Scientific understanding of past and future climate change has made substantial progress since finalization of the IPCC WG1 AR4. This paper reviews the new knowledge on climate change. It includes improved analysis of prehistoric climate shifts, updated observations of recent climate change, better attribution of the causes of observed climate change to anthropogenic and natural factors, improved understanding of carbon cycle feedbacks, and new projections of future changes in extreme weather events. As a result, many risks are now assessed to be larger than in the AR4, in particular the risk of large sea-level rise already in the current century and the risks from increases in extreme weather events.