TEN thousand years ago, the sea along the south coast of Ireland was some 120 feet below its present level. And in recent times, even before the worries about global warming came along, it has been rising at a rate of about one millimetre every year, so the citizens of Cork and its environs might be forgiven for wondering now and then what will happen if:
the bounded waters
Should lift their bosoms higher than the shores,
And make a sop of all this solid globe.
Changes in the apparent water level at a coastline result from two quite distinct effects. They mirror, naturally enough, the rise and fall of mean sea level on a global scale - the so-called eustatic change. As our climate has altered over the millennia, so the rhythmic growth and decay of the polar ice sheets has been paralleled by corresponding rises and falls in the level of water in the world's oceans.
Not only does the melting ice add more water to the global reservoir, but higher temperatures result in a thermal expansion of the sea, which in turn brings about a rise in the level of the water. Even in the past 100 years, there has been an increase in global sea level of about 10 cm for these reasons.
But there are also isoslatic changes to coastal sea level. These come about from changes in the elevation of the land itself, in most cases a delayed reaction to the last Ice Age. Land which has been depressed for centuries by millions of tons of ice tends to rise slowly when the ice retreats - in much the same way as a depressed cork bobs to the surface of a dish of water. And rising land, of course, brings about an apparent fall in the coastal water level.
Both eustatic and isostatic change have affected the level of the seas around Ireland's coasts. During the last ice age, the ice-cap covered only half the country, the southern half of Ireland stayed ice-free, while the northern half was buried under several hundred metres of the frozen mass.
In the intervening centuries, the south coast has been mainly influenced by the slowly rising trend in global sea level. In the north, however, this rise has been paralleled by an increase of similar proportions in the elevation of the land; although the coastal sea level around Ulster has oscillated a little over the past 10,000 years or so, the net change has been very small indeed.