A primary goal of stratospheric ozone research is to accurately calculate long-term ozone trends. That is, to determine whether stratospheric ozone amounts are consistently increasing or decreasing over long time periods. We are particularly concerned with long-term decreases of stratospheric ozone. Decreases in ozone amounts would allow more ultraviolet solar radiation to penetrate to the ground, which could lead to serious biological damage in plants, animals, and humans (see Chapter 1, Section 2.0).

In Chapter 3, and in more detail in Chapter 8, we discussed several of the natural processes that change the ozone amounts on short-term, seasonal, interannual, and long-term time scales. We saw that the most prominent variations of ozone resulted from the seasonal cycle, the Quasi-Biennial Oscillation (QBO), and the 11-year solar cycle. Interannual dynamical variability, the El Niño Southern Oscillation, and occasional volcanic eruptions were also discussed as sources of ozone variability. Short-term variations, such as the diurnal cycle and 27-day solar rotation cycle also vary ozone amounts, but these variations do not affect the calculation of the long-term trend.

We also learned in Chapter 8 that an ozone trend is a source of long-term variability. Observations from many regions of the globe clearly indicate a long-term decrease in ozone. Most people are aware of the severe ozone loss over Antarctica that is referred to as the "ozone hole," about which more will be said in Chapter 11. However, many are not aware that there are also significant decreases of stratospheric ozone at other latitudes, including highly populated regions such as the northern hemisphere midlatitudes. Scientists expect that these observed ozone decreases are in part a result of the release of manmade chlorofluorocarbon compounds (CFCs) into the atmosphere. Over the period of observed ozone decreases during the 1980s and into the mid-1990s, corresponding increases of CFCs released into the atmosphere have been measured (Montzka et al., 1996). These compounds eventually make their way into the stratosphere. As demonstrated first by aircraft and balloon measurements and later by measurements from instruments on the Upper Atmosphere Research Satellite (UARS), the subsequent increase of stratospheric chlorine concentrations cause the seasonal "ozone hole" over Antarctica (Solomon, 1990; Waters et al., 1993). However, the mechanism for middle latitude ozone loss is less certain. Other factors which may contribute to a long-term middle latitude ozone trend include a long-term trend in the stratospheric circulation pattern (resulting from unknown causes) or a long-period cycle in the ozone that we are not able to resolve because of our limited data record. We will explore the possible causes of observed systematic decreases of ozone in Chapter 10.

In this chapter, our goal is to characterize the magnitude of the ozone loss, regardless of its cause. In Section 2, we present evidence of long-term ozone decreases in satellite and ground based observations. We describe how statistical models are used to estimate the long-term trend from time series observations in Section 3, and we discuss the uncertainties associated with the ozone data sets and statistical models in Section 4. Results of current ozone trend research are found in Section 5, and the corresponding trends in surface UV are reviewed in Section 6.

1.1 Chapter Objectives

After reading this chapter you should understand