What causes an ice age? Although it is the positioning of the continents around the globe that essentially causes our current ice epoch (a longer period than an ice age). This does not necessarily explain however, the subsequent pattern of long ice ages interspersed with much briefer periods of much warmer weather patterns around the globe.

The first important point is that ice ages should be seen on Earth as the norm rather than the exception. That is to say with the present continental arrangements balanced against the heat source from the sun, it is to be expected that ice ages will occur. Huge glacial fields will stretch across most of northern Europe, including the whole of the British Isles, with temperatures in the region on average some 10C (18F) lower than now.

It follows from this that it requires relatively unusual circumstances to produce the necessary warmth to force the glaciers back. The answer appears to lie in the orbital patterns of the Earth around the sun. I have already mentioned two orbital features of the Earth, one its elliptical orbit, the other the tilt of the Earth in relation to its plane of orbit.

However, these eccentricities of orbit are not constant and fixed. The Earth's orbit changes from being almost a perfect circle to its elliptical form and then back again. This pattern can take around 100,000 years. Similarly, the Earth's tilt does not remain fixed at 66.5º to the plane but varies between 65.6º and 68.2º over a period of 40,000 years. At present the tilt is increasing.

There is a third quirk in the Earth's orbit, namely a 'wobble' in the Earth's axis of rotation caused by the pull of the moon and which has a cycle of approximately 24,000 years. The significance of these variations is that some sixty years ago a Yugoslavian geophysicist, Milankovitch, suggested that these alternations could account for the ice age/interlgacial pattern.

His argument was that only when these variations combined sufficiently to allow summers warm enough to melt the ice sheets could the Ice Age be halted and temporarily forced back. Subsequent analysis of climatic history suggests that this indeed is what has happened in practice and that it is these orbital variations which account for our weather pattern during the present ice epoch.

This theory, known as the Milankovitch model, is now widely accepted as a good explanation of the ice-age cycle though it does not explain what may trigger an ice age, or cause the mini-cycles of optima and little ice ages. The reasons behind these may be a combination of complex factors including the Earth's magnetism, changes in the sun's heating and variations in the atmosphere. These could be caused both by natural forces like earthquakes or even, some say, by meteor strikes; as well as by man-made activities which result in dust and fumes which could potentially influence future climatic trends.

Useful links
Ice Age Part 1