Environmental issues that may become priorities in the 21st century can be clustered in three groups - unforeseen events and scientific discoveries; sudden, unexpected transformations of old issues; and already well-known issues to which the present response is inadequate. The Scientific Committee on Problems of the Environment of the International Council for Science conducted a special survey for GEO-2000 on environmental issues that may require attention in the 21st century. Most of the responding scientists expect that the major environmental problems of the next century will stem from the continuation and aggravation of existing problems that currently do not receive enough policy attention. The issues cited most frequently are climate change, and the quantity and quality of water resources. Many scientists emphasized that the interlinkages between climate change and other environmental problems could be important. This includes the emerging scientific understanding of complex interactions in the atmosphere-biosphere-cryosphere-ocean system - which could lead to irreversible changes such as shifts in ocean currents and changes in biodiversity.