FIFTEEN years or so ago, when global warming first became the focus of attention, some of the predictions were apocalyptic. There were fears, for example, that the polar icecaps would quickly disappear, and that as a consequence the level of the sea would rise by 10 or 20 feet. The little rhyme of G. K. Chesterton, written half a century before, appeared prophetic:
I tell you naught for your comfort,
Yea, naught for your desire,
Save that the sky grow's darker yet
And the sea rises higher.
Today's assertions, however, tend to be more moderate. Improved computer models of the atmosphere have allowed estimates of any likely rise in global temperature due to the enhanced greenhouse effect to be revised significantly downwards, and in addition, our understanding of the consequences of such warming for the level of the world's oceans has greatly improved.
Of course the basis of the fears is sound. Melting ice unless it happens to be floating in the sea already must inevitably add more water to the global reservoir. As the world's 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 Bunt ice melts only very slowly, and there are other factors which would reduce the impact if the temperature of the world were to rise by a degree or two.
For one thing, with an increase in temperature more water vapour would evaporate from the oceans, and in the atmosphere it would condense as extra cloud. This increase in cloudiness would result in more precipitation, much of which would fall over the coldest regions of the world as snow. Each year, therefore, the ice caps of Antarctica and Greenland would get thicker in the middle even if they melted at the edges with the warmer seas.
Of course, a thicker Greenland icecap would be heavier than the present one, and the increased pressure from the centre would cause the glaciers to flow more quickly towards the sea. This would result in a greater rate of "calving", and a greater number of icebergs to add to the volume of the oceans.
Similar processes could take place in the antipodes on the Antarctic ice sheet. But even at worst, this would be a slow process; the best current estimates are that the two effects almost cancel out, and that the net effect of melting ice would be no more than an increase of a few centimetres in sea level in a hundred years.