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Greenhouse gases are responsible for warming, not the sun

Posted on 14 June 2012 by Bart Verheggen

Also published in European Energy Review (EER) and My view on climate change.

Comment on EER interview with Fritz Vahrenholt

Scientists working on climate on a daily basis must have been rather astonished by the interview with Professor Fritz Vahrenholt (European Energy Review, May 2, free registration required). Vahrenholt, chief of RWE Innogy, self-proclaimed climate expert and author of the book Die Kalte Sonne (The Cold Sun), claims that “the contribution of CO2 to global warming is being exaggerated”. These claims, however, do not stand up to scientific scrutiny. We assess his ideas in the light of the scientific literature on the role of the sun versus other climate forcing factors. The dominant influence of greenhouse gases follows not only from their basic physical properties, but also from their “fingerprint” in the observed warming. The sun, in contrast, has not exhibited any warming trend over the past 50 years. The sun is thus not responsible for the warming seen during this period. Greenhouse gases in all likelihood are.

First of all, we welcome the active participation of the business community in the discussion on climate change. Global warming and its effects may have consequences which business, e.g. the energy sector, should anticipate and adapt to. Furthermore, mitigation policies may affect the competitive advantages and business prospects of a variety of energy options. Investment portfolios should take that into account. That is not an easy task. The business consultant or director developing a climate change response strategy may be overwhelmed by the vast amount of –sometimes conflicting- scientific information available. Luckily, every couple of years an integrated assessment is made by the IPCC, the Intergovernmental Panel on Climate Change, primarily aimed at governments, but also quite valuable for the business community.

Of course, opinions differ regarding how well the IPCC assessment reflects the scientific understanding, with some claiming that IPCC overestimates the human contribution to global warming and the risks it poses, whereas others claim that these are underestimated. However, the main tenets of climate science, as described in working group 1 of the IPCC report, have proven to be robust: New research has confirmed the core conclusions, while the details are continuing to be filled in. Vahrenholt’s claim that the IPCC report is radically wrong is unfounded, and is mostly indicative of his views diverging from mainstream science.

This is not to say that no uncertainties remain; of course there are and in some cases they are inconveniently large. Inconvenient, not only because more research is required to further constrain these uncertainties, but also because the uncertainties go both ways. The human contribution to global warming could be somewhat smaller, or it could be somewhat larger than expected. Focussing only on aspects that downplay the anthropogenic contribution is closing one’s eyes for the whole picture.

Challenging the core tenets of climate science is easier said than done.  Varenholt’s extraordinary claim requires extraordinary evidence. The least one would expect of such a claim is that it be put to the scientific test. Spectacular theories and speculations abound in books and on the internet, but most of them were not offered to the scientific community, or did not stand up to scrutiny if they were. Scientists routinely check each other’s work via the peer review process, which can be seen as a first test of scientific validity. From further discussions in the literature and other scientific fora the relative robustness of competing ideas is assessed.  The most robust idea eventually gains acceptance. That is how science progresses. Surely the peer-review system is not perfect, but at least it is an organised process aimed at filtering what is possibly right from what is plainly wrong.  Such a mechanism is lacking in the public debate; and that adds much to the public confusion about this and other complex scientific topics.

Now, would the ideas of prof. Vahrenholt stand up to scientific scrutiny? On the basis of the interview, we expect: no, they would not. However, we would still encourage him to submit his ideas for scientific review. That is where the physical forces and feedbacks in the climate system should be discussed.  On the other hand, questions on how society and politics should respond cannot be answered by science, but should be discussed in the public and political debate. Unfortunately, Vahrenholt’s accusations like “we are being misinformed by the climate establishment” and “the whole purpose of the IPCC has been to get rid of the so-called Medieval Warm Period” betray him as being receptive to conspiracy theories, which are routinely echoed on the internet. In such a world view, any criticism by the scientific mainstream is of course only perceived as proof that his and similar views are being suppressed. This is often used as an excuse to not even try to submit one’s ideas to peer review. His book is criticized by scientists not because it would be politically incorrect, as Vahrenholt assumes, but because it is scientifically incorrect.

Now, if we take a closer look at the content of Vahrenholt’s theory, his key statements appear to be the following:

  1. The ‘hockey stick graph’ showing that global average temperatures are now higher than in the last 1500 or so years is flawed. The medieval warm period was about as warm as today, the Little Ice Age was globally very cold and both periods coincide with a more, respectively less active sun. Therefore, the IPCC underestimates the natural variations of our climate system.
  2. The IPCC fails to look at all the solar characteristics, focusing on sunspots and Total Solar Irradiation (TSI) only, while both fluctuations in the magnetic field and the amplifying feedbacks should also be taken into account.
  3. The sun has become more active over the past few hundred years, being responsible for a large part of the observed warming.
  4. The IPCC has underestimated the natural 60 years climate cycle dominated by the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO).
  5. Global warming has stopped since around 15 years.

On this basis, Vahrenholt claims that more warming is due to the sun and natural variability than the IPCC estimates, which automatically implies that less is due to CO2 than the IPCC says. 

To begin with the last statement, the idea that global warming has stopped is a common misunderstanding, based on confusing short term variability with the long term underlying trend. Natural fluctuations, as caused by e.g. the solar cycle, volcanic eruptions, or by the El Nino/La Nina phenomenon, can mask the underlying trend in surface temperatures for more than a decade. When these effects are accounted for, as e.g. Foster and Rahmstorf did in 2011, the underlying trend is seen to continue unabated. Moreover, other measurements, e.g. of ice extent and of ocean heat content, confirm that global warming is continuing.

The solar activity has been well-measured, particularly since 1979 using satellites, and before that by indirect (“proxy”) measures. From these, the solar activity is seen to have been relatively stable over the past 50 years. That means, that even if amplified strongly, the sun’s variations could still not explain the strong global warming that started halfway the 1970’s. Measurements of cosmic rays, a favourite candidate for a solar amplification mechanism, also show no trend since at least 50 years. The robust evidence needed to become a serious scientific competitor for the dominant greenhouse mechanism is sorely lacking. It is true however, that the sun gained strength over the first half of the 20th century, and thus contributed to warming seen during that time, as is also described in the IPCC report.

Figure showing temperature, CO2 and solar activity for the past 150 years, discussed in more detail here.

Various solar and climate physicists, like Lockwood, Haigh, Gray and others have published analyses indicating that the solar influence in the warming of the last half century is low or absent. These analyses include the magnetic field effects, which – in contrast to what Vahrenholt is saying – are not neglected by the IPCC. A few years ago, Pierce and Adams modeled the potential cloud forming effect of cosmic rays and found it wanting by more than an order of magnitude, even when the most favourable assumptions possible were made.

There is another indication to the statement that the sun’s role in warming is limited compared to the role of greenhouse gases: fingerprints. Each possible source of warming leaves a specific and characteristic fingerprint. For example, a solar fingerprint would be: warming throughout the atmosphere. A greenhouse gas fingerprint would be: warming at lower altitudes with simultaneous cooling of the stratosphere higher aloft, since in case of enhanced CO2 the stratosphere loses more infrared radiation than it receives from below. This is mainly a consequence of the temperature structure of the stratosphere. And guess what? Measurements clearly show a greenhouse gas fingerprint, not a solar one.

In addition, Vahrenholt claims that IPCC has underestimated natural variability, in particular a natural 60 year climate cycle manifesting in the purported Pacific Decadal Oscillation (PDO) and in the Atlantic Multidecadal Oscillation (AMO). However, Vahrenholt’s statement is based on curve fitting applied to a finite time series of (local) temperatures. It is well known that curve fitting to a series of chaotic signals can lead to apparent periodicities, but that these have no value for prediction unless they are supported by an underlying physical understanding. 

From this quick analysis, it seems unlikely that Vahrenholt’s claims would stand up to scientific scrutiny. They should be taken seriously, but only as an idea that deserves further research and assessment, rather than as a scientific fact or theory that rises to the level of robustness of basic climate science. In that respect, the physical science basis of the IPCC 2007 is still strong. Vahrenholt’s ideas do not change that conclusion.

Dr. Rob van Dorland (KNMI)

Dr. Bart Verheggen (PBL)

Postscript: Fritz Vahrenholt and his book co-author Sebastian Lüning responded at EER to our comment. Our complete reply to their response is published at Bart's blog.  Also see the previous SkS response to Vahrenholt.

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Comments 1 to 14:

  1. Just finished reading Bart's original paper, Varenholt's response, then Bart's response to that, and I must say, it's refreshing to see a thinly-disguised denier at least make some attempt to seem reasonable. Bart's rebuttal was precise, entirely devoid of anything even remotely smacking of *argumentum ad hominem* and to the point. Well-done, Bart! It is, at least in part, a shame that so many of us on the 'science-y' side have to spend so much time refuting these contrarians; a friend of mine at JPL-NASA does this constantly. I've learned so much here, and from him, that it is difficult to heap praise high enough! Thanks to the moderators here at SkS who do such a yeoman job, the contributors who bring ever-greater depth to the subject, and thanks to Drs. van Dorland and Verheggen for making clear how to combat these attacks on rational thought.
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  2. Thanks vrooomie. Glad to know our response landed well.
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  3. Just a grammar note: I suggest retitling this article to "Greenhouse gases, not the sun, are responsible for warming." The current title implies you're trying to argue that greenhouse gases are not responsible for warming the sun, which may be true but isn't the real subject.
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  4. I think that fingerprinting is a powerful attribution tool, but occasionally I run into someone who claims that the GHG signatures (upper atmospheric cooling, night/day and winter/summer trends) are caused by increased humidity, which is ultimately caused by the sun. Of course, this is contradicted by observations of the solar activity, but I am curious if there are other problems with this hypothesis? One answer might be that model results don't show the same fingerprints from solar forcing, even though they do show the effects of increased humidity (eg, ). But then again such people tend to dismiss model results. Any other ideas?
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  5. Hmmm, difficult stay away from political comments when Heartland is, at heart (sorry), a political institution; set up by big business and big oil, to look after their interests, and with the stated aim of casting doubt in the minds of the public about AGW. In order to do this, they hire scientists willing to plead their cause. Sadly, since the science is so rock solid, the only way they can do this is by twisting the science itself, by a combination of 'errors', 'mistakes', misrepresentation, and in some cases, outright falsification. These aren't accidental mistakes or errors. They are deliberate, so no way are these guys going to retract. They would lose their funding from Heartland. Hence the same scientists keep making the same sorts of 'errors'. They get called on it, refuse to admit it, then get called on it again in some other context, refuse to admit it, etc, and so the merry *Gish Gallop' continues. For example Bob Carter, the serial cherry picker, gets called on it time after time, but still we get 'GW stopped in 1998', or 'slight decline since 2002', or 'no warming since (insert future date here)'. Still he continues, because he is not out to persuade fellow scientists, he is out to bamboozle the general public, who are, in the main, scientifically challenged and do not even see the various cunning tricks that have been played on them. So while these 'scientists' need to be held to account scientifically, (and you guys are doing a great job), the actual battle is in the political area. The scientific battle was won long ago.
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  6. Just to hone a terse anti denialist argument for students, since the fingerprint of increased solar incident flux causing warming is warming throughout the atmosphere and the fingerprint for increased GHG warming is cooling in the stratosphere whilst warming on the surface, does this alone not shoot down the whole "The present is a reprise of the Med Warm period?" skeptic argument?
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  7. Wasn't sure where to put my query. This thread seems appropriate.

    A greenhouse gas fingerprint would be: warming at lower altitudes with simultaneous cooling of the stratosphere higher aloft, since in case of enhanced CO2 the stratosphere loses more infrared radiation than it receives from below. This is mainly a consequence of the temperature structure of the stratosphere. And guess what? Measurements clearly show a greenhouse gas fingerprint, not a solar one.

    While trying to explain why enhanced tropical tropospheric warming (the hotspot) is not a unique feature og greenhouse warming, I started wondering about the 'fingerprint' of stratospheric cooling under greenhouse warming.

    According to models/physics, warming from any source should cause increased water vapour in the atmosphere, which roughly doubles or triples the contribution from a doubling of CO2. Water vapour is a powerful greenhouse gas. If "enhanced CO2' causes cooling of the lower stratosphere by 'trapping' infrared radiation, why would we not expect the same effect from the enhancement of another greenhouse gas - water vapour?

    IOW, why would the stratospheric signature (cooling) not be seen with warming from other sources - like solar - when enhanced water vapour is anticipated regardless of the source of warming? Does water vapour mix differently in the atmosphere? Or is it that the cooling signature should only become evident from the combined radiative impact aloft of both water vapour and CO2?

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  8. Barry @7, the difference between the stratosphere and the troposphere is convection.  Because of the thinness of the atmosphere in the stratosphere, convection transfers energy far slower than does radiation, with the result that very little convection occurs.  The point in the atmosphere where this breakdown in convection occurs is the tropopause, a region of the atmosphere lying between the troposphere and the stratosphere.  Crucially, the temperature at the tropopause is well below freezing.  That means any water carried to the tropopause in its liquid form (as for example, in a thunderstorm) quickly freezes, and is unable to be carried to a greater altitude by diffusion.  It is also, of course, unable to be carried to a greater altitude by convection, which goes no higher.  This prevents the increased absolute humidity in the troposphere from resulting in an increased humidity in the stratosphere, preserving the unique greenhouse temperature signature.

    It should be noted that there has been a slight increase in humidity in the stratosphere, not from increased humidity in the troposphere, but as a waste product from the combustion of jet fuel.  As it is a direct anthropogenic effect, and is not temperature dependent for its strength, it is a forcing rather than a feedback and is included in IPCC modelling.

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  9. Tom, thanks for the reply.

    I might be a bit dim here, but you appear to be answering the question "why do we not see enhanced warming in the stratosphere from water vapour?"

    I'm interested in the effect of stratospheric cooling as a well-known signature of surface/tropospheric warming from enhanced CO2 in the troposphere.

    Water vapour, like CO2, is a greenhouse gas. Why would not enhanced water vapour in the troposphere - which amplifies surface warming from any source (WV feedback) - also cause the lower stratosphere to cool? On short time scales, wouldn't we see this effect with a strong persistent el Nino providing warming at the surface with attendant water vapour increase? I guess solar forcing would both heat the stratosphere and enhance the greenhouse cooling effect on the lower strat through increased water vapour, acting as a kind of negative feedback on stratospheric temps. I assume the solar heating of the stratosphere would outweigh the cooling effect on it from increased water vapour in the troposphere. But I've never seen that particular point addressed, so I'm curious. Makes me wonder about the this particular effect when the surface is warmed from different forcings.

    Slightly orthoganolly perhaps, the modeled impact of ozone depletion over the last century or so shows warming of the surface and cooling of the stratosphere. [ panel d) ]

    Which might make attribution of stratospheric cooling a bit complicated?

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  10. Tsk, spelling errors. I really need to keep more sensible hours.

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  11. Barry, some of what you may be looking for might be found here in the comment stream.

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  12. barry @9, you may be being misled by the explanation of stratospheric cooling in the main article here, which is IMO incorrect.  Essentially, the stratosphere is warmed by UV radiation absorbed by O3.  It is cooled by IR radiation from CO2, O3, and to a lesser extent, H2O and CH4.  If you double the quantity of CO2 and hold all else equal, you will effectively double the IR radiation from CO2.  That is because nearly all of the IR radiation emitted in the stratosphere either escapes to space or to the troposphere due to the thin atmosphere in the stratosphere.  Doubling the IR radiation results in more energy being radiated than is absorbed, thereby cooling the stratosphere until the IR radiation emitted matches that prior to the doubling of CO2.

    In the explanation given in the main post, it is assumed that increasing CO2 in the tropophere will reduce IR emissions from the troposphere to the stratosphere (which is true).  What is ignored is that the CO2 in the stratosphere also doubles, meaning it will absorb (approximately) twice as much radiation for a given amount of incoming radiation.  This effect approximately compensates for (and may more than compensate for) the reduced IR emission from the stratosphere.

    The question arises, could the mechanism described in the main post actually result in a decreased temperature in the stratosphere from reduced emission by H2O in the troposphere.  The answer is no.  H2O is restricted to too low an altitude by condensation due to falling temperatures.

    Below is a graph showing the cooling trends induced by various gases at various levels of the atmosphere.  Note, positive numbers represent a cooling trend.  Also note, this is produced by a Line By Line model, and as such presents straightforward physics that is very well understood; but it does not show any compensating adjustments to restore radiative equilibrium.

    You will notice the very strong cooling effect (deep blues and reds) caused by H20 in the upper troposphere, with altitude depending on the strength of absorption.  That cooling is likely caused by the reduced IR radiation from lower levels due to increased H2O which is not fully compensated at upper levels by increased absorption because the condensation of H2O limits the increase in watervapour at those levels.  In sharp contrast, the cooling in the troposphere from increased CO2 is limited because the CO2 concentration increases approximately equally at all levels.  There is even a slight warming at the tropopause where radiation that previously escaped to space is now traped in the atmosphere.

    In the stratosphere itself, the cooling effect of CO2 rises to the mesopause (the highest temperature region of the stratosphere), then falls to the thermopause, before rising again.  That is, it follows the temperature curve of the local gas - something predicted by the explanation I give above, but not predicted by the explanation given in the main post.  

    Although much weaker, a similar pattern can be seen in the stratospheric cooling from H2O.  That would be expected if the stratospheric cooling effect of H2O were due to increase H2O in the stratosphere from the combustion of fuels by jets in the stratosphere rather than from a shading effect from increased H2O in the troposphere.  (As noted in my post @8, increased tropospheric H2O cannot cause increased stratospheric H2O.) 

    Finally, and crucially, we see a different pattern from O3.  Reduced O3 will cause a warming of the stratosphere by the same mechanism that increased CO2 causes a warming (and with the same temperature dependent pattern).  However, the far stronger effect is a decreased absorption of UV radiation in the upper stratosphere, which cools by reducing the energy input.  That is accompanied by an increased absorption in the lower stratosphere as more UV reaches those levels (due to not being absorbed at a higher level).  The net result is a pattern of strong cooling in the mesophere and upper stratosphere, and a warming in the lower stratosphere and upper troposphere.  Due to the thinness of the atmosphere, and hence very low initial absorption of UV radiation (I presume), it appears that in the thermosphere the warming effect of reduced O3 due to reduced IR emissions dominates over the cooling effect due to reduced UV emissions.

    The distinctive cooling and warming patterns of CO2 and O3 with altituded allow us to see that there is a strong CO2 cooling in the stratosphere.  We can see that because cooling is weak in the lower stratosphere where CO2 and O3 have opposite effects, but very strong in the upper stratosphere where both cause a cooling trend.  Absent the effect of CO2, the stratosphere would cool in the upper stratosphere and warm in the lower stratosphere.  

    Interestingly, a third possible cause of a cooling stratosphere is reduced reflection of visible sunlight due to reduced aerosols or clouds.  That, however, would have its strongest effect in the lower stratosphere, and scale of rapidly with decreasing atmospheric density.  Thus the distinctive pattern of CO2 cooling in the stratosphere allows us to be certain that it is a major factor in the cooling of the stratosphere.


    Finaly, you will note in the stratospheric temperatures above any sign of the 1997 El Nino, while volcanic erruptions are clearly visible as a positive (warming) temperature spike in the lower stratosphere.  That spike is weakest where the negative trend is strongest showing clearly that the dominant influences on stratospheric cooling are reduced O3 and increased CO2.

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  13. Tom (and DSL), thanks for going to the trouble. I'll have to read this and the linked comments section carefully and more than once to get a grip on it.




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  14. The WUWT web site is championing the following paper as a possible "refutation" of AGW.  Has anyone read it?

    ZHAO X H, FENG X S. Periodicities of solar activity and the surface temperature variation of the Earth and their correlations (in Chinese). Chin Sci Bull (Chin Ver), 2014, 59: 1284, doi: 10.1360/972013-1089

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