Humankind is in the process of altering the global climate (IPCC, 1996a; Grieser et al., 2000). Anthropogenic greenhouse gas emissions are causing global warming at a rate without precedent over the past 10,000 years (IPCC, 1996a, b). Since the onset of industrialization, atmospheric greenhouse gas concentrations have risen significantly: Carbon dioxide by 30 per cent, methane by 145 per cent and oxides of nitrogen by 15 per cent (IPCC, 1996a).Almost threequarters of all anthropogenic emissions come from fossil fuel use (e.g. coal, mineral oil or natural gas) and about one-quarter from land-use change, notably as a consequence of the clearing of tropical forests (WBGU, 2000a). This has led since the late 19th century to a mean warming at the Earth’s surface of 0.3-0.6°C (IPCC, 1996a), whereby 1998 registered as the warmest year since records began in 1854 (Jones et al., 1999). Clear indications of climatic warming are given by, for instance, the shrinkage of mean sea-ice thickness in the Arctic by approx. 2m within the past 28 years (Johannessen et al., 1999) or widespread coral reef bleaching (Hoegh-Guldberg, 1999).