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What causes the tropospheric hot spot?

Posted on 27 June 2010 by John Cook

Previously, we examined how Jo Nova doesn't get the tropospheric hotspot. Recently, Jo posted a response. She points out various semantic errors in my post - how I misquote her and mistakenly describe an effect of a law of physics as a law of physics. Lest we get bogged down in minutiae, I'll concede the semantic mistakes. However, she goes on to repeat the same fundamental confusion - mistakenly thinking the tropospheric hot spot is a signature of the greenhouse effect. She's not alone - this is a common misconception. Here, I'll explain how the hot spot works in more detail (and hopefully with more clarity).

The tropospheric hot spot is due to changes in the lapse rate (Bengtsson 2009, Trenberth 2006, Ramaswamy 2006). As you get higher into the atmosphere, it gets colder. The rate of cooling is called the lapse rate. When the air cools enough for water vapor to condense, latent heat is released. The more moisture in the air, the more heat is released. As it's more moist in the tropics, the air cools at a slower rate compared to the poles. For example, it cools at around 4°C per kilometre at the equator but a much larger 8 to 9°C per kilometre at the subtropics.

When the surface warms, there's more evaporation and more moisture in the air. This decreases the lapse rate - there's less cooling aloft. This means warming aloft is greater than warming at the surface. This amplified trend is the hot spot. It's all to do with changes in the lapse rate, regardless of what's causing the warming. If the warming was caused by a brightening sun or reduced sulphate pollution, you'd still see a hot spot.

There's a figure in the IPCC 4th Assessment report that shows the "temperature signature" expected from the various forcings that drive climate. This figure is frequently misinterpreted. Let's have a close look:   


Figure 1: Atmospheric temperature change from 1890 to 1990 from (a) solar forcing, (b) volcanoes, (c) greenhouse gases, (d) ozone, (e) sulfate aerosols and (f) sum of all forcing (IPCC AR4).

The source of the confusion is box c, showing the modelled temperature change from greenhouse gases. Note the strong hot spot. Does this mean the greenhouse effect causes the hot spot? Not directly. Greenhouse gases cause surface warming which changes the lapse rate leading to the hot spot. The reason the hot spot in box c is so strong is because greenhouse warming is so strong compared to the other forcings.

The hot spot is not a unique greenhouse signature and finding the hot spot doesn't prove that humans are causing global warming. Observing the hot spot would tell us we have a good understanding of how the lapse rate changes. As the hot spot is well observed over short timescales (Trenberth 2006, Santer 2005), this increases our confidence that we're on track. That leaves the question of the long-term trend.

What does the full body of evidence tell us? We have satellite data plus weather balloon measurements of temperature and wind strength. The three satellite records from UAH, RSS and UWA give varied results. UAH show tropospheric trends less than surface warming, RSS are roughly the same and UWA show a hot spot. The difference between the three is how they adjust for effects like decaying satellite orbits. The conclusion from the U.S. Climate Change Science Program (co-authored by UAH's John Christy) is the most likely explanation for the discrepancy between model and satellite observations is measurement uncertainty.

Weather balloon measurements are influenced by effects like the daytime heating of the balloons. When these effects are adjusted for, the weather balloon data is broadly consistent with models  (Titchner 2009, Sherwood 2008, Haimberger 2008). Lastly, there is measurements of wind strength from weather balloons. The direct relationship between temperature and wind shear allows us to empirically obtain a temperature profile of the atmosphere. This method finds a hot spot (Allen 2008).

Looking at all this evidence, the conclusion is, well, a little unsatisfying - there is still much uncertainty in the long-term trend. It's hard when the short-term variability is nearly an order of magnitude greater than the long-term trend. Weather balloons and satellites do a good job of measuring short-term changes and indeed find a hot spot over monthly timescales. There is some evidence of a hot spot over timeframes of decades but there's still much work to be done in this department. Conversely, the data isn't conclusive enough to unequivocally say there is no hot spot.

The take-home message is that you first need to understand what's causing the hot spot. "Changes in the lapse rate" is not as sexy or intuitive as a greenhouse signature but that's the physical reality. Once you properly understand the cause, you can put the whole issue in proper context. As the hot spot is due to changes in the lapse rate, we expect to see a short-term hot spot. We do.

What about a long-term hot spot? With short-term observations confirming our understanding of the lapse rate, that leaves spurious long-term biases as the most likely culprit. However, as observations improve, if it turns out the long-term hot spot is not as strong as expected, the main question will be why do we see a short-term hot spot but not a long-term hot spot?

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Comments 51 to 55 out of 55:

  1. There was a documentary on the BBC last night called "Panorama: What's Up With the Weather?" In it Bjorn Lomborg and Prof. John Christy both happily accepted that CO2 has an effect on global temperatures. See here and here. Perhaps Nova's crowd will listen to them?
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  2. Yes Albatross, I've now corrected it, thanks. Damn my frazzled brain.
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  3. David #48 Thankyou for the reply I don't wish to argue anything, but I do wish to understand better. I'm personally not interested whether the IPCC has worded this well or not. The fact remains that if the surface has warmed, then the troposphere should have warmed by 2 to 2.5 times as much. Between satellites and radisondes this amount of warming should have been detected. It hasn't been. When you say "if the hot spot has not been found, I'd have to argue one of the 3 options you propose", I would contend that I don't have to argue any of that. It's the proponents of the hot spot (the IPCC) who have to argue one of those 3 options. Afterall, it is they who promoted the hot spot, anthropogenic or not. And they felt it was important enough to not only discuss in chp 9 but also in the faq's along with this in the synthesis report... "The observed pattern of tropospheric warming and stratospheric cooling is very likely due to the combined influences of GHG increases and stratospheric ozone depletion". (page 39) So, what tropospheric warming and what stratospheric cooling? So far they only exist in the models. The IPCC is quite clear in the passages that I cited. It is not the affect of warming that's distinguishable, it's the response to the warming. i.e. a troposphere that warms by 2 to 2.5 times that of the surface accompanied by a cooling stratosphere. Also, as much as I appreciate the charts you have posted, the AR4 makes it clear that the sun has had negligable affect on climate in the 2nd half of the 20thC, therefore, if a hot spot was to be detected, (along with a cooling stratosphere) it can only be a response to CO2 forcing, no?
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  4. Baa Humbug at 02:26 AM on 30 June, 2010 “So, what tropospheric warming and what stratospheric cooling? So far they only exist in the models”. This is from the AMSU satellite sensor so is a shorter time series. I have similar images for RSS and UAH which show more warming (since 1979), but I think the RSS one is even on Wikipedia.
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  5. Thanks Peter Short time series? How short? What is it's significance? The label says temperature trend. Trend since when? How does this relate to surface warming? These charts show warming of about 0.2-0.4. Not quite 2 to 2.5 times the surface warming.
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  6. Baa Humbug, can you explain where you got that "2 to 2.5" factor from? I don't know a lot about that side of things, but my understanding was that the models estimated an amplification factor of about 1.25 globally (1.4 over the oceans and 0.95 over land). But I could easily be misunderstanding either you or the literature (or both)! Also note that the top panel of Peter Hogarth's graphic shows the middle troposphere, not the lower troposphere.
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  7. Humbug, Here is the RSS image from the Wikipedia article Peter was referring to: . Time range is 1979-2010. From this you can see clearly that your assertion was incorrect, stratospheric cooling + troposheric warming is empirically measured, it doesn't just come from models. Also note that the tropospheric hotspot is a distinct concept from stratospheric cooling + tropospheric warming. The tropospheric hotspot is a greater warming of the troposphere in lower latitudes relative to higher latitudes. This is a prediction that follows from any warming, not just from CO2. Stratospheric cooling is the cooling of the stratosphere while the troposphere warms across all latitudes. This prediction is unique to warming from GHG's. Figure 1 in this post shows a combination of these two predictions.
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  8. The 2 to 2.5 was my understanding and I concede that I can't at this time put my finger on where this figure came from. So I'm happy to take your quoted figure of 1.25 However this 1.25 is GLOBAL MEAN. The hot spot is supposed to be most prominent over the oceans and over the tropics (as shown by chart C 30N to 30S) Unless the colours of my computer screen are off, the legend on these charts show a warming of 2 to 3 times than that of the surface ( 0.4 to 1.2) The charts kindly provided by Peter at #54 show tropospheric warming mainly in the high lattitudes, but very little warming along the equatorial band. like wise with the stratospheric cooling. There is a discrepency there (in relation to the hot spot hpotheses) that I can't explain, maybe someone else can.
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  9. It's 4.45am here so i need to excuse myself. Don't read anything into my silence please. And thanks to all those who replied
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  10. Baa Humbug @ #53 No problem. You say "Between satellites and radisondes this amount of warming should have been detected. It hasn't been." That is your view, and I can only refer you back to the article above and John's original article addressing Nova's claims. John points out that the amount of expected warming (or near to it) has been detected by weather balloons (Titchner 2009, Sherwood 2008, Haimberger 2008), wind shear measurements (Allen 2008), and the UWA satellite data. And as Prof. John Christy points out, the discrepancy between the UAH satellite observations and the models is most likely a result of measurement uncertainty. Regarding the 3 possible options for explaining the existing uncertainty, the IPCC does argue for one:
    Since the TAR, further evidence has accumulated that there has been a significant anthropogenic influence on free atmosphere temperature since widespread measurements became available from radiosondes in the late 1950s. The influence of greenhouse gases on tropospheric temperatures has been detected, as has the influence of stratospheric ozone depletion on stratospheric temperatures. The combination of a warming troposphere and a cooling stratosphere has likely led to an increase in the height of the tropopause and model-data comparisons show that greenhouse gases and stratospheric ozone changes are likely largely responsible (Figure 9.14). Whereas, on monthly and annual time scales, variations of temperature in the tropics at the surface are amplified aloft in both the MMD simulations and observations by consistent amounts, on longer time scales, simulations of differential tropical warming rates between the surface and the free atmosphere are inconsistent with some observational records. One possible explanation for the discrepancies on multi-annual but not shorter time scales is that amplification effects are controlled by different physical mechanisms, but a more probable explanation is that some observational records are contaminated by errors that affect their long-term trends.
    http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch9s9-4-5.html As you can see the IPCC raise the possibility that the models are incomplete (option 2), but suggests it is more likely that the discrepancies are a result of errors in the observational record (option 3). I'm inclined to agree with John when he says "Looking at all this evidence, the conclusion is, well, a little unsatisfying". However, if you take the view that the hot spot is absent, and you don't agree with the IPCC that its perceived absence is a result of measurement uncertainty (or possibly problems with the models), then you must take the view that the temperature record is unreliable. Something Roy Spencer (for one) disagrees with. When making judgements (which you sometimes must) it's important to look at the whole picture, and make the most reasoned call. In my view the most obvious call is that the contradictory evidence is a result of measurement uncertainty; any other judgements would involve larger leaps of logic. Regarding your final point:
    Also, as much as I appreciate the charts you have posted, the AR4 makes it clear that the sun has had negligable affect on climate in the 2nd half of the 20thC, therefore, if a hot spot was to be detected, (along with a cooling stratosphere) it can only be a response to CO2 forcing, no?
    Correct - if you first accept the IPCC's assertion that human activity is largely responsible for the rise in global temperatures - which I think is where all this confusion about the hot spot being a signature of AGW comes in. The IPCC, as you state, makes clear that anthropogenic forcings are the primary contemporary climate forcing. Therefore, if a hot spot is detected then it would be evidence of warming, which by the IPCC's definition must have been caused by human activity. Finding the hot spot doesn't prove that humans have caused global warming, in fact it doesn't prove anything, but it does show that the models are correct in what they anticipate the effects of any warming to be, and in the process strengthen the case that warming is occuring. The fact that the troposphere is warming and the stratosphere is cooling (as shown by observational data) does provide evidence that it is humans that are causing the current warming, as opposed to the sun's activities.
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  11. Thnx David Yes I also must agree that the results are a little unsatisfying. From the links in the original article, Titchner et al says (for instance) "our analysis using realistic validation experiments is unable to discount or confirm the presence of a tropical tropospheric lapse rate discrepancy between the radiosonde observations and climate model expectations". And Sherwood says "the agreement would improve if one were to remove the deep tropical stations whose behavior is inconsistent with the rest of the network. This reinforces similar previous findings of consistent trends (Fu et al. 2004; Mears and Wentz 2005; Sherwood et al. 2005; Vinnikov et al. 2006) but remains unsatisfying until errors are further reduced". Regarding the hot spot being a "signature" of GHG warming, I must respectfully disagree. It's not just the IPCC presenting a chart which clearly shows the distinctive signature of GHG warming (chart C in the article), four other organizations show very similar signature. (I hope I can do the image thing correctly) Zonally-averaged distributions of predicted temperature change in ºK at CO2 doubling (2xCO2 – control), as a function of latitude and pressure level, for four general-circulation models (Lee et al., 2007). Eyeballing each of these, one would be excused for concluding a tropospheric warming of up to 3 times that of the surface. The IPCC quote you provided is unequivocal IMHO, where it states, "The combination of a warming troposphere and a cooling stratosphere has likely led to an increase in the height of the tropopause and model-data comparisons show that greenhouse gases and stratospheric ozone changes are likely largely responsible". So if GHG's are largely responsible, then GHG's are the main cause of the distinct hot spot, which as the 6 graph chart shows in C and F, is much more powerful than other forcings, man made or natural (a,b,d,e) So I guess where I am at this moment is that a number of organizations including the IPCC have taken the trouble to distinguish GHG influence on the troposphere at the tropics. Why they chose to do that is theirs to answer. I await eagerly, some definitive papers on the unsatisfying troposphere t trends.
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  12. The quoted paragraph from the IPCC refers to troposphere warming AND stratosphere cooling and of GHG AND ozone. The graph you show are for doubling CO2 not for what we should currently observe.
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  13. Baa Humbug, You've made a few misleading statements there.
    Regarding the hot spot being a "signature" of GHG warming, I must respectfully disagree. It's not just the IPCC presenting a chart which clearly shows the distinctive signature of GHG warming...
    ...as modelled from 1890 to 1999. That's a rather important caveat. It is not a 'distinctive signature' of AGW at all. Please see my previous postings on this, at is a very important point. If it were a unique 'signature' of AGW, and you chose to believe it was absent, then it would be plausible to say that current warming is not being caused by human activity. But as it is not a unique signature, if you choose to believe it is absent, then that leads to the obvious conclusion that warming is not occuring, period. This is a much more difficult argument to make. This is the last time I will outline this distinction, as I'm starting to get bored of the sound of my own voice. The four images which you have provided reflect the same reality as the image I provided at #48, labelled 'For 2xCO2'. They demonstrate that a doubling of CO2 creates a tropospheric hotspot. As my other image at #48 shows, a 2% increase in solar activity (a forcing in the same order of magnitude as a doubling of ACO2) creates a similiar tropospheric hot spot. The comparison is the important bit, not the accepted fact that a doubling of CO2 should cause a tropospheric hot spot, as your images show.
    The IPCC quote you provided is unequivocal IMHO, where it states, "The combination of a warming troposphere and a cooling stratosphere has likely led to an increase in the height of the tropopause and model-data comparisons show that greenhouse gases and stratospheric ozone changes are likely largely responsible". So if GHG's are largely responsible, then GHG's are the main cause of the distinct hot spot, which as the 6 graph chart shows in C and F, is much more powerful than other forcings, man made or natural (a,b,d,e)
    As 'e' pointed out at #57:
    the tropospheric hotspot is a distinct concept from stratospheric cooling + tropospheric warming. The tropospheric hotspot is a greater warming of the troposphere in lower latitudes relative to higher latitudes. This is a prediction that follows from any warming, not just from CO2. Stratospheric cooling is the cooling of the stratosphere while the troposphere warms across all latitudes. This prediction is unique to warming from GHG's.
    I've also pointed out a number of times that a warming troposphere (distinct from a specific hot spot) and a cooling stratosphere, is a unique signature of AGW. The existence of both of these phenomena are supported by observational data. Although your logic breaks down, you are right in saying that (assuming we accept the IPCC's assertion that human activity is causing the majority of observed global warming) "GHG's are [or rather 'should be'] the main cause of the [current] distinct hot spot". As I've pointed out,if (big if) you conclude that the tropospheric hot spot does not exist, then you must argue against the observation of global warming from any source, as opposed to just AGW. There's actually very little in the IPCC document about the tropospheric hot spot - presumably as a result of the fact that it proves very little beyond the fact that the models are accurate (or not) in their predictions of atmospheric zonal temperature responses to warming (of any kind). The small, oft-quoted section that there is, is fairly misleading. Much greater 'air-time' is given to the warming troposphere/cooling stratosphere phenomonen, which is undeniably being observed, and is a signature unique to AGW. The brief section of the IPCC document which concerns us...
    The simulated responses to natural forcing are distinct from those due to the anthropogenic forcings described above. Solar forcing results in a general warming of the atmosphere (Figure 9.1a) with a pattern of surface warming that is similar to that expected from greenhouse gas warming, but in contrast to the response to greenhouse warming, the simulated solar-forced warming extends throughout the atmosphere.
    ...doesn't refer to the tropospheric hot spot at all, but rather to the fact that solar forcing results in a warming of the whole atmosphere, where as AGW forcing results in a warming of the troposphere and a cooling of the startosphere. The fact that it links to a picture of a number of charts showing zonal mean atmospheric temperature change from 1890 to 1999 (which happen to show the hot spot created by greenhouse gases during that period) is neither here nor there. This is a matter which has been mistakenly leapt on by 'sceptics', not least Jo Nova, who appear to consider it the lynch pin of AGW theory. Like you I look forward to some more definitive papers on the matter.
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  14. Hi David I'm sorry for the late reply. Firstly I wish to state that at no time have I commented to mislead people. If I have, it is inadvertant. Secondly I have 2 apologies to make. First, I have been using the term "hot spot" to mean a warming of the tropospohere and a cooling of the stratosphere. Maybe posting during the wee hours hasn't helped. I apologise for the confusion this has created. But with due respect, I have gone thru your previous comments and didn't find where you made this distinction until your last comment at #63 My second apology is to e who posted the RSS images at #57 In his/her comments he/she does clearly state the distinction. Both are my misunderstandings therefore my fault. Hope I didn't frustrate you too much. I'm not going to trouble you further until I resolve the following issues.. * The statement "Stratospheric cooling is the cooling of the stratosphere while the troposphere warms across all latitudes. This prediction is unique to warming from GHG's." made by e at#57. The charts presented by numerous organizations do not show warming across all latitudes for ghg forcing. They distinctly show warming at 30N to 30S at 8-10km aloft (yes along with stratospheric cooling) * The papers cited (Sherwood, Heimberger Titchner etc) go some ways to reconcile the radiosonde data with satellites, but by no means do they show the 2-3 times warming at the "hot spot" (accompanied by stratospheric cooling) * The ensemble of 6 charts we've been discussing. yes they are simulations from 1890 to 1999. This is the period man has been adding CO2 to the atmosphere. This is the period that has warmed (due to ghg's according to the IPCC). Chart C clearly shows a "hot spot" along with a cooling stratosphere. Therefore, our observations should show that. I don't believe they do. So again, like you, I look forward to future papers on this subject. Thnk you for your efforts.
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  15. Hi Baa Humbug, No worries, I don't doubt your integrity. I wouldn't disagree with the thrust of your second and third *s, but with regards to the first one, all the charts presented do show warming across all latitudes as a result of GHG forcing. A 0°C change would be shown as white on my charts and cream on yours. As you can see this is not the case. The charts do model more warming at 30N to 30S at 8-10km aloft, but that doesn't mean they don't also anticipate the troposphere as a whole to be warming.
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  16. Baa Humbug, I think the discussion of models and model predictions can be very misleading, as there is a very strong tendency on one side to stick to the modeling assumptions and think that the measurements are wrong. The IPCC is open to the possibility that their modeling is wrong, but they don't really believe it can be. On the other side, there are lots of flawed conclusions as to what really implies what. The basic phenomena here are: - Is the stratospere cooling or, at best, at constant temperature? Yes, it is. And this cooling is to be considered separately from anything that could occur elsewhere. Stratospheric cooling is cooling regardless of what might happen in the troposphere. - Is the troposphere, as a whole, warming? Yes, it is. This, too, is to be seen independently from what might happen further up or down. - Could this divergence be explained by the sun? No, not by any plausible model anyone has been able to produce. So far then, we have an observation of a GHG signature. Yes or no? - Is it conceivable that the lower troposphere is warming at about the same rate? Yes, it is. And in fact, that is what we seem to observe, with both the RSS and UAH temperature series. In fact, it is only during El Ninos we have, regularly, seen something else, with more warming of the troposphere. - Does this parallel warming contradict any laws of physics? Not necessarily, as there are several transport and conversion phenomena that could interact to produce the results we can observe. Whenever we have transport phenomena coupled with combinations of positive and negative feedbacks, as we have here, the results can become rather unpredictable, maybe even seemingly "non-determinstic".
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  17. " Is it conceivable that the lower troposphere is warming at about the same rate?" Should of course be: Is it conceivable that the lower troposphere and the surface is warming at about the same rate?
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  18. Baa Humbug > The charts presented by numerous organizations do not show warming across all latitudes for ghg forcing. They distinctly show warming at 30N to 30S at 8-10km aloft (yes along with stratospheric cooling) The models show enhanced warming at tropical latitudes (the hot spot), but there is also a clear overall warming of the lower atmosphere, which is also clearly visible in the satellite data. I think what you are really looking for is a chart showing what modeled results would like if a non-GHG forcing was causing the warming and comparing it to the actual model predictions and the empirical data. This isn't what the IPCC charts - or the other charts you cited - are doing, though that's the way they are often misinterpreted. I don't know if such a chart exists, but the key difference would be a cooling stratosphere, which should be visible with GHG warming but not solar or other warming. Both GHG and non-GHG charts would show a hotspot, which is why the hotspot is not a signature of GHG warming. The observed cooling in the stratosphere is therefore very strong evidence for the current warming being GHG driven, and a strong falsification of other attributions. If this cooling didn't exist, it would be a very strong falsification of GHG warming. In contrast, if the hot spot really didn't exist, it would shift our understanding of the process of global warming, but not the cause (since the hotspot is predicted for all causes of warming.)
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  19. Baa Humbug at 03:24 AM on 30 June, 2010 My apologies for not supplying details, this was actually an image from 1979 to 2005 composite MSU/AMSU data, (I’m afraid my image database is not as well indexed as it should be on older data, and I allowed the NASA filename - with amsu reference, to mislead me. The earliest AMSU was launched 1998). Also a belated thanks to e for posting the RSS image with the time series. This updated image might help visually clarify a number of points about trends and short term lower tropospheric latitudinal variations. The possible evolution of the short term tropospheric tropical hot spots (towards higher latitudes) is interesting.
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