5 -- CURRENT RESEARCH ON THE LONG-TERM OZONE TREND

Now that we've seen how the trends are calculated, let's look at the latest scientific results concerning the long-term ozone trend.

5.1 Total Column Ozone Trends as Function of Latitude and Season

Stolarski et al. [1991] provided the first global picture of seasonal total ozone trends estimated from the Version 6 Nimbus 7 TOMS data for November 1978 through May 1990. Since then many researchers have calculated long-term ozone trends from a number of satellite and ground based ozone instruments. In this section we present results from studies conducted at Goddard Space Flight Center. These results are consistent with other researchers' results and give a nearly global picture of the trends of total ozone.

Figure 9.07 shows the long-term trend estimated from Version 7 TOMS data (Nimbus 7 and Meteor 3 instruments) for the period November 1978 through October 1994 (McPeters et al., 1996). The V7 TOMS data are significantly improved as compared to Version 6, and will likely be the final version of the Nimbus-7 and Meteor-3 TOMS data sets.

Regions where trend values are greater than -2% per decade are colored. The black shaded regions denote the polar night (periods of extended darkness during the winter season near the poles) where measurements cannot be made. In the white regions, the long-term trend is not significantly different than a zero trend at the 2 uncertainty level (see Section 4). The first thing to note in Figure 9.07 is that the trends display a strong seasonal dependence, with the largest trends occurring during winter and spring of each hemisphere. Remember from Figures 9.06a-d that the trends in the northern midlatitudes showed a very large seasonal variation. Here we see that at 40°N, the trend varies from -6% per decade in early March to less than -2% per decade in the late summer and fall. A maximum depletion of greater than -8% per decade is estimated in the northern high latitudes during the northern winter, with smaller but significant trends during the summer and fall. Significant reductions are present in the southern hemisphere south of 40°S in all seasons, with depletions greater than -22% per decade in the high latitude southern hemisphere during October (the southern spring). This is directly associated with the Antarctic ozone hole. Throughout the tropics, the trends are nearly zero. In fact, as we saw in Figure 9.06c, the long-term, seasonally varying trends are slightly positive from January through April near the Equator.

The latitude dependent annual average trends are shown in Figure 9.08 for a number of data sets, including Nimbus-7/Meteor-3 TOMS; Nimbus-7 SBUV (Version 6 data); NOAA-11 SBUV/2 (Version 6.1.2 data), and a network of ground based Dobson photospectrometer measurements from November 1978 through February 1994. All these data sets are averaged into broad latitude bands (WMO, 1995). Results from all three systems -- TOMS, SBUV, and Dobson instruments -- agree. They show trends of less than -1% per decade in the equatorial zone, trends of -4 to -5% per decade at 45°N, trends of -3 to -4% per decade at 45°S, and trends of -8% per decade at 60°S.

These results are the trends of total ozone from the 1980s through the mid-1990s. What will happen now? As emissions of CFCs are reduced, we expect the ozone decreases to slow and eventually cease. But when might this happen? The only way to know is to continue to monitor the global ozone amounts. As of this writing, the Earth Probe TOMS instrument, launched in August 1996, makes global measurements of ozone. As the data become available, they are added to the current total ozone time series, so that updated trends can be calculated. Before we can use these data, we must establish the calibration relative to the previous instruments.

Another issue we must consider is how to model the long-term trend into the late 1990s and beyond. If the ozone does slow its decrease or even begin to increase, our assumption of a linear long-term trend will no longer be valid, and we must find a new proxy to represent the trend. Recall that the current linear proxy for the ozone trend is used in part because of the observed linear increase in stratospheric chlorine over the same period. Therefore, the future chlorine record is a promising proxy for future ozone changes. These issues are currently being studied by researchers in the ozone science field.

5.2 Profile Ozone Trends as Function of Latitude and Altitude

We've looked at the trend in total column ozone, but we would also like to know at what height most of the ozone decreases are taking place. The primary photochemical processes that affect ozone vary with altitude, so if we are to determine which processes deplete the ozone, we must know the altitude dependence of the ozone depletion. This is not easy, because measuring ozone as a function of altitude from space is difficult. Profile instruments must make measurements at more wavelengths to infer the ozone concentration at multiple levels of the atmosphere. Determining the instrument calibration for a profile ozone instrument is also substantially more difficult than it is for a total ozone instrument, because the measurement at each wavelength may have a different calibration error. In 1997, ozone researchers conducted a year long study to assess the current understanding of the trends of the ozone profile (SPARC/WMO, 1998). Here we present some of those results.

5.2.1 Profile ozone trends estimated from SBUV data -- Figure 9.09 shows the annual average trends as a function of latitude and altitude estimated from the combined Nimbus 7 SBUV (Version 6) and NOAA 11 SBUV/2 (Version 6.1.2) data set for the period November 1978 through October 1994 [SPARC/WMO, 1998; Hollandsworth et al., 1995]. We plot the trends from 65°S to 65°N, and from 25 km to just over 45 km. At higher latitudes, the SBUV data are not as reliable, so we cannot use these data to study trends of the polar ozone. In addition, the SBUV instrument is not designed to make measurements below 25 km.

Once again, the colored regions represent areas where the trends are greater than -2% per decade (significant at the 2 level). The maximum ozone depletion for both hemispheres occurs in the high latitudes, with trends of more than - 6% per decade between 40 and 45 km in the northern hemisphere and more than - 8% per decade in the southern hemisphere above 40 km. There is less depletion in the tropics (not statistically significant at the 2 level) and even a region of positive trends at the equator. In the lower stratosphere, from 25 to 30 km, the trends are generally near zero. Note that there is no indication of polar ozone loss associated with the ozone hole in this figure because the majority of the ozone loss in the ozone hole occurs at high southern latitudes below 25 km.

5.2.2 Profile ozone trends estimated from SAGE data -- Trends estimated in an independent analysis of satellite profile ozone data from SAGE (I/II) from 1979 through 1996 (Version 5.96) qualitatively agree with the SBUV(/2) trends in Figure 9.09. Both analyses indicate the largest trends are in the high latitudes above 40 km, and the trends of the tropical stratosphere and of the lower stratosphere at all latitudes are much less negative. However, the SAGE data show more depletion than the SBUV(/2) data above 30 km. Specifically, SAGE data indicate ozone trends of -8 to -10% per decade in both hemispheres at latitudes poleward of 45°, and trends of -4% per decade in the tropical upper stratosphere (compare that to the positive trends in Figure 9.09). Differences such as these are often due to inconsistencies in the calibration of the instruments. For instance, an outstanding problem of the SBUV/2 V6.1.2 calibration is believed to be contributing to the less negative trends of SBUV compared to SAGE. Not only do these analyses help us determine the ozone trend, but they also provide information on the quality of the data sets and the instrument calibration. Updated versions of these data sets, when available, should reconcile these differences and give a clear picture of the actual ozone trend as a function of altitude above 25 km.

5.2.3 Summary of profile ozone trends -- We would also like to know what is happening to the ozone below 25 km, but we must use data from other instruments to get this information. The SAGE data provides information down to 15 km, and data from ozonesondes, launched from the ground, provide information in the lowest layers of the stratosphere and in the troposphere. To estimate the trends at all altitudes in the stratosphere, we can combine the information from each of the separate instruments. Figure 9.10 shows one such calculation. Here trend estimates from SAGE, SBUV, Umkehr (another ground based instrument system), and ozonesondes from 1980-1996 were combined to estimate the profile ozone trend at northern midlatitudes.

The solid black line shows the estimated trend from all of these instruments; the thin line denotes the 1 trend uncertainty (68% probability that the actual trend lies within the solid lines), and the dashed line denoted the 2 trend uncertainty (95% probability that the actual trend lies within the dashed lines). Here are some conclusions we can draw from the data presented in Figure 9.10.

1. The trend is largest at 40 km. At 40 km, the trend is about -7.5% per decade. Above this altitude, the trends are decreasing to less than -5% per decade at 50 km. Above 47 km, the trend is not significantly different from zero (no trend) at the 2 level. The trend at this altitude is thought to be a result of gas phase photochemical ozone depletion.
2. The trend is at a minimum near 30 km. Between 40 km and 30 km, the ozone trend decreases until it reaches a minimum trend of about -2.5% per decade. Still, the trend is statistically significant at the 2 level at 30 km.
3. The trend increases again between 30 km and 15 km. This secondary maximum in the trend is nearly -7.5% per decade. Both SAGE data and ozonesonde data indicate this local trend maximum near 15 km. It is thought to be a result of heterogeneous chemistry on the surfaces of atmospheric aerosols.
4. The trend decreases in the troposphere. The only data in the upper troposphere are from ozonesondes, launched periodically from numerous stations in the 30-50°N latitude band. However, there is a significant range of results for the trends from individual stations in this band. Nevertheless, all show the general pattern of a decrease in the negative trend below 15 km into the troposphere.