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.