Perennial ground-based in situ observations of ODS show a clear decline since the implementation of the Montreal Protocol (Newman et al. 2007; Engel et al. 2018). However, the decreasing trend slowed down by about 50 per cent after 2012 for trichlorofluoromethane (CFC-11) (Montzka et al. 2018). There are indicators that the stratospheric O3 layer is starting to recover. Total atmospheric column O3 declined over most of the globe during the 1980s and early 1990s, but has remained stable since 2000, and there are indications of an increase in global-mean total column O3 over 2000-2013 (Figure 5.13) (WMO 2014). Since around 2000, measured concentrations of O3 in the upper stratosphere show an increasing trend, and modelling results indicate that decreasing ODSs and increasing GHGs, which increases stratospheric ozone by cooling the stratosphere, contributed equally to the increase in upper stratospheric ozone (WMO 2014; Harris et al. 2015; Chipperfield et al. 2017). Over Antarctica, positive trends for 2001-2013 were found for O3 concentrations in the lower stratosphere (about 10-20 km) for austral summer and for total column O3 for spring and summer (Kuttippurath and Nair 2017; Solomon et al. 2017). For the mid-latitudes (60°S and 60°N), there is no clear indication of O3 recovery for reasons that are not clear (Ball et al. 2018). As ODS concentrations continue to decline throughout the 21st century, stratospheric O3 concentrations are expected to rise, though the trends will be increasingly dominated by effects from rising GHG concentrations; thus, the time frame for stratospheric O3 to recover to 1960 levels is uncertain (Chipperfield et al. 2017).
Changes in ultraviolet (UV) radiation at the Earth’s surface in response to the recovery of stratospheric O3 have not yet been documented, because such changes are still masked by varying attenuation of UV radiation by O3, clouds, aerosols and other factors (Bais et al. 2018).
