By now you should have been left in no doubt that the Earth’s climate system is extremely complex. Our understanding of it is still not good enough for computer models to be constructed that can simulate changes observed over historical timescales. Yet we are faced with explaining changes over much longer timescales (Figure 6.1). These range from tens of millions of years as the Earth cooled from a greenhouse state during the Cretaceous to the icehouse condition of today, through tens of thousands of years as it oscillated between glacial and interglacial conditions, to changes spanning a few millennia or less such as stadials and interstadials, the Medieval warm period, and the Little Ice Age and subsequent global warming.