Age of Stratospheric Air: Progress on Processes, Observations, and Long-Term Trends

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. Transport strongly influences the composition of the stratosphere, the layer at around 10-50 km altitude. Air masses are transported upward into the stratosphere in the tropics, and get distributed globally by a hemisphere-wide overturning circulation. Stratospheric age of air is a measure of the transport times along this circulation. This metric is a crucial measure of the circulation strength, deducible from observations of certain stratospheric gases. Over the past two decades, scientific progress has been made in three main areas. First, we now understand better which processes influence transport times. Second, we have good knowledge on the global average distribution of age of air, thanks to more observational data and improved methods to deduce this metric. Third, a large focus has been on climate-change related trends in age of air. While it is well established and understood that global models predict an accelerating stratospheric circulation, the related decrease in age of air can only be confirmed observationally in the lower stratosphere. Continuous monitoring of stratospheric trace gases will be crucial to constrain circulation trends throughout the stratosphere. This is essential for understanding climate change effects, improving climate models, and assessing potential climate intervention strategies. Understanding of how individual processes impact stratospheric age of air has been improved through development of dedicated diagnostics The global age of air climatology is now well constrained by observations thanks to improved data records and age calculation methods Observed age trends confirm lower stratospheric model predictions but are too uncertain to confirm or dismiss middle stratospheric trends