INTRODUCTION

Many causes have been proposed over the past century for the onset and cyclicity of the Ice Ages. The causes of the Pleistocene Ice Ages can be grouped into four categories: factors external to the Earth–atmosphere system, factors with a strong geological expression, factors internal to the Earth–ocean–atmosphere system, and factors that have a significant randomness. External factors are mainly cyclic; however, by themselves, they cannot account for the temperature difference between glacial and interglacial periods. Some of these cycles, such as sunspots, have implications for recent climate change. Geological factors explain the overall cooling that has dominated the Pleistocene; but cannot explain the cyclicity of climatic fluctuations. Factors involving the Earth–ocean–atmosphere system are multi-faceted and, when linked to externally forced cycles, account best for the intensity and timing of glacial periods through positive and negative feedback mechanisms. The fourth factor acknowledges the fact that much climate change, even when associated with cycles, may be random. This chapter highlights the main causes that have been invoked under each of these general headings. The list is not meant to be comprehensive. Finally, while emphasis is placed upon large scale climate change, minor cycles and oscillations at the biennial-to-decade intervals, account for a higher degree of the noise in Pleistocene and recent climatic records.

EXTERNAL FACTORS

Changes in the sun's orbit within our galaxy

(Fairbridge, 1987 b)

A fundamental cyclicity of 200–300 million years appears to underlie large scale geological processes, such as mountain building and plate movements.