7.4.6.2 Past Changes in Stratospheric Ozone

Ozone losses have been largest in the polar lower stratosphere during later winter and spring. For example, the ozone hole over Antarctica has occurred every spring since the early 1980s (Fioletov et al., 2002). Antarctic ozone destruction is driven by climatologically low temperatures combined with high Cl and bromine amounts produced from photochemical breakdown of primarily anthropogenic CFCs and halons. Similar losses, smaller in magnitude, have occurred over the Arctic due to the same processes during cold winters. During warm winters, arctic ozone has been relatively unaffected (Tilmes et al., 2004). The antarctic lower stratosphere is nearly always cold enough to produce substantial ozone loss, but in the year 2002, a sudden stratospheric warming split the early ozone hole into two separate regions (e.g., Simmons et al., 2005). Temperatures were subsequently too high to produce further ozone loss. Following the later merging of the two separate regions back into a single vortex, the dynamical conditions were unsuitable for further ozone loss. This is not an indication of recovery in ozone amounts, but rather the result of a dynamical disturbance (e.g., Newman et al., 2004). A summary of recent stratospheric ozone changes is given in Chapter 2.