Observed ozone trends in the midlatitude lower stratosphere remain contradictory with results from climate model simulations, likely due to shortcomings in the representation of tracer transport and variability. But the exact nature of these limitations and the relative role of different mechanisms remains unclear, which hampers our ability to interpret observational changes. Moreover, while models project consistent changes in stratospheric transport in response to external forcings, trends derived from observations are inconclusive in some regions.
Large climate variability and the imperfect representation of transport processes in models are believed to be responsible for the lack of consistency in observed and modelled trends, but these uncertainties remain to be quantified. Because long-term trends are hard to constrain from observations of many trace gases, the models’ ability to simulate transport is best assessed looking at the natural year-to-year variability. New datasets of satellite observations offer a unique opportunity for model evaluation.
Since 2010 there has not not been a thorough evaluation of stratospheric transport in chemistry-climate models, even though several new generations of models have been produced. An updated evaluation based on high quality satellite data will help identify the most important biases to address in order to improve projections and attribution capabilities in future model generations, and assess the progress made over the last 15 years. The timing of this proposed activity is optimal to inform the 2026 WMO/UNEP Scientific Assessment of Ozone Depletion.