Outgoing longwave radiation

Something that often strikes me is that when I think I understand something quite well, there often turns out to be an aspect that I haven’t understood particularly well. I sometimes think that this is can be an important thing to realise; just because you think you have a good understanding of something doesn’t always make it so.

The context here is that a typical way to explain global warming is to point out that adding a greenhouse gas to the atmosphere reduces the outgoing longwave (OLR) flux, and that warming of the surface and troposphere then returns the system to energy balance. This tends to suggest that the OLR will go down when the greenhouse gases are added, and then slowly recover as the system returns to equilibrium.

Credit: Dewitte & Clerbaux (2018)

A particularly persistent climate “skeptic” has, on a number of occasions, promoted the figure on the right, which is from this paper. Because it shows a larger increase in OLR than would be expected if global warming simply involved the OLR recovering, it has been suggested that this disproves anthropogenic global warming (AGW).

As indicated at the beginning of this post, I was also somewhat confused by this. However, thanks to a comment from Chris Colose on this Realcimate post, I’ve cleared up my confusion. Chris highlights this paper which points out that the response to an initial external perturbation (such as increasing atmospheric CO2) involves both longwave and shortwave feedbacks.

Credit: Donohue et al. (2014)

In fact, if we increase atmospheric CO2, the expectation is that the shortwave feedback will lead to an increase in absorbed shortwave radiation (ASR). As illustrated by the figure on the left, this means that the OLR will end up increasing to above the level it had prior to the increase in atmospheric CO2. In fact, it potentially recovers in less than 20 years, which means that the subsequent warming is due to the increased ASR. To be clear, this does not mean that global warming isn’t due to increased atmospheric CO2, since the increased ASR is a response (feedback) to this increased atmospheric CO2.

I always find it quite fun to solve what was a bit of a puzzle (to me, at least). I would also argue that if someone thinks that they’ve found some supposedly obvious reason why a large number of other experts are wrong, it’s often best to take a step back and consider that there might be something that they’ve missed.

Links:
Realclimate comment – Chris Colose.
Shortwave and longwave radiative contributions to global warming under increasing CO2 – Donohoe et al. (2014).
Global warming due to increasing absorbed solar radiation – Trenberth & Fasullo (2009).

This entry was posted in Global warming, Greenhouse effect, Philosophy for Bloggers, The philosophy of science, The scientific method, Uncategorized and tagged , , , , . Bookmark the permalink.

37 Responses to Outgoing longwave radiation

  1. Eli Rabett says:

    As Eli has pointed out, the increase in absorbed solar in the atmosphere is the Foote Effect. The increase in OLR is the Tyndall effect

  2. JCH says:

    I have pointed this paper out many many times, and yet, Chris Colose gets the credit. Of course, he does know what he talking about and I just read a lot stuff.

  3. Eli,
    Yes, very good.

    JCH,
    Apologies if I’ve missed you pointing this out before.

  4. Everett F Sargent says:

    Increased ASR mostly goes into OHC?

  5. EFS,
    Yes, I would think so. The energy imbalance (most of which goes into the oceans) is an imbalance in the fluxes, and this would seem to be mostly ASR.

  6. John,
    Thanks. Apologies for missing that you’d shown this many years earlier.

  7. JCH says:

    I think the most recent were on Cesspool Etc., where there was a lot of excitement about Dewitte, and twitter with the yahoo from the GWPF. If I remember right, Chubbs responded here with some very insightful comments here, and it was awhile ago.

    I think the most interesting place this goes is OHC and Minnett’s theory.

  8. Eli Rabett says:

    The ASR goes mostly (almost entirely) into water vapor and clouds in the atmosphere. From there, of course, it gets redistributed by radiation into the oceans and surface. The dips in the solar spectrum in the NIR are almost all water vapor although there are weak O2 and O3 bands

  9. angech says:

    “The context here is that a typical way to explain global warming is to point out that adding a greenhouse gas to the atmosphere reduces the outgoing longwave (OLR) flux, and that warming of the surface and troposphere then returns the system to energy balance.”

    Another particularly recalcitrant climate “skeptic” has a giant blind spot re OLR.

    I am so pleased to see some more light thrown on this process by ATTP and Eli et al.
    The layering of the energy as it escapes back to space.
    The need for it eventually to equal what comes in.
    The time frames this happens over.
    Incredibly important.

    Thank you all for the input, I think Eli has an article up on his site with that graph. I will go have a relook
    “when I think I understand something quite well, there often turns out to be an aspect that I haven’t understood particularly well”
    What keeps us all going , I think.

  10. I dont think this is the correct way to look at the role of greenhouse gases;
    Greenhouse gases such as CO2 not only absorb outgoing long wave radiation of the Earth, they also absorb incoming infrared radiation of the sun. Extra CO2 will absorb extra radiation on both side of the equation. So the greenhouse effect is only a netto effect and as such must be quit small. This is not the way in which extra CO2 increases the temperature of the surface of the Earth. Extra greenhous gases will increase the temperature of the surface of the Earth but not via the mechanism of the enhanced greenhouse effect. The atmosphere of the Earth may not have a greenhouse effect at all. It is just a metaphore taking literally also by scientist.

  11. Raymond,
    I’m not entirely sure what you’re saying. Adding greenhouse gases changes the energy fluxes and – initially, at least – produce an energy imbalance which then leads to warming. There are many ways to describe this, some of which are better than others. Are you mostly criticising the term “greenhouse”?

  12. David B Benson says:

    Raymond Horstman — I recommend that you go to the Real Climate site. In the far upper left corner there is a button labeled Start Here. Possibly that will provide a satisfactory answer for you.

  13. Everett F Sargent says:

    Raymond Horstman says:
    “The aim of my research is to find out whether you can explain the average temperature of the Earth of about 15 ° C with a model that does not use things like the greenhouse effect. At the start of my research I had no idea if this would work. There was only one starting point. The energy radiated by the sun does not become evenly distributed over the surface of the earth. The solar energy is concentrated on only 1 half that I call the day side. The other half, the night side, gets nothing. From this self-evident beginning, I continued and looked into whether this was actually possible. The article seemed very plausible to me, but the reactions I received prompted me to continue with my research.”
    https://raymondhorstman.wordpress.com/over/

    So does not believe in GHG theory. Period. Full stop.

  14. izen says:

    My understanding has been that any decrease in OLR is a transient effect that is largely irrelevant to the warming from the increased CO2.
    That changes the thermal gradient within the vertical profile of the atmosphere. The amount of energy always has to balance going in and out. The increase in surface temperature is a result in the greater thermalisation of OLR from the surface in the lower layers of the atmosphere, not in a imbalance in the energy flux for the whole system.

    Where I get a bit hazy is how the extra CO2 increases the emissivity of the upper atmosphere (stratosphere?) causing cooling but the OLR still balances the incoming Joules.

  15. izen,
    I’m not quite sure what you mean. In order to accrue you need some kind of energy imbalance, which will then mean some imbalance in the fluxes. I don’t quite follow how this can be the case

    The increase in surface temperature is a result in the greater thermalisation of OLR from the surface in the lower layers of the atmosphere, not in a imbalance in the energy flux for the whole system.

  16. izen says:

    @-ATTP

    Yes, well I may have to go back and revise my basic understanding if the warming is dominated by changes in albedo caused by clouds.
    I had the concept it was this –

  17. izen,
    If there’s warming then I think you still need some kind of flux imbalance. My understanding is that quite soon after a perturbation (say, an increase in atmospheric CO2) the LW fluxes can return to balance, but the cloud feedback leads to an imbalance in the SW fluxes, which then dominates the subsequent warming.

  18. Phil says:

    Raymond Horstman’s comment can, if I’ve interpreted him correctly, be seen from the graph that Eli posted. If you look at the CO2 absorbtion (in the “major components”) section, you can see that CO2 absorbtions do extend into the frequencies of incoming short wave radiation and so increasing CO2 should, in theory, decrease the amount of incoming solar radiation. So Horstman is correct to say it is a “nett effect”. However he is wrong to infer that that must mean the nett effect must be small, because the effect of the different CO2 bands obviously depend on their positions within the envelope of the incoming short wave radiation from the Sun and the outgoing long wave radiation from the Earth, as shown at the top of Eli’s figure.

  19. dikranmarsupial says:

    Raymond Horstman wrote: “So the greenhouse effect is only a netto effect and as such must be quit small.”. I don’t see how that follows. The amount of outbound radiation in the IR is much higher than the amount of IR we receive from the sun, so there is no reason to think the net effect is small. Obviously CO2 can absorb down-welling IR emitted from GHGs higher in the atmosphere, but that is just redistribution of the atmosphere’s heat and does not directly cause warming (as it doesn’t directly lead to a planetary TOA imbalance).

  20. Dave_Geologist says:

    I think Raymond thinks that the incoming LWR from the Sun has the same spectrum and power as the outgoing LWR from Earth’s surface.

    If that’s the case the “Start Here” page on Real Climate is probably too difficult. Perhaps start with “temperature of the Earth” and “temperature of the Sun”, followed by some 19th century physics about black bodies. Yes I know they’re not black (see Eli’s graph), but baby steps at first.

  21. angech says:

    Everett “So does not believe in GHG theory. Period. Full stop.“ Good summary.

    Dave_Geologist says: “I think Raymond thinks that the incoming LWR from the Sun has the same spectrum and power as the outgoing LWR from Earth’s surface.“

    I think he is confusing incoming and outgoing LWR as incoming is heating and outgoing is cooling.

  22. dikranmarsupial says:

    Angech – no, he specifically distinguishes between IR that outbound from the Earth and that which is inbound from the Sun:

    “Greenhouse gases such as CO2 not only absorb outgoing long wave radiation of the Earth, they also absorb incoming infrared radiation of the sun.”

  23. Eli Rabett says:

    This is degenerating into the I want facts not theories denial.

    Solar radiation peaks at 500 nm and extends to about 2.5-3 microns. The 800 nm to 2.5 micron region is above the common limit for vision and is called the near IR or NIR. Molecular transitions in this region involve excitation of multiple vibrational quanta (overtones or combination bands) and in general are weak. In the Earth’s atmosphere most of the significant bands are involve water vapor and to a lesser extent CO2. Eli calls absorption of sunlight in this region the Foote Effect

    Atmospheric transition at longer wavelength in the IR or FIR (far IR) or sometimes thermal IR
    are at wavelengths where the surface, or ghg molecules emit IR involve changes of single quanta in vibrational modes of greenhouse gas molecules and when allowed by selection rules are generally strong. Eli and john Nielson Gammon call absorption if the thermal IR the Tyndall Effect.

    If somebunny starts talking about “Greenhouse gases such as CO2 not only absorb outgoing long wave radiation of the Earth, they also absorb incoming infrared radiation of the sun” start counting your fingers, they are dealing seconds.

  24. mrkenfabian says:

    Surely whatever incoming IR that is absorbed by the atmosphere … was going to be absorbed by the atmosphere anyway (or else reach the surface) and raising CO2 doesn’t cause the total incoming energy absorbed by the climate system to increase? Whereas the re-radiation – including out to space – of that absorbed energy is slowed by that raised CO2 concentration. More of it will be re-radiated downwards – and more will be carried by convection downwards than otherwise.

  25. Chubbs says:

    Colose refers to an Isaac Held blog (link below) which contains information that indicates how useful the OLR measurements are as a climate diagnostic. Per Held, OLR increasing when CO2 increases indicates that positive shortwave feedback (water vapor, melting ice and clouds) is larger than ocean heat uptake. Furthermore, the water vapor (absorbs in visible) and ice feedbacks are roughly as large as ocean heat uptake which allows cloud feedback to determine whether the OLR response is positive or negative.

    Held uses 5 CMIP5 models as an illustration. Four models show an increase in OLR as CO2 is ramped at 1%. The lone exception has low TCR (1.3) and negative cloud feedback. So the OLR measurements are a nice independent confirmation of positive cloud feedback and strong climate sensitivity.

    https://www.gfdl.noaa.gov/blog_held/46-how-can-outgoing-longwave-increase-as-co2-increases/

  26. Chubbs,

    Four models show an increase in OLR as CO2 is ramped at 1%. The lone exception has low TCR (1.3) and negative cloud feedback. So the OLR measurements are a nice independent confirmation of positive cloud feedback and strong climate sensitivity.

    That’s a good point. I hadn’t thought of that implication.

  27. Mack says:

    Issac Held hasn’t posted anything on his blog since 2016. Probably got tired of all the arguing.

  28. Mack,
    I vaguely recall there being a reason why he stopped, but I can’t remember what it was.

  29. Susan Anderson says:

    Moving sideways, as is my wont (enjoyed reading, and I love learning things and this (thanks Eli) was particularly informative though I can only grasp less than half of it)
    https://honors.agu.org/winners/isaac-m-held/
    https://www.gfdl.noaa.gov/isaac-held-homepage/

  30. JCH says:

    Held was being misrepresented by the usual suspects. I could tell he was going to quit before he quit. I can tell you the name of the person who likely caused him to quit. He’s a total jerk.

    The latest:

  31. an_older_code says:

    an interesting addendum to the “stay in your lane” blog post

    i suppose it’s worth pointing out that expertise in a whole raft of disciplines is getting deeper and narrower

  32. ab_older,

    i suppose it’s worth pointing out that expertise in a whole raft of disciplines is getting deeper and narrower

    I think that’s probably true and is probably a consequence of researchers delving more and more into the details of the areas in which they work, which means they have a deep understanding of their specific topic, but not a particularly good understanding of the broader area.

    I do wish that there would be more recognition of this; we should be more careful of turning people who have demonstrated good understanding of a specific topic into general public intellectuals.

  33. an_older_code says:

    @ ATTP

    to be clear my comment was in no way a criticism of you

    your post made me think of a recent article I read on Covid19 – that just seemed to nail so much of the misunderstandings re what science is and how it is done

    https://www.theatlantic.com/health/archive/2020/04/pandemic-confusing-uncertainty/610819/

    well worth reading

    it contains

    “The idea that there are no experts is overly glib. The issue is that modern expertise tends to be deep, but narrow. Even within epidemiology, someone who studies infectious diseases knows more about epidemics than, say, someone who studies nutrition. But pandemics demand both depth and breadth of expertise. ”

    and

    “Epidemiologists who are used to interacting with only their peers are racking up followers on Twitter. They have suddenly been thrust into political disputes. “People from partisan media outlets find this stuff and use a single study as a cudgel to beat the other side,” Bergstrom says. “The climate-change people are used to it, but we epidemiologists are not.”

  34. an_older,
    Thanks. I started reading that article, and then got distracted. I’ll have to go back and read it properly.

  35. Willard says:

    Good news everyone:

  36. Bob Loblaw says:

    One of the characteristics of longwave radiation in the atmosphere is that any simple algebraic expression of something like OLR at the top of the atmosphere is a very simplistic summary of the results of a complex process of emission and absorption. Rather than solving something simple like y=mx+b, you are looking at solving an infinite system of equations at each height from the surface to the top of the atmosphere.At each level, you have a combination of IR emission, longwave and shortwave (solar) absorption, condensation/evaporation/sublimation, plus thermal transfers related to atmospheric motion to consider.

    So, you really need a fairly complex model.

    Hansen et al (1981) “Climate Impact of Increasing Atmospheric Carbon Dioxide”, Science 213, 957-966 has a nice diagram in figure 4. The full paper is available here:

    https://pubs.giss.nasa.gov/abs/ha04600x.html

    With a one-dimensional radiative-convectiive model, figure 4 illustrates instantaneous changes due to doubling CO2, flux changes after a few months (time for the atmosphere to respond) and “many years later” (after the ocean mixed layer has time to respond). Initially OLR drops, but it eventually increases as the earth-atmosphere system responds.

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