Trends in solar UVR

The impact of the observed ozone decline on UV-B radiation at the Earth's surface is one of the key factors which motivated long-term measurements of UVR worldwide, in order to investigate if there is an associated increase in solar UV-B. However, cloudiness has a stronger modulating impact on UVR reaching the Earth's surface than ozone. Therefore statistical analyses have shown that several decades of continuous measurements with broadband detectors under all conditions of cloudiness are necessary to identify the actual changes due to ozone [51].

2.4.1 Long-term ozone changes

Based on measurements of the total column ozone content of the atmosphere from the ground as well as from satellites, a consistent picture of the current loss of stratospheric ozone can be derived. The most recent results are discussed in ref. [3]. Relative to the values in the 1970's, the ozone loss at the end of the 1990's is estimated to be about 50% in the Antarctic spring, where the "ozone hole" appears every year, and about 15% in the Arctic spring. In the mid-latitudes of the Southern hemisphere the loss is about 5% all the year round, while in the Northern hemisphere it is about 6% in winter/spring and about 3% in summer/fall. No significant trend in ozone has been found in the Equatorial regions. In the second half of the 1990's relatively little change in ozone has been observed in the mid-latitudes of both hemispheres.

2.4.2 Long-term UVR changes

Very few long-term UV measurement records are available, and most of these were obtained with Robertson-Berger type broadband detectors (see Section 2.3.1.1). The longest continuous measurements of solar UVR extend back to the middle of the 1970's [52]. In some cases the monitoring efforts were interrupted in the 1980's, i.e. with the network of 14 stations in the USA, operational from 1974 to 1985. A first analysis of these measurements had suggested a decrease in erythemally weighted UVR [53], recalculations have been introduced to consider specific calibration practices [54], resulting in no significant trend within these data. Annual measurement campaigns at Northern mid-latitudes (Switzerland) at a high mountain site largely unaffected by air pollution have shown a slight but significant increase of erythemally weighted UVR for cloud free days between 1981 and 1991. The change of 7 ± 3% per decade (1 standard deviation)

is in agreement (within the limits of uncertainty) with the observed ozone changes in this area [55].

As absorption by ozone is strongly dependent on wavelength in the UV-B-range, but the effects of clouds, aerosols and albedo have only weak spectral effects, spectral measurements have the advantage of allowing the various attenuation effects to be separated, and thus allow consequences of ozone changes to be more clearly identified [10]. However, records of spectral UV measurements are relatively short, the longest extending back to 1990, using Brewer single monochromator spectroradiometers. In general, deducing long-term changes from relatively short time series can be very dependent on the time interval selected.

In Reading, UK, the trend in erythemal UVR, after a first order correction for clouds and aerosols, was +4.3% from 1993-1997, compared to an ozone change of — 5.9% in the same period [56]. An updated analysis of these time series to the end of year 2000 shows no change in erythemal UV from 1993-2000 and a small increase in ozone, though the results are statistically insignificant. This illustrates the effect of changing time periods of analysis in a short data record. The past few years have seen near normal levels of ozone over Southern England, while in the early years of the record (1993-1996) the effects of Mount Pinatubo were still apparent and then there were several instances of very low ozone over Northern Europe. Measurements from other mid-latitude sites show similar results for similar periods of measurement. Spectral measurements in Thessaloniki, Greece, since 1990, indicate an increase of irradiance at 325 nm as well as at 305 nm on clear sky days. Assuming that the increase at 325 nm is a consequence of changing air pollution, then the irradiance at 305 nm can be corrected for this effect. The remaining increase at 305 nm is about 10% per decade, corresponding to an average decrease of total ozone by about 4.5% per decade at the same place [57], In Toronto, Canada, integrated daily doses in summer time between 1989 and 1997 show an increase of about 8-10% per decade at 305 nm and no increase at 324 nm, which again is in good agreement with the ozone decline of about 4.3% per decade during that time in Toronto [3]. Spectral measurements at high Northern latitudes (Sodankyla, Finland, 67° North) show for the time period from 1990 to 1998 especially for springtime a significant increase in irradiance at 305 nm of about 50% per decade, whereas the change at 325 nm is about 10% per decade. In summer and autumn, the increase at 305 nm is about 20% per decade and no significant change is observed at 325 nm [58].

Besides ground-based measurements of UVR, space-born estimates of UV levels can be used in producing estimates of long-term UV variations. An analysis of zonally averaged global UV data, derived using TOMS data from 1979 to 1992, shows an increase of erythemally weighted UV doses at 55° South by about 5.5 ±3.5% per decade (2 standard deviations) and at 55° North by about 4.5% per decade. In Equatorial regions between ±35°, no significant trend on the 95% level is found [59]. The largest increase of UVR has occurred in spring time at high latitudes, corresponding to the observed decrease in stratospheric ozone.

Reconstruction of previous levels of UVR and determination of long-term variations have also been modeled using long-term ozone data from ground-based measurements together with records of pyranometer data (total solar irradiance between 300 nm and 3000 nm) and cloud information from satellites or from synoptic data. A comparison of the monthly mean trends derived with this method, with broadband measurements in Northern Europe and with TOMS satellite data has shown good agreement [60], with trends of erythemally weighted UVR of 5-10% per decade in the last two decades. Similar agreement was found for other stations in Europe [61].

2.4.3 Future levels of UVR

2.4.3.1 Forecasting UVR

During the past few years, many countries have included a prediction of the UVR for the next one or two days in operational weather forecasting. The maximum value of erythemally weighted UVR, expressed as the UV index, is then broadcast to the public. The radiative transfer calculations are based on a prediction of total ozone amount and use parameters specific to the area of interest. This works relatively well as long as cloud-free situations are considered, showing the expected uncertainties due to uncertainties of ozone, aerosols and ground albedo [62]. If clouds are part of the expected weather then the success of the cloud forecast dominates the success of the UV forecast. In a comparison of measured and forecasted UV indices at several stations in Europe under all weather conditions, the agreement was better than ± 1 UV index value in about 60% of the cases [63], with very high deviations if cloudiness was wrongly forecasted.

2.4.3.2 Future UV scenarios

Predictions about UVR levels in future years or decades are of interest too. However, for realistic estimates using radiative transfer calculations it is necessary to know the future levels of cloudiness, ozone, aerosols and albedo. Chemical transport models (CTM) allow predictions of future ozone levels based on stratospheric chemical processes and on emission scenarios for the relevant halogen gases. They suggest that the maximum globally averaged ozone depletion would take place around year 2000 and a recovery of stratospheric ozone would occur around 2050 at the earliest, if the reduction of the halogen emissions strictly follows the Montreal protocol and all its amendments [3]. However, there is a strong feedback between ozone decline and stratospheric temperature, which in turn has a strong effect on stratospheric chemistry. Furthermore, general circulation models (GCM) suggest that the increasing atmospheric carbon dioxide content will lead to a cooling of the winter stratosphere [64]. Therefore it is necessary to study the whole climate-chemistry system with coupled GCM-CTMs in order to derive future ozone estimates. As a first approach to calculation of future U V levels, climatological means of cloudiness, aerosols and albedo can be used [65], which show peak values of UV irradiance occurring significantly later than in pure CTM calculations: they may take place around 2010-2020. Additional studies are needed to take into account also future changes in cloudiness and albedo as a consequence of climate change.

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