The WBGU derives a global CO2 reduction target by using an "inverse scenario" based on simplified models for climate dynamics and the carbon cycle. By first analysing the maximum stress levels caused by climate change that one can assume to be ecologically and economically bearable, a "tolerance window" for the future climatic development is deduced. In a further step, the set of admissible emission profiles is determined, i.e., those global CO2 emission functions which keep the climate system within the demarcated window. Among the so-defined family of emission options a specific strategy is finally singled out by feasibility criteria. The WBGU believes that such an integrated assessment of the climate change problem in the "backwards mode" has several advantages in comparison with the straightforward approach. It has to be emphasised, however, that the analysis employs a number of assumptions and approximations and therefore has the character of a "Gedankenexperiment".