During the Paris conference in 2015, nations of the world strengthened the United Nations Framework Convention on Climate Change by agreeing to holding ‘the increase in the global average temperature to well below 2 °C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5 °C’ (ref. 1). However, ‘pre-industrial’ was not defined. Here we investigate the implications of different choices of the pre-industrial baseline on the likelihood of exceeding these two temperature thresholds. We find that for the strongest mitigation scenario RCP2.6 and a medium scenario RCP4.5, the probability of exceeding the thresholds and timing of exceedance is highly dependent on the pre-industrial baseline; for example, the probability of crossing 1.5 °C by the end of the century under RCP2.6 varies from 61% to 88% depending on how the baseline is defined. In contrast, in the scenario with no mitigation, RCP8.5, both thresholds will almost certainly be exceeded by the middle of the century with the definition of the pre-industrial baseline of less importance. Allowable carbon emissions for threshold stabilization are similarly highly dependent on the pre-industrial baseline. For stabilization at 2 °C, allowable emissions decrease by as much as 40% when earlier than nineteenth-century climates are considered as a baseline.

在 2015 年巴黎会议期间，世界各国通过同意将“全球平均气温上升幅度控制在远低于工业化前水平 2°C 并努力限制气温”，从而加强了联合国气候变化框架公约增加到 1.5 °C'（参考文献 1）。然而，“前工业化”没有被定义。在这里，我们研究了工业前基线的不同选择对超过这两个温度阈值的可能性的影响。我们发现，对于最强的缓解情景 RCP2.6 和中等情景 RCP4.5，超过阈值的概率和超过时间高度取决于工业化前的基线；例如，在 RCP2 下，到本世纪末超过 1.5°C 的概率。6 根据基线的定义方式，从 61% 到 88% 不等。相比之下，在没有缓解措施的情况下，即 RCP8.5，这两个阈值几乎肯定会在本世纪中叶被超过，而工业化前基线的定义则不太重要。阈值稳定所允许的碳排放量同样高度依赖于工业化前的基线。为了稳定在 2 °C，当以 19 世纪之前的气候为基准时，允许排放量减少多达 40%。阈值稳定所允许的碳排放量同样高度依赖于工业化前的基线。为了稳定在 2 °C，当以 19 世纪之前的气候为基准时，允许排放量减少多达 40%。阈值稳定所允许的碳排放量同样高度依赖于工业化前的基线。为了稳定在 2 °C，当以 19 世纪之前的气候为基准时，允许排放量减少多达 40%。