This volume contains a selection of scientific papers which were presented at an international workshop on the impacts of climatic variability held in Wengen, Switzerland, September 1997. For the first time, an assessment is made of the interactions between physical and biological elements of the Earth System on the basis of shifts in extreme climatic conditions, rather than simply changes in mean atmospheric conditions which research has tended to focus on until recently. Natural ecosystems and forests are typical examples of systems which, while constrained within certain ranges of mean climate, can undergo rapid and often irreversible damage in the face of short-lived but intense extreme events.