The Weather in 100 Years

Predicting the Impact of Climate Change on Germany

By Axel Bojanowski

Part 5: How Robust are the Prognoses?

Despite having been computed by a mainframe computer that occupies several rooms at the Max Planck Institute in Hamburg, the climate models are subject to considerable uncertainty. For instance, it is unclear whether the underlying assumption of rising greenhouse gas emissions will in fact come true. Besides, vegetation and ground cover also affect the climate, and changes in these two factors cannot be predicted.

Because the effects of water shortages would be far more serious than the expected higher temperatures, precipitation forecasts are critical. However, the precipitation calculations are significantly less reliable than the temperature scenarios. Comparisons among various climate models have shown that predicting summer precipitation levels is especially unreliable.

"Only if we manage to predict the water content of the soil can we expect to see robust forecasts," says climatologist Joseph Egger of the Ludwig Maximilian University of Munich. But thunderstorms and clouds often extend across only a few kilometers, so that they fall through the grids in the models. Their effects have to be estimated.

The results of the regional climate calculations depend on the same models that calculate the global climate. However, radiation energy in the air, which determines the temperature, is very difficult to calculate. A lack of test data also means that many important climate processes are still poorly understood, such as the cycle among the land, air and ocean.

The MPI-M researchers are confident in their prognoses because they have aligned their model with climate in the past. Nevertheless, they are quick to point out that their scenario for Germany is not to be seen as an exact forecast, and they insist that making statements about the weather in a given year is impossible. "Our calculations," says MPI-M researcher Göttel, "only permit conclusions about the average weather over a period of 30 years."

Translated from the German by Christopher Sultan