2. What is a climate modeler actually working on - and how does he build a climate model?

In simple terms, there are two types of climate modelers: the climate model user who answers scientific questions using the climate model, and the model developer who deals with the question of how to improve a climate model - for example, to simulate a physical process in the atmosphere. For this purpose the developer can change the model like any other software using programming codes.

Completely newly developed climate models are very rare, because the development of a model together with all important physical, chemical and biological processes is extremely complex and takes several years to complete. There are around 20 well-established climate models which have been developed by large research institutes and computing centres worldwide. The climate model user can perform exciting simulation experiments. To give a prominent example, he or she can observe how the climate changes when the atmospheric carbondioxid concentration is doubled.

The AWI physicist Helge Goessling does both. He works as a climate model user and as a developer. His main focus is sea ice in the polar regions. "For some questions, I simply use the climate model as a tool", says Goessling. "I can change the greenhouse gas concentration or the extent of the sea ice and observe how the climate reacts."

In model development, on the other hand, he changes the climate model. In a programming language that makes the computer understand the climate system, he can change how the model behaves. For example, in his doctoral thesis he modified the program code in such a way that the model records where the atmospheric water stems from. In other cases, climate modellers rewrite the program code to calculate certain physical processes in different ways. A particular challenge is the realistic simulation of cloud formation, because many aspects such as temperature, wind and evaporation play a role. By comparing different formulations one can find out how the physical process is simulated most realistically - and improve the climate model bit by bit.