Global Effects of Ozone Depletion
Josh Burrell (programmer), Allen Hankins (writer), Erin Britt (researcher)
Ozone depletion in the earth’s atmosphere is under constant scrutiny by the American
press and people. Perhaps the concern is warranted; the ozone layer protects life on
earth by absorbing 97-99% of the damaging ultraviolet radiation from the sun (U.S.
Environmental Protection Agency, 2006). Over the past twenty years, the stratospheric
ozone has decreased approximately 3%
per decade. Possible
concerns have been raised about loss of polar caps, DNA damage, increased human health
risks, and environmental process disturbance. Emerging research has helped clarify
effects of global ozone depletion. Three areas that have received recent scientific
attention include UV effects upon macroalgae, decomposition, and nitrogen fixation.
Ozone is depleted both by natural and industrial sources. One natural source of depletion is volatile organohalogens, which are produced by marine macroalgae (Laturnus, et. al., 2004). Scientists are not sure why macroalgae produce organohalogens, but many believe it is due to stress. UV radiation is a known source of plant stress. If this is the case, then plants would likely produce more organohalogens if exposed to increased UV radiation. To test this hypothesis, a group of Swedish scientists applied increased levels of radiation to five marine macroalgal species. After four hours, the levels of organohalogens produced by the macroalgae were measured. The scientists found that “exposure to ultraviolet radiation rendered a significant increase in the concentration of four or more of the volatile organohalogens for each of the studied macroalgal species” (Laturnus, et. al., 2004).
Although the findings appear quite disturbing initially, the increases in chloroform and methyl iodine are relatively minor. Macroalgae (natural sources) account for less than .2% of the global input of chloroform and methyl iodine (Laturnus, et. al., 2004). However, approximately 90% of chloroform comes from natural sources. Therefore, if increased UV radiation has a similar effect on other natural chloroform-emitting sources, the increase in organohalogens could be much more significant. If such an increase were significant, it would create a vicious circle – increased UV radiation causes stress to living organisms; increased stress causes a higher production of organohalogens; organohalogens breakdown stratospheric ozone; lower levels of ozone increase earth’s surface radiation.
Increased surface radiation has also been linked indirectly to increased decomposition time. Barley that has been exposed to increased levels of UV-B radiation has been shown to have increased levels of lignin (a binding agent) and cellulose (Pancotto, et. al., 2005). Microbes are unable to breakdown lignin and cellulose as quickly as soluble carbohydrates. As a result, decomposition takes longer. Scientists speculated that barley litter exposed to higher levels of UV radiation would decompose quicker than barley exposed to ambient levels of radiation (regardless of exposure to UV radiation during growth). However, a group of American and Argentine scientists found the slight decrease in decomposition time to be insignificant compared to the overall increase due to higher levels of cellulose and lignin. It is important to note, however, that the results of this study do not confirm other similar studies, likely due to varying plant types and levels of UV exposure. To date, the only regions of the earth that are receiving significant levels of increased UV exposure are near the poles where there is virtually no organically abundant plant environments to study. Thus, at this point, Pancotto’s experiment is only helpful in modeling possible consequences of continued ozone depletion on the earth’s ecosystem.
A third area in which concerns have been raised is UV radiation’s effect upon plant DNA and nitrogen fixation. A study of deciduous trees’ sensitivity to increased levels of UV-B radiation found that sensitivity varies by species, but most are highly resistant (Julkunen-Tiitto, et. al., 2005). Short-term studies show that plants adapt by producing antioxidants to inhibit destructive oxidative reactions. They also produce phenolic compounds which have the ability to absorb UV-B radiation, thus reducing its destructive effects upon DNA relatively quickly. However, plants require more time to adapt their nitrogen fixation processes. Cultures of Anabaena, Nostoc, and Scytonema showed a “significant reduction of biomass and overall productivity” as a result of increased UV-B (Solheim, et. al., 2004). S. uncinata showed a significant reduction of nitrogen input into the soil after three and four years of increased UV-B exposure. However, after six years, the reduction in nitrogen fixation was much smaller, and eventually insignificant. Scientists believe that the plants followed this pattern because they eventually adapted to higher levels of radiation and were able to assume their previous levels of nitrogen fixation. Although this adaptation is slower than that of antioxidant and phenolic compound production, it showed complete recovery of fixation.
It is clear that the increased levels of UV radiation associated with ozone depletion have potentially far-reaching effects. However, recent scientific and statistical analysis shows that stratospheric ozone depletion in regions outside of the poles has stopped (Yang, et. al., 2006). Changes in stratospheric ozone levels are best represented by a combination of three satellites and two observation networks: the Stratospheric Aerosol and Gas Experiment I and II (SAGE I/II) instruments; the version-19 Halogen Occultation Experiment (HALOE) data; the Total Ozone Mapping Spectrometer (TOMS)/SBUV data; and 36 Dobson/Brewer ground-based observation stations. A combined analysis of this wealth of data found that “the thickness of Earth’s stratospheric ozone layer stopped declining after about 1997” (Yang, et. al., 2006). The most likely cause is international efforts to reduce the production and release of ozone-depleting compounds into the environment since the 1987 Montreal Protocol.
Ozone depletion affects life around the globe in many ways. Higher levels of UV radiation increase the production of ozone-destroying organohalogens in macroalgae and decrease decomposition speed and nitrogen fixation in studied plants. However, recent studies suggest that the earth’s ozone is no longer depleting in areas outside of the polar regions. If the stratospheric ozone continues this trend, the earth may have already seen the brunt of decreased ozone levels and may be on the path to recovery.
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