Vahrenholt: I don't claim that I know precisely whether the sun is responsible for a 40, 50 or 60 percent share of global warming. But it's nonsense for the IPCC to claim that the sun has nothing to do with it.
SPIEGEL: On balance, you predict a global cooling of 0.2 to 0.3 degrees Celsius by 2035. Why such a risky prediction?
Vahrenholt: If you want to revitalize the deadlocked debate, you have to have the courage to name a number. And we derive this number from scientific studies on climate history to date.
SPIEGEL: So your contention that we are wrong about global warming is merely a provocation?
Vahrenholt: No. I mean it very seriously, and I know that dozens of solar researchers agree with me. I am perfectly aware of the defamation I will have to listen to in the near future. The climate debate also has some of the trappings of an inquisition. I'm curious to see which truth ministry will now initiate proceedings against me. Perhaps it'll be the Potsdam Institute for Climate Impact Research, which is headed by Hans Joachim Schellnhuber, an adviser to the chancellor.
SPIEGEL: You claim that the standstill in global warming since 2000 has been caused in large part by a simultaneous decline in solar activity. But, in fact, the sun behaved relatively normally until the middle of the century, only becoming noticeably quieter after that. How does this fit together?
Vahrenholt: There are two effects: the declining solar activity, as well as the fluctuations in ocean currents, such as the 60-year Pacific oscillation, which was in a positive warm phase from 1977 to 2000 and, since 2000, has led to cooling as a result of its decline. Their contribution to the change in temperature has also been wrongly attributed to CO2. Most of all, however, the last sunspot cycle was weaker than the one before it. This is why the sun's magnetic field has continued to weaken since 2000. As a result, this magnetic field doesn't shield us against cosmic radiation quite as well, which in turn leads to stronger cloud formation and, therefore, cooling. What else has to happen before the IPCC at least mentions these relationships in its reports?
SPIEGEL: What you neglect to mention is that it hasn't been proven yet that cosmic radiation, which is shielded by the sun at varying degrees of effectiveness, truly leads to more cooling clouds on Earth. So far, it is only a hypothesis.
Vahrenholt: It's more than that. The Cloud Experiment, headed by physicist Jasper Kirkby, has been underway at the CERN particle research center near Geneva since 2006. The initial results of tests conducted in a chamber in which the earth's atmosphere was simulated showed that cosmic particles do indeed lead to the formation of aerosol particles for clouds.
SPIEGEL: But the aerosols demonstrated in the Cloud Experiment are much too small. They would have to grow before they could actually serve as condensation germs for clouds. Whether this happens in nature is still an open question. You present this as a fact.
Vahrenholt: You will find many correlations between cloud cover and cosmic radiation in the book. I'd like to know why the IPCC doesn't thoroughly examine this mechanism. My guess is that the answer to this question would jeopardize the entire foundation of the IPCC predictions.
SPIEGEL: Nevertheless, you should be more careful with prognoses on future solar activity. In 2009, US scientists predicted that there would be no sunspots for years. In fact, they returned in 2010. The truth is that we are experiencing rather normal solar activity at the moment.
Vahrenholt: The solar cycle is everything but normal. NASA scientists predict that this cycle will indeed be the weakest of the last 80 years. Not only did it start two years too late, but it's also very weak. And, besides, you can't just count sunspots. Cosmic particles continue to rain down on us because the sun's magnetic field is hardly shielding us.
SPIEGEL: It's true that there will be a large solar minimum sometime in the next 500 years. But no one knows exactly when. The probability that this will occur in the next 40 years is less than 10 percent. But, in your book, you predict: "It is clear that the sun will be responsible for colder periods in the first half of this century." Do you know more than all astrophysicists combined?
Vahrenholt: The probability of a large solar minimum, as it occurred during the Little Ice Age, is indeed less than 10 percent. But we are at the beginning of a lighter decline in solar activity of the kind we see every 87 and every 210 years. I've spoken with many solar physicists who expect this to happen.
SPIEGEL: We know many other solar scientists who question this. Another maximum is just as statistically likely as a minimum. Predicting what the sun will do in the coming decades borders on fortune-telling.
Vahrenholt: I know only one German solar scientist who has expressed such doubt. Various American and British solar research groups believe that weak solar cycles are ahead. I take this seriously and expect only cooling from the sun until 2050.
SPIEGEL: And what will you do if temperatures continue to rise, after all?
Vahrenholt: Then I'll give SPIEGEL an interview in 2020 and publicly admit that I've made a mistake. But I'm convinced that it won't be necessary.
SPIEGEL: Do you seriously believe that all 2,000 scientists involved in the IPCC are deluded or staying true to the official line?
Vahrenholt: It's not like that. However, I am critical of the role played by the handful of lead authors who take on the final editing of the report. They claim that they are using 18,000 publications evaluated by their peers. But 5,000 of them are so-called gray literature, which are not peer-reviewed sources. These mistakes come out in the end, just like the absurd claim that there will no longer be any glaciers in the Himalayas in 30 years. Such exaggerations don't surprise me. Of the 34 supposedly independent members who write the synthesis report for politicians, almost a third are associated with environmental organizations like Greenpeace or the WWF. Strange, isn't it?
SPIEGEL: Why are you taking on the role of the climate rebel with such passion? Where does this rage come from?
Vahrenholt: For years, I disseminated the hypotheses of the IPCC, and I feel duped. Renewable energy is near and dear to me, and I've been fighting for its expansion for more than 30 years. My concern is that if citizens discover that the people who warn of a climate disaster are only telling half the truth, they will no longer be prepared to pay higher electricity costs for wind and solar (energy). Then the conversion of our energy supply will lack the necessary acceptance.
SPIEGEL: If we take your book to its logical conclusion, it will be unnecessary to reduce CO2 emissions at all.
Vahrenholt: No. Even a temperature increase of only one degree would be a noticeable change. But I am indeed saying that climate change is manageable because the cooling effects of the sun and the ocean currents give us enough time to prepare. In any case, it will be easy for us in Germany to adjust.
SPIEGEL: So, is it a mistake to concentrate exclusively on the reduction of carbon dioxide?
Vahrenholt: Yes. In addition to carbon dioxide, we also have black soot, for example. It creates 55 percent of the warming effect of CO2, but it could be filtered out with little effort within a few years, especially in emerging and developing countries. And, in doing so, we would achieve huge benefits for human health.
SPIEGEL: Would the expansion of wind energy have proceeded as quickly without concerns over the climate?
Vahrenholt: It was a driving force. But it was mainly engineering skills that brought wind energy to a profitable level. Again, I want us to continue stressing renewable energy, which we have to make competitive. I just think that we should proceed sensibly: Wind power and biomass are fine in Germany, but no solar panels, please! They're better off in Africa and southern Europe. It's crazy to install 50 percent of worldwide solar panels in "Solar Country Germany" for fear of the supposed climate disaster and to spend 8 billion ($10.4 billion) a year on it!
SPIEGEL: But aren't you shooting yourself in the foot when you say that climate change isn't really that bad? How do you intend to keep justifying emissions trading if you feel that greenhouse gases are irrelevant?
Vahrenholt: All I'm saying is that CO2 is a climate gas, but that its effect is only half as strong as the IPCC claims. Nevertheless, we still have to reduce CO2 emissions through worldwide emissions trading. And there are also other reasons to burn fewer fossil fuels. We don't have that much coal, oil and gas left in the world, so we have to economize more. We also have to become less dependent on imports from totalitarian countries.
SPIEGEL: Surveys show that fear of the climate catastrophe has declined. Are you preaching to the choir with your all-clear?
Vahrenholt: The fear mongers are still shaping the political debate. According to the German Advisory Council on Global Change, environmentally minded countries should forcibly bring about reduced consumption for the sake of protecting the climate. This takes us in the direction of an environmental dictatorship. And the fearmongering is also beginning to take effect. When I was in a restaurant recently, I overheard a woman at the next table telling her children that it's wrong to eat an Argentine steak -- because of the climate. That's when I ask myself: How could we have come to this point?
SPIEGEL: Mr. Vahrenholt, thank you for this interview.