Continental drift influences the oceans and hence the atmosphere in many ways. It shifts land masses across latitudes and climate zones and opens and closes gateways, controlling the circulation of surface and deep water. It raises mountains that cast rain shadows and make deserts. Erosion grinds the mountains down and again the climate changes. Plates move and hotspots rise, and the resulting sea level changes alter the climate as they increase or decrease the size of shallow seas. Volcanoes exhale carbon dioxide and enhance the green-house effect, but weathering consumes carbon dioxide and the earth grows cooler.

To address the question left open at the end of Chapter 5, “Why do ice ages occur?”, we must examine the 100 million years of history that have led to the present ice-house world. It is too early to do this with finality, because we are continually becoming aware of other forces that drive the climate, fully understand only a few, and are a long way from being able to say whether one is more important than another or how they interact. In this regard, the past ten years have brought a descent from confidence into humility, but that is not rare in the course of scientific progress. I start with the oceans in the naive belief that we understand them best.