Age of stratospheric air is a well established metric for the stratospheric transport circulation. Rooted in a robust theoretical framework, this approach offers the benefit of being deducible from observations of trace gases. Given potential climate-induced changes, observational constraints on stratospheric circulation are crucial. In the past two decades, scientific progress has been made in three main areas: (a) Enhanced process understanding and the development of process diagnostics led to better quantification of individual transport processes from observations and to a better understanding of model deficits. (b) The global age of air climatology is now well constrained by observations thanks to improved quality and quantity of data, including global satellite data, and through improved and consistent age calculation methods. (c) It is well established and understood that global models predict a decrease in age, that is, an accelerating stratospheric circulation, in response to forcing by greenhouse gases and ozone depleting substances. Observational records now confirm long-term forced trends in mean age in the lower stratosphere. However, in the mid-stratosphere, uncertainties in observational records are too large to confirm or disprove the model predictions. Continuous monitoring of stratospheric trace gases and further improved methods to derive age from those tracers will be crucial to better constrain variability and long-term trends from observations. Future work on mean age as a metric for stratospheric transport will be important due to its potential to enhance the understanding of stratospheric composition changes, address climate model biases, and assess the impacts of proposed climate geoengineering methods.

中文翻译：

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平流层空气年龄：过程、观测和长期趋势的进展


平流层空气的年龄是平流层输送环流的公认指标。该方法植根于稳健的理论框架，具有从痕量气体的观察中推断出来的优势。鉴于潜在的气候诱发变化，对平流层环流的观测限制至关重要。在过去的二十年中，在三个主要领域取得了科学进展：（a） 加强对过程的理解和过程诊断的发展，使人们从观察中更好地量化了各个运输过程，并更好地理解了模型缺陷。（b） 由于数据（包括全球卫星数据）的质量和数量的提高，以及通过改进和一致的年龄计算方法，全球空气气候学的年龄现在受到观测的很大限制。（c） 众所周知，全球模型预测了年龄的减少，即平流层环流加速，以响应温室气体和臭氧消耗物质的强迫。观测记录现在证实了平流层低层平均年龄的长期强迫趋势。然而，在平流层中部，观测记录的不确定性太大，无法确认或反驳模型预测。对平流层痕量气体的持续监测并进一步改进从这些示踪剂中得出年龄的方法，对于更好地限制观测的变率和长期趋势至关重要。未来将平均年龄作为平流层传输指标的工作将非常重要，因为它有可能增强对平流层成分变化的理解、解决气候模型偏差以及评估拟议的气候地球工程方法的影响。