In the 1960s, when long, continuous sediment cores were first recovered from the bottom of the oceans, it was realised how many ice ages there have been. ‘Great ice ages’ looks at what influenced their waxing and waning. Glacial–interglacial cycles are the fundamental characteristic of the Quaternary Period (the last 2.5 million years). Movements in the continental ice sheet are initiated by changes in the Earth's orbit. The Earth wobbles on its axis, changing the amount of solar energy received by different parts of the Earth. These small changes force climate alterations. It is the Earth's climate reaction which translates small changes in regional solar energy into climate variability.