There are three reasons why sea level ought to rise in response to global warming. One is the thermal expansion of the ocean itself, which, if present trends continued, might be sufficient to account for perhaps 30 cm in 100 years.
A second is the melting of the world's glaciers, which have been shrinking for more than a century and adding their meltwaters to the oceans; sea level may have risen by as much as five cm in the last 100 years from this cause. And the third reason for rising sea levels in a greenhouse world is the melting of Antarctic ice.
But all these are counterbalanced by the theory that as the world gets warmer, more snow can be expected to fall in the Antarctic regions. As a consequence Antarctic ice should increase in bulk, not shrink, and the source of this extra frozen water can only be evaporation from the oceans - and so the level of the seas should fall.
So what is actually happening? Most scientists agree that sea level is rising at the moment, but establishing this fact incontrovertibly is anything but easy. Tide gauges at ports around the world have been measuring sea level for many decades, but unfortunately the land to which these gauges are attached is in many cases moving up and down itself.
Scandinavia, for example is still springing back after being crushed by massive glaciers during the last Ice Age, so the sea level around Stockholm appears to be falling at a rate of about four millimetres a year. The opposite is the case in Honolulu. And the eastern seaboard of North America is slowly sinking, thereby accelerating any rise in the level of the sea that might be evident there.
The best that can be done is to try to calculate for each site the likely movement up or down of the adjacent land, and then subtract this amount from tide-gauge measurements. Using this approach, it would seem as if global sea level has been rising at a rate of about two millimetres a year for the last few decades.
A potentially more accurate method now available is to use radar altimeter measurements from satellites: because the position of a satellite in space is known precisely, measurements of the vertical distance to the sea below allow sea level to be calculated.
Data from such instruments appear to confirm the landbased assessment of a two millimetres a year rise in global sea level. What we do not know for sure, however, is how this trend will increase or diminish in response to changes in the global climate.