SPIEGEL: Just since the turn of the millennium, humanity has emitted another 400 billion metric tons of CO2 into the atmosphere, yet temperatures havenât 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. Weâre 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 hasnât happened. In fact, the increase over the last 15 years was just 0.06 degrees Celsius (0.11 degrees Fahrenheit) â 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 weâre 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 â 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 wouldnât 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.
