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9 The Ozone Layer and Ultraviolet Radiation The first unmistakable sign of human-induced change in the global environment arrived in 1985 when a team of British scientists publishecl findings that stunned the world community of atmospheric chemists. Joseph Farman, of the British Meteoro- Togical Survey, and colleagues reported in the scientific journal Nature that concentrations of stratospheric ozone above Antarc- tica had plunged more than 40 percent from 1960s baseline levels during October, the first month of spring in the Southern Hemisphere, between 1977 and 1984. Most scientists greeted the news with disbelief. Existing the- ory simply had not predicted it. It meant that for several months of the year a hole forms in the ozone layer, which protects an- imals and plants from ultraviolet solar radiation. Suddenly it seemed that the chemical processes known to deplete ozone high in the earth's atmosphere were working faster and more efficiently than predicted. The discovery brought home a critical fact about the planet. No matter how much we learn about the workings of the earth system, the unexpected can always occur. Ground-based observations conducted by the National Oce- anic and Atmospheric Administration (NOAA) since 1964 had 103
104 THE FACES OF GLOBAL ENVIRONMENTAL CHANGE not revealed the drop. Measurements taken by the Total Ozone Mapping Spectrometer aboard the Nimbus 7 satellite operated by the National Aeronautics and Space Administration (NASA) since 1978 reflected the change but had not yet been analyzed. When researchers scrutinized the data, Farman's findings were confirmed, but tough questions remained. What processes were causing the hole? Would the thinning of the ozone layer spread to other latitudes, or was it confined to the Antarctic? To gather more information about antarctic ozone chemistry and the ozone hole anct its causes, a team of scientists led by atmospheric chemist Susan Solomon, of NOAA in Boulder, Col- orado, headed off in 1986 on the first National Ozone Expedition to the Antarctic. By 1987 they and other teams of researchers had learned that the ozone over Antarctica had been reduced by more than 50 percent of values recorded in 1979, the first Oc- tober of satellite operation, and that at altitudes between 15 and 20 kilometers, depletion was as great as 95 percent. In 1988 tem- peratures (which influence processes in the stratosphere) were milder than in 1987, and in October the ozone declined by about 15 percent of 1979 values (aIreacly 20 percent below the baseline values of the 1960s). in 1989 temperatures dropped again, and ozone levels matched the severe depletion of 1987. CHEMISTRY OF THE OZONE LAYER Until the hole was discovered, scientists were fairly sure that they understood the chemical processes at work in the ozone layer. Oxygen molecules (O2), abundant throughout the atmosphere, are split apart into individual atoms (O+O) when energized by radiation from the sun. These atoms are free to collide with other O2 molecules to form ozone (Oily. The partic- ular configuration of the ozone molecules allows them to absorb the sun's radiation in ultraviolet wavelengths that are harmful to life if they penetrate to the earth's surface. The ozone molecules formed by collision are partially re- moved by other naturally occurring chemical reactions, and so
THE OZONE LAYER AND ULTRAVIOLET RADIATION 60 Ozone is ford In varying concentrations from the earth's surface to a height of some 60 kilometers. Its concentration ~n- creases sharply In the stratosphere. Max- ~mum ozone concentrations occur at a height of 25 to 35 kilometers but even here never exceed about 10 parts per mil- lion by volume. (Adapted from U.N. En- v~ronment Programme. 1987. The Ozone Layer, Fig. 2, p. 9. Copyright ~ 1987, U.N. Environment Programme.) 105 Troposphere o 0 2 4 6 8 10 OZONE CONCENTRATION (ppm) the overall concentration of stratospheric ozone remains con- stant. High above the stratosphere, the density of gases is so low that oxygen atoms rarely find other molecules to collide with, and ozone does not form in abundance. Below the ozone layer, too little solar radiation penetrates to allow appreciable amounts of ozone to form. Thus most of the worId's ozone is in a stratospheric layer bulging with ozone at altitudes from 10 to 35 kilometers. Closer to the ground, in the troposphere, ozone procluced through a series of chemical reactions involving hydrocarbons and nitrogen oxide emissions from vehicles and industrial activ- ity is an effective greenhouse gas (in addition to having adverse impacts on human health at high concentrations). Thus ozone plays two very different roles in global environmental change: one in the stratosphere as a shielct against harmful ultraviolet radiation, and another nearer the ground in the troposphere as a greenhouse gas and a health hazard. It is now known that in addition to the naturally occurring chemical reactions in the stratosphere, certain reactions involv- ing chemical species of industrial origin, including chlorine and bromine, also chemically destroy ozone molecules. Atmospheric chemists F. Sherwood Rowland, of the University of California at Irvine, and Mario T. Molina, now at Massachusetts institute of
106 THE FACES OF GLOBAL ENVIRONMENTAL CHANGE Technology, first hypothesized the link between natural ozone in the stratosphere and chlorine released into the atmosphere from inclustrial sources. In 1973 they began to wonder: What happens to the industrially produced chlorinated molecules that are releasecT into the lower atmosphere ancT for which no nat- ural mechanisms for removal are known? The only long-lived natural source of chlorine in the earth's atmosphere is methyl chioricle, which comes from the ocean and is present in the atmosphere at low levels. The researchers hypothesized in 1974 that increasing con- centrations of chIorofluorocarbons (CFCs), synthetic compounds that are chemically very stable in the lower atmosphere, rise unchanged through the lowest atmospheric layer, the tropo- sphere. Even though CFCs are produced mostly in the indus- trialized countries of Europe and North America- where they are used in a wide variety of applications such as for solvents and refrigerants they mix throughout the Tower atmosphere, so that there are as many CFC molecules over Antarctica as over Colorado or Washington, D.C. The researchers surmised that upon reaching the stratosphere, the CFCs encounter high- energy ultraviolet light, which breaks them down, releasing their chlorine atoms. The chlorine atoms can then engage with ozone in a catalytic reaction in which each chlorine fragment can destroy up to 100,000 ozone molecules before other chemi- cal processes remove the chlorine from the atmosphere. The hypothesis was borne out and improved by measure- ments and observations. In 1970 chlorine was present in the stratosphere at 1.2 parts per billion, and at about 3 parts per billion in 1985. Were CFC use to continue at the 1985 rates (an eventuality precluded in 1987 by an international agreement known as the Montreal Protocol, described below), the strato- sphere would contain about 3.2 parts per billion of total chlorine in the year 2050; current models of the chemistry and physics of the stratosphere suggest that at this concentration, total global ozone would drop by 5 percent. Rowland and Molina believed that most of the chlorine molecules that reached the stratosphere would form relatively
THE OZONE LAYER AND ULTRAVIOLET RADIATION 107 inactive and harmless compounds. The ozone depletion would occur gradually, they hypothesized, and might not be detected for many years. As ozone was lost, more ultraviolet radiation would reach the earth's surface. The researchers said two of the CFCs-CFC-~l, which is widely used as a blowing agent in plastic foam, and CFC-12, mostly used as a refrigerant were particularly likely to destroy ozone because of their widespread use. These two CFCs alone are increasing in the atmosphere at an annual rate of about 5 percent. They are part of a class of chemicals known as halocarbons, many of which attack and destroy stratospheric ozone and also contribute to global warm- ing as greenhouse gases. Another chIorofluorocarbon, CFC-~13, is used as a solvent for cleaning electronic circuitry. Its atmo- spheric concentration is going up at an annual rate of about I] percent. Scientists are beginning to eye concentrations of still other synthetic halocarbons with suspicion. These include car- bon tetrachIoride, which is used as a cleaning fluid and in CFC production; methyl chloroform, used in solvents and adhesives; and halon 1301 and halon 121, which are used in fire extinguish- ers. Bromine, a chemical element that is related to chlorine and which is released from compounds used in fumigants and some fire extinguishers, is accumulating rapidly in the atmosphere. Bromine is believed to cause 10 to 30 percent of the antarctic ozone depletion. STUDYING THE ANTARCTIC OZONE HOLE In the year before the discovery of the ozone hole, scien- tists were estimating that increasing use of chIorofluorocarbons might cause reductions in the total ozone at high latitudes by about one percent in the 1980s and by 5 to 10 percent 50 to 100 years from now. "While those numbers were disturbing," Solomon said, "they were nevertheless small enough that it was hard to argue that they were even real. They were not being observed." That viewpoint changed along with scientists' faith in their
108 400 _ · - ~ 300 o rot o 0 200 100 THE FACES OF GLOBAL ENVIRONMENTAL CHANGE 400 . ^ / / ~ / _ 300 / c o o ~ 200 / - . ) · · ~ - South Pole ~ October Meon .~ . 60 1970 1980 1990 Year 1' / \\ ~-ID-~- /~ TOM S Oct.7, 1987 Em_ at_ Halley Boy October Mean 100, , 1 , 1 , I , /1950 1960 1970 1980 1990 Year \a, 300 o o ~ \200 _ 0 \ \ 100\ _ 1960 . S yowa October Mean ·` · ~ 1 1 ~1 1 1 1970 1980 1990 Year Observational data that first indicated the existence of the antarctic ozone hole. (DU, Dobson units; TOMS, total ozone mapping spectrometer.) (Reprinted, by permission, from Global Change and Our Common Future. Copyright A) 1989, National Academy Press, Washington, D.C.) models in the mid-19SOs as observations poured in from the coldest place on earth. Now, many scientists describe the antarc- tic ozone hole as the first unequivocal evidence of ozone loss due to man-made chlorine and one of the first clearly definable effects of human-~nduced global change. When the antarctic ozone hole was first discovered, little was known about the antarctic stratosphere beyond the ozone measurements themselves. Virtually no data were available on the other chemical compounds present in the stratosphere, nor was there detailed meteorological information. This information was gathered rapidly by means of aircraft and state-of-the-art instrumentation. In short order, scientists were able to measure a broad range of atmospheric compounds, including chlorine monoxide, chlorine dioxide, hydrochloric and nitric acid, nitric oxide and nitrogen dioxide, and nitrous oxide. They found that
THE OZONE LAYER AND ULTRAVIOLET RADIATION 109 the ozone levels dip at about the same latitudes where levels of chlorine monoxide ascend. As one researcher quipped, "These measurements are better than a smoking gun. This is more like seeing the guy pull the trigger." Scientists can now calculate how much ozone would be lost with a given amount of chlorine monoxide. The answer is strikingly similar to the levels of ozone depletion observed. Scientists are convinced that the elevated levels of chlorine and bromine account for much, if not all, of the antarctic ozone depletion. For most of the year, the atmosphere over Antarctica has fairly high ozone concentrations. The ozone molecules are formed over the tropics and are delivered along with chlorine to the Antarctic, as well as to the Arctic, via atmospheric mo- tions. In Antarctica, a circulation pattern known as the antarctic polar vortex traps the ozone over the South Pole for several months. It is within this vortex that scientists have measured such shockingly low ozone concentrations during the first two weeks of October, shortly after the beginning of the Southern Hemisphere spring. The explanation for the decrease lies in the combination of ozone-destroying chemistry and weather conditions that favor formation of the high, thin clouds known as polar stratospheric clouds (PSCs). The stratosphere is extremely dry, and the ice crystals that make up the clouds form only when temperatures drop to -80°C (-~12°F) or lower. The clouds foster a basic change in stratospheric chemistry by allowing reactions to occur on surfaces rather than between gas molecules. The chemical reactions that take place on these surfaces convert chlorine from forms that do not react with ozone to other, less stable forms that readily break up in the presence of sunlight and go on to destroy ozone. Both cold temperatures and sunlight are critical to the process leading to ozone depletion in the Antarctic. Antarctic ozone is depleted not during the winter, when temperatures are coldest and the South Pole is immersed in darkness, but in the southern spring, after sunlight returns but temperatures are still low. Researchers describe a process something like this: Usu
110 THE FACES OF GLOBAL ENVIRONMENTAL CHANGE ally chlorine in the stratosphere becomes trapped in so-called reservoir compounds, such as hydrogen chloride and chlorine nitrate, which themselves do not destroy ozone. Once the strato- sphere becomes cold enough that cloud particles freeze, the ice crystals provide surfaces on which reactions can occur: chlorine nitrate (ClONO2) reacts with hydrochloric acid (MCI) present on the ice surface, producing molecular chlorine (CI:) and nitric acid (HNO31. The nitric acid remains bound to the ice, and the molecular chlorine is quickly broken down Into atomic chlorine (CI). The chlorine atoms react with ozone (03), destroying it through the production of chlorine monoxide (ClO) and molec- ular oxygen (O:~. In a vicious cycle, the chlorine monoxide undergoes further reactions that re-form a chlorine atom, which is then free to destroy another ozone molecule. As researchers improve their understanding of the antarctic ozone hole, it seems less ominous than it did at first for most of the rest of the world. Over the mid-latitudes in the Southern Hemisphere, however, the hole may be spreading. Recent re- search suggests that in the late spring, when the antarctic vortex breaks up, the winds transport the polar, ozone-depleted air into lower latitudes. The record low ozone values found over the Antarctic in October 1987 were followed by record low levels over Australia and New Zealand that December as the South- ern Hemisphere summer began. NASA's Ozone Trends Panel reports that the effect may persist year round and that since 1979 ozone levels at all latitudes south of 60°S have decreased by 5 percent or more. For the most part, the hole has not spread outside of Antarc- tica and the lower Southern Hemisphere because it is limited by the seasons and the frigid temperatures required for the formation of the ice-laden polar stratospheric clouds. Yet, the insights gained during several years of intense data-gathering have raised concern about ozone in the stratosphere over the rest of the globe.
THE OZONE LAYER AND ULTRAVIOLET RADIATION OZONE DEPLETION IN OTHER LATITUDES 111 With ozone levels over the South Pole dropping up to 50 percent or more for several months each year, scientists are eager to know whether the same processes are operating to cleplete ozone over the Arctic. Results gathered by scores of atmospheric scientists using sensors aboard airplanes and balloons suggest that the arctic stratosphere differs from the antarctic stratosphere in a number of important ways that make a northern ozone hole of the same magnitude unlikely. Measurements from satellites and grouncI-based stations re- veal ozone losses of about 5 to 10 percent at northern high latitudes during the arctic winter. This is much smaller than in the Antarctic for several reasons. For one, the arctic stratosphere generally warms up much earlier in the spring than does the antarctic, and the average temperatures are warmer. This means that cold temperatures and the sunlight necessary for the forma- tion of polar stratospheric clouds and the ozone depletion they promote-overlap for a much shorter interval. Another factor is that the arctic vortex is not as tight as the antarctic vortex. As Rowland explains, air drifts across the pole, through the polar darkness, undergoes some polar stratospheric cloud chemistry, emerges into sunlight still in arctic winter-and loses a little ozone. Then the vortex warms up, and the ozone loss in the air mass stops. Meanwhile, another air mass is coming, and the process of successive small losses is repeated throughout the winter. So far, the timing of the warming in the Arctic has of- ferecl some protection against wholesale ozone depletion. But researchers worry that this may not always be the case. in the winter of 1988-1989, the arctic winter was unusually cold- the coldest for at least 25 years. In January 1989 the polar vortex was relatively stable, giving rise to conditions similar to those in the antarctic winter stratosphere. In late January near the Arctic Circle at Kiruna, Sweden, researchers measured an ozone deficit very similar to the initial stages of ozone depletion in early September in the Antarctic. With so much extra chlorine
112 THE FACES OF GLOBAL ENVIRONMENTAL CHANGE in the stratosphere, repeated occurrences of such winters could cause sudden ozone reductions over the Arctic and perhaps over much of the Northern Hemisphere. Although the unusual chemistry of polar stratospheric clouds has made the Antarctic ozone layer more vulnerable than the rest of the atmosphere, there is the particularly trou- bling possibility that similar chemical reactions couIcl occur in warmer latitudes. Temperatures outside the polar regions are 20° to 30°C too warm for ice clouds to form, but droplets of sul- furic acid and water can support reactions involving the same chlorine reservoir compounds that deplete stratospheric ozone over the Antarctic and may help to explain part of the 3 percent ozone decrease observed over the Northern Hemisphere in the past two decades. One prospect is that sulfurous particles emitted by a large volcanic eruption could team up with chlorine compounds to accelerate ozone destruction. Solomon and David I. Hofrnann, of the University of Wyoming, describe a sharp drop in strato- spheric ozone at mid-latitudes in 1982 after E! Chichon erupted in Mexico, vaulting tons of volcanic debris into the upper at- mosphere. At the time, the ozone drop was unexplained; atmo- spheric chemists still thought in terms of gases, not surfaces of particles. in light of the recent ozone studies, it seems likely that the sudden increase in the availability of surfaces provided by the volcanic debris allowect the industrially produced chlorine compounds to break down into chlorine atoms that could then destroy ozone, though more slowly than in the Antarctic. Many factors other than industrial chemicals affect the con- centration of stratospheric ozone. Ozone ebbs and flows along with the cycle of sunspots. This solar cycle affects ozone be- cause during the height of sunspot activity ultraviolet radiation increases at wavelengths that can split apart an oxygen molecule to form a molecule of ozone, causing a change of a few (! to 2) percent in ozone concentrations. The solar cycle was wind- ing down between 1979 and 1986, but it is currently increasing. The upswing in sunspot activity will lead to ozone production that could partially cancel the chlorine-caused decline, but this
THE OZONE LAYER AND ULTRAVIOLET RADIATION 113 will be temporary. The researchers warn against a sense of false security: After 1991 ozone could decrease again. They also sus- pect that ozone responds to a 26- or 27-month cycle of varying wind direction in which shifts in winds from the equatorial stratosphere change the flow of ozone to the poles. Still other factors fuel concern for the global ozone layer. Rowland and colleagues report that the amount of water in the normally arid stratosphere could increase by 25 percent by the middle of the next century (because of water vapor produced with oxidation of increasing amounts of methane in the atmo- sphere) and contribute to increased cloud formation. EFFECTS ON LIFE The ozone layer is essential to life because it shields it from damaging ultraviolet radiation. ironically, much less is known about the biological effects of increased ultraviolet radiation than about the chemical processes of ozone depletion in the atmosphere. Researchers are trying to learn how humans, veg- etation, anct aquatic ecosystems each may be affected by ozone depletion. Scientists do know that direct exposure to ultraviolet radi- ation can damage the human immune system, cause cataracts, and increase the incidence of skin cancer. The EPA estimated in 1986 that the incidence of skin cancers would rise 2 percent for each ~ percent depletion of stratospheric ozone. (Today, mostly because of lifestyles that encourage skin exposure to strong sun- light, there are about 300,000 to 400,000 new cases of skin cancer each year in the United States.) As part of the effort to understand the effects on vegetation and crops, researchers have tested more than 200 plant species, two thirds of which show sensitivity to increased ultraviolet exposure. Soybeans, one of civilization's staple food crops, is particularly susceptible to ozone damage, as are members of the bean and pea, squash and melon, and cabbage families. Plant responses to ultraviolet radiation include reduced leaf size,
114 THE FACES OF GLOBAL ENVIRONMENTAL CHANGE stunted growth, poor seed quality, and increased susceptibility to weeds, disease, and pests. Scientists are also in the early stages of understanding how ultraviolet radiation might affect marine ecosystems and ani- mals. Concern about these systems begins with phytoplankton, microscopic marine algae that form the base of the marine food web. Studies in the tropics have shown that significant amounts of ultraviolet radiation can kill them, while lesser amounts can slow photosynthesis and thus productivity. In Antarctica, this could affect Frill, tiny crustaceans a notch up the food chain, and then fish, birds, and marine mammals including seals and whales. While water provides some protection from radiation, crude estimates indicate that ultraviolet radiation can penetrate to depths of 10 to 20 meters. Some phytoplankton are known to be tolerant of ultraviolet radiation, whereas others cannot toler- ate any. A likely response will be for tolerant species to replace sensitive ones, though no one knows how this would affect the fish that eat them. NATIONS JOINING TO PROTECT THE OZONE LAYER The strong scientific consensus that CFCs deplete the ozone layer prompted nations to come together in unprecedented co- operation. The Montreal Protocol on Substances That Deplete the Ozone Layer, negotiated in September 1987, calls for a 50 percent reduction in CFC production from 1936 levels by 1999. Forty-nine nations including Canada, the United States, Japan, and many nations in Europe, which together consume 80 per- cent of the chemicals controlled have ratified the protocol. An important factor in the discussion leading to the protocol was the recognition that, because the chlorine compounds are so stable, CFC molecules emitted today will exist to deplete ozone for a century or more. The average lifetime of CFC-~l, for instance, is believed to be about 75 years and for CFC-12, 110 to 140 years. With a 100-year average lifetime, Rowland explains, 37 percent of the CFCs will still be in the stratosphere after 100
THE OZONE LAYER AND ULTRAVIOLET RADIATION 115 years, about 13 percent after 200 years, and about 4 percent after 300 years. Researchers agree that CFC concentrations will continue to increase for 10 to 20 years after we stop releasing them to the atmosphere because they will escape from existing reservoirs such as automobile air conditioners, and because of the lag between emission, arrival in the high stratosphere, and decomposition. Thus, if the nations that ratified the protocol comply with the terms established, average global ozone Tosses will still continue, but at a slower rate. These facts, and the growing bocly of scientific data on the threat to the ozone layer, are prompting nations to consider a 100 percent reduction in CFC production by the year 2000. The protocol is a clelicate balance between the most up- to-date scientific information, reliable inclustrial expertise, and committed political leadership, all supported by strong and in- formed public interest. The Montreal Protocol may prove to be a mode! for actions that span national boundaries and in- terests as the world addresses common environmental issues such as greenhouse warming and other forms of global change. It is perhaps the best illustration of the emerging role of sci- entific information and scientists in discussions about policies to manage global change. As Norway's former Prime Minister and chairperson of the World Commission on Environment and Development Gro Harlem BrundtIand explains, "The scientist's chair is now firmly drawn up to the negotiating table, right next to that of the politician, the corporate manager, the lawyer, the economist, and the civic leader."