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Climate Hustle

New study finds a hot spot in the atmosphere

Posted on 15 May 2015 by John Abraham

A new study, just published in Environmental Research Letters by Steven Sherwood and Nidhi Nashant, has answered a number of questions about the rate at which the Earth is warming. Once again, the mainstream science regarding warming of the atmosphere is shown to be correct. This new study also helps to answer a debate amongst a number of scientists about temperature variations throughout different parts of the atmosphere.

When someone says “The Earth is warming”, the first questions to ask are (1) what parts of the Earth? and (2) over what time period? The Earth’s climate system is large; it includes oceans, the atmosphere, land surface, ice areas, etc.

When scientists use the phrase “global warming” they are often talking about increases to the amount of energy stored in oceans or increases to the temperature of the atmosphere closest to the ground. By either of these measures, climate change has led to a progressive increase in temperatures over the past four decades. But what about other parts of the climate system? What is happening to them?

One important area to consider is the troposphere. It is the bottom portion of the atmosphere where most weather occurs. Tropospheric temperatures can be taken by satellites, by weather balloons, or other instruments. In the past, both satellites and weather balloons reported no warming or even a cooling.

However, that original work was shown to be faulty and now even the most strident sceptics admit that the troposphere is warming. But obtaining an accurate estimate of the rate of warming is difficult. Changes to instruments, errors in measurements, short term fluctuations all can conspire to hide the “real” temperature.

This is where the new study comes in. The authors develop a new method to account for natural variability, long-term trends, and instruments in the temperature measurement. They make three conclusions.

First, warming of the atmosphere in the tropical regions of the globe hasn’t changed much since the late 1950s. Temperatures have increased smoothly and follow what is called the moist-adiabatic rate (temperature decrease of humid air with elevation). This result is in very close agreement with climate computer models and it contradicts the view that there is a slowdown in climate change.

Second, the vertical height of the tropics that has warmed is a bit smaller than the models predict. Finally, there is a change in observed cooling in the stratosphere – the layer of the atmosphere above the troposphere.

Taken together, these results show that the tropospheric warming has continued as predicted by scientists years ago.

Embedded in this research is a conclusion about the so-called “tropospheric hot spot”. This “hot spot” refers to expectations that as global warming progresses, the troposphere will warm faster than the Earth surface. The hot spot is really hard to detect; it requires high quality measurements at both the surface and throughout the troposphere. Past studies which could not detect a hot spot were often used by climate contrarians to call into question our simulation models and even our basic understanding of the atmosphere.

But this new study finds a clear signal of the hotspot. In fact, the temperature in the troposphere is rising roughly 80% faster than the temperature at the Earth’s surface (within the tropics region). This finding agrees very well with climate models which predicted a 64% difference.

And this is exactly how models are supposed to work. Models can be used to predict changes that will occur in the future. Once we make measurements, we can compare them with the models. If the two disagree, it either means our models are wrong, our measurements are wrong, or both are wrong. More often than not, the models have been found to be vindicated.

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Comments

Comments 1 to 14:

  1. Mike Mann has an excellent description of this new study's relevance to AGW.  In brief,

    What's the punchline? Well, if global warming really *were* due to a (natural) decrease in volcanic activity over time (rather than due to an anthropogenic increase in greenhouse gases), then we would expect to see an increase in global surface temperatures WITHOUT any mid-tropospheric "hot spot".

    In the end, then, the confirmation of a "hot spot" in this latest study by Sherwood and Nishant isn't completely irrelevant to the issue of human-caused climate change. While it may not be a unique fingerprint of anthropogenic greenhouse gas increases, it does nonetheless potentially allow us to rule out at least one possible suspect (changes in volcanic activity). It turns out that anthropogenic changes in ozone (both tropospheric, as a surface pollutant, and stratospheric, as a result of stratospheric ozone depletion) are another potential "forcing" of climate change that does not have a clear "hot spot" signature as part of its fingerprint.

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  2. Interesting point in the comments at HotWhopper, that the term "tropospheric hot spot" seems not to be used scientifically.  Implications of failure to use that term are more than merely semantic.

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  3. Tom Dayton @1, in this case Michael Mann is wrong.  As the RealClimate team have explained, the tropospheric hotspot is a result of the lapse rate feedback, ie, the change in decrease in temperature with altitude due to the increased heat capacity of the atmosphere resulting from a higher absolute humidity with increased warming.  The RealClimate team illustrate this by comparing expected warming and cooling by latitude and altitude for a doubling of CO2, and an equivalent increase in solar forcing:

     

    The patterns are broadly similar, although the CO2 forcing results in a stronger arctic warming (but note the difference in scales).

    Unfortunately, it is not possible to similarly compare the result of CO2 forcing to a loss of background volcanic aerosols as the background level of aerosols is not that large.  We can however compare the cooling due to a loss of 50% of CO2, and due to a 1/3 rd Pinatubo level volcanic forcing:

    As you can see, for these negative forcings, the patterns in the tropics are very similar.  At the poles, however, the increased volcanic aerosols result in stratospheric cooling, unlike the stratospheric warming resulting from reduced CO2.  Further, the peak arctic cooling is further south with volcanic aerosols.  Again the scales are different, but the forcings are not SFAIK of equivalent strength.  The patterns, however, are the important feature.

    The polar stratospheric cooling with increased volcanic forcing may be partly due to the location of the aerosol cloud, which is not global (and was tropical in Pinatubo's case) and ergo will have different regional effects.  It is, however, at least partly due to a decreased warming of the stratosphere from upwelling IR radiation due to the cooler troposphere not being compensated by increases short wave radiation reflected of the stratospheric aerosols due to the low insolation at the poles.  Ergo, it is at least in part a feature we would expect of any volcanic cooling, although the exact boundaries of warming and cooling zones will vary depending on the latitude of the volcano causing the cooling.

    The important thing about this comparison for our discussion is that there is definitely a tropospheric cool spot for both reduced CO2 and the volcanic forcing.  As can be seen by comparing the doubled CO2 and halved CO2, the patterns of change in temperature are the same for both.  It is just that the signs of the change are different.  The same would be true for a reduced volcanic forcing.  That is, it would show a similar pattern to the increased volcanic forcing, only with warming where the volcanic forcing shows cooling, and vice versa.  Ergo a reduction of background volcanic aerosols would definitely result in a tropospheric hotspot, contra Mann.

    I believe Mann has been decieved on this because:

    1) He fails to note that the observed heating for Pinatubo through radiative absorption and re-emission of the aerosols is primarilly above the 100 mb line, ie, the tropopause:

    2)  And because he focuses only on radiative heat transfer, ignoring the convective heat transfer effects that drive the change in lapse rate.

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  4. Tom Curtis, I asked Mike Mann to reply to your comment. Here's what he wrote over on his FaceBook post:

    I've explained (and shown) carefully in the post above that there is no hotspot associated with a global warming trend resulting from changes in volcanic forcing (this is also true for ozone forcing-related trends). The problem with the RealClimate post is that it doesn't acknowledge this exception to (Gavin's) argument. The argument works for GHG, it works for El Nino, it works for Solar. IT DOES NOT WORK for volcanic forcing (or ozone forcing, or any forcing which has a similarly complex vertical radiative forcing profile associated with it). I have confirmed my take on this with Ben Santer.

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  5. Mike Mann on FaceBook followed up in his reply to Tom Curtis's above comment:


    I did take a look at the piece. First of all, I am not ignoring convective transport of heat. What I am saying is that the convective transport of heat (which would normally lead to a "cold spot" in this case) is offset by the radiative forcing at least partly, eroding (if not erasing entirely) the cold spot. Now, I don't know the details of the calculation he's showing, because he hasn't stated where it comes from. I'd really like to see the details, i.e. how the forcing was implemented, etc. because it appears to be at odds, in terms of the pattern of response, with that shown in Santer's work.This figure is from Santer et al (2013) PNAS. Look at the amplitude of the response to the volcanic forcing events in the mid-to-upper troposphere vs the surface, e.g. Pinatubo, we see about -0.25C peak cooling for "TMT" (mid-to-upper tropos) and about -0.5C peak cooling in the lower troposphere. I don't see how that can be consistent w/ what Tom is showing in his figure.

    Santer et al. (2013) PNAS

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  6. Tom Curtis, I'm being go-between you and Mike Mann, because though he is popping over here to read your comments, he can't post here because he does not have an SkS account and doesn't want to get one because he already is too busy. (I don't buy that excuse, because I know the only way he can accomplish all the things he already accomplishes is by having an army of clones of himself.  What's one more clone?)  He wrote on FaceBook:


    Tom: in fact, I've having a lot of trouble reconciling the trends shown in (the other Tom's) post at SkS and this plot from Santer et al '13. Tom's plots show maximum amplitudes in the way upper troposphere, while Santer's corresponding plots show greatest amplitudes in the lower-to-mid troposphere. In Santer's plot, the SIGN of the temperature anomaly associated with volcanic forcing actually reverses at the mid-troposphere (roughly 450 mb)! Perhaps there is something I'm missing or misunderstanding about the plots Tom has posted. They certainly don't seem consistent w/ Santer et al. The "hot spots" (or "cold spots" if you like, depending on sign of forcing) are located at greatly different altitudes.


    Santer et al. (2013) PNAS

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  7. Tom Curtis:  Mike Mann continued at FaceBook:

    I suspect the model simulations shown by Tom may have a very different convective parameterization than the models employed in the CMIP5 average shown by Santer et al. The latter is consistent with maximum precipitation rate (and latent heating) in the lower-to-mid troposphere, with greater stability (and less latent heating) in the upper troposphere, while the results shown by Tom suggest a great degree of convective heating and instability all the way to the tropopause. The convective dynamics described by the two situations are greatly different.

    I'm interested to hear Tom's response. I open to being disproved on this, but thusfar I'm troubled by the inconsistencies in what he has posted vs. what is shown in the peer-reviewed literature by Santer and others...

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  8. Tom Dayton @4-7:

    1) The model used was the GISS Model E vIII, as can be found out by tracing back the links through the Real Climate link I provided.  As such I suspect it is the same model that was used for the second figure in Mann's original article, with Mann's figure differing in being rapped around the hemicircle, and not being to scale relative to the real thickness of the atmosphere.  Importantly, Mann's figure also differs in showing the effects of historic forcings (which are very different in magnitude between the various forcings) rather than the effects of forcings of approximately equivalent magnitude:

     

    2)  Santer et al (2013) shows historical forcings from Jan 1979 to Dec 2012.  Santer's graph shows an overall positive volcanic forcing, with a negative forcing in southern polar latitudes, and no forcing in northern polar latitudes.  That is, it shows a very different regional pattern (and a far smaller forcing) than does Pinatubo.  Consequently I would hesitate to draw any inference from the differences in pattern between GISS model E and the CMIP5 mean.  Such differences are as likely to be due to the different geographical pattern of the forcing as to any difference in model physics.

    3)  Each of the TMT (Channel T2), TTS (Channel T3), TLS (Channel T4) include a portion of the lower stratosphere in their weighting functions:

     The result is that all are downgraded by opposite signed signals contaminating the data.  This is particularly the case for the TMT (T3) channel which obtains approximately a third of its data from the stratosphere, and hence with opposite sign to the upper tropospheric signal.  (This is also why certain deniers prefer to use the TMT channel as representative of tropospheric temperature trends.)  As Santer et al graph by anomalies for these channels, I presume they have taken a weighted temperature function by altitude to match the weighting of the satellite channels, and will suffer the same problem.  This by itself would lead us to expect a reduced TMT temperature response relative to the TLT response as shown by Santer et al, even with the GISS Model E temperature response.

    4)  Beyond that, it is possible that the CMIP5 models show a different response to volcanic forcing, and that consequently the Real Climate article and GISS model E data are out of date.  If Mann could get Gavin Schmidt to comment on that point, I would be very interested.  As it stands, however, I do not find his counter argument convincing, and nor is this topic close enough to Mann's core areas of expertise that I would take him as an authority on this aspect of AGW.

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  9. Tom Curtis:  Mike Mann read your most recent response, and replied:

    After having read Tom's response to my response at SkS, I'm now not inclined to take the critique too seriously. First of all, his argument that there is little tropospheric IR signal in the Robock radiative forcing series is just wrong. The positive IR radiative forcing anomalies near 300mb (well into the troposphere) at the equator are similar in magnitude to the negative short-wave anomalies at the surface. This is not neglible. He has misinterpreted Figure 1 shown above--this is an adaptation of the key IPCC AR4 figure on fingerprint detection (based on simulations w/ PCM by Santer et al '03). I'm surprised he doesn't recognize it, since it is actually featured in main post at SkS on the hot spot. Over the shorter more recent interval shown in Santer et al '13, the volcanic surface forcing is indeed positive (that is a completely different interval and thus very different drend from the 20th century trend analyzed in Santer et al '03). But the sign of the net surface forcing, whether negative (for the 20th century trend) or positive (for the more recent trend), isn't the point. The point involves the vertical pattern of the response. And there is no way to reconcile that pattern of response w/ the GISS result Tom shows in his plot. Fully recognizing the complicating issue of the TMT/TLT vertical weighting functions (which Santer et al take some pains to deal with in their interpolation scheme) there are real discrepancies here. Just compare the GHG signal in Santer et al '13 (which shows a hot spot) and the volcanic (which doesn't, and shows a change of sign at 500 mb, which is very close to the 595 mb PEAK of the TMT weighting function--if there were a mid-tropospheric hotspot, it should be seen in TMT). That discrepancy cannot be explained away by the vertical weighting functions. So I think I'll go with the CMIP5 results shown in Santer et al '13 over the results of a single (GISS) model, which is certain to have its own peculiarities w.r.t. convective parameterizations, etc. Suffice it to say, I feel comfortable stating that my original post stands without need for any modification. And since Tom C chose to question my qualifications for talking about climate dynamics and climate models, I'll let the roughly 90 of my 170+ publications that deal with climate modeling and climate dynamics speak for themselves.

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  10. Mike Mann updated his post on the hot spot.  This is the last time I'm going to copy over his posts and comments.  Please go to his FaceBook page if you want to read and comment more.


    UPDATE to "The Tropical 'Hot Spot' and Global Warming"

    In response to some interesting comments in the comment thread of my previous post, I've decided to add some additional discussion (and two new figures). Figure 1 is a modified version (from Karl et al '09) of a figure that appeared in the CCSP SAP 1.1 report "Temperature Trends in the Lower Atmosphere" in 2006 that compares the PCM model-predicted patterns of response to the various forcings (based on Santer et al 2000). What I like about this particular figure is that the tropopause (border between troposphere and stratosphere) is explicitly shown, which helps to guide the interpretation of the results. The difference in the relative vertical patterns of response to the various forcings is clear here. Greenhouse gases and Anthropogenic Sulphate aerosols show a clear mid-tropospheric amplification. Volcanic forcing does not. Note that the peak warming in the response to greenhouse gases alone (i.e. the center of the "hot spot") is very close to the tropopause, but the response to all forcings combined shows a peak "hot spot" that is well below the tropopause. That is true because the other forcings do not follow the same vertical pattern of response as the GHG forcing alone.

    Figure 2 is from the same report, and it is especially instructive as it shows the response of 4 different models (trend from 1979-1999) to the same imposed late 20th century forcing changes (greenhouse gas increases are the single largest forcing contribution). A "hot spot" of sorts is observed in each case, but note the dramatic variation in the vertical pattern and in the "shape" of the hot spot. GFDL for example has a very well defined hot spot peak at 300mb (and both PCM and CCM3.0 have peaks just above 300mb), while GISS-EH (the model used in the Skeptical Science post mentioned earlier) has a peak that is centered higher up, closer to 200mb. This speaks to the importance of varying convective and cloud parameterizations that govern the dynamical responses. There is no simple, universal response common to all models subject to all forcings. And I will leave it there...

    Figure 1:


    Karl et al., 2009

    Figure 2:

    CCSP SAP 1.1 report

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  11. Tom Curtis @3,

    Looks like Mike Mann did not even change the details of his hypothesis based on your input. And as it turns out MM hypothesis is backed by Ben Santer, your inquiry to the authority of Gavin Schmidt on the subject is unlikely to change anything. Meanwhile Chris Colose offered to run GISS model you claim about volcanic hot spot is based on, to run it and check the apparent discrepancy: we are all eager to see the results, thanks Chris!

    Meanwhile, pardon my punt, I don't see the extraordinary evidence you need to provide in support of your extraordinary claim taking on the top scientists. If particular, your third image is just a hot spot cooling due to a loss of 50% of CO2, as seen in the NASA link you cite. Where is the "1/3 rd Pinatubo level volcanic forcing" picture where you claim "the patterns in the tropics are very similar"? Maybe you've seen but missed that picture. Please provide it for the benefit of us better understanding your point.

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  12. Tom Dayton @9 & 10, Michael Mann writes:

    "Just compare the GHG signal in Santer et al '13 (which shows a hot spot) and the volcanic (which doesn't, and shows a change of sign at 500 mb, which is very close to the 595 mb PEAK of the TMT weighting function--if there were a mid-tropospheric hotspot, it should be seen in TMT)."

    My emphasis.

    Below is a figure showing the trend warming rates from 1979-2012 in the troposphere by altitude for all CMIP5 RCP8.5 models:

    The source of the diagram is John Christy, which is dubious in some respects, but I presume he has not misrepresented the model outputs.  That being the case, the CMIP5 models show a clear upper tropospheric hotspot with a multimodel mean peak warming at 250 mb.  Despite this the trends from 1979-current are:

    GISS 0.158 C/decade

    RSS TLT 0.121 C/decade

    RSS TMT 0.077 C/decade

    RSS TTS 0.011 C/decade (from 1987)

    RSS TLS -0.269 C/decade.

    Clearly the trends decline as the mean altitude of measurement rises, with the TMT trend being less than either the TLT or surface trends, and greater than any trends above it.  It follows that if Mann's claim highlighted above is true, then there is no tropospheric hotspot contrary to the model predictions.  In other words, if Mann's highlighted claim is true, the models have been falsified on a fundamental issue that is vital to estimates of climate sensitivity and future warming.  Michael Mann may want to claim in response that his claim is only applicable to volcanic warming, and not warming in general.  If so that is just the rankest special pleading.  If not he either has to accept my claim @8, point (3), above, or radically revise his reliance on models, and his view as to the dangers of AGW.

    Further, the GISS Model E vIII 1880-2000 all forcing response by altitude is shown below:

    As you can see, the peak warming is around 350 mb (337.5 mb).  That is in fact lower than the CMIP5 multimodel mean, and also lower than the CMIP5 versions of the GISS Model E (six versions, shown with dotted lines of various shades in the first figure above).  That discrepancy may be simply due to the different forcing periods, but if anything the evidence is that the altitude of peak warming has shifted upwards from CMIP3 to CMIP5, not downwards as would be required for the GISS model E images I have been using to be in need of significant correction.  

    As an aside, there is certainly no reason to prefer the data from cone CMIP3 era model, ie the Parallel Climate Model (PCM) as shown in the IPCC reports and used by Mann over another model of the same vintage (GISS Model E vIII).  As a further aside, the peak cooling for the one third Pinatubo forcing shown @3 is also at 337.5 mb).  This point should be irrelevant in that I originally rebutted Mann's claim that,  "if global warming really *were* due to a (natural) decrease in volcanic activity over time ... then we would expect to see an increase in global surface temperatures WITHOUT any mid-tropospheric "hot spot"".  Mann now appears to be trying to make the issue about the exact altitude of peak warming, which shifts the goal posts.  He also misrepresents the altitude of the peak warming (or cooling) in the GISS model, claiming (@10) it is about 140 mb higher in the atmosphere than actually shown by the model.

    3) Fairly abviously, the PCM models shown @10 do not use a period with significant volcanic forcing, with the consequent that any coldspot is too small to register for the volcanic forcing given scale.  The same also applies with sign reversed for the solar forcing.  That is telling in that solar forcing is definitely one of the forcings which does show a hotspot, but shows no hotspot in the PCM figure.  As Mann accepts that solar forcing generates a hotspot, he must attribute the lack of a visible hotspot in the solar forcing panel to the small quantity of warming relative to the temperature scale used.  He cannot therefore consistently argue that that is not also the case with respect to the volcanic forcing.

    4)  Finally, with respect to figure 2 from Santer et al (2013) (see your post @6 above), Santer et al write:

    "Zonal-mean atmospheric temperature trends in CMIP-5 models (A and D–G) and observations (H and I). Trends were calculated after first regridding model and observational TLS, TMT, and TLT anomaly data to a Graphic latitude/longitude grid, and then computing zonal averages. Results are plotted in “MSU space,” at the approximate peaks of the TLS, TMT, and TLT global-mean MSU weighting functions (74, 595, and 740 hPa, respectively)."

    My emphasis.

    It is not explained what "MSU Space" is, although it is almost certainly not a linear function of temperature with altitude.  More likely, at each altitude, the value shown is the weighted average of the TMT, TLS and TLT trends, with the weighting determined by the relative weight of each channel at that altitude as shown in my post @8, although different algorithms with similar effect could also be used.  As such, it loses vertical structure.  That is because it first reduces the vertical structure to just three values, and then tries to recompose it from those three values.  It is, in effect, a complex smoothing of the data.  As such it will not more show the tropospheric hotspot than will the TMT channel (for reasons given above).  What it will show is what will be found by attempts to reconstruct the vertical temperature signal from the MSU or AMSU channel outputs.  That, of course, is very useful for comparison with satellite data, but renders the graphs positively misleading about the detailed vertical temperature structure of the atmosphere.

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  13. chriskoz @11, in my comment @3, the first two images are the images used in the RealClimate post to show that both CO2 and solar forcing result in a tropospheric hotspot.  The third image is, as you show the effect of halving CO2 (after 100 years).  The fourth image, for comparison, is the effect of maintaining 1/3rd Pinatubo forcing for 100 years.

    I would certainly be interested in, and gratefull for, Chris' rerunning of the GISS model for the equivalent experiment.

    I cannot comment on Ben Santer's input as I have not seen it.  Could you provide a link.  Further, if he is commenting on the topic somewhere, and you can respond, could you ask his opinion of my point 4 immediately above.  Specifically, what is the algorithm for "MSU Space"?  Does converting to it have the effect I postulate?  Also does he have any graphs of absolute temperature values with altitude in MSU space as the test of the effect of using MSU space?

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  14. Yay, my first post. As most people here probably know, this isn't the first study to find the hot spot. The hot spot has been found since at least 2004 in the NOAA satellite data analysis. So I wanted to give a brief list of studies that found the hot spot. This list is by no means exhaustive, since it does not include at least 6 other papers that provided evidence of the hot spot. But it should be a helpful list nonetheless.

    Here's the list, along with the data sources for the papers (I think your article is on paper #6):

    In satellite data:
    #1 : "Contribution of stratospheric cooling to satellite-inferred tropospheric temperature trends"
    #2 : "Temperature trends at the surface and in the troposphere"
    #3 : "Removing diurnal cycle contamination in satellite-derived tropospheric temperatures: understanding tropical tropospheric trend discrepancies", table 4
    #4 : "Comparing tropospheric warming in climate models and satellite data", figure 9B

    In radiosonde (weather balloon) data:
    #5 : "Internal variability in simulated and observed tropical tropospheric temperature trends", figures 2c and 4c
    #6 : "Atmospheric changes through 2012 as shown by iteratively homogenized radiosonde temperature and wind data (IUKv2)", figure 1 and 2
    #7 : "New estimates of tropical mean temperature trend profiles from zonal mean historical radiosonde and pilot balloon wind shear observations", figure 9
    #8 : "Reexamining the warming in the tropical upper troposphere: Models versus radiosonde observations", figure 3 and table 1

    In re-analyses:
    #9 : "Detection and analysis of an amplified warming of the Sahara Desert", figure 7
    #10 : "Westward shift of western North Pacific tropical cyclogenesis", figure 4b
    #11 : "Influence of tropical tropopause layer cooling on Atlantic hurricane activity", figure 4
    #12 : "Estimating low-frequency variability and trends in atmospheric temperature using ERA-Interim", figure 23 and page 351

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