uchicago.edu/~archer/cgimodels/radiation.html interface to MODTRAN.
If you want the “lookig up” and some other features you may need to do the same.
re #75. Ray Ladbury states that “as we raise the ghg content of the atmosphere, we effectively raise the altitude at which IR photons can escape.”
As I understand it, this altitude or level is referred to as the effective radiating level and is measured by examining brightness temperatures of various parts of the IR spectrum from space. Thus, IR travelling through the atmospheric window has a brightness temperature of 288K, that in the water vapour absorbing bands of 275K and that passing through the CO2 band of 215K.
My question is this. As we continue to pump more CO2 into the atmosphere, the climate remains in disequilibrium with more incoming than outgoing energy. It seems to me that, in theory, this could be demonstrated by plotting a falling brightness temperature for IR in the CO2 absorbing band against increasing atmospheric CO2 concentration. Have these measures been made? If not, why not? Is it becauswe we haven’t had access to the appropriate equipment for long enough to show a trend?
I would appreciate help with the above but, while you are at it, perhaps someone could also explain the logical difficulties I have with my understanding of brightness temperatures and their relationship with altitude. My (possibly dodgy) logic tells me that a lower brightness temperature would indicate less effluent energy per se rather than emission height. (I accept that there will be less radiation at the colder temperatures of the higher atmosphere so I may be quibbling over terminology).
Finally, I am uncertain why climatologists are so confident that water vapour is a positive feedback. (If it is, I assume that the brightness temperature of IR in the water vapour absorbing bands should also be dropping). I accept that extra surface warmth will increase atmospheric water vapour but will also increase convection which will carry more energy high into the atmosphere above the water vapour level. Water vapour will also presumably translate into more cloud. This, in turn, could increase albedo and absorb some solar energy before it got to the surface. As clouds act as black body radiators in the IR range, clouds could make it more likely that IR would escape to space than to reach the surface. It seems to me (in my ignorance), therefore, that it is quite complicated to decide whether water vapour will be a positive or negative feedback.
Re #91
Douglas,
I am not sure how they measure radiation temperatures in practice but in principle if the gas has two or more emission bands you compare their relativ intensities. As gases get hotter they tend to emit more in all bands but the increase is greater at shorter wavelengths. Comparing two or more wavelengths, and after a fair amount of calibrating and you can give a value to the effective radiation temperature of the gas.
If you are inside the gas and the radiation is in thermal equilibrium with the gas then this is the gas temperature. If you are outside the gas it can be more an indicative than an absolute value.
Perhaps someone who knows how it is done in practice will come along and correct me.
The absolute brightness is not the important part it is the ratios between different wavelengths. Much like a furnace changes colour when you look through a spy hole. It is the colour not the intensity that indicates the temperature. I have done this and it works.
Best Wishes
Alexander Harvey
Rod B., You are indeed close. Basically, all the IR from the surface in the H2O and CO2 bands gets absorbed. Most of the energy goes into collisions with other atmospheric molecules (mostly not ghgs). Some small fraction is emitted from excited ghgs (excited either by collision or by IR). Some goes up, some down. Repeat ad infinitum or ad nauseum, whichever comes first. Eventually, you reach a level where the amount of a particular ghg is small enough that the chances a photon is absorbed are small. Water vapor peters out. Then much higher, CO2 peters out. The ghgs at this level radiate at their effective temperature in the relevant IR line, and that ~5% is what you see from space. I’m sure Gavin, Ray, et al. are wincing at my oversimplification, but in the limit of the spherical horse, that’s how I understand it.
John Galt (#78) wrote:
John,
Skeptics point out a lot of things.
They’ve pointed out that CO2 actually has no effect upon the climate system, that what is actually changing the climate system is some sort of delayed reaction to the rise in solar temperatures from the earlier part of this century, that can’t be explained except by mysterious heat retention in the ocean but which we’ve picked up somehow in the rise of rivers. They’ve pointed out that greenhouse gases don’t work because the hot air rises up before it gets a chance to radiate, that there is no such thing as average temperature, that the average temperature hasn’t been rising, that its really magnetic fields thats causing the temperature to rise and even that carbon dioxide can’t be in the upper troposphere or the stratosphere because its heavier than air and when it comes as exhaust, it goes sideways then falls immediately to the ground.
As I have said, they point out a lot of things, and its usually horse-hockey. And by the way: I didn’t make any of the above up. Its all stuff that’s been published – although the last was in a letter to the editor – by someone who claimed to be a high school science teacher. And all of this is something I’ve run into this year.
But it might help to know their source.
However, from what I have seen of the accounting, we pretty-well know its us thats put it up there. Pretty much all of it, down to within a few percent. Although nature has been kind so far in sweeping the majority of it under the rug – or to be more precise – into the oceans and even trees. But the oceans might not be such a good idea. And it has a higher content of a lighter isotope of carbon in just the right percentage to show that it came from fossil fuels.
PS
Just out of curiosity: who are you?
Ray (David Archer, Gavin …) — one question I keep coming back to.
We see weather satellite photographs in many different infrared wavelengths. Some show land/water/low clouds; some show water vapor. Other similar imagery shows concentrations of CO2 at different locations and heights in the atmosphere.
Those images are made with the photons that do move directly from source to satellite — right?
And photographs in the infrared of the limb of the Earth, like photographs in visible light, are showing the scattered light and infrared from the gases of the atmosphere. Right?
John Galt,
By all means, natural sources of CO2 emission dwarf those of humans. They always have and will continue to do so. However, until somebody figures out a way to tell those pesky plants, animals, etc. to quit decaying, the only source we can control is anthropogenic. What is more, the ~5% emitted by humans is actually a huge amount, and the fact that human emissions have increased exponentially (compared to stasis in natural sources) is very worrying. I actually find it hard to believe that the folks who propagate such statements are unaware of how misleading they are, but I would be willing to allow them to cop to whatever they think is the lesser charge: mendacity or willful ignorance.
Alexander Harvey (#89) wrote:
I am aware of the fact that it becomes easier for the radiation to escape as the peaks become narrower due to diminished temperature, and likewise, diminished air density makes the paths longer with upwelling radiation. But I don’t see this as a violation of Kirchoff’s Law: that absorptivity and emissivity are equal under Local Thermodynamic Equilibrium – as Kirchoff’s Law describes the local behavior.
Alexander Harvey (#89) wrote:
As I understand it, brightness temperature deals with how much radiation is being emitted at a particular wavelength. It says nothing about how much is being absorbed, and therefore would not necessitate a violation of Kirchoff’s law under and circumstances, high pressure or low pressure, warm or cold – for any greenhouse gas.
Alexander Harvey (#89) wrote:
Top of the Atmosphere. If this is CO2, by 30 km it is already in non-LTE, although the temperatures don’t really begin to diverge significantly until about 50 km.
Anyway, thank you for the response. At the very least, we seem to pretty much understand things the same way – except with respect to Kirchoff’s Law. But perhaps there is something I am just not seeing and I have misunderstood Kirchoff’s Law. Something which Eli said would seem to suggest so.
Take care.
Hank,
This is a SWAG, but I think that in order to provide images, they have to be direct from the source to the detector. Now, I’m trying to think how I’d image at different levels. One way would be to take advantage of line broadening and image in the tails of the line. You could do this either using a band-gap engineered detector, or more likely by subtracting two filtered images. I’d be really interested to know whether my guess is close.
In reference to comment #89 Alexander states:
“I do not know the sensitivity of outbound radiation to temperatures in the upper troposphere but based on MODTRAN my best guess is that a temperature rise of 1C might contribute 0.75 W/m2.
On the other hand increasing greenhouse gases will I think mean that more energy would need to be supplied to the upper atmosphere to power the thermal radiation output there. These are areas that I know little about but would be interested to know more.”
If I understand correctly what process Alexander is interested in The following is from “Global Warming The Complete Briefing ” third edition by John Houghton – U of Cambridge Press, 2004
He describes the process in the upper atmosphere as follows:”Let us imagine, for instance, that the amount of carbon dioxide in the atmosphere suddenly doubled everything else remaining the same(figure 2.8).What would happen to the numbers in the radiation budget presented earlier? The solar radiation would not be affected.The greater amount of carbon dioxide in the atmosphere means that the thermal radiation emitted from it will originate on average from a higher and colder level than before. The thermal radiation budget will therefore be reduced,the amount of reduction being about 4 watts per square metre( a more precise value is 3.7).
” This causes a net imbalance in the overall budget of 4 watts per square metre.More is coming in than going out. To restore the balance the surface will warm up.If nothing changes apart from the temperature -in other words, the clouds, the water vapour, the ice and snow cover and so on are the same as before – the temperature turns out to be about 1.2 C.”
The figure referred to shows 240 W/M^2 incoming (net solar radiation) and 240 leaving. When the carbon dioxide is suddenly doubled the amount leaving is reduced by 4 watts per square meter to 236. Balance is restored to 240 and if nothing else changes apart from the temperature of the surface and lower atmosphere, the surface rises by 1.2 C. If feedbacks are accounted for the average temperature of the surface rises by about 2.5C.
Hopefully, this will add to further clarification of the energy balance process.
The responses to “John Galt” have been many and excellent, but perhaps I can add a tiny bit of value. There is a low-level and a high-level answer to the “CO2 is mostly of natural origin” objection. Here’s the low-level answer:
If we add up the human and natural components of the carbon cycle, we find that sources add up to 217.1 gigatons of carbon (GtC) annually, and that sinks add up to 213.8 GtC. The annual difference of 3.3 GtC is what causes the steady increase in atmospheric CO2. Now, human emissions contribute 5.5 GtC annually, and other human activities (cutting trees, etc.) another 1.6 GtC. Of the gross 7.1 GtC caused by humans, 3.2 GtC stays in the atmosphere, and 2 GtC are absorbed by the oceans. That leaves 1.9 GtC unaccounted for. The mechanisms taking up the missing carbon are under study — leading candidates include more rapid absorption than expected into vegetation and soils. Bottom line: it’s not surprising that humans are causing increases in atmospheric CO2, it’s surprising that we are not adding more.
The fact that the added carbon is overwhelmingly of human origin has been well established by isotopic analysis, among other methods.
And here’s the high-level answer:
If you really think that climatologists, upon hearing this, will clap their foreheads and cry out “Of course! We forgot all about natural CO2! How could we have been so foolish?”, then you and I are inhabiting different realities.