The papers in this Theme Issue were written by participants in the Four degrees conference, at the invitation of the editors. In many cases, authors were asked to combine their separate conference contributions into a joint paper synthesizing multiple viewpoints and results. We appreciate their enthusiasm in taking on this challenge. Now, as we go to press with the Theme Issue a year on from the conference, the results of the research presented in its papers seem even more relevant than a year ago. We have no comprehensive climate-change ‘deal’, and Yvo de Boer, in his final remarks as head of the UNFCCC before stepping down in 2010, said that it could take up to 10 years for negotiations to deliver a robust and effective agreement. Voluntary and non-nation state emissions-reduction commitments might give us some breathing space, but still take us closer to four degrees than is comfortable. The papers in this issue that look at impacts and adaptation challenges in a four degrees world are sobering: the possible impacts are large, in some cases, transformational, and the challenges in understanding and developing responses to these impacts considerable. Hopefully, this Theme Issue will stimulate much-needed further research that explores the implications of and solutions to high-end climate warming.