Since the industrial revolution the surface temperature of the Earth has increased by about 1°C and climate scientists are overwhelmingly of the opinion that this largely due to the effect of gases released into the atmosphere by human activities. How can we be sure that this is the case? Does it matter? What can we say about the future? In this talk Jo Haigh will outline the evidence for climate change, in temperature and other measures, globally and regionally in the context of natural variations in climate, and she will discuss what is well known and what less certain. She will describe how the climate reflects a delicate balance between the energy coming in from the Sun and the heat energy leaving Earth for space, and how this can be disrupted by increasing concentrations of “greenhouse gases”, especially carbon dioxide (CO2). She will show how basic physical science can be used to construct computer models of the climate which can then be employed to investigate how climate processes work and what increasing CO2 may mean for the future. The talk will conclude with a discussion around different approaches to tackling climate change and where the world is heading following the recent United Nations climate conferences.