For every one degree rise in ocean water temperatures, sea levels must rise by the best part of a metre simply because seawater expands as it warms up. This is quite separate from the additional (and potentially greater) rise in sea level that would happen if the Antarctic and Greenland ice caps were to begin to melt.
Sea level has fluctuated by hundreds of metres over geological time, but the prospect of several metres' rise over the next few centuries ought to cause real concern to us all, and not just those who live in low-lying oceanic islands (such as Kiribati) or in coastal areas already prone to flooding (such as much of Bangladesh).
DAVID A. ROTHERY
Department of Earth Sciences
The Open University