Human activities, principally through emissions of greenhouse gases, have unequivocally caused global warming, with global surface temperature reaching 1.1⁰C above 1850–1900 in 2011–2020.’ These words are from the Summary for Policy Makers of the Synthesis Report of the IPCC’s 6th Assessment Report. It is the aim of this book to help you understand exactly how, and why, the collective body of scientists has come to these, and many other, conclusions. Addressing this aim requires that you understand two facets of the nature of our planet. Firstly, you need to understand how our planet ’behaves’ under what we might choose to call ’normal conditions’. Secondly, you need to understand how human activity has moved us away from that normal. The following chapters will take you through both of these in the necessary detail. In this chapter we present a quick overview, as well as introducing you to the IPCC.