Evaluations of impacts from increased ultraviolet radiation at a given location must consider the time of year and the latitude. Ultraviolet radiation naturally varies with time of year, latitude, and altitude. For example, four times as much ultraviolet radiation reaches the earth's surface at Philadelphia at 10 AM in the summer than in the winter. Southern latitudes receive more ultraviolet radiation than northern latitudes. (Zurer, 1993).
However, it is clear that reductions in ozone levels will lead to higher levels of UVB reaching the Earth's surface. The sun's output of UVB does not change; rather, less ozone means less protection, and hence more UVB reaches the Earth. In Antarctica, studies have shown that the amount of UVB measured at the surface can double during the annual ozone hole (Figure 4). A United Nations Environmental Program (UNEP) assessment estimates that for every 1 percent decrease in ozone, biologically damaging ultraviolet radiation will increase 1.3 percent. Another study recently confirmed the relationship between reduced ozone and increased UVB levels in Canada during the past several years.
Oceanic Carbon Cycle
The effects on aquatic ecosystems, especially on phytoplankton and larvae of higher organisms, are of particular concern. Marine phytoplankton play a fundamental role both in the food chain as well as the oceanic carbon cycle by which atmospheric carbon dioxide is converted into oxygen. See Figure 13. Approximately 30 percent of the world’s animal protein for human consumption comes from the sea (Tevini, 1983). The base of the marine food chain are the phytoplankton organisms which are concentrated in high latitudes where reductions in stratospheric ozone are predicted to cause the greatest increase in the amount of UV-B radiation reaching the Earth’s surface. Equatorial regions contain densities of phytoplankton approximately 10 to 100 times smaller than the circumpolar regions (UNEP, 1994). Additional concentrations of phytoplankton occur in upwelling areas along the continental shelves. Investigations in Antarctica indicate that current UV-B radiation levels already affect phytoplankton productivity (UNEP, 1994; Tevini, 1993). Current UV-B radiation levels are also limiting factors for early developmental stages of fish, shrimp, crab, amphibians, and other animals (UNEP, 1994).
Quantitative estimates of the potential effects of increased UV-B radiation on the marine ecosystem are questionable given the current state of knowledge. A complicating factor is that small changes could cause nonlinear (multiplicative) reactions. One study estimated that a 16 percent reduction in stratospheric ozone levels would produce a five percent loss of phytoplankton productivity, leading to a loss of approximately seven million tons of fish from the annual fisheries harvest (UNEP, 1994).
Terrestrial plants vary considerably in their response to UV-B radiation between species and even between cultivars of the same species. Plants have several mechanisms to ameliorate or repair adverse effects from UV-B radiation, and may acclimate to a certain extent to increased UV-B radiation levels. In agriculture, reduction in stratospheric ozone will require the use of UV-B tolerant cultivars and the development of new ones. Scientific evidence indicates that there will be an adverse effect on crops, but the magnitude of these effects cannot be estimated given the current state of knowledge (UNEP, 1994; Tevini, 1993).
The risks of increased UVB due to stratospheric ozone depletion includes damage to crops and aquatic organisms, increased formation of ground-level smog, and accelerated weathering of outdoor plastics.
Another concern relates to the global warming potential associated with decreases in stratospheric ozone. However, recent modeling studies conclude that decreases in stratospheric ozone serve to cool the global climate (WMO, 1994).
In addition to the above effects on the biosphere, increased UVB can have direct effects on humans including increased skin cancer, cataracts, and suppression of the human immune response system.
Laboratory and epidemiological studies demonstrate that UVB causes nonmelanoma skin cancer and plays a major role in malignant melanoma development. A major effort over the last several decades has been to understand the results of human epidemiological studies that have investigated the relationship between various forms of skin cancer and increased UV-B radiation. The USEPA has used the results of these studies to support its rulemaking on the protection of stratospheric ozone, concluding that it may be reasonably anticipated that an increase in UV-B radiation caused by a decrease in the ozone column would result in increased incidences of cutaneous malignant melanomas (potentially mortal skin cancers). In addition to the conclusions reached by USEPA, other analyses have been published which acknowledge the adverse relationship between reduced stratospheric ozone and increased cancer incidences (Shea, 1988; Van Der Leun, 1986).
Non-melanoma skin cancers mainly include basal cell carcinoma and squamous cell carcinoma. The mortality rate from non-melanoma skin cancer is less than or equal to one percent in areas with good medical care (UNEP, 1994; Tevini, 1993). An estimated 1.2 million cases occur worldwide annually (UNEP, 1994). The development of non-melanoma skin cancer is correlated strongly to exposure to sunlight and sufficient scientific information is available to roughly forecast the effects of increase UV-B radiation. A one percent decrease in stratospheric ozone is estimated to cause an increase of approximately 2.3 percent in non-melanoma skin cancer (UNEP, 1994; Tevini, 1993).
For melanoma skin cancer, sufficient scientific information is not available to project increased incidences. The incidence of melanoma skin cancer is lower than non-melanoma skin cancer by a factor of ten (Tevini, 1993) for an estimated 120,000 cases worldwide annually. However, the mortality is much higher, approximately 25 percent in areas with good medical care (Tevini, 1993). Rather than cumulative exposure to UV-B radiation, studies suggest that melanoma may be produced by severe episodic exposures (sunburn). These results are inconclusive. Earlier estimates by USEPA (1987) were that each one percent decrease in stratospheric ozone would increase the incidence of melanomas by one to two percent and mortality by 0.8 to 1.5 percent. Because of the many uncertainties involved, these estimates are considered questionable (Tevini, 1993).
Potential human health effects on the eyes include increased incidence of "snowblindness" and cataracts. The medical term for snowblindness is photokeratitis, an acute inflammation of the superficial layers of the eyes. The effect is dose related and can cause lasting damage in severe cases. Increased UV radiation will likely increase incidences. However, eye protection is available and a single incident is usually sufficient to encourage use of protective sunglasses (UNEP, 1994; Tevini, 1993). Sufficient information is available with regard to cataracts to roughly forecast increases. An approximate 0.5 percent increase in cataracts would occur for each one percent drop in stratospheric ozone (UNEP, 1994; Tevini, 1993). An estimated 17 million people in the world are blind due to cataracts (Tevini, 1993). Based on calculations presented in the cited reference (Tevini, 1993), the number of additional blindness annually due to cataracts is estimated as 680,000. A one percent drop in stratospheric ozone therefore could cause an additional 3,400 cases of blindness due to cataracts each year.
Laboratory studies have also shown that increased exposure UVB weakens the immune response system. A reduction in the efficiency of the immune system could lead to increases in cancers and infectious disease. The complexity of the immune system, which is comprised of several subsystems that help and suppress each other, and the complex reactions of different types of diseases to UV-radiation prevent any quantitative predictions of the effects of increased UV-B radiation given the current state of scientific knowledge (UNEP, 1994; Tevini, 1993).
All sunlight contains some UVB, even with normal ozone levels. It is always important to limit exposure to the sun. However, ozone depletion will increase the amount of UVB, which will then increase the risk of health effects.
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