Australia State of the Environment Report 2001 (Theme Report)
Lead Author: Dr Peter Manins, Environmental Consulting and Research Unit, CSIRO Atmospheric Research, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06746 9
Stratospheric Ozone (continued)
The level of UV radiation reaching the ground is controlled largely by the amount of ozone in the atmosphere (90% being in the stratosphere) and the degree of cloud cover. Data from various locations around the world show clearly that decreases in stratospheric ozone lead to increases in UV radiation under clear skies. However, the long-term trends of an expected increase in UV radiation following the near-global reduction of stratospheric ozone have proved elusive to detect, largely because of a lack of high-quality data on UV and coincident cloud cover.
In SoE (1996), the reported Australian UV radiation climatology was based on a 1978 model study that used 1960s Southern Hemisphere average ozone data from the Dobson network and surface observations of Australian cloud cover. Since SoE (1996) there have been significant advances in the description of UV radiation climatology in the Australian region. From 1979 to 1992, the satellite-based TOMS data on UV radiation exposure show good agreement with the Australian network of ground-based broad-band UV radiation detectors (Udelhofen et al. 1999). Statistically significant increases of erythemal (skin-reddening) UV radiation of 5% per decade were found for the Australian continent in summer. The summer UV radiation increases in the tropics were 10% per decade because of simultaneous depletion of ozone and decreases in cloud cover. At mid-latitudes, no significant annual UV radiation increases were found because of increasing levels of cloud cover.
Another UV radiation climatology has been derived from an Australian UV analysis and forecast model using TOMS ozone data (1979-1993), a new UV surface albedo scheme (Deschamps-Lemus 2000) and independent satellite-based estimates of total cloud cover. The clear-sky erythemal UV Index (1 unit=25 mW/m 2) is the erythemal UV dose at local noon under clear-sky conditions (Figure 76). The average summer UV Index values for Australian capital cities are 9.8 (Hobart), 11.0 (Melbourne), 11.6 (Adelaide), 11.8 (Sydney), 12.6 (Perth), 13.0 (Brisbane) and 13.6 (Darwin). The UV Index shows little variation with longitude, apart from a distinct local maximum (>14) in the region of the highly reflective (high albedo) Great Sandy Desert in north-west Australia. Similar high UV levels are observed in the region of the New Guinea highlands.
Figure 76: The average summer (Dec.-Feb.) clear-sky erythemal UV Index for the Australian region, 1979 to 1993.
Source: Bureau of Meteorology; Deschamps-Lemus (2000)
One of the best sites for the determination of long-term trends in UV radiation is Lauder, South Island of New Zealand (45S), a National Institute for Waer and Atmosphere-operated research station. The site has a 22-year Dobson ozone record, like several sites in Australia, and an 11-year record of spectral UV radiation measurements (Figure 77), unlike the Australian sites. There is a clear correlation in the Lauder data between declining ozone and rising UV radiation levels on clear-sky days. The Lauder ozone data have been used to derive, via a radiation model, a 22-year UV Index record. There is good agreement between the model-derived and the measured UV Index from 1989 to 1999, suggesting that the trend deduced from the 22-year model-derived UV Index record (about 10% per decade) is reliable.
Figure 77: (a) Mean summer (Dec.-Feb.) total ozone (DU) for Lauder, New Zealand (45S, blue) and Melbourne (38S, red); (b) Estimated (black) and measured (red) UV Index under clear-sky conditions.>
Source: McKenzie et al. (1999, 2000); Bureau of Meteorology
The Melbourne and Lauder 22-year ozone records show similar long-term trends (-4% and -4.5% per decade, respectively) and therefore the clear-sky summer UV Index record at Melbourne is likely to be similar to that recorded at Lauder (about 10% per decade). The non-linear relationship between ozone depletion and UV increases is because the amount of UV radiation increase for a specified decrease in ozone is wavelength-dependent.
Data from Australia and New Zealand show that skin-reddening UV levels appear to be rising by about 10% per decade. This means that the average exposure time for an individual in Australia to develop sunburn has been reduced by about 20% from 1980. This information should reinforce the message from community health programs encouraging reduction in exposure to UV radiation.