‘What a silly question!’ I hear you say. Any fool knows the weather matters. So why pose the question? The answer lies in the weasel-word ‘really’. The whole issue of the potential impact of climatic change, whether natural, or as a result of human activities, depends on how sensitive our economic and social structures are to such changes. Only by asking direct questions about what has been the real impact in the past and how much future developments are likely to take us into new territory can we assess whether the various options for action are worth the effort. This also takes us into difficult areas associated with our ability to forecast and how societies respond to both unexpected changes and to apparently believable forecasts. All these matters have been the subject of an immense amount of expert analysis: UN-sponsored programmes have crawled over issues and drawn on the expertise of a vast number of specialists in the field of meteorology, climate change and its impact on economic and social systems; environmental movements have pressed vigorously to get action on alleviating the worst predictions before they become reality; and leading politicians have nailed their colours to the climatic-change mast.