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3. Why does the ice extent expand and contract?
There are three factors that contribute to the development of glacial conditions in Ireland every few thousand years. These are all related to the Earths position and geometry as an orbiting planet in the Solar System.1. The 'eccentricity' factor: As the Earth orbits the sun, its orbit is not perfectly circular but resembles that of an ellipse, or stretched out circle. This means that over a cycle of 100,000 years the orbit brings the Earth further away from the sun when the elliptical orbit is at its most 'eccentric'. Being further away from the sun at these times obviously means we don't get as much heat from the sun. This has huge effects on Earth surface temperatures as a whole.
2. The 'tilt' factor: The Earth's axis is tilted and the northern hemisphere leans towards the sun in June and away from it in December. The angle of tilt is not stable, either meaning the Earth 'leans' towards the sun more at some times than it does on others. This means that if the tilt angle is higher the temperature range may be greater. Approximately every 41,000 years, the tilt is closest to vertical. Sunlight then strikes the poles at a sharper angle, and seasonal variations in temperature are reduced.
3. The 'wobble' factor: The Earth's axis wobbles like a spinning top, wobbling along a circular path every 23,000 years. This means that in the case of the northern hemisphere its summer occurs either when the Earth is furthest from the sun on its elliptical orbit, or when it is closest.
These three cycles combined means that the Earth goes through predictable sequences of temperature variations of global cooling and warming. During cooling periods snow and ice that falls in winter does not melt during the summer and slowly accumulates. The greatest and longest lasting ice ages occur when all three cycles coincide.
