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Terrestrial Plant Life

10) What will be the effects of an increased UV-B radiation on crop and forest yields?

There are some UV-B-sensitive varieties of crops that experience reductions in yield. However, there are also UV-B-tolerant varieties, providing the opportunity to breed and genetically engineer UV-B tolerant varieties. For commercial forests, tree breeding and genetic engineering may be used to improve UV-B tolerance. For unmanaged or natural forests, these methods are not an option. While many forest tree species appear to be UV-B tolerant, there is some evidence that UV-B effects, sometimes detrimental, can slowly accumulate from year to year. If this finding is a general phenomenon, this would be cause for concern since it would greatly complicate breeding efforts in commercial forests and negatively affect natural forests.

11) Can plants protect themselves against increased UV-B?

Yes, partly. Plants already have reasonable UV shielding; for most plants only a small proportion of the UV-B radiation striking a leaf actually penetrates very far into the inner tissues. Also, when exposed to an enhanced UV-B level, many species of plants can increase the UV-absorbing pigments in their tissues. Other adaptations include increased thickness of leaves which reduces the proportion of inner tissues exposed to UV-B radiation. Several repair mechanisms also exist in plants, as is the case for other organisms. This includes repair systems for DNA damage or oxidant injury. The net damage a plant experiences is the result of the balance among damage, protection and repair processes. For many plants, the net damage is negligible.

Location-specific Issues

12) Is the increase in UV-B radiation caused by ozone depletion equivalent to that incurred by moving several hundred kilometres towards the equator?

Yes, but this comparison does not nullify the serious impact of an ozone depletion, as is sometimes suggested by questions like this. The suggestion is based on a fallacy, namely, comparing a personal risk perception with the effect on a population. An elevation of say 10% in risk would not be noticeable for the person involved. For a population it is quite different. With regard to skin cancer such an increase could mean 100-200 extra cases a year per million people. This would be an important public health effect. However, movements of entire populations, or even ecosystems, do not usually occur in a human lifetime, and the comparison is therefore inappropriate.

13) Can organisms adjust to a changed UV environment?

Yes, many organisms can respond physiologically with changes such as development of UV screening compounds and additional layers of protective tissues. However, there are genetic limitations to the degree to which these physiological adjustments can take place for each organism. Some can adjust more effectively than others. Over long periods of time and several generations of populations, there is the possibility that genetic adaptation can develop as well. However, in organisms with moderately long life spans and small population sizes, the genetic adaptation is likely to be very slow.

14) Does ozone depletion pose any danger in the tropics?

Probably not. Increases in UV-B radiation are unlikely, since no significant trend in stratospheric ozone has been observed in the tropics. However, viewing the biosphere as a unit, there may be indirect effects of ozone depletion at other latitudes on tropical ecosystems. If ozone were to be depleted in the tropics, this would constitute a serious danger because of the naturally occurring high levels of UV-B radiation due to the high solar angles and already relatively low normal stratospheric ozone levels.

15) Do we need to worry about relatively small increases in UV-B due to ozone depletion, when natural variability is so much larger?

Yes. The change in UV-B from ozone depletion is systematically upward. The natural variability (e.g., from time of day, or clouds) can be larger, but goes in both directions, up and down. While the evidence for ozone depletion is very strong, there is little evidence for long-term changes in cloud cover.

Many detrimental effects of UV-B are proportional to the cumulative UV-B exposure. For example, skin cancer results from the total exposure accumulated over many years under both sunny and cloudy conditions. Any systematic increase in UV-B radiation will increase incidence among a population (as well as individual risk) regardless of the natural variability of the UV-B radiation.

16) Does one get higher UV exposures at higher elevations?

Yes. Higher elevations have less atmosphere overhead, as evidenced by the thinner air and lower atmospheric pressure. The increase in sun-burning UV radiation is typically about 5-10% for each kilometre of elevation, the exact number depending on the specific wavelength, solar angle, reflections, and other local conditions. Frequently, other factors besides thickness of the atmosphere cause even larger differences in UV radiation between elevations. Snow is more common at higher elevations, and reflections from it can lead to very large increases in exposure.

Lower locations tend to have more haze and more polluted atmosphere which can block some UV radiation.

17) Does air pollution protect one from UV-B radiation?

Yes, but at a high price. Air pollution is generally undesirable due to the numerous other serious problems associated with it, including respiratory illness, eye irritation, and damage to vegetation. While most of the atmospheric ozone resides in the stratosphere, some ozone is also made in the troposphere by the chemical interactions of pollutants such as nitrogen oxides and hydrocarbons. This tropospheric ozone is a component of the photochemical smog found in many polluted areas. Airborne particles (smoke, dust, sulphate aerosols) can also block UV radiation, but they can also increase the amount of scattered light (haze) and therefore increase the UV exposure of side-facing surfaces (e.g., face, eyes).

No single value can be given for the amount of UV-B reduction by pollution, because pollution events tend to be highly variable and local. Comparisons of measurements made in industrialised regions of the Northern Hemisphere (e.g., central Europe) and in very clean locations at similar latitudes in the Southern Hemisphere (e.g., New Zealand) suggest pollution-related UV-B reductions can be important.


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