The evidence for a long-term downward trend of stratospheric ozone comes from many independent measurements of total column ozone and ozone profiles (see Chapter 7 for information on these different types of measurements) that have been made over the last several decades. Ground based ozone measurements at various locations began as early as 1926. By the late 1950s, a network of ozone monitoring instruments were in place, including instruments in the Arctic and Antarctic polar regions. Over the past 20 years, we have been monitoring ozone levels on a global scale from space. These data sets give us a very good picture of the long-term changes in the ozone layer. For example, the Total Ozone Mapping Spectrometer (TOMS) instruments have been providing global coverage of total ozone nearly continuously since late 1978. Currently, the TOMS data record consists of measurements from four instruments, each flown on a different satellite.

2.1 Dobson Ozone Measurements at Arosa, Switzerland

The longest record of total column ozone is from ground based Dobson measurements at Arosa, Switzerland. Figure 9.01a shows the time series of total ozone amounts at Arosa from the beginning of the record in 1926 through 1997 (Staehelin et al., 1998). The ozone measurements at Arosa show a strong seasonal cycle, with a range of about 100 Dobson Units (DU; see Chapter 1, Section 4.1 for the definition of a Dobson Unit). The dashed lines on the plot show the estimated trend of this data set for two time periods: 1926-1973 and 1973-1993. While a decrease in the latter period is perceptible, it is small relative to the seasonal cycle in the data.

However, if we smooth the data to remove the seasonal cycle, the trend in the data after 1973 is more prominent. Figure 9.01b shows the same data, but in this figure a 1-year moving average was applied to the data. That is, each point on the graph is the average of one year of data centered about that point. This is done to smooth variations on seasonal and shorter time scales in the data. The solid blue line shows the smoothed Dobson total ozone time series, and the green line shows total ozone data measured by the Nimbus 7 and Meteor 3 TOMS instruments over Arosa.

One of the first things we note in this plot is that the TOMS measurements are typically 5-10 DU larger than the Dobson measurements. It is not unusual for different types of instruments to have small offsets in the measurements they make, as all remote sensing techniques have some uncertainty associated with them. However, in this case the offset does not concern us, because it is much smaller than the size of the feature we are studying.

After it was smoothed to remove seasonal variations, the data record showed the total ozone amount over Arosa varied between 20 and 40 DU as a result of variations on time scales longer than the seasonal cycle. From the beginning of the record until the early 1970s, the data show interannual (year to year) variations, but little long-term change, or trend. However, after about 1973, the ozone amounts started to decline (dotted line). The TOMS ozone measurements show similar declines over the latter period.

2.2 TOMS Global Ozone Measurements

While we can look at other similar examples of ozone decreases at specific locations, one might ask if perhaps the ozone has simply been transported from where we are making our measurement to some other place. To answer this question, we will look at a time series of global average ozone from satellite data.

Note that the global average of TOMS satellite ozone data is not strictly global, because the TOMS instrument does not measure ozone all the way to the poles. In this Chapter, the global average is defined as the average from 60°N to 60°S. This is the largest latitude range where TOMS has year round coverage, and covers 87% of the surface of the globe. For our purposes, this average is representative of a true global average.

This is essentially a measure of all the ozone in the atmosphere. If it is the case that the ozone is merely moving from one place to another (which we know happens because of natural variations such as the seasonal cycle and QBO, for instance), then the total amount of ozone in the atmosphere should remain constant.

Figure 9.02a shows the global average, weekly average time series of total ozone from the Nimbus 7 TOMS instrument. Here the data have not been smoothed to remove the seasonal cycle; only variations with time scales less than one week are removed by the averaging. Even without removing the seasonal cycle, it is clear that the total amount of ozone above Earth has decreased since the late 1970s. In 1979 the ozone values ranged from 290 DU to 300 DU, but by 1993 the ozone values ranged from 275 DU to 285 DU. This translates to a 5% net loss in the total amount of ozone in the atmosphere, and an ~ 10% increase in UV radiation reaching the ground (see Chapter 1).

Figure 9.02b shows another way to look at the same data. Here we compare the monthly averages of global TOMS data in the early 1990s to the monthly average of all the global data from 1979 to 1990. The red line shows the 1979-1990 average global ozone as it varies through the year, and the white shading indicates the range of global average values over this period. Notice that the pattern is repeated three times so we can compare to the three years of recent TOMS data. The Nimbus 7 TOMS data from January 1992 to April 1993 and Meteor 3 TOMS data from January 1992 to June 1994 are shown in Figure 9.02b by the yellow and blue lines, respectively. Throughout the period, the amount of ozone in the 1990s is as low as or lower than the lowest amounts from the 1979-1990 period (given by the white shading). This shows the significant decrease of global ozone in the early to mid-1990s relative to the rest of the record. In the next section, we will apply statistical techniques to estimate the trend of global average ozone.