Long-term increase in summer UV radiation

Do you have the impression that you get sunburnt more easily in recent summers? If so, you are probably right. A study by NIWA scientists, which appears in the 10 September issue of the international magazine Science, demonstrates that long-term increases in peak summertime UV radiation have occurred in recent years as a result of ozone depletion.

Last summer the peak summertime levels of sunburning UV measured at NIWA's laboratory in Central Otago were about 12% more than 10 years ago when measurements began. Larger increases were seen for DNA-damaging UV and for plant-damaging UV, whereas UV-A radiation, which is not affected by ozone, showed no change, in agreement with model calculations. These results provide the strongest evidence yet of man-induced increases in UV, in a region where baseline levels of UV were already relatively high.

Based on these measurements and their agreement with model calculations, we estimate that sunburning UV has increased by more than 15% at this site since ozone depletion began in the late 1970s. This is important because even relatively small increases in UV radiation can have serious impacts on human health, the biosphere, and materials. For example, each 1% increase in sunburning UV leads to increases of about 2% in non-melanoma skin cancers.

The UV radiation in the Southern Hemisphere has historically been more intense because of its lower ozone, its cleaner atmosphere, and because the closest Earth–Sun separation occurs during the southern summer. Even in the 1970s, before substantial ozone depletion had occurred, erythemal UV radiation at 45°S was probably substantially greater than during the 1990s at 45°N. In addition, in the Southern Hemisphere ozone depletion has occurred all year round, whereas in the Northern Hemisphere the depletions have been less severe in the summer. The resultant relatively intense UV radiation may be a factor contributing to the high rates of skin cancer in New Zealand, although other factors such as lifestyle and skin-type are also important.

During last summer, the UV index exceeded 12.5 several times (and was more in the north of the country). Although these UV intensities are large compared with corresponding latitudes of the Northern Hemisphere, they are not extreme by global standards. Half of the area of the globe lies between 30°S and 30°N, and all unpolluted locations in this band would receive more extreme UV radiation. Although there are only few measurements to confirm this, we know it to be true because of the higher solar elevations and the lower ozone amounts experienced there.

The future outlook is uncertain. Although the stratospheric loading of ozone-depleting substances is now close to the maximum expected under the present control regime, there is concern about possible interactions between ozone depletion and global warming which could delay the recovery of ozone by decades. Full recovery cannot occur until the middle of the 21st century at the earliest.

Reference: "Increased summertime UV Radiation in New Zealand in response to ozone loss", by Richard McKenzie, Brian Connor, Greg Bodeker, Science, Vol 285, 10 September, 1999