We have examined the potential causes of ozone variability on short-term, seasonal, interannual, and long-term time scales. To best analyze ozone data for long-term trends we must have an understanding of the underlying variability in ozone.

Variability in ozone can result from a number of causes. Solar UV radiation that produces ozone is present during the day but not at night. It varies with the 27-day solar rotation cycle and with the 11-year sunspot cycle. The atmospheric circulation affects ozone on many time scales. These include the regular seasonal variations, the 27-30-month quasi-biennial oscillation, and the less regular El Niño/Southern Oscillation. Sporadic major volcanic eruptions can also cause a short-term variation of ozone. Finally, the chemical effect of the long-term increase in chlorine from CFCs should lead to a long-term decrease in ozone that can be separated from the other causes of variability.

The upper stratosphere and lower stratosphere respond to the causes of variability in a manner dependent on the time scale for ozone photochemistry. In the upper stratosphere, time scales for photochemistry are short and ozone responds to the daily and 27-day variations of UV radiation from the Sun. Ozone responds to short-term and seasonal variations in temperature via a photochemical adjustment that results in ozone concentrations being anticorrelated with temperature. In the lower stratosphere, photochemical time constants are long (months to years) and the ozone concentration does not respond to short-term variations in solar UV radiation. The column amount of ozone does respond to short-term dynamical variations such as weather systems. These move ozone around from one place to another without photochemically producing or destroying it. Variations and long-term trends in the ozone column are dominated by variations in the lower stratosphere because the bulk of the ozone is located there.

Chapter 9 will examine how to use this information on variability to do the best possible job of determining the magnitude of the trend in ozone and determining the uncertainty in that trend.