In Part One we looked at a few basic numbers and how to compare “apples with oranges” – or the solar radiation in vs the earth’s longwave radiation going out.
And in Part One I said:
Energy radiated out from the climate system must balance the energy received from the sun. This is energy balance. If it’s not true then the earth will be heating up or cooling down.
Why hasn’t the Outgoing Longwave Radiation (OLR) increased?
In a discussion on another blog when I commented about CO2 actually creating a “radiative forcing” – shorthand for “it adds a certain amount of W/m^2 at the earth’s surface” – one commenter asked (paraphrasing because I can’t remember the exact words):
If that’s true – if CO2 creates extra energy at the earth’s surface – why has OLR not increased in 20 years?
This is a great question and inspired a mental note to add a post which includes this question.
Hopefully, most readers of this blog will know the answer. And understanding this answer is the key to understanding an important element of climate science.
Energy Balance and Imbalance
It isn’t some “divine” hand that commands that Energy in = Energy out.
Instead, if energy in > energy out, the system warms up.
And conversly, if energy in < energy out, the system cools down.
So if extra CO2 increases surface temperature… pause a second… backup, for new readers of this blog:
First, check out the CO2 series if it seems like some crazy idea that CO2 in the atmosphere can increase the amount of radiation at the earth’s surface. 10,000 physicists over 100 years are probably right, but depending on what and where you have been reading I can understand the challenge..
Second, we like to use weasel words like “all other things being equal” to deal with the fact that the climate is a massive mix of cause and effect. The only way that science can usually progress is to separate out one factor at a time and try and understand it..
So, if extra CO2 increases surface temperature – all other things being equal, why hasn’t energy out of the system increased?
Because the system will accumulate energy until energy balance is restored?
More or less correct. No, definitely correct – probably an axiom – and probably describes what we see.
Higher Surface Temperature – Same OLR – Does that make sense?
The question that the original commenter was asking was a very good one. He (or she) was trying to get something clear – if surface temperature has increased why hasn’t OLR increased?
Here’s a graphic which has caused much head scratching for non-physicists: (And I can understand why).
Upward Longwave Radiation, Numbers from Kiehl & Trenberth (1997)
For those new to the blog or to climate science concepts, “Longwave” means energy originally radiated from the earth’s surface (check out CO2 – An Insignificant Trace Gas – Part One for a little more on this).
Where’s the energy going? Everyone asks.
Some of it is being absorbed and re-radiated. Of this, some is re-radiated up. No real change there. And some is re-radiated down.
The downwards radiation, which we can measure – see Part Six – Visualization, is what increases the surface temperature.
Add some CO2 – and, all other things being equal, or weasel words to that effect, there will be more absorption of longwave radiation in the atmosphere, and more re-radiation back down to the surface – so clearly, less OLR.
In fact, that’s the explanation in a nutshell. If you add CO2, as an immediate effect less longwave radiation leaves the top of atmosphere (TOA). Therefore, more energy comes in than leaves, therefore, temperatures increase.
Eventually, energy balance is restored when higher temperatures at the surface finally mean that enough longwave radiation is leaving through the top of atmosphere.
If you are new to this, you might be saying “What?”
So, take a minute and read the post again. Or even – come back tomorrow and re-read it.
New concepts are hard to absorb inside five minutes.
Conclusion
This post has tried to look at energy balance from a couple of perspectives. Picture the whole climate system and think about energy in and energy out.
The idea is very illuminating.
The energy balance at TOA (top of atmosphere) is the “driver” for whether the earth heats or cools.
In the next post we will learn the annoying fact that we can’t measure the actual values accurately enough.. Which is also why even if there is an energy imbalance for an extended period, it is hard to measure.
Update – Part Three in the series on how the earth radiates energy from its atmosphere and what happens when the amount of “greenhouse” gas is increased. (And not, as promised, on measurement issues..)
The Science of Doom 
Will you have a post on how the global OLR is measured? I can imagine the difficulty in measuring the global average OLR. Its accuracy is likely to be questionable, just as the average global temperatures are. The claimed changes in global average temperature is a very small percent of absolute (degrees K) temperature. With OLR, the accuracy capability of the global measuring method may be well outside the small percentage change, if any.
John Phillips:
Probably the next post will touch on that.
Love the site, deserves more traffic.
I wonder if I could ask you, Dr. Phillips, to help clarify something for me. I recently read Dr. Hansen’s book, in which he argues that we should look not to models to see what the earth will be like if we continue on the present path, but at the climate record, to other times when we had similar CO2 levels.
As you know, “peoplewhodontagreewithus-ists” make much of the fact that CO2 increases in response to rising temperatures. (They harp on the time lag, too, but that part doesn’t trouble me.)
What I am wondering is, how do we associate past warming with past CO2 levels, given the complex relationship between the two?
Excuse me if you’ve covered this (I looked, a bit). What Dr. Hansen said made a lot of sense, and I believe, given what we know about GHGs, that that level of CO2 has to have played a major role in driving temperatures, but I don’t know how to prove it to my own satisfaction and the satisfaction (if only!) of “skeptics.”
[…] climate is in overall equilibrium, the energy radiated out will match the incoming energy. See The Earth’s Energy Budget – Part Two and also Part One might be of […]
Robert:
Your comment got trapped in the spam queue because it breached a point of etiquette. I resurrected it with a minor edit. I’ll take a look at your question shortly, but I’m not “Dr. Phillips”..
Robert:
Thanks for kind comment.
As to your question – it seems like a massively complex problem – which you also allude to. Climate effects are inter-related and non-linear.
There’s also the argument “CO2 levels have been much higher in times of similar temperatures, so there is nothing to worry about..” – I haven’t dug into that to find out even how strong the evidence is – because many other conditions would have been different.
We have had times of higher overall temperatures that have been more moderated than present day distributions – lower temperatures in the tropics and higher temperatures at the poles than present. But circumstances were much different – continents in different locations and who knows what else.
The Ghosts of Climates Past are a “gift that keeps on giving” for both sides of the discussion.
I do plan to cover some aspects of the more recent past – like the last million years.
Forgive my ignorance; but would out going radiation not be less until an equilibrium was reached?
Alf:
You are right. I thought that’s what I said.
Increase CO2, less outgoing radiation.. finally a new equilibrium at higher temperatures.
I might have labored the point so much that the key point was obscured..
Robert,
I’m just another reader of the blog like you, and have no PhD.
Considering the fluency and authority with which you review the science, you can understand my confusion. Thanks for your response.
Hi — great site.
I’m still missing something, though. If you add more CO2, then more energy will be absorbed and re-radiated in all directions — including up. Is there some latency between the absorption and re-radiation? If not, I still can’t see how the TOA radiation doesn’t increase “all things being equal”
Tim W.
Tim:
Of course, there’s no latency, (except of interest to people who love quantum mechanics) between absorption and re-radiation.
Well, let’s suppose a “layer” of atmosphere absorbs 1W/m^2 of energy. (And let’s suppose it’s a new change, someone poured in some CO2 to that layer, or water vapor, depending on your favorite greenhouse gas)
Now that energy increases the temperature of that layer. So the layer radiates energy out.
Lets simplify and say 1 W/m^2 is immediately radiated out-
0.5 W/m^2 upwards and 0.5 W/m^2 downwards
So at TOA we have lost 0.5 W/m^2, which is now heating up the surface (redirected)
But instantaneously there won’t be any temperature increase. Why not?
Because of the specific heat capacity of the oceans, the atmosphere and so on. Like turning the heat on the stove, the water takes time to heat up.
More energy comes into the system than out so the whole system has to heat up – where the heat ends up depends completely on the characteristics of that system, a very complex problem.
But simplifying it down we end up with an increased temperature at the surface. Eventually the temperature increase at the surface is high enough that the “lost” 0.5W/m^2 is now made up for and energy equilibrium is now re-established.
“All things being equal” of course.
Science,
I am a bit dense so bear with me. If the earth is at equilibrium and radiating 265 W/m^2 at the TOA and we add a bit of CO2, the added CO2 absorbs energy (say 1 W/m^2). Now that 1 W/m^2 is re-emmitted with 0,5 going down (as a temperature increase) and the other 0.5 going up. Why isn’t the emission then 265.5 W/m^2 at the TOA?
I have the same question
ps. never mind! The lightbulb just went on. 🙂
[…] Another post to check out – The Earth’s Energy Budget – Part Two […]
[…] 21, 2010 by scienceofdoom In the previous article in this series, The Earth’s Energy Budget – Part Two we looked at outgoing longwave radiation (OLR) and energy imbalance. At the end of the article I […]
Wouldn’t the main reason you don’t see OLR increase is that, from space, you are (mostly) seeing the radiation associated with the “top” of the troposphere?
Since that temperature isn’t expected to change (much), you wouldn’t expect the OLR to increase….
Carrick:
This post really tries to cover the basic idea of energy balance.
Once we move deeper into the subject it all gets more complicated. Like “how does the equilibrium get restored”?
There’s more in Part Three – with the concept of where the atmosphere radiates from.
[…] by energy leaving from the top of the atmosphere – otherwise temperature will increase (see The Earth’s Energy Budget – Part Two). And if one “layer” of the atmosphere totally absorbs it will still radiate energy to […]
[…] Update – new post The Earth’s Energy Budget – Part Two […]
[…] Part Two – which explained energy balance a little more […]
[…] In long term equilibrium energy in = energy out. However, we want to know what happens if something disturbs the system. For example, if increased CO2 reduces OLR then heat will be added to the climate system until eventually OLR rises to match the old value – but with a higher temperature in the climate. The same is the case with any other forcing. (See The Earth’s Energy Budget – Part Two). […]
[…] The most important point to understand is that the atmosphere and surface are heated by the sun via radiation, and they cool to space via radiation. While all of the components of the climate are inter-related, the fundamental consideration is that if cooling to space reduces then the climate will heat up (assuming constant solar radiation). Which part of the climate, at what speed, in what order? These are all important questions but first understand that if the climate system radiates less energy to space then the climate system will heat up. See The Earth’s Energy Budget – Part Two. […]
[…] The Earth’s Energy Budget – Part Two – the important concept of energy balance at top of atmosphere. […]
Say you assume the U.S.Standard Atmosphere, with tropopause starting at a little over 11 km and stratosphere maybe 20 km.
(a) At what altitude does the top of the atmosphere start?
(b) What is the physical definition of TOA? Perhaps when collision frequency is low enough that one can no longer assume local thermodynamic equilibrium?
Douglas,
In the context of this article, it is just the limit of outgoing longwave radiation as you move further away from the earth.
TOA is where OLR (outgoing longwave radiation) doesn’t increase any more, and where DLR (downward longwave radiation) is zero.
Doug: You can answer some of your questions using the online MODTRAN calculator at: http://climatemodels.uchicago.edu/modtran/
Looking down from between 30 and 40 km, OLR increases about 1 W/m2. Above that the changes are in the tenths of a W/m2 for a change of 10 km. The highest this program allows is 70 km. It reports DLR for looking up from 60 km as 0.17 W/m2.
My assumption is that the TOA is the altitude above which nothing changes significantly – but “significantly” means different things in different situations. 30 km (10.9 mb) is above 99% of the atmosphere.
Hi Frank,
But the ultimate answer you have to get for total outward flux in standard atmosphere is about 239 watts/square meter because that is what you get if you turn off the greenhouse effect., assume equilibrium with the sun’s input, put in cloud albedo and then deduce an earth surface temperature of 255 degrees K.
In fact, using satellite data analysis, Trenberth, Fasilla, and Kiehle
do obtain 239 watts/square meter. see: http://earthobservatory.nasa.gov/Features/OrbitsCatalog/
On the other hand, if you set U. Chicago Modtran with default settings except use a U.S. Standard atmosphere rather than a tropical atmosphere,, it calculates upward flux at 70 km of 260 watts per meter squared. This is entirely too large?
Thank you for your response!
Douglas
Douglas,
The US Standard atmosphere is just one “representative” atmosphere. As far as I know, it wasn’t designed to produce the global annually averaged OLR.
There are four factors that affect OLR from each location:
a) surface temperature
b) temperature profile of the atmosphere
c) each “greenhouse” gas concentration through that profile
d) clouds
And we could say e) the absorbing characteristics of each greenhouse gas across wavelengths, but e) is effectively a fixed material property, while a-d are factors that change around the earth and from day to day.
If you look at “clear sky” OLR (globally annually averaged) you get a value around 270 W/m2 (I’m going from memory because it isn’t critical to understanding the various effects), while the “all sky” (clear sky + cloudy sky) value is 239 W/m2.
Douglas,
Just to add to that, the OLR varies significantly across the earth’s surface and by season.
For example, January 2009 (averaged for the month), measured by CERES:
You can see the month by month results for the whole year at Understanding Atmospheric Radiation and the “Greenhouse” Effect – Part One.
Douglas: Our host has provided you with a better reply to your comment than I could have. I’ll simply add that most atmospheric profiles in MODTRAN also don’t calculate exactly a 3.7 W/m2 reduction in OLR when CO2 is doubled for exactly the same reason.
I wish MODTRAN allowed one to choose a planetary average atmospheric profile that consisted of an appropriately weighted average of radiation transfer through the atmospheric profiles you can select.
Is there some way I can construct a graph of my own related to this topic and either post it in Science of Doom or send it to Science of Doom for further discussion?
Douglas, you can email it to me and I’ll put it up. scienceofdoom you know what goes here gmail.com.
Dear Science of Doom
I think I have found something interesting between your recent reply to my question and something I already found out. I will send you two attachments, but perhaps there would be something better to do than post these. So lets exchange a couple more e mails before posting. See attachment 1 and 2. Attachment 1 is of ModtranChicago set for only CO2 as greenhouse gas looking down from 20 km. The settings are shown in the attachment.
I wanted to check something out and partially digitized my graph with UnScanIT in point by point mode. The way I did this was to construct my own Planck distribution for the regions of the curve above and below the ditch shaped minimum then splice these into the digitized result. There is a little scatter. The curve so shown is in attachment two. You will notice that my curve extends from 5 wn to 2000 wn, whereas the ModtranChicago curve goes from about 100 wn to 1500 wn. I integrated my curve using the trapezoid method and got 346 watts/square meter, as opposed to the result ModtranChicago gave of 322 watts/ square meter. Could it be that this significant error is because ModtranChicago really cuts off below 100 wn and above 1500 wn? I checked this out.
I then used the button on the ModtranChicago output that accesses the”:Raw Model Output” of the computer run. Here is a quote from an on line pamphlet I have been writing which hopes to translate the nomenclature and point of view of atmospheric science into a form intelligible for solid state physicists. *Upon examination of the âRaw Model Outputâ feature of Modran, it became evident that the Modtran integration only extends to 1500 wn. **If I repeat my integration under the orange curve in Fig. 9, but stop at 1500 wn the output flux obtained is then 329 watts/meter squared. The Modtran âRaw Model Outputâ shows that the Modtran integration is started at a lowest frequency of 100 wn, instead of 5 wn. If I then integrate from 100 wn to 1500 wn, as does Modtran, the output flux I obtain is 327 watts/meter squared, compared with the Modtran output of 322 watts/meter squared. This is a difference of less than 2 percent, which is probably all one can expect considering some error in the *point by point digitizing of the data shown in Fig. 7. My conclusion is, therefore: The Modtran output was low by about 20 watts/meter squared! I checked this by looking at the “Blackbody radiator” feature of SpectralCalc. If I integrate between 1500 wn and 2000 wn using SpectralCalc for 288 deg K and emissivity one, I get a spectral radiance of 5.622 watts per square meter per steradian, which multiplied by pi steradians yields 18 watts per meter squared. Using SpectralCalc between 5 wn and 100 wn adds two more watts per square meter to make 20 watts per square meter. The SpectralCalc result is independent of any digitizing error on my part.
I then explored further. I set Modtran for 288 degrees K and looked at the output down from zero altitude. ModtranChicago then yields 360 watts/meter squared. But the result from the Stefan – Boltzmann law with an “effective emissivity” of earth of 1 at 288 deg K is 390 watts/ meter squared. Then dividing 360 by 390 is 0.923. But dividing 380 (with the 20 w/sq m correction) by 390 gives “effective emissivity for thermal IR” of 0.974. (I don’t know if there is such a thing as “effective emissivity”. But there should be? Because of course there must be reflection /scatter if emissivity is less than one for earth’s surface and not all of this escapes…)
Now…..If you take Modtran and run it at 70 km looking down for U.S. standard atmosphere and leave all the defaults in except change to U.S. Standard atmosphere, you have all the greenhouse gases at the usual level but you *choose zero cloud cover.* Then you get OLR from Modtran of 260 watts/meter squared. *With the 20 watts per meter correction you get 270 watts per meter which you recall as the OLR for a clear sky!*
There is a Journal of American Physics which publishes material that is useful for teaching about various physics topics. Would you be interested in co authoring this with me? I would write initial version and pay for putting it into LATEX.
As far as posting those figures, here is my fear. There are people who would want to run with this with the ridiculous point of view that there is a small error in David Archer’s wonderful program and therefore Global Warming is a Hoax. Rather the point of view should be that this program is a marvelous teaching tool which we are now improving so that it even gets the clear sky OLR right.
Peace,
Douglas Pease (You can find me on Researchgate)
On Mon, Jan 2, 2017 at 2:28 PM, The Science of Doom wrote:
> scienceofdoom commented: “Douglas, you can email it to me and I’ll put it > up. scienceofdoom you know what goes here gmail.com.” >
Douglas,
I didn’t get any attachments.
I wouldn’t worry. Everything out in the open and subject to scrutiny is the best medicine. That is what this blog tries to do.
From your first paragraph, and from Frank’s comment it seems you have solved the question on the discrepancy of upwards flux higher in the atmosphere.
As for the question about the upwards flux from the surface, I don’t know the limits of the Modtran program, I have barely played with it at all.
I did write a Matlab program which used line by line calculations, using the HITRAN database, for various “greenhouse” gases and was able to reproduce a lot of results found in textbooks and papers. You can see these results (and the code) in the series Visualizing Atmospheric Radiation.
The results are never an exact match, for all kinds of technical reasons – the temperature profile, which GHGs are used, whether better line shapes (Voigt vs Lorenz) are used for the stratosphere/upper troposphere, what surface emissivity is used, what water vapor continuum formula is used.. it’s a long list and this is why there are periodic inter-comparisons between GCMs and LBL (“line by line”) calculations. For example, Radiative forcing by well-mixed greenhouse gases: Estimates from climate models in the IPCC AR4,WD Collins et al (2006).
Well, that is very kind offer, but unfortunately I don’t have a lot of spare time generally – as you can see from the complete lack of articles I have written in some time. I would like to write more and have been reading up, but I think my “climate time” will need to go into this blog if it goes anywhere.
Thank you so much, Science!
Looking at the words I sent you, even if the graphs were lost in cyberspace, I see something that makes this better. The ModtranChicago 260 watts per meter squared plus 20 watts per meter squared correction actually gives 280 watts per meter squared. Multiply this by 0.97 which is my “effective emissivity” at surface, gives 271.6 watts per meter squared, very close to the “clear sky emissivity” number that you remembered.
So the refinement in my above paragraph makes the corrected ModtranChicago results looking down from zero altitude consistent with the corrected ModtranChicago results looking down from 70 km and do give an OLR at 70 km of about 270 watts per meter squared.
Now my next step is to dig into the literature to find a reference that gives the “Clear Sky OLR” so as to verify the approximately 270 watts per squared OLR as seen from a satellite, which is a number you recalled from somewhere.
Douglas,
What’s your goal? Or, asked another way, what is the problem you are try to solve?
Calculating the clear sky OLR from one surface (with one temperature and one emissivity) with one atmospheric temperature profile and one GHG concentration (for each of water vapor, CO2, CH4, N2O, O3) is one problem.
Calculating the average (global annual) clear sky OLR is another problem – which relies on either the measurements from CERES, or on a “reanalysis” where you have the above list of parameters across the globe, across the year and are able to do the calculation and produce an average.
I found a cool reference for this. X. Huang, et al, in Journal of Climate, Vol 26, p. 478, 2012. Their table three. Entitled “Near Global Annual Mean Band Fluxes and OLR Derived From collocated AIRES and CERES Measurements….(and other stuff as well.)” Bottom Line…..They use baseline AIRES value of 273.74 watts / meter squared and then do plus various somethings or other on order of ~ +3 watts per meter squared — – 2 watts per meter squared trying to see how the various satellites and satellite corrections do or do not agree.
But for my purposes I think all I need is that the answer is indeed something like 274 watts per meter squared.( Previously I had corresponded with David Archer on another Modtran issue I found….if you compute the CO2 climate sensitivity from Modtran versus altitude the plot has sharp little discontinuities that look like round off errors to me.) He pointed out that the basic program is left over from the days that the funding was strictly to design better heat seeking missiles. But if I curve fit the plot I get something that makes sense and the discontinuities do not really change the over all picture.
So by a heat seeking missile program that is a great teaching tool, something around 274 watts per meter squared is fine and all one should expect..
I found a paper in my files that showed CERES and AIRS data on clear sky OLR, Interannual variability of outgoing longwave radiation as observed by AIRS and CERES, Joel Susskind et al (2012):
It’s a technical paper looking at the differences between different “editions” of the data from two different satellites – the kind of stuff that climate scientists do routinely (as far as I can tell from reading 1000+ papers).
It’s also important to note that OLR from CERES and AIRS is produced differently (CERES measures, AIRS calculates), and also that “clear sky” is a judgement call. What cloud fraction (as measured by what satellite? or calculated by what algorithm from multiple satellites?) is deemed to be “clear sky”? 1% 5%
On a somewhat different topic I noticed the comment of Robert above.
“What I am wondering is, how do we associate past warming with past CO2 levels, given the complex relationship between the two?”
Robert, there is an on line lecture by one Richard Alley of Penn State (Bjerknes Lecture) that is still available on the web. One takeaway I got from this….if you go back past the ice ages, say past 800,000 years….then it is not as accurate to determine correlation between CO2 levels and temperature.. i.e. nothing like an ice core for doing that. For geologic times millions of years ago the experimental climate folks have to use other “proxies” to investigate the relationship between CO2 and temperature.
However, the most modern work seems to show this correlation even for geologic times well before the ice age cycles.
Douglass: I ran into some discrepancies trying to look at transmission through the first 10 m (0.01 km) of atmosphere and commented somewhere here. I believe DeWitt told me that surface emissivity is not 100%. I also taught me about “Show Raw Model Output” Button which I had ignored before. That clearly shows that they only integrate from 100 cm-1 to 1500 cm-1
“Add some CO2 – and, all other things being equal, or weasel words to that effect, there will be more absorption of longwave radiation in the atmosphere, and more re-radiation back down to the surface – so clearly, less OLR.
In fact, that’s the explanation in a nutshell. If you add CO2, as an immediate effect less longwave radiation leaves the top of atmosphere (TOA). Therefore, more energy comes in than leaves, therefore, temperatures increase.
If you are new to this, you might be saying “What?” ”
I´m not new to this, but “what” is exactly my response after my jaw broke when it dropped to the floor.
You write that co2 cause less radiation to be emitted from the top of the atmosphere, which means that it absorbs more from dropping in temperature, and that this is a cause of rising temperature in the surface mass.
Where in your textbooks do you find an explanation showing how a cold gas that decrease its emitted intensity when in contact with a surface that is hotter, when surrounded by 3 Kelvin vacuum, is cause of a rising temperature?
You can start by trying to find an example without the vacuum, since it is a rather special situation.
“Eventually, energy balance is restored when higher temperatures at the surface finally mean that enough longwave radiation is leaving through the top of atmosphere.”
You mistake this situation for what happens when a hot surface warming a cold fluid increase in temperature and the emitted intensity gets higher. Then there is a delay as the energy distributes through the volume. There can not be a rising temperature from the gas dropping in temperature. Why is it so hard to keep warm and cold separate?
If I take any 5-year old and take them out in the sun, and first spray him/her with a mist of water, then pour a bucket of water over the head, then put some dry ice in the kids hand, and ask them which is hot, the sun or the dry ice and water, all of them can give the right answer. What is your problem?
Lasse
The issue here is about how much LWR penetrates the atmosphere and leaves the earth system into space. Nothing to do with hot vs cold bodies.
Most of the LWR emitted from the earth to space originates in the atmosphere, not from the surface. Emission is non-linearly proportional to temperature. That’s why it’s important that the temperature in the troposphere declines with altitude.
SoD: “Why hasn’t the Outgoing Longwave Radiation (OLR) increased?”
This is a question that can be answered when you are working with an idealized atmosphere. And you can give good answers.
But the real question is:
“Why has the Outgoing Longwave Radiation (OLR) increased?”
From Decadal Changes of Earth’s Outgoing Longwave Radiation
by Steven Dewitte and Nicolas Clerbaux, 2018″Abstract: The Earth Radiation Budget (ERB) at the top of the atmosphere quantifies how the earth gains energy from the sun and loses energy to space. Its monitoring is of fundamental importance for understanding ongoing climate change. In this paper, decadal changes of the Outgoing Longwave
Radiation (OLR) as measured by the Clouds and Earth’s Radiant Energy System from 2000 to 2018, the Earth Radiation Budget Experiment from 1985 to 1998, and the High-resolution Infrared Radiation Sounder from 1985 to 2018 are analysed. The OLR has been rising since 1985, and correlates well with the rising global temperature.”
And here is the answer:
“The spatial analysis of the regional OLR change from 1985–2000 to 2001–2017 shows a ‘clear-sky effect’ mixed a ‘tropical cloud’ effect.
Concerning the clear sky effect, we see regional changes of the OLR which are correlated with surface temperature changes. In the Arctic—where the strongest temperature increase occurs—we also see a strong increase in the OLR. In general, in the Northern Hemisphere—where the surface temperature increase is stronger than in the Southern Hemisphere—we also see an OLR increase.
Concerning the tropical cloud effect, we see regional patterns in the changes of the OLR, which are suggesting a relative strengthening of La Niña conditions compared to El Niño conditions. These changes imply societally important regional changes in precipitation. The relative La Niña strengthening can also be seen in the ‘cumulative MEI index’ that we have introduced.”
The Steven Dewitte and Nicolas Clerbaux paper is mostly silenced, and when it is discussed the outcome is mostly confusion. People seemingly isn`t used to think of ocean circulation as the greatest energy transport on earth, and that discharge of energy at higher latitudes, results in OLR increase. Neither are they used to think that more precipitation over tropics are thinning clouds. This could be destructive for “settled science” as it places some questions about water vapor and cloud feedbacks. The spatial analysis point to the geography of climate change and feedbacks.
I have seen one scientist who had his departure in geographical and historical data. Professor Bill Gray left a manuscript when he passed away, on greenhouse warming and climate variability. I think S Dewitte and N Clerbaux confirm some of his ideas, and can show that he had a deeper understanding than most other climate scientists. He attributed much of climate change to the strength of ocean currents and overturning, and to thinning tropical clouds by precipitation.
Click to access Bill-Gray-Climate-Change.pdf
26 April 2006 Bill Gray was executed at RealClimate. The big guns came foreward. Gavin Schmidt, Michael Mann, Ray Pierrehumbert and Stefan Rahmstorf. with the post: “Gray and Muddy Thinking about Global Warming”. And a discussion strictly controlled by gavin, stefan, raypierre and mike.
It would be strange if all Gray`s ideas are right. But I think he made an important contribution, and had a voice worth listening to.
To the Case of increasing TOA outgoing longwave radiation. ”
On the decadal increase in the tropical mean outgoing longwave
radiation for the period 1984–2000″
D. Hatzidimitriou, I. Vardavas, K. G. Pavlakis, N. Hatzianastassiou, C. Matsoukas and E. Drakakis.
“Conclusions. To summarize, our model calculations, which are based on ISCCP-D2 cloud climatologies, and temperature and humidity profile information from NCEP/NCAR reanalysis show that there has been an increase of OLR at TOA of 1.9±0.2 Wm−2 /decade between 1984–2000. This decrease is lower than the decrease displayed by the ERBE S-10N (WFOV NF edition 2) non-scanner OLR time-series, or by the results of Wielicki et al. (2002a, b). Analysis of the interannual and long-term variability of the various parameters determining the OLR at TOA, showed that the most important contribution to the observed trend comes from a decrease in high-level cloud cover over the period 1984–2000, followed by an apparent drying of the upper troposphere and a decrease of low-level cloudiness. Opposite but small trends are introduced by a decrease in low-level cloud top pressure, an apparent cooling of the lower stratosphere (at the 50 mbar level) and a small decadal increase in mid-level cloud cover.”
Much of the same explanation. Climate change of clouds and wate vapor. How is it possible to think that clouds and water vapor give a positive feedback on longwave radiation equivalent to 3,5 times the CO2 forcing?
NK: I tried to post a response about these old papers. It is my impression that these improbably large trends from the pre-CERES era are now presumed to be due to changing and drifting instrumentation. I did a citation search on a widely cited paper Science paper by Weilicki (2002) with large offsetting trends in OLR and SWR similar to the ones you cited. I found the following 2012 paper by Loeb et al with a Figure 1 that aligns all of the data up to 2010 with no overall trend.
https://link.springer.com/article/10.1007/s10712-012-9175-1
Without access to an expert, it is difficult to know what surprising observations from the past are still considered valid today after collecting another 15+ years of data, but a citation search is one place to start.
On the other hand, Nic Lewis got started by digging into an early paper by Forster showing an ECS lower than most other publications and trying to understand why others obtained a much higher ECS from similar information.
Frank, thank you for your thoughts on this. What struck my mind was that a very recent paper had this increase of TOA LW radiation. And that it was analysed for a long period, 1984 to 2016. Steven Dewitte and Nicolas Clerbaux paper is from 2018. For me this was rather astonishing. So I wondered that it wasn`t followed up by other scientists. It goes into the basics of GHG warming, and questions the consensus myth. It looks like cloud and water vapor feedbacks are more than offset by lapse rate feedback. I think that other papers on this subject have too short periods to tell much about trends. And even Steven Dewitte and Nicolas Clerbaux have a period with some natural variation.
NK: Thanks for bringing up Dewitte&Clerbaux (2018). When you copied the abstract, you appear to have missed the most important sentences about LWR feedback. The crucial finding is Figure 4, showing dOLR/dTs = -2.93+/-0.6 W/m2/K (95% ci) as determined by observations from space. That would an ECS of about 1.25 K/doubling before including SWR feedback. Figure 4 shows observations from three sources that are superficially consistent with this conclusion.
The paper is marked “editorial”, was accepted 4 days after it was received and published 4 days after that. Probably not peer-reviewed.
I tried to look up dOLR/dTs for CMIP5 models, but AR5 and two papers it cites show only dOLR/dTs and dOSR/dTs from clear skies and a net CRE for LWR and SWR combined. So I couldn’t do a proper comparison. The author’s citation (#15) is to a 1990! paper on climate models showing an average dOLR/dTs of -2.2 W/m2.
Two other comparisons I am aware of are: -2.2 W/m2/K seen in seasonal warming (which overweights the extra-tropics) and -4.0 W/m2/K for the tropics in Mauritsen and Stevens paper on an iris effect. In other words, this isn’t out of line with other estimates for dOLR/dTs made from observations.
“We conclude there exists a “longwave cloud thinning effect”: as the earth warms, it contains less clouds, and becomes a more effective radiator. This cloud thinning effect is underestimated in most of the models.” Sound a lot like the iris effect postulated by Mauritsen and Stevens. In their simple model of an iris effect, however, cloud thinning is accompanied by a large positive SWR feedback.
Do you have a link for Clerbaux and Dewitt?
Here is one link:
https://www.researchgate.net/publication/327874661_Decadal_Changes_of_Earth's_Outgoing_Longwave_Radiation
Frank: “I tried to look up dOLR/dTs for CMIP5 models, but AR5 and two papers it cites show only dOLR/dTs and dOSR/dTs from clear skies and a net CRE for LWR and SWR combined.”
It looks like you have to go into secret AR5 archives to get data on model dOLR.
I read the paper recently. It cites Trenberth and Loeb, etc. This is a consensus paper.
From: “Attribution of the present‐day total greenhouse effect”
Gavin A. Schmidt, Reto A. Ruedy, Ron L. Miller, and Andy A. Lacis, 2010.
“In the clear‐sky case, they found (after accounting for overlaps) that water vapor, CO2, O3 and others provided 60%,
26%, 8% and 6% of the net LW absorption, respectively,
with similar percentages in cloudy skies. The all‐sky percent
contributions can be estimated (within a percent) to be 41%,
31%, 18% and 9% for water vapor, clouds, CO2 and everything else.”
So, if the TOA outgoing LW radiation is increasing over time, with increased CO2, does it mean that 41% of of this is attributed to change in water vapor, 31% attributed to change in clouds, 18% attributed to Change in CO2 and 9% attributed to everything else?
The numbers above was from Kiehl and Trenberth, 1997.
Schmidt et al wrote this. “The relative contributions of atmospheric long‐wave absorbers to the present‐day global greenhouse effect are among the most misquoted statistics in public discussions of climate change. Much of the interest in these values is however due to an implicit assumption that these contributions are directly relevant for the question of climate sensitivity. Motivated by the need for a clear reference for this issue, we review the existing literature and use the Goddard Institute for Space Studies ModelE radiation module to provide an overview of the role of each absorber at the present‐day and under doubled CO2. With a straightforward scheme for allocating overlaps, we find that water vapor is the dominant contributor (∼50% of the effect), followed by clouds (∼25%) and then CO2 with ∼20%. All other absorbers play only minor roles.”
It is interesting that Held in his blog analyses the case of increasing TOA outgoing longwave radiation. “How can outgoing longwave flux increase as CO2 increases?”
https://www.gfdl.noaa.gov/blog_held/46-how-can-outgoing-longwave-increase-as-co2-increases/
He talks of longwave (L) and shortwave (S) perturbations. It is the effects of change in LongWaveRadiation and ShortWaveRadiation. These pertuberations are considered positive and heating ,N, is the sum of them. And then he goes on, writing: “But in most of these models, L is actually decreasing over time, cooling the atmosphere-ocean system. It is an increase in the net incoming shortwave (S) that appears to be heating the system — in all but one case. “
NK: Schmidt is focused on the pre-industrial GHE. The enhanced GHE appears more important. We are expecting a doubling or tripling of CO2, but water vapor may increase roughly 7%/degC of warming. If you go to the online MODTRAN and increase the water vapor scale from 1.00 to 1.07, OLR drops by about 1 W/m2, except through clear tropical skies where the decrease is 1.6 W/m2. Clouds produce a smaller decrease in OLR and higher cloud have more effect than lower ones. So the decrease in OLR associated with the increased humidity expected for 3K of warming is roughly comparable to the decrease OLR from a doubling of CO2.
Modtran doesn’t take into account the negative lapse rate feedback produced by rising humidity, which negates roughly half of the feedback expected from rising water vapor alone. If so, we expect roughly comparable decreases in OLR for a doubling of CO2 and from a 50% increase in humidity due to 6K of warming.
Thank you for the comment Frank.
I wonder what you think about Held`s finding that most models have an decreased LW perturberation, which means increased OLR, and show a cooling of the atmosphere-ocean system over time, with rising CO2. I read this as a general LW negative feedback, but don`t understand the dynamics behind it.
I have got the feeling that it is difficult for the “settled” climate scientist society to acknowledge some iris effect. Dressler, Schmidt and others have invested some prestige to debunk Lindzen, so it seems like an official RealClimate policy to hold back on negative feedbacks. And the followers follow.
NK: I’ve become more convinced that our planet has the ability to cool itself by emitting more LWR with warming than climate models predict. Satellites say LWR feedback is -2.9 W/m2/K, seasonal warming shows a robust -2.2 W/m2/K (mostly outside the tropics), Lindzen&Choi and Mauritsen& Stevens say topical LWR feedback is -5 or -4 W/m2/K. To get high climate sensitivity, warming must “punch holes in clouds” and let in more SWR: aka positive SWR cloud feedback. (Most climate scientists discuss the cloud radiative effect (CRE), but I think this incorporates changes in cloud fraction.)
The genius of our host can be seen in his insistence on understanding the basics. His first post on clouds shows key results from ERBE: cloudy skies emit 18 W/m2 less LWR (cold cloud tops), but reflect 30 W/m2 more SWR. On the average, if cloud fraction goes down with warming, we should expect to see an increase in emission of LWR and a bigger decrease in reflection of SWR. Then SOD shows us maps showing how this average values vary geographically. If LWR feedback in the tropics is far more negative from cloudy skies than clear skies because cloud fraction is going down, then most likely it is going to be associated with strongly positive SWR cloud feedback.
https://scienceofdoom.com/2010/05/30/clouds-and-water-vapor-part-one/
When CMIP5 models were forced in AMIP experiments with observed rising SST from 1979-2008, they exhibited an average ECS of 1.5 K due to a low climate feedback parameter 2.3+/-0.7 W/m2/K. It seems to me that the observed warming must have been in regions where is didn’t “punch large holes in clouds”. That heat apparently does “punch holes in clouds” during normal model runs where the AOGCM controls changes in SSTs. Or perhaps the difference is which clouds change. Figure 5.6 shows us which regions have clouds that are (on the average) most and least cooling.
In any case, the situation is ridiculously complicated.
Thank you Frank. What you say seems very reasonable to me.
From the history of the greenhouse effect. There were surprising things that happened in the atmosphere, according to scientists in 2014. From MIT News: “But in a paper out this week in the Proceedings of the National Academy of Sciences, MIT researchers show that this canonical view of global warming is only half the story.”
http://news.mit.edu/2014/global-warming-increased-solar-radiation-1110
““So there are two types of radiation important to climate, and one of them gets affected by CO2, but it’s the other one that’s directly driving global warming — that’s the surprising thing,” says Armour, who is a postdoc in MIT’s Department of Earth, Atmospheric and Planetary Sciences.”
“The study sorts out another tricky climate-modeling issue — namely, the substantial disagreement between different models in when shortwave radiation takes over the heavy lifting in global warming.”
“The finding was a curiosity, conflicting with the basic understanding of global warming,” says lead author Aaron Donohoe, a former MIT postdoc who is now a research associate at the University of Washington’s Applied Physics Laboratory. “It made us think that there must be something really weird going in the models in the years after CO2 was added. We wanted to resolve the paradox that climate models show warming via enhanced shortwave radiation, not decreased longwave radiation.”
“Donohoe, along with MIT postdoc Kyle Armour and others at Washington, spent many a late night throwing out guesses as to why climate models generate this illogical finding before realizing that it makes perfect sense — but for reasons no one had clarified and laid down in the literature.”
“I think the default assumption would be to see the outgoing longwave radiation decrease as greenhouse gases rise, but that’s probably not going to happen,” Donohoe says. “We would actually see the absorption of shortwave radiation increase. Will we actually ever see the longwave trapping effects of CO2 in future observations? I think the answer is probably no.”
Why was this so surprising? There had been some sceptical voices to the canonical wiev. Why couldn`t they just say that the scientific consensus on greenhuose warming was a big misunderstanding, and that the canonical view was just plain wrong?
So how big is the contribution from longwave backradiation compared to SW warming? 1%, 10%, 100%?. How much can the “control knob” CO2 manage to control?
NK: “As 93% of this warming goes into oceans, the behavour of oceans will be the critical point.”
To a first approximation, the deep ocean is simply a large heat sink. ECS is the reciprocal of the climate feedback parameter: dOLR/dTs + dOSR/dTs, meaning that steady state climate change depends on forcing and the climate feedback parameter. Chaotic fluctuations in the currents linking the surface and the deep ocean may be the most important source of internal/unforced climate variability, but (on paper) they don’t change long-term steady state temperature. For energy balance models:
TCR = F_2x*(dT/dF) ECS = F_2x*(dT/(dF-dQ))
TCR/ECS = 1 – dQ/dF
where dQ is ocean heat uptake. So, the only thing a larger ocean heat uptake does is increase the amount of time it takes to approach steady-state and not steady-state itself.
Figure 2B in the paper we were discussing shows the differences in ocean heat uptake between various IPCC models, expressed as the effective depth of the mixed layer that contains all of the heat ocean absorbs over time (assuming a uniform heat accumulation in that mixed layer). That value ranges +/- 25%.
https://www.pnas.org/content/111/47/16700
We can track transport of heat by physical mixing through two proxies: CFCs and C-14 from atmospheric testing of atomic bombs, but I haven’ found any good comparisons between observations and hindcasts.
NK wrote: “Why couldn`t they just say that the scientific consensus on greenhouse warming was a big misunderstanding, and that the canonical view was just plain wrong?”
The canonical view isn’t wrong; it is just poorly expressed – because the consensus has a very CO2-centric view. Consider ANY forcing, an imbalance at the TOA that is imposed by a permanent change (not the temporary changes associated with internal variability in chaotic systems). If forcing is positive, the retained energy is going to warm the planet until the balance at the TOA is restored by an increase in emission of LWR OR reflection of SWR. The planet’s radiative imbalance (I) is given by the equation below where OLR is negative:
I = ASR + OLR
dI/dTs = dASR/dTs + dOLR/dTs
Taking the derivative gives us the climate feedback parameter dI/dTs (W/m2/K), which is what we need to know to figure out how much warming (deltaT) will correct the imbalance caused by a forcing F:
F + (dI/dTs)*deltaT = 0
Equilibrium warming depends only on the climate feedback parameter, not how it is partitioned between its LWR and SWR components. The “canonical view” misses the importance, generality and physics of the climate feedback parameter by discussing its reciprocal K/(W/m2) and converting W/m2 to doublings of CO2 to get ECS.
dI/dTs must be negative (or you have a runaway GHE) and the biggest negative term is Planck feedback (-3.3 W/m2/K for a graybody model). If your forcing is from the sun, you certainly expect OLR to increase, despite the fact that the forcing is in the SWR channel.
The forcing caused by rising GHGs is no different, except that we have no way to physically measure that forcing. Instead, we fall back on radiative transfer calculations, which require us to specify the temperature and composition of the atmosphere through which radiation is traveling. Thus we do calculations imagining an abrupt doubling of CO2. However, long before CO2 can double, the planet has begun to warm, changing both OLR and ASR.
If dASR/dTs were zero, what would we measure at the TOA? For an instantaneous doubling of CO2, we would expect OLR to drop 3.6 W/m2 and gradually return to normal as the planet warms.
Now lets suppose dASR/dTs and dOLR/dTs are both equal magnitude and have the same (negative) sign. Balance at the TOA will be restored by a 1.8 W/m2 increase in OLR and a 1.8 W/m2 increase in reflected SWR (technically both values are negative, since they are heat lost by the planet). In this case only half of the drop in OLR (1.8 W/m2) will be recovered by steady state warming; the rest will come from decreasing ASR (1.8 W/m2).
Now let’s suppose that dASR/dTs has the opposite sign as dOLR/dTs and half the magnitude. That would mean that for every 2 W/m2 more OLR lost to space by warming, 1 additional W/m2 of ASR would be coming in. To get a NET 3.6 W/m2 out, we need 7.2 W/m2 of increased OLR (technically a negative number) and 3.6 W/m2 more solar radiation will be getting past the clouds. Halfway through this warming process, OLR will have increased 3.6 W/m2 – restoring “balance” in the LWR channel – and ASR will have increased 1.8 W/m2 – making it appear as it ASR is now doing all of the warming. However, this isn’t right, because the planet IS WARMER at this halfway point and would be emitting 1.8 W/m2 more LWR simply because of its higher temperature, and leaving half of the forcing from rising GHGs in place.
You cited a press release by MIT. The paper can be found at the link below. Figure 3 shows us how mutually inconsistent climate models are in partitioning the climate feedback parameter into its dOLR/dTs and dASR/dTs components.
https://www.pnas.org/content/111/47/16700
Figure 3A in the paper shows how much varies AOGCMs disagree with each other about dASR/dTs and dOLR/dTs. The surprise in the figure is the square symbol and dashed circle claiming to be observational estimates for dASR/dTs and dOLR/dTs. These observational estimates are calculated in the Supplementary Material. Those values are derived from CERES data for 2000-13, using the same kind of linear regressions as Dessler (2018) except that OLR and SWR are analyzed separately. dOLR/dTs is -2.0 +/- 0.4 W/m2/K (one STD, r^2 = 0.28) and dOSR/dTs = +0.8 +/- 0.4 W/m2 (r^2 = 5%). Of course, the data from seasonal warming indicates to me that monthly SWR isn’t a simply function of Ts and that some components are lagged, explaining why the r^2 for SWR is so low and invalidating the slope as a measure of SWR feedback.
Thank you for an informative answer, Frank.
Thoughts to the SW warming: (The energy budget in reality). As 93% of this warming goes into oceans, the behavour of oceans will be the critical point. Most of the warming goes out again in the night. Another part of the energy has a seasonal outlet. Some great cycles have an outlet of some years, as Pacific and Atlantic currents. And still others will have decadal and even millennial cycles, as ocean overturning and water sinking. So how can this energy transport be simulated in models? My guess is that informed guessing comes closer than mathematics.
And how do the CO2 level influence this energy transport?
And I wonder if changing wind pattern has more impact on energy transport than changing temperatures.
“The energy balance at TOA (top of atmosphere) is the “driver” for whether the earth heats or cools.”
No, you don’t heat a system by less heat emission from the coldest part in it. If that was true the heated floor in my house would warm up if I cooled the outside of the walls. You’re confusing thermal insulation with cold heat absorbing gases. They are opposites. Thermal insulation reduces heat transfer by minimizing absorption from the heat source, which leads to less absorbed heat in surrounding cold air. GHGs increase absorption from the heat source. Lower temperature=more heat absorbed, higher temperature=less heat absorbed. So if GHGs increase absorption, they cool.
Planck, theory of heat radiation:
“But the empirical law that the emission of any volume-element depends entirely on what takes place inside of this element holds true in all cases (Prevost’s principle).”
That emission by the surface entirely depends on what happens inside the emitting surface is in no way compatible with the claim that the top of the external atmosphere “drives” surface emission. So, you’re saying Planck is wrong.
Next page:
“A body A at 100◦ C. emits toward a body B at 0◦ C. exactly the same amount of radiation as toward an equally large and similarly situated body B’ at 1000◦ C. The fact that the body A is cooled by B and heated by B’ is due entirely to the fact that B is a weaker, B’ a stronger emitter than A.”
Here Planck clearly says that a cool body cools a warm body, and that emission at a temperature is independent of there being an atmosphere i between the surface and 3K space. Not a single word about any warming by a cold body.
Page 6 & 7:
https://archive.org/details/theoryofheatradi00planrich/page/6
Since Planck refers to Prevosts principle, here it is:
“Prevost also showed that the emission from a body is logically determined solely by its own internal state. The causal effect of thermodynamic absorption on thermodynamic (spontaneous) emission is not direct, but is only indirect as it affects the internal state of the body.”
Again, this makes a greenhouse effect impossible, because surface emission depends only on the internal state. Also, absorption is not a cause of emission, they’re only relative through the internal state. A cold volume of gas can absorb much more heat than it emits. Just like if you place a bucket of water outside in the winter with a heater in it, it will absorb much more heat than it emits. Steady states with gradients works like that. So if there’s less emission at TOA the system is cooling. Because lower temperature=less emission. Lower temperature never causes warming, but lower temperature causes more heat absorption. Which is what co2 does, and that’s the reason it’s an efficient industrial coolant.
Never wondered why the greenhouse gases is claimed to warm the surface, but the same gases cool everything ON the surface?
It’s backwards physics, Gavin.
TIA,
Frank is being nice when you don’t actually deserve it. Your argument assumes its conclusion, i.e. the logical fallacy known as begging the question. You clearly haven’t actually paid any attention to the posts at this site where all your errors are addressed in great detail. I suspect that you are not actually interested in learning anything new. If that is the case, then please go find somewhere else to preach your nonsense.
DeWitt: Would there be a GHE is the atmosphere behaved like a blackbody – as TIA assumes after quoting Planck? That depends on what kind of model one constructs. Shell models do produce a GHE. However the fundamental assumption behind the derivation of blackbody radiation is that absorption and emission are in equilibrium. Which is somewhat equivalent to saying that absorption is saturated, and that more CO2 can’t change the intensity of the radiation passing through the atmosphere. (You and I know that most radiative forcing occurs on the shoulders of the strong CO2 absorption where absorption and emission aren’t in equilibrium.)
There are a lot of booby traps between simple, but inaccurate, explanations for the GHE and a proper understanding of the GHE. You, SOD and others here generously helped me past them to a state of (hopefully) greater enlightenment (based on what I read in textbooks on atmospheric radiation). We have seen many others who can’t or won’t overcome their confirmation bias and assimilate any new information that contradicts their current beliefs. It is not their fault: Most have been taught that the scientific method is “just another way of knowing” (along with reason, emotion, faith, imagination, intuition, memory, and language) and that research is the accumulation of information that agrees with one’s preconceptions.
Frank,
A black body is by definition (I think) opaque at all wavelengths and must be in at least local thermal equilibrium. If there is no internal energy source, then I think you might end up with an isothermal atmosphere at low altitudes. But there are problems for a gaseous atmosphere where pressure drops with altitude. At some altitude you would no longer have a black body even if the individual molecules absorbed perfectly at all wavelengths because the individual molecules would no longer be in thermal equilibrium. I think that would mean the apparent temperature would increase with altitude as in the stratosphere because the molecules would absorb solar radiation and not be able to efficiently transfer the energy to other molecules. But I’m speculating. It’s a thought experiment as it has no relation to reality.
DeWitt: If we think about radiation passing through an optically thin layer of atmosphere of thickness ds:
dI = noB(lambda,T)*ds – noI*ds
If the radiation entering that layer has blackbody spectral intensity for the local temperature, the change in intensity, dI, must be zero. If radiation traveling vertically through one or more such layers doesn’t decrease in intensity with increasing altitude, there is no GHE. Absorption and emission are equal for these layers and whenever blackbody radiation is emitted.
TIA wrote: “this makes a greenhouse effect impossible, because surface emission depends only on the internal state”.
Suppose the “surface emission” TIA is discussing is the emission from a layer of atmosphere and if the “internal state” refers to temperature. (This may not be correct, of course.) If the radiation exiting the surface doesn’t depend on the density of GHG molecules (n) inside the layer, it must be emitting radiation of blackbody intensity. So anyone who believed that the atmosphere emits BB radiation could conclude that GHE is impossible.
I’ve come to the conclusion that my education on the interaction of radiation and matter is a patchwork of concepts that should have been rigorously derived in P-chem. A rigorous derivation would start with Einstein coefficients and move on to line broadening, which I think gives an oscillator strength and cross-sections at a range of wavelengths, rather than single lines. However, cross-sections are measured empirical and probably used to calculate Einstein coefficients. Then you ask if the matter is in LTE. If yes, you can replace the emission cross-section with o*B(lambda,T), where o is the absorption cross-section, affording Schwarzschild’s equation. This equation basically says that radiation gradually approaches blackbody intensity as it travels through a homogeneous medium, and the rate of approach depends on the density of absorbing/emitting molecules and their cross-section. Eventually, you ask if the radiation has passed far enough through that medium for absorption to have come into equilibrium with emission at the local temperature. If yes, you have blackbody radiation and Planck’s Law. Such equilibrium may be common inside of dense materials such as solid and liquids, but is always lost high enough in an atmosphere which changes temperature and density with altitude. Then you recognize that emissivity/absorptivity is likely less than unity (probably due to reflection at surfaces) and have gray bodies. Gases don’t have surfaces, so their emissivity should be unity. You also need to ask if the matter can scatter radiation. Finally, if the source of the radiation is much hotter than the matter it interacts with, then you can ignore the emissivity term of Schwarzschild’s equation and obtain Beer’s Law.
Many appear to focus on laws that apply to a limited set of situations without understanding those limitations. And few mention by name the central player most important to climate, Schwarzschild’s equation.
Frank,
Einstein coefficients can be calculated from first principles, at least for certain conditions:
http://scienceworld.wolfram.com/physics/EinsteinCoefficients.html
According to the law of conservation of energy, if an object gains more energy by all mechanisms of energy transfer that it loses by all mechanisms of energy transfer, the difference becomes internal energy – a higher temperature. Right?
If the object in question is our climate system and we make the boundary of our climate system the “top of the atmosphere” or TOA, the only mechanism by which our climate system can gain or loss energy is by radiation, greatly simplifying the problem. Then we can replace the word “gain” with net absorption (incoming SWR minus reflected SWR) and “lose” with emission (of thermal IR). Our climate system is effectively isolated from the warmer mantle by the insulation of kilometers of crust (which allow an upward flux of only 0.1 W/m2).
Therefore, the law of conservation of energy DEMANDS that – when there is a positive net flux of radiation downward ACROSS the TOA – the planet must be getting warmer somewhere BELOW the TOA. This is why SOD said that the energy balance at the TOA is the driver of whether the earth [warms] or cools.
As soon as you mention “the coldest part of the system”, you began to confuse heat transfer WITHIN our climate system with heat transfer ACROSS the TOA. DLR is heat transfer WITHIN our climate system. So are convection and latent heat. When we consider the energy budget at the TOA, we avoid dealing with all of those complications.
ThermalIntelligenceAgency wrote: “Since Planck refers to Prevosts principle, here it is: “Prevost also showed that the emission from a body is logically determined solely by its own internal state. The causal effect of thermodynamic absorption on thermodynamic (spontaneous) emission is not direct, but is only indirect as it affects the internal state of the body.”
Planck is discussing emission of radiation from a black [or gray] body, a solid or liquid with a surface. When he derived Planck’s Law (which led to the Stefan-Boltzmann equation), Planck started by ASSUMING radiation in equilibrium with quantized oscillators – meaning that absorption and emission were equal. He also assumed that these oscillators absorbed and emitted all wavelengths. He also assumed a Boltzmann distribution of energy between excited and ground states, making emission temperature dependent. These assumptions are often reasonable for solids and liquids, but radiation in our atmosphere is not in equilibrium with the quantized oscillators – GHGs – in our atmosphere. GHGs don’t absorb and emit at all wavelengths. Some wavelengths of thermal IR pass through the atmosphere unchanged. On the other hand, the wavelength most strongly absorbed by CO2 has 90% of incoming photons absorbed within 1 m in the lower atmosphere – AND replaced by emission of an equal number of photons. There is equilibrium at this wavelength. At other wavelengths, photons can travel an average of a vertical kilometer between where they are emitted and absorbed and it will be about 6.5 degC colder there. Since emission depends on temperature, the number of photons traveling downward will be less than the number traveling upward. Again, there is no equilibrium. As one goes higher in the atmosphere, the density of GHGs decreases and there is less opportunity for absorption and equilibrium to come into equilibrium.
So the statements you are quoting from Planck do NOT apply to emission of radiation by the atmosphere. Emission from a layer of atmosphere depends on the temperature of that layer AND the DENSITY OF GHG’s in that layer AND the intensity of radiation entering that layer from behind AND the absorption of IR by GHGs in that layer. This is called Schwarzschild’s equation for radiative transfer through an atmosphere:
https://en.wikipedia.org/wiki/Schwarzschild%27s_equation_for_radiative_transfer
You wrongly concluded: “Again, this makes a greenhouse effect impossible, because surface emission depends only on the internal state (temperature).” The emission from a layer of atmosphere depends on its temperature and the density of the GHGs it contains and their absorption cross-section at the wavelength of interest.
OLR is NOT the same. When the earth’s average temperature rises, it must emit more radiation, otherwise the Stefan Boltzman law is violated. It doesn’t matter how many times energy bounces between earth’s surface and TOA. Just consider the total radiation 1M km away from earth. It must go up. You can see that in Figure 1b of the famous Harries 2001 paper showing the difference in spectrum from a 1970 satellite and a 1996 one. That graph does indeed show less radiation at the CO2 and CH4 bands in 1996, but other frequencies are higher to make up for that. The net energy radiation is higher. And not coincidentally, that additional energy radiation is about 2.4 W/m2, pretty close to the CO2 forcing (minus some slow heat sinks like deep ocean water, ice, etc), as you would expect at equilibrium.
oops, please ignore the last sentence, the 2.4 number is not at TOA.
OK, i get it now. I found it on another page: at equilibrium incoming energy must match outgoing, so net energy of OLR must be the same since solar incoming has not changed.
Tomasz: A non-paywalled version of the Harries 2001 paper is available here. However the labels on the 1970 and 1997 spectra were switched in Figure 1. The brightnesses temperature for CH4 at 1305 cm-1 was about 5 K lower in 1996 than in 1970, consistent with the idea that the average photon emitted at this wavelength came from higher in the atmosphere. However, the full explanation may not be this simple.
Click to access Harries%202001%20GHG%20forcing%20change.pdf
Let’s consider the simplest situation: a single GHG with a single absorption band. If we instantaneously double the concentration of that GHG, the brightness temperature of that band will drop and then gradually return to the initial value as the planet warms and the radiative imbalance goes to zero. In the real world, we are performing a slow increase in the GHG concentration accompanied by a slow rise in temperature. In a 1% pa transient climate response experiment, the radiative imbalance rises for a few decades and then reaches a plateau where the increased forcing is compensated by the increased warming. During the plateau phase, the brightness temperature would remain constant.
Our real planet has multiple GHGs that are rising at different rates, the troposphere is warming, the upper troposphere is warming faster, the stratosphere is cooling from ozone destruction and rising CO2 and there was a strong El Nino in 1997. The simulated spectra were calculated using re-analysis data for the actual temperature and GHG data. Radiative transfer calculation are pretty accurate, so the simulated and observed difference spectra were similar. (If they had let an AOGCM predict atmospheric conditions and spectra for the same years, the agreement might not be so good. The spectrometers collected data for the 667 cm-1 CO2 band, but the authors didn’t show that part of the spectrum in Figure 1; they stopped at 700 cm-1. Many photons around 667 cm-1 that escape to space are emitted from altitudes where the temperature is rising, not falling, with altitude. I’ve wondered if this part of the spectrum was omitted to avoid dealing with this complication. Finally, in climate models, positive SWR feedback causes the planet to emit more LWR at steady state after forcing than it did before forcing. This is most apparent in abrupt 4X experiments.
Yes, I see that. They did not show the full IR spectrum so that’s why it looks like there is more energy leaving. I have retracted my original comment.
Firstly thanks for all your careful work in presenting the physics in such an easy to consume format. Well done. I refer people to it often.
Perhaps the following thought experiment could be expanded here (and perhaps even quantified?) as I think it serves as an easy to understand proof (if I can be so bold?) of the atmospheric ‘greenhouse’ effect. It is a bit of a back to front way of looking at it and it goes like this…
Satellites provide a view of the outgoing spectrum with troughs corresponding to the absorption properties of ‘greenhouse’ gases. Observed, measured and undeniable.
But what if you had the magical power to instantly disable all the absorption these gases are responsible for. In an instant the troughs in the outgoing spectrum would disappear – and accordingly, the energy escaping to space would increase dramatically. From a simple energy balance perspective, the incoming energy has not changed but the outgoing is suddenly significantly larger than the incoming. This can mean only one thing – cooling. The earth would cool. Thus, the absorption properties of ‘greenhouse’ gases are indeed helping hold the temperature of the planet higher than it would be without them.
Cheers
Dave,
You can see how that works at Professor Archer’s MODTRAN web page here:
http://climatemodels.uchicago.edu/modtran/
Set all the inputs to zero and almost all the absorption goes away. There’s still a little absorption peaking at about 1550 cm-1. I’m not sure what that’s from. For the tropical atmosphere outgoing emission at 70 km altitude goes from 298.52 W/m² to 443.682 W/m².
Thanks DeWitt, that certainly makes it easy to quantify!
I also found the SOD Hoover Incident which is essentially the same as the thought experiment that I proposed for a simplistic proof of the atmospheric ‘greenhouse’ effect.
Cheers