Seeking the cause of climate change is both an exciting activity and a difficult task, as borne out by the long history of enquiry and the scarcity of firm conclusions. While numerous hypotheses have been advanced, either to explain local or temporally short changes or to provide a global framework for change throughout geologic time, it is safe to say that, although elements of truth exist in many of these hypotheses, no single one takes account of all the variables as they are known at present. To help us to understand the climate system, recent studies have utilized an integrated approach that considers the atmosphere, the hydrosphere, the biosphere and the solid Earth. This approach has tended, justifiably, to be historical in scope in order to make use of geological data bearing on the temporal and spatial variability of processes central to the system. The flood of new information means that there is a constant need to re-evaluate concepts and, given the growing awareness of the complexities of the climate system, it is likely that many more attempts will need to be made before we have a thorough understanding of how the system has worked in geological history.

The search for cyclical climate change

The palaeoclimate information collated here illustrates that the history of climate over the past 600 m.y. was not a simple trend of cooling or warming but was characterized by alternating periods of warm and cool climates. These periods we have defined as Cool or Warm Modes, based on the presence of cool or warm climate indicators as presented in the previous chapters.