In the nineteenth century geologists discovered evidence that large parts of the northern hemisphere had once been covered by gigantic ice sheets. Scientists have now learnt that the waxing and waning of these ice sheets are just one aspect of global climatic change, and that the planet has been in the past both hotter and colder than it is today.The complex interactions between variations in the Earth's orbit around the Sun, the movements of tectonic plates, the planet's atmosphere and ocean currents, can result in large and rapid swings in the Earth's climate