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.

《联合国气候变化框架公约》各缔约方在上一次于巴黎举行的2015年会议上，同意将全球平均温度的升高幅度保持在比工业化前水平高2°C以下的水平。但是，在科学文献中很少证实如何实现这个目标。我们审查并记录该目标的历史以及迅速出现的科学证据以支持该目标。该目标最初是在1975年由美国经济学家诺德豪斯（Nordhaus）在评估了近1000年和1000万年的气候变化范围之后提出的。1980年，他的结论得到了古生态学家的支持，他们基于重新定殖的植被在最后一次冰川融化后冰盖撤退之后，计算得出树木物种可以应对每世纪2°C的温度升高。Vellinga和Swart在1990年的第二届世界气候大会上提出了更为详尽的目标，包括可忍受的温度和海平面变化速率。该目标通过交通信号灯进行了说明：全球气温每升高1°C就会亮起橙色光，2°C遇到红色。这些概念首先导致了1989年的《诺德韦克部长宣言》，后来导致了UNFCCC的1992年目标，即防止人为干扰气候系统。尽管科学证据有限，但欧洲环境部长们于1996年在政治上认可了2°C的目标，此后又在UNFCCC的2009年哥本哈根峰会上浮出水面。全球温度上升1°C时会亮橙色，蓝色2°C时会亮红色。这些概念首先导致了1989年的《诺德韦克部长宣言》，后来导致了UNFCCC的1992年目标，即防止人为干扰气候系统。尽管科学证据有限，但欧洲环境部长们于1996年在政治上认可了2°C的目标，此后又在UNFCCC的2009年哥本哈根峰会上浮出水面。全球温度上升1°C会亮橙色，2°C会亮红色。这些概念首先导致了1989年的《诺德韦克部长宣言》，后来导致了UNFCCC的1992年目标，即防止人为干扰气候系统。尽管科学证据有限，但欧洲环境部长们于1996年在政治上认可了2°C的目标，此后又在UNFCCC的2009年哥本哈根峰会上浮出水面。

2000年，对“远低于2°C”的目标进行了科学的进一步分析，并将其作为IPCC在其综合章节中评估危险气候变化的第三份评估报告的一部分。生成的“燃烧余烬”图表明，温度超过2°C时，适应能力迅速下降，脆弱性增加，特别是对于独特的生态系统和极端事件。自这份评估报告以来，关于观测到的气候变化影响的证据表明，当今的脆弱性可能更大。最近，有报道说海平面上升的速度越来越快，极端事件也越来越频繁。2015年《巴黎协定》的措辞（即“远低于2°C”）可能考虑了所有这些见解。