Antarctica, ozone levels

Over the past 3 decades, scientists have concluded that this protective shield has been damaged. Each year, an ozone hole forms over Antarctica. Ozone levels there can fall to 60% below normal. Even over the United States, stratospheric ozone levels are about 3% below normal in siunmer and 5% below normal in winter. 

The discovery of ozone holes over Antarctica in the mid-1980s was strong observational evidence to support the Rowland and Molina hypothesis. The atmosphere over the south pole is complex because of the long periods of total darkness and sunlight and the presence of a polar vortex and polar stratospheric clouds. However, researchers have found evidence to support the role of CIO in the rapid depletion of stratospheric ozone over the south pole. Figure 11-3 shows the profile of ozone and CIO measured at an altitude of 18 km on an aircraft flight from southern Chile toward the south pole on September 21, 1987. One month earlier the ozone levels were fairly uniform around 2 ppm (vol). 

Mario Molina and Sherwood Rowland used Crutzen s work and other data in 1974 to build a model of the stratosphere that explained how chlorofluorocarbons could threaten the ozone layer. In 1985, ozone levels over Antarctica were indeed found to be decreasing and had dropped to the lowest ever observed by the year 2000, the hole had reached Chile. These losses are now known to be global in extent and it has been postulated that they may be contributing to global warming in the Southern Hemisphere. 

The substantial concentration of ozone in the stratosphere can be significantly depleted by comparatively small amounts of other substances. The significantly depleted ozone level in polar regions (mostly over Antarctica) is referred to as the ozone hole. 

FIGURE 9.18 A false- color satellite image of the ozone hole over Antarctica on September 26, 2002. The lowest ozone concentrations are represented by the black and violet regions, where ozone levels are up to 50% lower than normal. 

A computer-generated image of part of the Southern Hemisphere on October 17, 1994, reveals the ozone hole" (black and purple areas) over Antarctica and the tip of South America. Relatively law ozone levels (blue and green areas) extend into much of South America as well as Central America. Normal ozone levels are shown in yellow, orange, and red. The ozone hole is not stationary but moves about as a result of air currents. (Courtesy NASA)... 

Unfortunately, these compounds were found to destroy stratospheric ozone.133 This is the layer that protects us from ultraviolet light. Without it, or with less of it, there will be more skin cancers and cataracts. The decline has been worldwide, but most significantly in the polar regions. The ozone hole in Antarctica was 25 million km2 in September 1998 (i.e., 2.5 times the area of Europe).134 Ozone levels at the North Pole have fallen 40% since 1982.135 A 15-year study shows an increase in the wavelengths that can damage DNA of 8% per decade at 40-de-grees north latitude, the latitude of Philadelphia, Pennsylvania and Madrid, Spain. 

While not a true hole in the sense that some column ozone remains even in the most extreme depletions observed in the mid 1990s (when October ozone minima were near 100 Dobson Units over the South Pole, or depletion of about two-thirds of the historical levels, see Hofmann et al, 1997), the descriptor captures the fact that the peak depletion is sharply limited to Antarctic latitudes. Dobson (1968 and references therein) noted that there is less ozone naturally present over Antarctica than over the Arctic in winter and much of the spring, but this climatological difference between the natural ozone levels over the poles of the two hemispheres should not be confused with the abrupt decline that began near the mid-1970s as depicted in Figure 6.9. Newman (1994) discusses these and other historical measurements of total ozone and shows that the Antarctic ozone hole began only in the last few decades. 

Figure 1.5 Satellite photos confirmed the British Antarctic Survey team s measurements that the ozone layer was thinning over Antarctica. On this satellite map, the area over Antarctica appears pink, purple, and black. The color-key on the right indicates that the ozone level ranges from 125 to about 200 Dobson Units, which is well below the normal level of 300 Dobson units. 

Figure 1.18 shows the ozone hole over Antarctica in September 2005. The ozone thinning over Antarctica reached its maximum for the year during this month. If you compare the color-coded key to the satellite image, you can see that the ozone level is between 110 and 200 DU. Notice the area surrounding the ozone hole. Much of this area has ozone levels around 300 DU, which is considered normal. 

How do ozone levels vary throughout the year in Antarctica The National Oceanic and Atmospheric Administration (NOAA) continues to monitor the concentration of ozone in the stratosphere over Antarctica. 

Interpreting ozone data The value of using graphs to visualize data is illustrated by Figure 2.17. These important ozone measurements were taken at the Halley Research Station in Antarctica. The graph shows how ozone levels vary from August to April. The independent and dependent variables are the month and the total ozone, respectively. 

The most severe depletion of ozone in the ozone hole was observed in the 2000-20001 season in the Antarctica. The area of the hole was over 28 million sq.m, and the ozone levels were about 40 percent of the normally expected level ... 

For many years, scientists suspected that significant ozone depletion might also occur over the Arctic circle. However, the Arctic polar vortex is much weaker, and the temperatures there are not as cold as in Antarctica, preventing the formation of PSCs. Scientists suspected that a particularly cold winter, like those in Antarctica, might result in significant ozone loss. In 1995, the Arctic circle experienced the coldest winter in recent history. Measurements of stratospheric ozone in the spring of 1995 showed record low levels (Figure 11-9). In some locations, ozone levels were as much as 40% below normal. 

Is the depletion of ozone over the poles a foreshadowing of what may happen worldwide Although not as dramatic as the decreases seen in Antarctica, global stratospheric ozone levels have also fallen. A United Nations Environment Program Study called Scientific Assessment of Ozone Depletion concludes that ozone in the mid-northern latitudes has decreased about 6% since 1979. The closer to the equator, the smaller the observed decrease in ozone becomes. These decreases are more troublesome than polar ozone depletion because they occur over populated areas. The evidence for ozone depletion has brought worldwide cooperation for the phaseout of chlorofluorocarbons and other ozone-depleting compounds. This cooperation, which is explained in the next section, has halted any further decreases in global ozone. 

A The chlorine in chlorofluorocarbons caused the ozone hole over Antarctica. The dark blue color indicates depressed ozone levels. 

The Montreal Protocol of July 1987 resulted in an international treaty in which the industrialized nations agreed to halt the production of most ozone-destroying chlorofluorocarbons by the year 2000. This deadline was hastily changed to 1996, in February 1992, after a U.S. National Aeronautics and Space Administration (NASA) satellite and high-altitude sampling aircraft found levels of chlorine monoxide over North America that were 5i % greater than that measured over Antarctica. 

Thus, the mean temperature of the atmosphere, which is about 20°C at sea level, falls steadily to about —55° at an altitude of 10 km and then rises to almost 0°C at 50 km before dropping steadily again to about —90° at 90 km. Concern was expressed in 1974 that interaction of ozone with man-made chlorofluorocarbons would deplete the equilibrium concentration of ozone with potentially disastrous consequences, and this was dramatically confirmed by the discovery of a seasonally recurring ozone hole above Antarctica in 1985. A less prominent ozone hole was subsequently detected above the Arctic Ocean. The detailed physical and chemical conditions required to generate these large seasonal depletions of ozone are extremely complex but the main features have now been elucidated (see p. 848). Several accounts of various aspects of the emerging story, and of the consequent international governmental actions to... 

Consequences of Ozone Depletion. Ozone depletion over Antarctica is causing renewed concern about the consequences of increased levels of UV reaching the earth s biosphere. One area of concern involves the free-floating microscopic plants, known collectively as phytoplankton (the grass of the sea), which through the process of photosynthesis, fix carbon dioxide into living organic matter. Phytoplankton forms the basis of the marine food chain on which zooplankton (animal plankton) and all other components of the ecosystem depend for their sustenance. 

Effect of UV on Productivity of the Southern Ocean. Has ozone depletion over Antarctica affected the productivity of the Southern Ocean There is no easy answer. First, one has to take into account the fact that the drastic decrease of ozone over Antarctica has been reported as recently as 1976, a relatively short time in the evolution of the organisms to develop mechanisms to cope with elevated UV. One of the most vexing problems in studying the effects of UV radiation on productivity, is a dearth of historical data on the level of UV. Without these baselines, normal fluctuations could easily be interpreted as decline in productivity. Second, there is a host of biotic and abiotic factors that play significant roles in governing the productivity of the Southern Ocean (40). Ultraviolet radiation is but one more complicating factor to be considered in an already stressful environment. 

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