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.
Compared to 1964–1980 total column ozone, ground-based and space-based observations show that present-day (circa 2008–2013) ozone columns are:
lower by about 2% for the near-global average (60°S–60°N), compared to 2.5% reported in the last Assessment;
lower by about 3.5% in the Northern Hemisphere (35°N–60°N), as reported in the last Assessment;
lower by about 6% in the Southern Hemisphere (35°S–60°S), as reported in the last Assessment. The larger depletion in the Southern Hemisphere is linked to the Antarctic ozone hole; and
almost unchanged in the tropics (20°S–20°N), as in the last Assessment.
Ground- and space-based observations indicate that near-global (60°S–60°N) column ozone has increased by around 1% ± 1.7% (2 sigma) between 2000 and 2013. However, there is substantial disagreement among the data sets about the magnitude and statistical significance of this increase. Two out of three independent data sets show increases at the upper end; one recently updated data set shows an increase at the lower end. The CCMVal-2 multi-model mean predicts a 1% increase between 2000 and 2013 for the near-global (60°S–60°N) column ozone.
Total column ozone (dominated by lower stratospheric ozone) displays large, dynamically forced year-to-year variability in the middle and high latitudes, exemplified by unusually high ozone in 2010 and low ozone in 2011 in the Northern Hemisphere, and low ozone in 2006 in the Southern Hemisphere. The recent decline (15% since 1997) in concentrations of ODSs, as described by Equivalent Effective Stratospheric Chlorine (EESC), is expected to have had only a small impact on total ozone recovery (approximately 3 Dobson units (DU), or 1%, since 2000). Separation of the small recent ODS-related ozone increase from the large natural variability (up to 15 DU or 5% change from one year to the next) can currently not be made with a high level of confidence.
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.
Measurements show a statistically significant increase in upper stratospheric ozone (35–45 km altitude) in middle latitudes and the tropics since around 2000. Following a large observed decline of 5–8% per decade through the 1980s and middle 1990s, ozone has increased by 2.5–5% per decade over the 2000 to 2013 period.
About half of the upper stratospheric ozone increase after 2000 can be attributed to the decline of ODS since the late 1990s. Increasing CO2 concentrations have led to a cooling of the upper stratosphere. CCM simulations reveal that, between the 1980s and the present this has contributed to an increase in ozone concentrations. Before the middle 1990s, this ozone increase was substantially smaller than the ozone decrease caused by ODS increases. From 2000 to 2013, the ozone increase arising from the decline in ODS concentrations is of comparable magnitude to that caused by upper stratospheric cooling.
As reported in the last Assessment (WMO, 2011), CCMs consistently show a long-term decline of ozone in the lowermost tropical stratosphere by up to 20% between 1960 and 2060. This modeled ozone decline is caused by an increase in the strength of upwelling in the tropical lower stratosphere. This increased upwelling is associated with a strengthening Brewer-Dobson circulation caused by GHG-induced climate change.
In-situ and space-based observations reveal that ozone concentrations in the lowermost tropical stratosphere have declined by as much as 10% between 1984 and 2005. There are several additional data sets available since 2002. Continued ozone decreases are not detected in the presence of large natural variability during 2002–2013. This observed behavior is consistent with that computed in CCMs, which also show periods of strong interannual and decadal variability.
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.
Estimates of the likely return dates of total column ozone concentrations to their 1980 values have not changed since the last Assessment. The best estimates are:
by midcentury for global mean annually averaged ozone;
between 2015 and 2030 for annually averaged Northern Hemisphere midlatitude ozone;
between 2030 and 2040 for annually averaged Southern Hemisphere midlatitude ozone; and
for annual average tropical column ozone, slowly increasing until the middle of the 21st century, before leveling off at values about 0–3% below 1980s columns.
The updated lifetimes estimated for ODSs in the SPARC lifetimes report have no significant impact on model projections of future ozone evolution.
Projections of future ozone levels depend substantially on the assumed scenario of greenhouse gas (GHG) emissions, especially in the later half of the 21st century. Six chemistry-climate model simulations show that projected total ozone columns in 2100 differ by up to 20 DU or 7% in the global average, by up to 40 DU or 12% in midlatitudes, and by up to 10 DU or 4% in the tropics between minimum and maximum radiative forcing Representative Concentration Pathway scenarios for future CO2, N2O, and CH4 emissions. These new estimates of scenario uncertainty are broadly consistent with previous estimates from different models and scenarios reported in the last Assessment. Our confidence in the magnitude of this scenario uncertainty remains low because of the small number of models and scenarios assessed.
Part of the scenario uncertainty in future column ozone is due to differences in emissions of N2O and CH4 between different scenarios. Increases of stratospheric N2O and CH4 impact the chemical cycles relevant for ozone. Higher N2O emissions tend to reduce column ozone, whereas higher CH4 tends to increase column ozone, each by a few percent from 2020 to 2100. The magnitude of these effects on ozone is comparable to what is expected from stratospheric cooling by CO2 increases. The influence of each individual trace gas on ozone also depends on emissions of the others, meaning that their impacts on ozone are strongly scenario dependent.
Given that ODS levels remain high, a large enhancement of stratospheric sulfate aerosol in the next decade, e.g., due to a volcanic eruption of the same size as Mt. Pinatubo, could result in chemical losses of at least 2% in total ozone columns over much of the globe. Confidence in this conclusion is strengthened because the long-standing puzzle about the midlatitude hemispheric asymmetry in the midlatitude ozone response to Mt. Pinatubo aerosols is now much better understood. Studies have shown that enhanced ozone transport in the Brewer-Dobson circulation more than compensated the enhanced chemical loss in the Southern Hemisphere.