Learning about the Tail of the Probability Density Function for Equilibrium Climate Sensitivity

人类活动引起的气候变化的重要性主要取决于平衡的气候敏感性(以下称为气候敏感性，即工业化前大气二氧化碳浓度翻倍导致的全球地表气温变化)。政府间气候变化专门委员会(IPCC)历来对气候敏感性的评估范围为1.5至4.5摄氏度。采购经理人指数表明，气候敏感性很有可能超出了这个范围，概率密度函数(Pdf)有一个厚实的上尾。政府间气候变化专门委员会的最新报告提供了一些关于敏感性的pdf，所有这些都有厚厚的上尾巴。这项研究的重点是厚的上尾巴。测试真实的气候系统是否位于这个厚厚的上尾巴不仅是一个诱人的科学难题，而且还可以提供巨大的经济价值，因为它可以为改进的气候政策的设计提供信息。最近的一项经济分析表明，就效益/成本比而言，当前气候敏感度估计的厚尾可能主导着拟议的气候变化战略的排名。这背后的直觉是，决策者可能会相当重视不同气候变化战略的代价高昂的结果，这些结果与当前气候敏感性估计的厚重上尾巴被揭示为系统的真实状态有关。PI将使用他们简单的气候模式和更新的观测数据来检验五个假设：(1)自1998年以来增加的近地表气温观测将减少气候敏感性的90%可信区间；(2)包括非硫酸盐气溶胶辐射强迫将增加气候敏感性的90%可信区间，这是因为由此产生的负辐射强迫将大于单独使用硫酸盐气溶胶；(3)包括海洋热含量数据将降低气候敏感性的90%可信区间；(4)这三个数据集的综合影响将减少气候敏感性的90%可信区间；(5)气候噪声引起的气候敏感性的不确定性可以通过长期了解气候敏感性来减少。此外，PIS将研究气候变异性和古气候信息如何影响气候敏感度pdf的估计。他们将使用的贝叶斯平均方法将使他们能够了解不同的辐射强迫模型，并将其相关的事后预测能力纳入气候敏感度分布的估计。这项研究的学术价值在于，它将使用更多的观测数据，从频率和贝叶斯的角度改进对气候敏感性的估计：更新的地表气温、海洋热含量、古气候数据和非硫酸盐气溶胶的辐射强迫。根据《联合国气候变化框架公约》第2条的要求，这一知识将使决策者了解温室气体人为排放的减少幅度和速度，这些温室气体的减少是“……防止对气候系统造成危险的人为干扰”所必需的。这也是该研究最重要的更广泛的影响。第二个更广泛的影响将是其他科学家利用这些结果来研究人类引起的气候变化的影响和政策影响。第三个更广泛的影响将是公众宣传和沟通。PI将通过他对公众、商界领袖和政策制定者的演讲来继续这一点。

The importance of human-induced climate change depends critically on the equilibrium climate sensitivity (referred to as climate sensitivity hereafter, i.e., the change in global surface air temperature resulting from a doubling of the pre industrial atmospheric carbon dioxide concentration). The Intergovernmental Panel on Climate Change (IPCC) historically assessed the climate sensitivity at a range from 1.5 to 4.5 degrees Celsius. The PIs document that there is a significant likelihood that the climate sensitivity lies outside this range, with probability density functions (pdfs) that have a thick upper tail. The most recent IPCC report presents a number of pdfs for the sensitivity, all of which have a thick upper tail. This research focuses on the thick upper tail. Testing whether or not the real climate system is located in this thick upper tail is not just a tantalizing scientific puzzle, but can also provide large economic value, as it can inform the design of improved climate policies. A recent economic analysis shows that the thick upper tail of current climate sensitivity estimates can dominate the ranking of proposed climate-change strategies in terms of their benefit/cost ratio. The intuition behind this is that a decision maker may give considerable weight to the very costly outcomes of different climate change strategies that are associated with the cases where the thick upper tail of current climate sensitivity estimates is revealed to be the true state of the system.The PIs will use their simple climate model and updated observational data to test five hypotheses: (1) The inclusion of the additional near-surface air temperature observations since 1998 will reduce the 90% confidence interval of climate sensitivity; (2) The inclusion of non-sulfate aerosol radiative forcing will increase the 90% confidence interval of climate sensitivity, this because the resulting negative radiative forcing will be greater than that for sulfate aerosol alone; (3) The inclusion of oceanic heat-content data will reduce the 90% confidence interval of climate sensitivity; (4) The combined effect of these three datasets will be a reduction of the 90% confidence interval of climate sensitivity; (5) The uncertainty in climate sensitivity due to climatic noise can be reduced by learning about climate sensitivity overtime. In addition, the PIs will examine how climate variability and paleoclimatic information influence the estimation of the pdf of climate sensitivity. The Bayesian averaging method that they will use will allow them to learn about different radiative forcing models and incorporate their associated hindcast abilities into the estimation of the distribution of climate sensitivity. The intellectual merit of the research is that it will produce improved estimates of climate sensitivity from both the Frequency and Bayesian points of view using additional observations: updated surface air temperatures, ocean heat content, paleoclimatic data, and radiative forcing from non-sulfate aerosols. This knowledge will inform decision-makers about the magnitude and rate of reductions in the anthropogenic emission of greenhouse gases that are needed to "...prevent dangerous anthropogenic interference with the climate system", as required by Article 2 of the UN Framework Convention on Climate Change. This is also the most important broader impact of the research. The second broader impact will be the use of the results by other scientists to study the impacts and policy implications of human-induced climatic change. The third broader impact will be public outreach and communication. The PI will continue this via his lectures to the public, business leaders and policymakers.