Skeptics/Denialists Part 2: Hotspots and Repetition

In Part 1 I discussed the difference between skeptics and denialists. Not a few hours after I did that, I read a post over at Deltoid where Tim Lambert talks about a popular subject: Hotspots (no, nothing to do with geology).

Back in July, one David Evans published a piece in The Australian where he made a series of awful arguments against AGW, but the article was most famous for him coining the term ‘hotspot’ and claiming that such a hotspot doesn’t exist. The “hotspot” is the amplifying tropical tropospheric temperature relative to the surface. The argument goes like “models predict this hotspot with rising CO2, but it’s not there, so AGW must be wrong.”  This is an important issue since formal attribution of temperature trends has always been based on spatio-temporal patterns, so it is necessary to discuss the observed and modeled evidence as well as their relevance and implications.

As Tim Lambert has shown, David Evans is very confused, and now it appears he doesn’t even understand what his own argument was. I will copy David Evan’s reply to Tim Lambert with piece-by-piece clarifications,

Tim (Post 14):

With your last reply, we have now identified the source of your disagreement: You say the hotspot is not part of the signature of enhanced greenhouse warming, I say it is.

I do not really like how either party has phrased this.  You should get enhanced tropospheric warming with increased CO2, but that is not the complete argument.  Tropospheric warming in the tropics is a signature of greenhouse warming, but it is more accurate to say that it is not a unique signature (i.e., you get this “hotspot” with all types of forcings).    The ‘hotspot’ arises due to the moist adiabat.  In the extra-tropics you do not don’t expect the lapse rate changes to be so dominated by moist convective effects.  Some people have claimed this signature exists because “greenhouse gases do their absorbing at higher altitudes” but they also absorb outside the tropics so this cannot really be correct.  The IR heating change is pretty uniformly distributed, in fact. Moreover, the whole troposphere is well mixed in heat, and is more or less constrained by convection to stay near the moist adiabat. The below graphs are model runs from GISTEMP, the first being a 2x CO2 scenario with a 100 yr. response, and the second being a 1.02x Solar increase case.



As seen, until you get to the stratosphere the responses are very similar.

Please look at Chapter 9 of the IPCC Assessment Report 4, 2007:

Click to access AR4WG1_Print_Ch09.pdf

On page 675 is Figure 9.1, which shows “Zonal mean atmospheric temperature change from 1890 to 1999 (°C per century) as simulated by the PCM model” from five possible causes of global warming. Part (c) shows the signature of “well-mixed greenhouse gases”. Part (f) shows the warming pattern from “the sum of all forcings” (considered by the model). Both (c) and (f) show a large, intense hotspot over the tropics at about 190km — it is the most dominant feature in both diagrams

No disagreement, but again, it’s not unique. Climate forcings such as increasing solar irradiance, increasing, decreasing surface albedo, decreasing low cloud cover, will warm the ground and troposphere and will also warm the stratosphere. On the other hand, increasing GHG’s uniformily in the vertical, decreasing high-level ozone, and increasing stratospheric water vapor will act to “invert” the temperature anomaly profile above the tropopause and cause a cooling and contraction of the stratosphere while the surface and troposphere warm.

Here is another source, at the US CCSP 2006, chapter 1: On page 25, Figure 1.3, there are “PCM simulations of the vertical profile of temperature change due to various” for Jan 1958 to Dec 1999, with similar results. The “Well mixed greenhouse gases” and “All forcings” signatures both show an intense, dominant tropical hotspot at 10km.

Or look at Figure 14 on page 59 in, which shows the zonal mean distributions of temperature change for a CO2 doubling, as predicted by four atmospheric general-circulation models, for all forcings. Again, the hotspot is prominent in each one.

Again, same thing. Tim Lambert probably led David Evans to believe there should not be an amplicative effect in the tropical troposphere, but this is not the case, just that the argument needs to be completed to all forcings. We can just as well say the hotspot for increasing the sun is missing, but the most unique fingerprint is that the “cold spot in the stratosphere” for the sun is missing. Indeed, even the mesosphere and thermosphere cool with increased CO2. The lapse rate in the troposphere is set by convection and other vertical mixing processes; in the stratosphere, the lapse rate is set by radiation balance, so the reasons why the stratosphere cool depend on upwelling and emitted radiation but also on ozone depletion. The stratospheric balance is heating by solar absorption vs. infrared cooling. If you increase the greenhouse gas concentration then the stratosphere is being shielded more strongly from upwelling radiation from below by the increased opacity in the 15 micron region and you get more cooling balancing the same solar heating, hence the stratosphere can get cooler.

Tim, you quote the Executive Summary of the IPCC AR4 Chapter 9, from page 665, “The observed pattern of tropospheric warming and stratospheric cooling is very likely due to the influence of anthropogenic forcing, particularly greenhouse gases and stratospheric ozone depletion.”

Tim this does not say anything about a hotspot, or imply that the signature of enhanced greenhouse warming does not include a hotspot. As noted in my article, the combination of broad stratospheric cooling and broad tropospheric warming is one of the two features of enhanced greenhouse warming (the other is the hotspot), and is the sole feature of ozone depletion (see the first two sources quoted above for verification). And this combination has been observed — which is what your quote is commenting on.

Keep in mind that the lower stratospheric is influenced heavily by ozone depletion, whereas the mid-stratosphere and above is influenced by greenhouse gas increases.

Tim, your quote attributes the observed stratospheric-cooling and tropospheric-warming to extra greenhouse bases and ozone depletion, just as I do. And no mention of the hotspot.

Everyone agrees about this “hotspot.”

Now Tim, look at the sentence after the one you quoted: “The combination of a warming troposphere and a cooling stratosphere has likely led to an increase in the height of the tropopause.” That increase in the height of the tropopause IS the hotspot! The hotspot arises in AGW theory because an increase in greenhouse gases (of CO2 due to humans, and of water vapor due to increasing temperature) pushes the top of the tropopause higher, thus replacing cold stratosphere with warmer troposphere at the top of the troposphere — which is at about 10km over the tropics. (Btw, it now appears that the atmosphere just drops out water vapor as it is replaced by CO2, to keep the total greenhouse effect about constant. This is the nub of the argument, and where AGW went wrong.)

None of this is correct. Warming the troposphere or cooling the stratosphere tends to increase tropopause height. If this were the whole argument, the tropopause height increase is real. But the “hotspot” refers to the fact that the surface-to-troposphere temperature gradient decreases. Interestingly, since the greenhouse effect depends on the lapse rate, models produce a negative feedback to this change in temperature gradient. If David Evans is correct about the hotspot not being real, then the most intuitive implication would be toward a heightened climate sensitivity.  As Tim Lambert also mentions, water vapor certainly does not decrease with more CO2.  I believe this came from Ferenc Miskolczi’s flawed paper on atmospheric optical depth and the greenhouse effect, but like most neo-Galilean pieces, it didn’t work, and neither did that particular claim.

In other words Tim, a fuller quote reveals that the IPCC AR4 is saying that it thinks a hotspot should be there. But it does not claim to have found it — if they could they would have explicitly claimed it, and trumpeted it around the world. The IPCC is encouraged by the observed stratospheric-cooling and tropospheric-warming because if it hadn’t been observed either then there is no hope for AGW. They are playing up the positives: at least half the signature of enhanced greenhouse warming might have been found! But notice that they acknowledge that the half signature observed could be due to ozone depletion. The IPCC quotes are fully consistent with ozone depletion as the sole cause of the observed stratospheric-cooling and tropospheric-warming.

This is nonsense. Much of the cooling above the tropopause is due to GHG’s, while MSU4/TLS lower down does respond to ozone changes which are levelling off. This paper may be instructive.

Sorry Tim, the text in these reports is pored over by teams of bureaucrats and parsed endlessly, which is why it ends up so dense and so hard to read. It is easy to be misled.

Misled indeed.

Tim, climate scientists have been debating for years over the (some would say apparent!) non-observation of the hotspot. If the answer was as simple as noting that the hotpot is not part of the signature of enhanced greenhouse warming then they could all have saved themselves a lot of time and work.

Now this is a real issue. The real issue is what is actually going on in the tropical troposphere, because there’s a lot of uncertanties in both observations and modelling. Part of the reason involves sampling issues, contamination of stratospheric cooling and incoming sunlight with sensing devices, and standarization problems. There’s also a lot of “noise” in the troposphere.The troposphere is certainly warming, but the issue of how big the signal is becomes a difficult problem. In the extratropics, observations are as expected. Based on many recent papers, such as Tropical vertical temperature trends: A real discrepancy? by Thorne et al, Robust tropospheric warming revealed by iteratively homogenized radiosonde data, by Sherwood et al, and the recent Santer et al. paper, it is not obvious a real model-observation discrepancy exists, so to ignore these papers and the uncertainty in the data is not going to get Evans very far. Realclimate has a much more thorough discussion of those, and more papers on this topic here, Part 2, and Part 3

How Evans continues to go about this “hotspot” issue should be very revealing.


13 responses to “Skeptics/Denialists Part 2: Hotspots and Repetition

  1. Hi, Chris. I like your response above and, in fact, I tried to make many of the same points over there at Club Troppo (including the one about the lack of hotspot most naively tending to support a higher climate sensitivity) before I then read your post here. (Reading what I wrote, it may be hard to believe that I didn’t read your post first but really I didn’t!)

    One more comment I would make in response to Dave Evans talking about Fig. 9.1 in the IPCC AR4 report: I have found in previous discussions that a lot of misunderstanding on this occurs because people don’t know how to read contour plots very well. I.e., a lot of people look at that figure and think that they don’t see a similar “hot spot” for the case of solar forcing. However, what they are failing to understand is that the contours have simply not been drawn finally enough to see much of the structure at all for the solar forcing. In fact, all you can strictly say from looking at that plot is that the magnification of the surface warming for solar forcing as you go up in the tropical atmosphere is greater than 1 and less than infinity!

    Of course, the plots from the GISS climate model that you reproduce here get around this problem by artificially increasing the solar forcing from the value that we believe it to have been over the 20th century to a value that produces a similar surface warming to that from greenhouse gases…and then the fact that the “hot spot” is there for solar forcing (and is of almost exactly the same magnitude as for greenhouse gas forcing) becomes readily apparent.

  2. Chris:The way the troposphere deals with too much warming, is that it will eventually back up into the bottom of the stratosphere. Sort of just piling on.
    Then this deprives the stratophere of even less heat and so it becomes even cooler, like a desert. How would you explain it?Thanks,KIPP

  3. Gavin’s inline reply here (on my pointer to Deltoid) is worth reading and includes a link to an earlier discussion:

    The AGU collects history (up to about 1990) of thinking here:

    The annual $20 for an AGU membership gets you the weekly EOS newsletter and monthly Physics Today; in the December 2008 issue of PT there’s a 2-page “Physicist’s Guide to the Upper Atmosphere” worth reading . IT’s part of their “Quick Study” series.

    Kipp — I suggest getting that from your local library, they can certainly borrow it via Interlibrary Loan if it’s not on the shelf.

    It’s two fairly dense pages of small print with a couple of references. It’s probably as few words as can possibly be used to actually give you a useful answer.

  4. A librarian can find this for you

    Physics Today / Volume 61 / Issue 12 / QUICK STUDY
    Physics Today – December 2008
    A physicist’s tour of the upper atmosphere
    John T. Emmert
    US Naval Research Laboratory, Washington, DC

  5. Chris:Thinking about this again, I would guess that perhaps the troposhere could expand to it’s outer limit from the warmth, pushing up against the stratosphere. As a photgrapher, I love science but was a philosophy major.
    It’s only been a year and I have read several books on Climate Change,Chemistry,Geology, and geophysics. I know that I don’t know much.

  6. From Chris’ last link:

    “The bottom line is that while there is remaining uncertainty in the tropical trends over the last 30 years, there is no clear discrepancy between what the models expect and the observations.”

    So there may not be a real problem here. And Chris suggests that if there is a problem, it may be with the data instead of the modeling.

    Worst-case scenario in terms of demonstrating a lack of general scientific understanding is that the data is right, the discrepancy is real, and the modeling is therefore flawed. The flaw isn’t a lack of an anthropogenic signature, but the lack of a high-tropo signature that models predict for anthro or natural forcing. Skeptics like Evans would be making a much more plausible if more limited argument if they said this possibility was all they were suggesting.

    It would be interesting if modelers could play a “what if” game and tell us whether this flaw if proved true means modeling is entirely useless or whether it’s just one prediction, and they can construct other models that don’t require this outcome.

    I know David Evans personally, from a lot of interaction as we set up our climate bet against each other. He’s a nice guy, reasonable in all matters that we negotiated. It will be interesting to see how he reacts over time.

  7. Hank Roberts:Thanks for your input.I will start to subscribe to AGU as well as find the piece you referenced. I also want to buy the Oxford Dictionary of Physics when Amazon gets them in. Do you know any other sources or books that might help me. If you ever want to pick off a couple of really dumb skeptics(deniers), go over to Accuweather Global Warming blog. They are all residing there for some reason. It’s sort of like shooting skunks with your bb gun when you were a kid. You know, when you have a fight with the wife or kids just go over to Accuweather and nail a denier.It’s good for you.Thanks Again, appreciate your time,best;Kipp

  8. Chris:Did I read that the tropapause is measured, and it does grow by the mixture of GHG’s, and water vapor.KIPP

  9. Chris:Here it is! Thanks. Contributions of Anthropogenic and Natural Forcing to Recent Tropopause Height Changes
    B. D. Santer,1* M. F. Wehner,2 T. M. L. Wigley,3 R. Sausen,4 G. A. Meehl,3 K. E. Taylor,1 C. Ammann,3 J. Arblaster,3 W. M. Washington,3 J. S. Boyle,1 W. Brüggemann5
    Observations indicate that the height of the tropopause—the boundary between the stratosphere and troposphere—has increased by several hundred meters since 1979. Comparable increases are evident in climate model experiments. The latter show that human-induced changes in ozone and well-mixed greenhouse gases account for 80% of the simulated rise in tropopause height over 1979–1999. Their primary contributions are through cooling of the stratosphere (caused by ozone) and warming of the troposphere (caused by well-mixed greenhouse gases). A model-predicted fingerprint of tropopause height changes is statistically detectable in two different observational (“reanalysis”) data sets. This positive detection result allows us to attribute overall tropopause height changes to a combination of anthropogenic and natural external forcings, with the anthropogenic component predominating.


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  11. Hey Chris! What’s up man. Shoot me an email sometime – [got it]

    About the Stratosphere cooling, when you have higher ammounts of greenhouse gases it absorbs more of the irradiance and it doesn’t re-radiate back to space right? We know that. But because of this, the stratosphere should exhibit a cooling trend because there is less irradiance escaping through the atmosphere. The layers of the atmosphere are also in a constant change in flux.

    Response- Eventually all the radiation that enters the Earth-atmosphere system makes it back out to space, and eventually a radiative equilibrium is reached so the absorbed incoming solar = the outgoing radiation at the top-of-atmoshere. It’s just greenhouse gases close some extra bits of the atmospheric window (i.e, where it goes out to space) and forces radiation to come from the higher, colder layers of the troposphere. The stratosphere cooling arises because with more CO2 it becomes a less efficent absorber but a better emitter. Ozone depletion also plays a role in the lower stratosphere.– chris

  12. Look, this is silly. 4AR shows the hotspot distinctly and solely for greenhouse warming. It ain’t there in the observations, and those (including Santer) trying to fiddle the observations ignore their near-total uniformity — hundreds of thousands of radiosondes of different types at different times of day all over the world giving essentially the same answer. The AGW rabble-rousers are trying to tell us, in effect, that nearly forty years after putting men on the moon we can’t make a decent thermometer. Give me a break.

    If indeed the GISTEMP model shows such a hotspot for solar warming, then apparently it disappeared completely in the different model averages used by the IPCC report, which proves only that GISTEMP is even more ludicrously wrong than the others. The hotspot is simply not there, just as ARGO’s 3000 bathythermographs can’t find a trace of the by now 4 x 10^22 J of heat the modelers tell us is hiding in the ocean ready to jump out and bite us with more warming at any moment.

    After more than 20 years and $50 billion, the modelers still have no conception of hydrology as a primary driver of climate, as Lindzen wrote back in 1990. Which is slightly odd, since most humans know the basic facts about clouds and rain by the time they reach the age of six.

    Response– Either that, or the other option is that you don’t understand the AR4 model results, you have no interest improving the observational data, you don’t understand the basic thermodynamics of the moist adiabatic lapse rate, and you’re a conspiracy nut. I pick choice b.– chris

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