The IPCC Fourth Assessment Report (AR4) published in 2007 presents the most complete and authoritative assessment of the status of scientific knowledge on all aspects of climate change. This paper presents an updated assessment of the risks from anthropogenic climate change, based on a comprehensive review of the pertinent scientific literature published since finalisation of the AR4. Many risks are now assessed as stronger than in the AR4, including the risk of large sea-level rise already in the current century, the amplification of global warming due to biological and geological carbon-cycle feedbacks, a large magnitude of “committed warming” currently concealed by a strong aerosol mask, substantial increases in climate variability and extreme weather events, and the risks to marine ecosystems from climate change and ocean acidification. Some topics remain the subject of intense scientific debate, such as past and future changes in tropical cyclone activity and the risk of large-scale Amazon forest dieback. The rise in greenhouse gas emissions and concentrations has accelerated recently, and it is expected to accelerate further in the absence of targeted policy interventions. Taken together, these findings point to an increased urgency of implementing mitigation policies as well as comprehensive and equitable adaptation policies.