Scientific Assessment of Ozone Depletion: 2014

Chapter 2 Scientific Summary

2014 Ozone Assessment cover
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Scientific Summary Chapter 2: Update on Global Ozone: Past, Present, and Future

Past Changes in Total Column Ozone

This chapter deals with the evolution of global ozone outside of the polar regions. The increase of ozone- depleting substance (ODS) concentrations caused the large ozone decline observed from 1980 to the mid- 1990s. Since the late 1990s, concentrations of ODSs have been declining due to the successful implementation of the Montreal Protocol. As reported in the last Assessment, global ozone levels have remained stable since 2000. Ozone columns observed in the last four years have largely remained in the range observed since 2000.

Over the next decades we expect increasing global-mean stratospheric ozone columns, as ODSs decline further. Climate change and emissions of greenhouse gases, especially carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), also affect the evolution of global stratospheric ozone, particularly in the second half of the 21st century, when ODS concentrations are expected to be low.

Past Changes in Ozone Profiles

Additional and improved data sets have strengthened our ability to assess ozone profile changes over the last 10 to 15 years. Data from the upper stratosphere now confirm the significance of ozone increases that were already suggested in the last Assessment. Large ozone variability in the lower stratosphere complicates the identification of long-term ozone changes in this region. Chemistry-climate model (CCM) simulations that include realistic time variations of greenhouse gas (GHG) and ODS concentrations capture changes in the ozone profile that agree quite well with those observed. These CCM simulations provide a means of attributing changes in ozone to different processes.

Future Ozone Changes

The chemistry-climate model simulations used in the last Assessment are still the main source for projection of future ozone levels and the dates of return of ozone to 1980 levels. Declining ODS concentrations, upper stratospheric cooling because of increased CO2, and the possible strengthening of the Brewer-Dobson circulation from climate change are all likely to affect recovery of global column ozone, with different relative contributions in various latitude regions.