Surprisingly, a book co-written by a well-known former German  environmental activist presents a strong revisionist story and is able  to capture both headlines and spot on the best-seller list. I read the book “The  cold sun: why the climate catastrophe is not happen” by Fritz  Vahrenholt and Sebastian Lüning (VL) for a coherent revisionist  argument.

The book was written with strong conviction and a very pervasive  style. It is richly illustrated with charts displaying the time series  of climatic and other variables. Not satisfied with the inability of  climate models to predict shorter term climate variability on an annual  to decadal scale, VL argue that the influence of solar activity and  cosmic rays has been underestimated and are responsible for much of the  warming observed from 1980 to 2000. As a trained chemist, V understands  the absorption of heat radiation by CO2 and acknowledges the greenhouse  effect caused by it (ca. 1.1 °C for a doubling of the CO2  concentration). However, VL dispute feedbacks that produce a stronger  climate effect and claims that temperature variations are driven mainly  by solar activity. The core of the argument:

  1. No water-vapor feedback. In climate models, the warming caused by  additional CO2 leads to the increased evaporation of water and a higher  concentration of heat-trapping water vapor in the atmosphere. Through  the water-vapor feedback mechanism, the warming effect of a doubling of  the CO2 concentration is commonly assumed to be 2.8-4.5°C instead of  1.1°C for the direct effect of CO2.  VL dispute the existence of the  water vapor feedback mechanism. Without such a feedback, alternative  explanations are needed for the already observed temperature increase.  VL argue that such an explanation is offered by an increased solar  activity since 1900, measured in the increase in total solar energy  reaching the earth and especially increased UV radiation in the number  of sunspots, the doubling of the solar magnetic field, and the resulting  reduction by 20% in the cosmic radiation which is shielded from earth  through the increased magnetic field. To explain the increased  temperature, they offer, in particular, two potential mechanisms.
  2. Increased UV radiation. The sun’s total energy output varies  cyclically by ca. 0.1%, where the strongest cycle is the 11 year cycle.  VL recognize that the variation in energy output cause only tiny  variations in global temperature. (1/4 of 0.1% of the global avg.  temperature following the Stefan-Boltzmann law,  or 0.07°C ). The 10% of the total energy flux emitted in the  ultraviolet part of the solar spectrum, in the meantime, varies by as  much as 2-3%. VL now suggest that the effect of the UV radiation  explains the observed increase in surface temperature.
  3. Less cloud-seeding through reduced cosmic radiation. Low clouds can  have a cooling effect on the surface. VL suggest that reduced cosmic  radiation due to an increased solar magnetic field causes the formation  of fewer cloud condensation nuclei, which again cause fewer clouds and  hence an increased temperature.

Water vapor feedback examined

I was quite surprised by the bold claim that the water vapor feedback  mechanism does not exist. Already in Middle School I had learned about  the strong temperature dependence of the ability of air to hold water and  the need to define a relative humidity, which expresses the water vapor  content of air relative to that capacity. When the relative humidity  reaches 100%, water condenses and clouds form. It is not only that a  warmer atmosphere can hold more water, the evaporation rate also  increases with increased temperature, ensuring the production of more water vapor.  For there to be no water vapor feedback, relative humidity must  decrease as climate warms. Mechanisms must exist to explain the quicker  condensation of water vapor in a relatively dryer atmosphere. Even  though the lack of a water vapor feedback is central to their  hypothesis, the claim that a warmer atmosphere will not hold more water  is supported only with reference to three scientific papers. One paper  reports measurement of the water vapor in the stratosphere over Bolder,  Colorado, which shows a reduction of water vapor since 2000. The  stratosphere, lying above most of the greenhouse gases, is not expected  to have increased temperatures as a result of the greenhouse effect. The  second paper cited,Lindzen and Choi,  does not explicitly address the water vapor feedback but feedbacks more  broadly and offers no basis at all for dismissing the existence of the  water vapor feedback (For a discussion, see here). Only one of  the three papers actually reports measurements of water vapor (by  radiosondes) and sets it in connection with temperature, providing some  support to VL’s theory. VL neglect to tell the reader that the paper  itself reports contradicting satellite measurements and cautions the  reader regarding the interpretation of the results. VL also fail to  inform their readers of many other measurements thatconform to the theory of a water vapor feedback also in the tropics, e.g. fromsatellites. A recent review, while acknowledging remaining uncertainty, supports a water-vapor feedback.
Alternative Explanations for Warming

Given that the solar cycles, while very regular, lead only to tiny  variations in the total solar output which, based on an energy balance,  will have very little influence on global temperature, VL suggest the  variation in certain UV wavelengths or in the magnetic field could be  responsible for the observed temperature changes.
The problem with UV radiation is that it is mostly absorbed in the  stratosphere, 10-50 km above ground, as VL recognize. Therefore,  additional energy emitted as UV is hence not amplified by the greenhouse  effect. VL do not present any plausible mechanism by which increased UV  radiation could influence the climate more strongly than accounted for  in the present climate models. Of course, increased UV radiation  increases the concentration of ozone, a strong greenhouse gas. However,  the increases are too small to have a significant effect and changes in  the ozone concentration are included in climate models.Measurements show  that the ozone concentration has gone down 1980-2000, the period of the  most rapid warming, due to a combination of the effect of  ozone-depleting chemicals (by 3%) and the solar cycle (by 1.5%). The  amplification and feedbacks VL suggest for UV are the same feedbacks  which they deny for CO2. The suggested feedback mechanisms amplifying  the effect of UV radiation directly contradict their discussion of cloud  cover by Lindzen and Choi.

The seeding of clouds by cosmic rays has been suggested as an  important driver for the global climate by Svensmark. Potential  mechanisms behind this theory are currently being investigated by the CLOUD experiment  as CERN and by a number of other scientists. VL present Svensmark’s  hypothesis as already confirmed. This is not the case, and the CLOUD  experiment at CERN has been set up to test Svensmark’s hypothesis.

There are two potential mechanisms discussed by atmospheric scientists by which cosmic rays could influence cloud formation and hence temperature. TheCLOUD results published  so far investigated only a single connection in a long causal chain:  Whether ions formed from cosmic rays could influence the formation of  cloud condensation nuclei. Yes, there is an influence. Further steps  need to be investigated, such as whether an increased number of cloud  condensation nuclei lead to the formation of more cloud droplets and how  this could potentially influence cloud properties. The experiment at  CERN also showed that the concentration of pollutants such as sulfuric  acid, and ammonia has a large influence on the formation of cloud  condensation nuclei.
If cosmic radiation had a significant influence on the climate through  the mechanism investigated in the CLOUD experiment, the changes in  emissions of conventional pollutants in the 20th century must have had a  strong influence on the climate as well. None of the regression charts  presented by VL includes an influence of air pollution, and nor does  their alternative explanation.

On close inspection of VL’s figures, abstracting away suggestive  trend lines and arrows, one can in fact see that there can be no strong  correlation between cosmic rays and global temperature. The figure shown  above presents a smoothed development of temperature and cosmic rays  (from a discussion of the techniques for data presentation and analysis used by VL).Other observations contradicting Svensmark’s theory have recently been published.

No Coherent Alternative Theory

In “The cold sun: Why the climate catastrophe is not happening”,  Vahrenholt and Lüning present a new truth. Through a very selective and  biased presentation of the available scientific literature and through  the extensive use of suggestive correlations in absence of known  mechanisms, they give the impression to possess a new theory that can  better explain the evolution of the climate of the past century. Looking  at the underlying science, I find mechanisms by which the sun could  potentially influence the climate as strongly as suggested are not well  understood and most likely not as effective as suggested. The level of  uncertainty about the processes addressed by VL is much higher than the  uncertainty about the effect of CO2 and greenhouse gases. The level of  cosmic radiation has oscillated but not decreased over the past 50 years and as such does not present a likely cause for the warming.

Despite its best efforts and effective rhetoric, the book is not able  challenge the role of CO2 and other anthropogenic factors as a  important factors influencing our climate.