We assess evidence relevant to Earth's equilibrium climate sensitivity per doubling of atmospheric CO2, characterized by an effective sensitivity S. This evidence includes feedback process understanding, the historical climate record, and the paleoclimate record. An S value lower than 2 K is difficult to reconcile with any of the three lines of evidence. The amount of cooling during the Last Glacial Maximum provides strong evidence against values of S greater than 4.5 K. Other lines of evidence in combination also show that this is relatively unlikely. We use a Bayesian approach to produce a probability density function (PDF) for S given all the evidence, including tests of robustness to difficult-to-quantify uncertainties and different priors. The 66% range is 2.6-3.9 K for our Baseline calculation and remains within 2.3-4.5 K under the robustness tests; corresponding 5-95% ranges are 2.3-4.7 K, bounded by 2.0-5.7 K (although such high-confidence ranges should be regarded more cautiously). This indicates a stronger constraint on S than reported in past assessments, by lifting the low end of the range. This narrowing occurs because the three lines of evidence agree and are judged to be largely independent and because of greater confidence in understanding feedback processes and in combining evidence. We identify promising avenues for further narrowing the range in S, in particular using comprehensive models and process understanding to address limitations in the traditional forcing-feedback paradigm for interpreting past changes.

中文翻译：

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使用多种证据评估地球气候敏感性


我们评估与大气二氧化碳增加一倍的地球平衡气候敏感性相关的证据，其特征是有效敏感性 S。这些证据包括反馈过程理解、历史气候记录和古气候记录。 S 值低于 2 K 很难与这三种证据中的任何一种相一致。末次盛冰期期间的冷却量提供了强有力的证据，证明 S 值大于 4.5 K。其他证据组合也表明这种情况相对不太可能。我们使用贝叶斯方法在给定所有证据的情况下生成 S 的概率密度函数 (PDF)，包括对难以量化的不确定性和不同先验的稳健性测试。我们的基线计算的 66% 范围为 2.6-3.9 K，并且在稳健性测试下保持在 2.3-4.5 K 范围内；相应的 5-95% 范围为 2.3-4.7 K，以 2.0-5.7 K 为界（尽管应更加谨慎地看待此类高置信度范围）。这表明通过提高范围的低端，对 S 的限制比过去评估中报告的更强。出现这种缩小范围是因为这三条证据一致并且被认为在很大程度上是独立的，并且因为对理解反馈过程和合并证据更有信心。我们确定了进一步缩小 S 范围的有希望的途径，特别是使用综合模型和过程理解来解决解释过去变化的传统强迫反馈范式的局限性。