The case of Vahrenholt and Lüning is a clear-cut one: They've handpicked the theories that best back their thesis -- the primary one being that the sun has been getting weaker since 2005 and will continue to do so in the coming decades. Together with other natural influences, this cooling sun will supposedly lessen the warming effect created by man-made greenhouse gas emissions.
Vahrenholt's thesis isn't exactly fresh out of the oven: Five years ago, Danish sun researcher Henrik Svensmark of the Danish National Space Institute wrote about the theory of the impending cooling in his book "The Chilling Stars." Svensmark, together with other sun researchers, back the theory that the impact of the sun on the climate has been underestimated. The experts' argument mainly involves the sun's indirect effects, primarily the claim that the sun's rays control cloud formation.
These critics of the prevailing climate science may indeed be right, and their theories may be plausible, but practical evidence is lacking. The IPCC classifies knowledge about the extent of the sun's effect on the climate as "low" to "very low." But it's not as if the body is trying to avoid the topic.
Are the Answers in the Pacific?
Experts are divided over predictions that activity from the sun will diminish. Some are indeed expecting a weak sun phase beginning in 2020. Still, most studies indicate that even something like the Maunder Minimum -- which is believed to have caused the "Little Ice Age" in the Middle Ages -- would result in a cooling of the global average temperature at ground level by a maximum of half a degree. However, the output of greenhouse gases could significantly surpass the effect. The UN climate report is open about this, saying there's a "low" level of knowledge in this area.
Vahrenholt and Lüning, on the other hand, are sticking to their bold thesis, saying several solar cycles will lead to a minimum. "Various American and British solar research groups believe that weak solar cycles are ahead," Vahrenholt told SPIEGEL. What he neglects to say is that the effects of the different cycles on the climate are not well known. What makes it even more difficult is that in the last century the Earth's temperature often did not correspond to the sun's strength.
That's why the two authors maintain that the cyclical changes in ocean currents will also ensure lower temperatures. For example, they cite the Pacific Decadal Oscillation, which has the capacity to significantly change the global climate over a period of many years. However, the Pacific climate cycle has not run parallel with the global average temperature over much of the 20th century either. Why should it do so in the future? Ultimately, Vahrenholt and Lünging are doing little more than guessing what might happen with the climate.
The Myth of Consensus
On the other side of the debate are a number of prominent climate researchers who always repeat the same mantra, namely that the scientific community has long been united that Planet Earth is facing dangerous warming. For the most part, there is also consensus in the view that humans are heating up the climate through the emission of greenhouse gases. But the extent to which that is happening, as well as the expected consequences, are both disputed.