This chapter presents a selection of topics on the physics of climate change. By way of introduction, in Section 8.1 we briefly discuss greenhouse gases and the radiative forcing associated with several drivers of climate change. Then in Sections 8.2 and 8.3 we introduce a very simple time-dependent, ‘energy balance’, climate model. This model illustrates some key concepts that arise in the study of the physics of the Earth's climate and its response to external forcing, and in the diagnosis of the highly complex models that are used to simulate the climate of the past and present and to predict future climate. In Section 8.4 we examine some elementary aspects of the important topic of climate feedbacks. Finally, in Section 8.5 we use another simple model to examine the basic physics of the process by which the radiative forcing due to carbon dioxide increases with its concentration.

Our emphasis in this chapter is on the underlying physical principles of climate change; we shall not discuss in any detail the climate-change projections by the current range of complex general circulation models. Comprehensive information on these projections is provided for example by the Assessment Reports of the Intergovernmental Panel on Climate Change (IPCC). The most recent such report is that of Solomon et al. (2007), which includes useful summaries for non-specialists and a glossary of technical terms.