Future UV Radiation Levels
The prediction of future UV radiation levels must be considered according to the time scales of interest. On short time scales, of order of a few days or a week, UV radiation forecasts incur all of the difficulties of forecasting weather (especially clouds); of estimating atmospheric profiles of ozone and other gases and particles, some anthropogenic; and of accounting for a variety of possible other local factors including surfaces (elevation, orientation, reflectivity). These factors make accurate UV forecasts impractical beyond a few days. Next-day forecasts based on meteorological analyses are now being made with some success in a number of countries. In most cases, the results are disseminated to the public, with UV radiation levels expressed as a dimensionless UV index. International standardization was reached (WMO, 1994b; ICNIRP, 1995) on the method of calculation of the index, which is defined as the UV irradiance, in units of W m-2, weighted by the erythemal action spectrum of McKinlay and Diffey (1987), then multiplied by 40. Using this scale, a UV index of 10 or more may be considered "extreme".
Long-term UV predictions (years, decades, or longer) are exceedingly difficult and uncertain, and therefore only appropriate in a statistical sense of averages, variabilities, and broad geographical patterns. Even then, many assumptions must be made not only about the future state of the ozone layer, but also about possible long-term changes in clouds, tropospheric pollutants, and changes in surface albedo. In considering future biological effects of UV changes, it is also necessary to allow for uncertain long-term changes in ecosystem size and composition and - specifically for humans - changes in behavior, migration and demographics.
Predictions of future ozone amounts
are in themselves also very difficult. Natural perturbations such as major
volcanic eruptions are unpredictable, though their importance to stratospheric
ozone was clearly demonstrated in the aftermath of the 1991 Mt. Pinatubo
eruption. Large uncertainties exist concerning the interactions of stratospheric
chemistry with expanding human activities, e.g. the increasing emissions
of so-called greenhouse gases and the associated changes in global climate,
the effluents from growing fleets of subsonic and supersonic aircraft,
and the changes in tropospheric air quality and self-cleaning (oxidizing)
capacity. Their interactions with stratospheric ozone are current subjects
of active research and are still not well quantified (WMO, 1998). A recent
study, for example, suggests that the recovery of the ozone layer may be
delayed significantly by interactions with increasing green-house gas concentrations
(Schindell et al., 1998).
|Fig. 1.5. Scenario for future changes in ozone and erythemally-weighted UV radiation at the Earth’s surface, at 45° N and 45° S. UV radiation changes are estimated from ozone changes, which in turn are estimated from changes in atmospheric amounts of ozone-destroying substances (halocarbons). All other factors are assumed constant. Future scenarios shown are based on current control measures (Montreal 1997 Amendments), with scenario A1 (baseline, solid curves) accounting for the fact that production of some ozone-depleting substances is currently already below the allowed maximum, while in scenario A3 (dashed curves) production is at the maximum allowed level. Dotted curves are the zero-emission limit (starting in the year 2000) and only illustrate the minimum delay time imposed by atmospheric processes (from Madronich et al., 1998).|
With a clear understanding of these uncertainties, it is nevertheless of interest to examine the implications of current international regulations to the future of the ozone layer, and consequently to the future of UV radiation. The 1987 Montreal Protocol and its subsequent adjustments and amendments limit the production and emission of ozone-destroying substances, primarily halocarbons. The atmospheric concentrations of these chemicals had been increasing throughout the 1970s and 1980s, but observations in the last few years (e.g. Montzka et al., 1996) show a marked slowing of growth and even decreases in many of these compounds as a result of implementation of the Protocol (WMO, 1998). Figure 1.5 shows the temporal change of ozone and surface UV radiation (at 45° N and 45° S) computed in correspondence to the halocarbon loading of the atmosphere. This calculation assumes that changes in UV radiation are due solely to ozone changes, which in turn are assumed to respond only to atmospheric halocarbon loading. The quantitative relation between ozone and halocarbon changes is based on the measured changes in both quantities through the 1980s (Daniel et al., 1996). The future scenarios shown in the figure are based on current control measures (Montreal 1997 Amendments), with scenario A1 accounting for the fact that production of some ozone-depleting substances is currently already below the allowed maximum, while under scenario A3 production is at the maximum allowed level. In either case the UV radiation is expected to return to normal (pre-1980) levels by the middle of the next century. Scenario A2 shows the ozone/UV recovery if there is no emission after the year 2000; while this scenario is obviously unrealistic, it illustrates the natural time scale for the removal of the halocarbons already present in the atmosphere, and is therefore a fundamental limit to the rate of recovery.
Given the numerous uncertainties listed
above, it is unlikely that future UV radiation changes will follow precisely
any scenario presented in Figure 1.5. Two features of this figure are nonetheless
noteworthy. First, the return to pre-ozone depletion levels will take several
decades even under the most optimistic scenarios of compliance with international
regulations of ozone-depleting substances. Second, and perhaps most important,
is to note that in the present half-decade (1995-2000) ozone reductions
are the largest since ozone observations began. The observed slowing and
even turnover of the rate of growth of some atmospheric halocarbons is
highly significant, but large uncertainties, stemming from both future
human activities and the imperfect understanding of the complexity of the
atmosphere, leave open the question of the extent and timing of the return
to natural levels of stratospheric ozone and surface UV radiation.