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Earth's protective ozone layer in the atmosphere shields the surface from the Sun's ultraviolet rays, which can cause cancer. Because Australia is found near the edge of the ozone "hole" observed above Antarctica, the ozone layer above Australia is thinner than at the same latitudes in the Northern Hemisphere. But the ozone problem is a global one. Ozone levels have plummeted above much of the planet since the 1970s. Ozone gas is a molecular "cousin" of the oxygen that we breathe. The familiar oxygen molecule O 2 has two atoms of oxygen chemically bonded together. By contrast, a molecule of ozone O 3 is made up of three oxygen atoms. In the atmosphere, ozone is much rarer than oxygen. Out of every 10 million air molecules, 2 million are oxygen but only three are ozone.
Some ozone is found in the troposphere, the layer of the atmosphere which extends about seven miles upward from Earth's surface. At ground level, ozone is a health hazard—it's a major component of smog. It causes problems for people with asthma and other respiratory ailments. Most of the ozone in the atmosphere—about 90%—is found in the stratosphere, which extends 7 to 30 miles above Earth's surface. The stratospheric ozone layer is like a gaseous ocean floating above the Earth. Its depth varies with location. Ozone is constantly produced and destroyed in a natural cycle, with the overall level remaining roughly the same. Generally, the ozone layer is thinnest around the equator and densest towards the poles. We couldn't survive without stratospheric ozone: It absorbs ultraviolet radiation, which has been linked to an increase of skin cancer rates and cataracts. In the late 1970s, scientists began observing an alarming thinning of the protective ozone layer around the globe. What caused its rapid depletion? To find out, we need to take a closer look at some chemical reactions that happen in the stratosphere. |
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Earth's Ozone Shield
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| The Role of CFCs |
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By the mid-1970s, evidence was mounting that CFCs were causing a big problem. Chlorine compounds in CFCs—unlike those from sources such as swimming pools—are so stable that they do not dissolve in rainwater. As a result, they are not cleansed by rain from the troposphere, and winds eventually drive them high into the stratosphere. The evidence showed that CFCs were contributing to the depletion of stratospheric ozone. Only ultraviolet radiation from the Sun can break down the chemical bonds in CFCs. The chemical reaction of UV light and CFCs releases a large number of ozone-destroying chlorine atoms. Each chlorine atom can destroy over 100,000 ozone molecules. A modest amount—about 15%—of the chlorine in the stratosphere comes from natural sources, such as forest fires and certain marine organisms. But the remaining 85% comes from CFCs and other chlorine-containing chemicals. The increase in CFCs over the past two decades has disrupted the natural ozone balance. Now more ozone is removed than can be naturally replaced. Soon after the link between CFCs and ozone-depletion was established, the global community moved quickly to control the spread of the chemicals. The use of CFCs in aerosol sprays was banned in the United States and several other countries. But overall use of CFCs continued to rise around the globe in the 1980s.
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Since the middle of the twentieth century, a group of chemicals known as chlorofluorocarbons, or CFCs, had been considered miracle substances. This group of chlorine-containing compounds were stable, cheap to produce, nonflammable, and relatively non-toxic. They proved especially useful in refrigerators, air-conditioners, solvents, and aerosol sprays. CFCs were too good to be true—literally.
| The Ozone Hole Grows |
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The abundance of CFCs (and other ozone destroying chemicals such as bromine compounds) led to a rapid decline in ozone levels nearly everywhere on the planet throughout the 1980s and 1990s. Scientists observed that the ozone had thinned 2% to 4% per decade around the globe at middle latitudes, which include Australia. The depletion above Australia was worse because even more ozone-depleted air moved northward from Antarctica, where the most serious ozone loss occurred. The ozone above Antarctica became so thin that scientists began using the term ozone "hole" to describe its alarming depletion. The hole naturally expands and contracts with the seasons. It reaches its maximum size in the Antarctic spring from September to November, when sunlight bursts through the atmosphere and breaks down CFCs. By 1994, the average total Antarctic ozone level was less than half what it was in the 1970s. By 1998, Antarctic ozone levels reached record lows. The hole had extended to 10.5 million square miles (27 million square kilometers), covering the entire Antarctic land mass and even the southern tip of South America. Over some parts of Antarctica, the total ozone column was depleted by up to 60% during the September through November period. The graph to the right shows October average minimum levels in Dobson Units (DU) of Antarctic ozone from 1956 to 1996. The colored dots represent three different data sources. |
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Radiation on the Rise
The amount of ultraviolet radiation reaching a particular spot on Earth at any moment depends on several other factors, including the position of the Sun above the horizon and the amount of cloud cover and pollution. But declining ozone levels have been blamed for the steady increase in ultraviolet rays reaching the surface across much of the planet. Every region of Australia saw an increase in average daily levels of ultraviolet radiation from 1979 through the mid-1990s, especially in the areas where the biggest ozone losses were recorded. A dramatic rise in skin cancer rates triggered a nationwide Australian campaign to raise awareness about the link between UV radiation and skin cancer. The flood of ads encouraging use of sunscreen, hats, and protective clothing has already paid off. Recent data suggests that the nation's skin cancer rate has started to decline. Look at the graph showing the relationship between UV radiation and stratospheric ozone levels for five locations around the world. Then complete the table below.
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A Slow Recovery
Recent evidence suggests that these measures have already made a difference. Global levels of ozone were greater in 1999 than in 1998. The ozone hole over Antarctica actually shrunk last year. At its maximum, the hole covered an area of about 9.2 million square miles in 1999, down from the 1998 total of 10.5 million square miles.
Despite the encouraging signs of healing, complete recovery of the ozone is expected to be a slow process. With continued global cooperation, the amount of CFCs in the stratosphere should continue to decline. Existing CFCs will slowly be removed from the air by natural processes. Scientists estimate that amount of ozone in the stratosphere will slowly return to normal pre-1980 levels sometime around the middle of this century. In the meantime—wherever you live or travel to—keep lots of sunscreen handy.
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