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 20^th 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.