Computer modeling pervades today all fields of science. For the study of com plex systems, such as the environment, it has become an indispensable tool. But it is also a tool that is often misunderstood and misinterpreted. These dangers are particularly pronounced in the environmental sciences, an area of interest and concern not only to scientists, but also to the general public, the media, policy makers and powerful interest groups. We cannot experiment with our planet. The only quantitative tool available for the assessment of the impact of our actions today on the future environment and...