7.4.6.3 Future Changes in Stratospheric Ozone
The evolution of stratospheric ozone over the next few decades will depend on natural, including solar and volcanic activity (e.g., Steinbrecht et al., 2004; Dameris et al., 2005), and human-caused factors such as stratospheric halogen loading, which is expected to decrease over future decades (WMO, 2003; IPCC/TEAP, 2005). The evolution of ozone will also depend on changes in many stratospheric constituents: it is expected that the reduction of ozone-depleting substances in the 21st century will cause ozone to increase via chemical processes (Austin et al., 2003). However, this increase could be strongly affected by temperature changes (due to LLGHGs), other chemical changes (e.g., due to water vapour) and transport changes. Coupled Chemistry-Climate Models (CCMs) provide tools to simulate future atmospheric composition and climate. For this purpose, a set of consistent model forcings has been prescribed as part of the CCM Validation Activity for Stratospheric Processes and their Role in Climate (SPARC CCMVal; Eyring et al., 2005). Forcings include natural and anthropogenic emissions based on existing scenarios, atmospheric observations and the Kyoto and Montreal Protocols and Amendments. The simulations follow the IPCC SRES scenario A1B (IPCC, 2000) and changes in halocarbons as prescribed in Table 4B-2 of WMO (2003). Figure 7.18 shows the late winter minimum total column ozone poleward of 60° for various transient CCM reference simulations compared with observations. Antarctic ozone follows mainly the behaviour of Cl and bromine in the models. The peak depletion simulated by the CCMs occurs around the year 2000 followed by a slow increase with minimum values remaining constant between 2000 and 2010 in many models. Most models predict that antarctic ozone amounts will increase to 1980 values close to the time when modelled halogen amounts decrease to 1980 values, lagging the recovery in mid-latitudes due to the delay associated with transport of stratospheric air to polar regions. The late return to pre-1980 values by about 2065 in the Atmospheric Model with Transport and Chemistry (AMTRAC) model (Austin and Wilson, 2006) is consistent with an empirical model study based on observations (Newman et al., 2006). Moreover, increased atmospheric fluxes of CFCs have recently been reported (Hurst et al., 2006), which may point to a still later recovery. The CCMs do not predict consistent values for minimum arctic column ozone, with some models showing large discrepancies with observations. In all CCMs that have been run long enough, arctic ozone increases to 1980 values before antarctic ozone does, by as much as 30 years (e.g., Austin and Wilson 2006). This delay in the Antarctic arises from an increased Brewer-Dobson circulation (Butchart and Scaife, 2001; Butchart et al., 2006) combined with a reduction in stratospheric temperatures.