The Ireland of 18,000 years ago was not the green and pleasant land we know today. It was a barren waste, languishing under several hundred feet of ice. The only sounds to be heard were the whistling of the freezing wind, and the rumblings of deep crevasses as they opened and closed across the rough surface of that huge solid ocean. We were in the middle of an Ice Age.
The Arctic and Antarctic ice-caps at present cover about 10 per cent of the Earth's surface. But they vary in extent: at least four times in the past two million years or so, the ice sheets have expanded to about three times their current size. They have covered the plains of northern Europe and filled the shallow continental seas around it. In between - in the so-called interglacial periods - the Earth returns to a more congenial regime, like that which we enjoy at present.
The most important causes of this climatic pendulum have their origins in the idiosyncrasies of the orbit of the Earth. Our path around the sun is not a perfect circle; it is an ellipse - a kind of stretched-out circle rather reminiscent of an oval. Moreover, the shape of this ellipse is not a constant; it goes from being a very pronounced ellipse - when the astronomers say its eccentricity is high - to very near a perfect circle, and then extends itself again.
The change from low eccentricity to high eccentricity and back again takes about 100,000 years, so every 100,000 years the orbit is such that it carries the Earth unusually far away from the sun for part of the year - which gives the coldest weather possible in the hemisphere in which it happens to be winter.
Then, in 1920, a Yugoslav physicist called Milutin Milankovich found another part of this climatic jig-saw puzzle: he recognised the role of axial tilt. We all know that the Earth's axis is tilted at an angle of 23 1/2 degrees to the plane of the Earth's orbit around the sun - but this figure is not constant either.
The axial tilt varies from about 25 to 22 degrees, a change in what astronomers call the obliquity of the ecliptic and which has a cycle of about 41,000 years - the so-called Milankovich cycle. Since it is the axial tilt that gives us seasons, it follows that the amount of the tilt affects the severity of winters. And when axial tilt and orbital eccentricity combine to do their worst - we get an Ice Age.