Long-term Ozone Trends and Uncertainties in the Stratosphere (LOTUS)

WMO/UNEP Assessments on the state of the ozone layer require an accurate evaluation of both total ozone and ozone profile long term trends. These trend results are of utmost importance in order to evaluate the success of the Montreal Protocol with regards to the recovery of the ozone layer and the effect of climate change on this recovery, in the main regions of the stratosphere (polar, mid-latitudes, tropics). A previous activity sponsored by SPARC, IO3C, IGACO-O3 and NDACC (SI2N) aimed at the evaluation of trends from long term ozone profile records (ground-based and satellites, including merged satellite data records). This activity successfully provided estimates of ozone profile decreasing trend in the period 1979 - 1997 and recovery trend in the period 1998 – 2012, from a variety of long term records. The activity was timed with preparation of the WMO/UNEP 2014 Ozone Assessment, but ended after the release of the report. The SI2N team devised an approach to determine significance of ozone trends as a function of altitude, which was different from those published in the WMO 2014 Ozone Assessment report. Reason for the discrepancy was a different evaluation of uncertainties in both trend studies and more particularly the consideration of satellite drifts in the SI2N study1. Indeed, the latter study concluded that “The [trend] significance will become clearer as (i) more years are added to the observational record, (ii) further improvements are made to the historic ozone record (e.g. through algorithm development), and (iii) the data merging techniques are refined, particularly through a more rigorous treatment of uncertainties”.

For the WMO/UNEP 2018 Ozone Assessment, a clear understanding of ozone trends and their significance as a function of altitude and latitude is still needed, nearly 20 years after the peak of ozone depleting substances in the stratosphere. Since the end of SI2N, new merged satellite data sets and long awaited homogenized ozonesonde data series are being produced. There is thus a strong interest in the scientific community to understand limitations in determining significance of ozone recovery. In order to address the issues left pending after the end of SI2N, a comprehensive evaluation of all long term data sets available together with their relative drifts is needed. Evaluation of error propagation in ozone trend calculation is also required.

The objectives of the LOTUS project are:

  • to update and extend stratospheric observations to recent years
  • improve our understanding of crucial yet poorly known sources of uncertainties
  • to investigate how uncertainties interact and propagate through the different stages of analysis chain
  • to re-evaluate current best practice(s) and possibly establish more suitable alternatives

LOTUS activities are targeted at providing support and input to the next WMO/UNEP Ozone Assessment. Several new satellite and ground-based ozone profile data sets will be updated and made available to the community. A SPARC Report will accompany this data release and discuss the progress made on several key open issues identified by previous assessments, most notably on the understanding of uncertainties in the trend analysis chain. An important part of the report will be devoted to an update of the assessment of long-term trends and their significance, in different regions of the stratosphere. A section will list further requirements on data sets and analysis methods that have been identified during the project. In addition, a SPARC Newsletter will be written after project kick-off, after the workshop and after the project closure.

1 Harris N., et al., Past changes in the vertical distribution of ozone – Part 3: Analysis and interpretation of trends. Atmos. Chem. Phys., 15, 9965–9982, 2015, doi: 10.5194/acp-15-9965-2015