Southern Hemisphere. Relatively low sampling frequencies (typically weekly) and calibration issues have made it difficult to use these observations to quantify long-term trends of ozone and its vertical distribution, in both the troposphere and the stratosphere (Logan, 1999). This uncertainty in ozone trends and our ability to describe them in models is the main difficulty in quantifying the radiative forcing of ozone in the past and making projections for the future.

A global climatology of total ozone columns extending back to 1979 is available from the Total Ozone Mapping Spectrometer (TOMS; see Figure 6-1) and other sensors, and has been used extensively and successfully for trend analyses (WMO, 2003). A similarly long, although sparser, record is available for the vertical distribution of ozone down to the lower stratosphere from the Stratospheric Aerosol and Gas Experiment (SAGE) and

FIGURE 6-1 Total column ozone observed on January 25, 2005, by the Total Ozone Mapping Spectrometer (TOMS) aboard the Earth Probe satellite. SOURCE: NASA Goddard Space Flight Center.

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