The Montreal Protocol, strengthened by its Amendments and adjustments, has successfully controlled the production and consumption of ODSs that act to destroy the ozone layer (Q15 ). As a result, atmospheric abundances of ODSs have peaked and are now decreasing (Q7 and Q16 ). By 2012, equivalent effective stratospheric chlorine (EESC; the total chlorine and bromine abundances in the stratosphere) had declined by 15% at midlatitudes from peak values of around 15 years ago. This raises the question, is global ozone increasing in response to the observed EESC decreases?
Identifying an ozone increase is not easy, because ODS levels are not the only factor that determines global ozone levels. For example, the global ozone minimum was observed half a decade before the EESC maximum was reached. The difference in the timing resulted from the strong global ozone response to enhanced stratospheric aerosol loading after the Mount Pinatubo eruption in 1991, which led to increased ozone depletion for several years. Observed global ozone increases through the 1990s were therefore a result of the steady removal of the aerosol from the stratosphere, and not a sign of decreasing ODSs (see Q14 ). Another factor complicating the identification of ozone recovery in different regions of the atmosphere is the year-to-year variations of the stratospheric circulation. These variations lead to ozone variability in most regions of the atmosphere that is currently still larger than the signal expected from the observed EESC decreases. Finally, greenhouse gas increases (such as carbon dioxide, CO2) affect ozone by decreasing stratospheric temperatures (which slows down ozone depletion rates) and by strengthening the stratospheric circulation (which enhances the transport of ozone from the tropics to higher latitudes). It is therefore difficult to attribute observed ozone changes to these different factors.
Observations now show a clear 5% increase of ozone in the upper stratosphere (42 km) over the 2000-2013 period. Model simulations that allow for separation of the different factors suggest that about half of this increase results from a cooling in this region due to CO2 increases, while the other half results from EESC decreases. Also, total column ozone declined over most of the globe during the 1980s and early 1990s (by about 2.5% averaged over 60°S to 60°N). It has remained relatively unchanged since 2000, with indications of a small increase in total column ozone in recent years. Models suggest that this small increase is likely due to EESC decreases. These findings based on both models and observations suggest that there are initial signs of ozone recovery.
Because of their long lifetime, the impact on stratospheric ozone of the most prominent ODSs (CFC-11 and CFC-12) will continue for many decades after emissions have ceased. Assuming continued compliance with the Montreal Protocol, EESC will continue to decline over the coming decades and will return to pre-1980 levels around midcentury. With the exception of the tropics (see Q20 ), climate change is expected to accelerate the return of the ozone layer to pre-1980 levels. However, as long as ODS levels remain elevated in the atmosphere, the possibility of extreme low-ozone events due to volcanic eruptions or cold winter conditions persists into the second half of the 21st century.