SPIEGEL: Just since the turn of the millennium, humanity has emitted another 400 billion metric tons of CO2 into the atmosphere, yet temperatures haven't risen in nearly 15 years. What can explain this?
Storch: So far, no one has been able to provide a compelling answer to why climate change seems to be taking a break. We're facing a puzzle. Recent CO2 emissions have actually risen even more steeply than we feared. As a result, according to most climate models, we should have seen temperatures rise by around 0.25 degrees Celsius (0.45 degrees Fahrenheit) over the past 10 years. That hasn't happened. In fact, the increase over the last 15 years was just 0.06 degrees Celsius (0.11 degrees Fahrenheit) -- a value very close to zero. This is a serious scientific problem that the Intergovernmental Panel on Climate Change (IPCC) will have to confront when it presents its next Assessment Report late next year.
SPIEGEL: Do the computer models with which physicists simulate the future climate ever show the sort of long standstill in temperature change that we're observing right now?
Storch: Yes, but only extremely rarely. At my institute, we analyzed how often such a 15-year stagnation in global warming occurred in the simulations. The answer was: in under 2 percent of all the times we ran the simulation. In other words, over 98 percent of forecasts show CO2 emissions as high as we have had in recent years leading to more of a temperature increase.
SPIEGEL: How long will it still be possible to reconcile such a pause in global warming with established climate forecasts?
Storch: If things continue as they have been, in five years, at the latest, we will need to acknowledge that something is fundamentally wrong with our climate models. A 20-year pause in global warming does not occur in a single modeled scenario. But even today, we are finding it very difficult to reconcile actual temperature trends with our expectations.
SPIEGEL: What could be wrong with the models?
Storch: There are two conceivable explanations -- and neither is very pleasant for us. The first possibility is that less global warming is occurring than expected because greenhouse gases, especially CO2, have less of an effect than we have assumed. This wouldn't mean that there is no man-made greenhouse effect, but simply that our effect on climate events is not as great as we have believed. The other possibility is that, in our simulations, we have underestimated how much the climate fluctuates owing to natural causes.
SPIEGEL: That sounds quite embarrassing for your profession, if you have to go back and adjust your models to fit with reality…
Storch: Why? That's how the process of scientific discovery works. There is no last word in research, and that includes climate research. It's never the truth that we offer, but only our best possible approximation of reality. But that often gets forgotten in the way the public perceives and describes our work.
SPIEGEL: But it has been climate researchers themselves who have feigned a degree of certainty even though it doesn't actually exist. For example, the IPCC announced with 95 percent certainty that humans contribute to climate change.
Storch: And there are good reasons for that statement. We could no longer explain the considerable rise in global temperatures observed between the early 1970s and the late 1990s with natural causes. My team at the Max Planck Institute for Meteorology, in Hamburg, was able to provide evidence in 1995 of humans' influence on climate events. Of course, that evidence presupposed that we had correctly assessed the amount of natural climate fluctuation. Now that we have a new development, we may need to make adjustments.
SPIEGEL: In which areas do you need to improve the models?
Storch: Among other things, there is evidence that the oceans have absorbed more heat than we initially calculated. Temperatures at depths greater than 700 meters (2,300 feet) appear to have increased more than ever before. The only unfortunate thing is that our simulations failed to predict this effect.
SPIEGEL: That doesn't exactly inspire confidence.
Storch: Certainly the greatest mistake of climate researchers has been giving the impression that they are declaring the definitive truth. The end result is foolishness along the lines of the climate protection brochures recently published by Germany's Federal Environmental Agency under the title "Sie erwärmt sich doch" ("The Earth is getting warmer"). Pamphlets like that aren't going to convince any skeptics. It's not a bad thing to make mistakes and have to correct them. The only thing that was bad was acting beforehand as if we were infallible. By doing so, we have gambled away the most important asset we have as scientists: the public's trust. We went through something similar with deforestation, too -- and then we didn't hear much about the topic for a long time.
SPIEGEL: Does this throw the entire theory of global warming into doubt?
Storch: I don't believe so. We still have compelling evidence of a man-made greenhouse effect. There is very little doubt about it. But if global warming continues to stagnate, doubts will obviously grow stronger.
SPIEGEL: Do scientists still predict that sea levels will rise?
Storch: In principle, yes. Unfortunately, though, our simulations aren't yet capable of showing whether and how fast ice sheets in Greenland and Antarctica will melt -- and that is a very significant factor in how much sea levels will actually rise. For this reason, the IPCC's predictions have been conservative. And, considering the uncertainties, I think this is correct.
SPIEGEL: And how good are the long-term forecasts concerning temperature and precipitation?
Storch: Those are also still difficult. For example, according to the models, the Mediterranean region will grow drier all year round. At the moment, however, there is actually more rain there in the fall months than there used to be. We will need to observe further developments closely in the coming years. Temperature increases are also very much dependent on clouds, which can both amplify and mitigate the greenhouse effect. For as long as I've been working in this field, for over 30 years, there has unfortunately been very little progress made in the simulation of clouds.
SPIEGEL: Despite all these problem areas, do you still believe global warming will continue?
Storch: Yes, we are certainly going to see an increase of 2 degrees Celsius (3.6 degrees Fahrenheit) or more -- and by the end of this century, mind you. That's what my instinct tells me, since I don't know exactly how emission levels will develop. Other climate researchers might have a different instinct. Our models certainly include a great number of highly subjective assumptions. Natural science is also a social process, and one far more influenced by the spirit of the times than non-scientists can imagine. You can expect many more surprises.
SPIEGEL: What exactly are politicians supposed to do with such vague predictions?
Storch: Whether it ends up being one, two or three degrees, the exact figure is ultimately not the important thing. Quite apart from our climate simulations, there is a general societal consensus that we should be more conservative with fossil fuels. Also, the more serious effects of climate change won't affect us for at least 30 years. We have enough time to prepare ourselves.
SPIEGEL: In a SPIEGEL interview 10 years ago, you said, "We need to allay people's fear of climate change." You also said, "We'll manage this." At the time, you were harshly criticized for these comments. Do you still take such a laidback stance toward global warming?
Storch: Yes, I do. I was accused of believing it was unnecessary to reduce greenhouse gas emissions. This is not the case. I simply meant that it is no longer possible in any case to completely prevent further warming, and thus it would be wise of us to prepare for the inevitable, for example by building higher ocean dikes. And I have the impression that I'm no longer quite as alone in having this opinion as I was then. The climate debate is no longer an all-or-nothing debate -- except perhaps in the case of colleagues such as a certain employee of Schellnhuber's, whose verbal attacks against anyone who expresses doubt continue to breathe new life into the climate change denial camp.
