ABSTRACT
In this chapter we ask the question: ‘For how long and over what extent have relatively warm climatic conditions, and the biological communities characteristic of these, existed on the earth’s surface?’ The widely used Koppen-Geiger climate classification system arbitrarily defines tropical climates as those having no month with mean temperatures below 18°C. While in Chapter 3 we choose to define tropical climates in a more dynamic way, the Koppen-Geiger definition provides a convenient first approximation of where tropical climates exist today; this is generally within (although not universal to) the latitudinal zone enclosed by the tropics of Cancer and Capricorn (Figure 3.1). As we shall see, this pattern has changed considerably through time. These changes raise questions about presumptions of stability and fragility for tropical ecosystems that have been based on an assumed lack of climate change in the tropics. This historical record also sheds new light on the enigma of the origins of tropical biotic diversity.
