Quantifying stratospheric ozone trends: Complications due to stratospheric cooling

[1] Recent studies suggest that ozone turnaround (the second stage of ozone recovery) is near. Determining precisely when this occurs, however, will be complicated by greenhouse gas‐induced stratospheric cooling as ozone trends derived from profile data in different units and/or vertical co‐ordinates will not agree. Stratospheric cooling leads to simultaneous trends in air density and layer thicknesses, confounding the interpretation of ozone trends. A simple model suggests that instruments measuring ozone in different units may differ as to the onset of turnaround by a decade, with some indicting a continued decline while others an increase. This concept was illustrated by examining the long‐term (1979–2005) ozone trends in the SAGE (Stratospheric Aerosol and Gas Experiment) and SBUV (Solar Backscatter Ultraviolet) time series. Trends from SAGE, which measures number density as a function of altitude, and SBUV, which measures partial column as a function of pressure, are known to differ by 4–6%/decade in the upper stratosphere. It is shown that this long‐ standing difference can be reconciled to within 2%/decade when the trend in temperature is properly accounted for.