Changes in biologically active ultraviolet radiation reaching the

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Earth’s surface

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National Institute of Water and Atmospheric Research

2003

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Since publication of the 1998 UNEP Assessment, there has been continued rapid expansion of the

literature on UV-B radiation. Many measurements have demonstrated the inverse relationship

between column ozone amount and UV radiation, and in a few cases long-term increases due to

ozone decreases have been identi.ed. The quantity, quality and availability of ground-based UV

measurements relevant to assessing the environmental impacts of ozone changes continue to

improve. Recent studies have contributed to delineating regional and temporal di.erences due to

aerosols, clouds, and ozone. Improvements in radiative transfer modelling capability now enable

more accurate characterization of clouds, snow-cover, and topographical e.ects.

A standardized scale for reporting UV to the public has gained wide acceptance. There has been

increased use of satellite data to estimate geographic variability and trends in UV. Progress has

been made in assessing the utility of satellite retrievals of UV radiation by comparison with

measurements at the Earth’s surface. Global climatologies of UV radiation are now available on the

Internet.

Anthropogenic aerosols play a more important role in attenuating UV irradiances than has been

assumed previously, and this will have implications for the accuracy of UV retrievals from satellite

data. Progress has been made inferring historical levels of UV radiation using measurements of

ozone (from satellites or from ground-based networks) in conjunction with measurements of total

solar radiation obtained from extensive meteorological networks.

We cannot yet be sure whether global ozone has reached a minimum. Atmospheric chlorine

concentrations are beginning to decrease. However, bromine concentrations are still increasing.

While these halogen concentrations remain high, the ozone layer remains vulnerable to further

depletion from events such as volcanic eruptions that inject material into the stratosphere.

Interactions between global warming and ozone depletion could delay ozone recovery by several

years, and this topic remains an area of intense research interest.

Future changes in greenhouse gases will a.ect the future evolution of ozone through chemical,

radiative, and dynamic processes. In this highly coupled system, an evaluation of the relative

importance of these processes is di.cult; studies are ongoing. A reliable assessment of these

e.ects on total column ozone is limited by uncertainties in lower stratospheric response to these

changes.

At several sites, changes in UV di.er from those expected from ozone changes alone, possibly as a

result of long-term changes in aerosols, snow cover, or clouds. This indicates a possible interaction

between climate change and UV radiation. Cloud re.ectance measured by satellite has shown a

long-term increase at some locations, especially in the Antarctic region, but also in Central Europe,

which would tend to reduce the UV radiation.

Even with the expected decreases in atmospheric chlorine, it will be several years before the

beginning of an ozone recovery can be unambiguously identi.ed at individual locations. Because

UV-B is more variable than ozone, any identi.cation of its recovery would be further delayed.