The UNFCCC parties in their last 2015-meeting in Paris agreed to hold the increase in the global average temperature well below 2 °C above pre-industrial levels. However, how this target came about is rarely substantiated in the scientific literature. We review and document the history of this target and the rapidly emerging scientific evidence to support it. The target was initially proposed after evaluating observed ranges of climate variation over the last 1000 and 1 000 000 years by an US economist, Nordhaus, in 1975. His conclusion was supported in 1980 by paleo-ecologists, who, on basis of the recolonizing vegetation after the retreat of the ice-sheets after the last glaciation, calculated that tree species could cope with a 2 °C temperature increase per century. A more elaborated target including tolerable rates of temperature and sea-level change was presented by Vellinga and Swart at the Second World Climate Conference in 1990. The target was illustrated by means of a traffic light: 1 °C global temperature rise meets an orange light, 2 °C meets red. These notions led first to the 1989 Noordwijk Ministerial Declaration and later to the UNFCCC's 1992 objective to prevent dangerous anthropogenic interference with the climate system. Although the scientific evidence was limited, the European environment ministers in 1996 endorsed the 2 °C target politically, after which it surfaced again in UNFCCC's 2009 Copenhagen summit. The ‘well below 2 °C’ target was scientifically further analysed in 2000 and motivated as part of the IPCC's third assessment report in its synthesis chapter, which assessed dangerous climate change. The resulting ‘burning ember’ diagram indicated that beyond 2 °C warming adaptation possibilities rapidly deteriorated and vulnerabilities increased, especially for unique ecosystems and extreme events. The evidence that emerged since this assessment report, on observed climate-change impacts show that vulnerabilities nowadays likely are larger. Recently, accelerated and higher levels of sea-level rise and more frequent extreme events are reported. All these insights were likely considered in the wording (i.e. ‘well below 2 °C’) of the 2015 Paris Agreement.