Büntgen et al. have published a paper in Science
(10.1126/science.1197175) which correlates the climate of the past 2500 years to social and political events. The data is based on various sources, mainly tree ring data but also eye witness reports.
The authors say they are aware that it is difficult to establish causality between climate and history. Nevertheless, many speculations are made on the basis of this data. They make an explicit link to present changes in the climate and to political reluctance as regards mitigation strategies. What I find interesting is the assertion that civilizations blossomed when it was warm (but should this not lead to a welcoming of warming?). DER SPIEGEL has an elaborate write up of this paper which also emphasises the point about warm periods being beneficial to political and social stability. Therefore, the point seems to be that not warming as such is the climate risk of the future, but change, such as cooler temperatures and more rain. Do we see a re-definition of dangerous climate change?