Expected Changes in Irradiances, Particularly UV-B, in the Biosphere
Expected changes in UV radiation reaching the biosphere are fundamentally tied in with expected changes in stratospheric ozone. Predictions of changes in stratospheric ozone have a high degree of uncertainty associated with them. Present chemical models predict the decrease of a few percent over the next decade. Resultant trends in UV will be difficult to detect with present and currently proposed monitoring efforts. Changes in UV levels, however, are not limited to linear trend observations. Change in variance and episodic activity may also occur and may be detected by monitoring instruments.
Concern over possible changes in UV reaching the biosphere have brought about monitoring efforts of two types: efforts to monitor ozone concentrations and efforts to monitor absolute UV radiation directly. Both approaches are continued for the future and offer separate information about UV reaching the biosphere. There are benefits and limitations to both types of monitoring. Satellite monitoring of ozone allows for uniform assessment of world-wide changes in maximal UV doses. Radiative transfer models to estimate UV dosage based on ozone are reliable, however, only in clear sky conditions. Surface UV measurements allow direct measurement of biologically-active doses although devising instrumentation for UV monitoring is more difficult and offers more limitations than does ozone monitoring.
One of the most severe limitations to UV monitoring is that only a small number of geographic locations can be monitored. Aerosols, gaseous pollutants, and clouds affect the amount of UV reaching the biosphere. The effects of gaseous pollutants may alter observed UV trends by several percent per decade. Local changes in clouds can alter observed trends at a single location even more. Trends in the amounts of tropospheric pollutants, aerosols, and clouds can introduce trends in UV measurement that cannot be extrapolated to a broad geographic region, nor can they be extrapolated for predictions on future UV levels.
General conclusions cannot be drawn from UV measurements from a single location. Assessment of global changes in UV must be make from the aggregation of many monitoring sites. The use of co-located measurements of tropospheric aerosols, gases, and clouds will help develop radiative transfer modeling so that estimates may be made on world-wide changes in UV radiation.