The most intense radiation from the Sun is emitted at visible wavelengths, and our atmosphere permits it to reach the ground. That is the colored sunlight that our eyes respond to. The Sun emits lesser amounts of invisible, short-wavelength radiation, which is partially or totally absorbed in the atmosphere.
Even though the total amount of invisible solar radiation is substantially less than the visible emission, the individual short-wavelength rays are more energetic. That is why we get sunburns from the ultraviolet radiation that manages to get through the atmosphere, and you need to be protected from the Sun when climbing at high altitudes where the air is thinner and more ultraviolet penetrates the atmosphere. The greater energy of radiation at shorter wavelengths also explains why X-rays, generated by machines, can see through your skin and muscles to detect your bones.
When absorbed in our air, the invisible short-wavelength radiation from the Sun transfers its energy to the atoms and molecules there. The solar ultraviolet radiation is largely absorbed in the stratosphere, located between 10 thousand and 50 thousand meters above the Earthís surface. This is where the ozone layer is continuously replenished and destroyed.
When ultraviolet rays strike a molecule of the ordinary diatomic oxygen that we breathe, denoted by O2, they split it into its two component oxygen atoms, or two O. Some of the freed oxygen atoms then bump into, and become attached with, an oxygen molecule, creating an ozone molecule, abbreviated O3, that has three oxygen atoms instead of two. The Sunís ultraviolet rays thereby produce a globe-circling layer of ozone in the stratosphere.
Although the ozone is present to the extent of only about 10 parts per million, the ozone layer is critical to life below. It protects us by absorbing most of the Sun's ultraviolet emission and keeping its destructive rays from reaching the ground. If there were no ozone shield, plants, animals and humans could not even exist on land.
The amount of ozone in the stratosphere resembles the level of water in a leaky bucket. When water is poured into the bucket, it rises until the amount of water poured in each minute equals the amount leaking out. A steady state has then been reached, and the amount of water in the bucket stops rising and it will stay at the same as long as you keep pouring water in at the same rate. However, if you pour the water in at a different rate, or punch a few more holes in the bucket, the steady-state level of water in the bucket changes.
Solar ultraviolet radiation supplies ozone to the stratosphere from above, like pouring water into a bucket, at a rate that depends on the varying ultraviolet output of the Sun. We have recently been punching holes in the ozone layer from below, with chemicals used in our everyday lives.
Man-made chemicals, called chloroflurocarbons, are consuming the protective ozone layer, eating holes in it and making it thinner. They are synthetic chemicals, entirely of human origin with no counterparts in nature.
The name of the chemicals is a giveaway to their composition. Each molecule has been constructed in company laboratories by linking atoms of chlorine, fluorine and carbon. The shorthand CFC notation abbreviates some of them. A number sometimes follows, providing a complex description of the number of atoms in each molecule, the most widely used being CFC-11 and CFC-12.
Beginning in 1930, the biggest producer of CFCs, the Du Pont Company, manufactured and marketed them under the name Freons. They have been widely used in refrigerators, plastic foams, spray-can propellants, automobile air-conditioning systems, and the cleaning of circuit boards used in televisions and computers.
The hardy chemicals don't interact chemically to form other substances. They are so inert and stable that once entering the atmosphere the CFC molecules can survive for more than a century, permitting them to drift and waft up into the ozone layer in the stratosphere. Although more than 20 million tons of CFCs have been released into the air, their combined concentration isnít very significant, only about one CFC molecule for every two billion molecules in the air. Yet, even these seemingly insignificant amounts can have enormous impact.
In 1974, Mario J. Molina (1943-) and F. Sherwood Rowland (1927-), two chemists who were then at the University of California at Irvine, showed that the chlorine in the CFCs can destroy enormous amounts of ozone. Once arriving in the stratosphere, the Sun's ultraviolet rays will split chlorine atoms out of the CFCs, and the liberated chlorine sets off a self-sustaining chain reaction that destroys the ozone. A single chlorine atom will react with an ozone molecule, taking one oxygen atom to form chlorine monoxide; the ozone is thereby returned to a normal oxygen molecule and its ultraviolet absorbing capability is largely removed. Moreover, when the chlorine monoxide encounters a free oxygen atom, the chlorine is set free to strike again. Each chlorine atom thus acts as a catalyst, destroying about 10,000 ozone molecules before it finally combines permanently with hydrogen.
Molina and Rowland were awarded the Nobel Prize in Chemistry in 1995 for their "contribution to our salvation from a global environmental problem that could have catastrophic consequences". They shared the prize with the German chemist, Paul Crutzen (1933-), who showed how the rate of ozone depletion could be accelerated by other chemical reactions in the atmosphere.
The ozone layer is itself invisible. But you can determine its ozone content by measuring the amount of solar ultraviolet radiation getting through the layer and reaching the ground. When there is more ozone, greater amounts of ultraviolet are absorbed in the stratosphere and less reaches the ground, and when the ozone layer is depleted, more of the Sunís ultraviolet rays strike the Earthís surface.
The British scientist G. M. B. Dobson (1889-1976) pioneered measurements of the airís ozone content about half a century ago. When his instrument was installed at Halley Bay, Antarctica, in 1957-58, Dobson found that the ozone abundance in polar spring (September-November) was noticeably less than that above other parts of the world.
Other British scientists continuously monitored the southern polar skies for 27 years; always detecting a springtime loss that became steadily larger as the years went on. By 1985 the ozone loss above Antarctica had nearly doubled when compared to the earlier measurements in the 1960s, and it extended all the way to the top of South America, where another British monitoring station detected it. A continent-sized hole had opened up in the sky Ė the ozone hole (Fig.1).
This unexpected discovery astounded space-age scientists who had not detected any ozone hole using satellites that had been monitoring the ozone layer from above. Their computers had been programmed to automatically reject large ozone depletion, apparently because their models did not predict such huge losses. So the now-famous ozone hole had been discarded as an anomaly, perhaps caused by an instrumental error. After reanalyzing the satellite data, the scientists confirmed the existence of an ozone hole in the local springtime above the South Pole.
We now know that whirling winds concentrate ozone-destroying chemicals, the CFCs, within a vast, towering vortex above Antarctica, resembling the eye of an immense hurricane. Each year the gaping hole opens up during Antarctic spring when the sunlight triggers ozone-destroying chemical reactions; the hole starts to close up in the early polar fall when the long sunless winter begins. Ozone-depleted air is dispersed globally, and the ozone is slowly restored, filling the hole until the cycle repeats in the following year.
The sudden and frightening discovery of an enormous ozone hole in 1985 sparked public awareness of the fragile ozone layer. In the meantime, the scientific community had been actively investigating Molina and Rowlandís theory that synthetic chemicals, known as the CFCs, could be destroying the ozone layer. Although global models of the expected ozone depletion initially led to widely varying estimates of the potential threat, affecting the scientistís credibility and dampening public concern, a coordinated international investigation eventually led to a unified assessment of the problem.
A group of approximately 150 scientific experts reported in 1986 that atmospheric accumulations of CFC-11 and CFC-12 had nearly doubled from 1975 to 1985. The continued release of the synthetic chemicals at the 1980 rate could, through the action of their chlorine, they said, reduce the ozone layer by about 9 percent on a global average by the last half of the 21st century, with even greater seasonal and latitudinal differences. As a result, higher levels of dangerous ultraviolet radiation could reach heavily populated regions of the Northern Hemisphere.
Who cares if chemicals are punching a few holes in the sky and letting a little more sunlight reach the ground? The U.S. Environmental Protection Agency, or EPA, cared. In 1986 it published a report of the many serious consequences of ozone depletion. A thinner ozone layer lets more solar ultraviolet radiation through to the ground, where it can produce severe biological harm. The most energetic ultraviolet rays reduce the effectiveness of the human immune system, increasing human vulnerability to infections and cancer.
The EPA estimated that there could be over 150 million new cases of skin cancer in the United States alone among people currently alive or born by the year 2075, resulting in over 3 million deaths. The dangerous ultraviolet would also produce eye cataracts, distorting the vision of about 18 million people in the same population and blinding many of them. Added to this was the potential of widespread genetic damage to crops and forests, if nothing was done to stem the production of ozone-destroying chemicals.
Faced with the evidence of vanishing ozone, the global increase of atmospheric CFCs, and the prospect of widespread skin cancer and eye cataracts, international diplomats forged an accord in 1987 to limit and eventually ban the production of the substances that deplete the ozone layer (Focus OD.1). The treaty, known as the Montreal Protocol, has led to substantial reductions in ozone-destroying chemicals made by humans.
Focus OD.1. The Montreal Protocol
Growing scientific, political and public awareness of ozone depletion eventually resulted in international negotiations to limit the manufacture and use of chemicals that destroy atmospheric ozone. A scientific consensus in the 1980s indicated that the destruction of the ozone layer by these synthetic chemicals had already begun, and that this would ultimately affect the health of large numbers of people if the production of the chemicals were not curtailed. The press and television also played a vital role in informing the public about the dangers, and the discovery of the ozone hole above Antarctica attracted added attention to the problem.
On 16 September 1987, representatives of 24 nations signed the Montreal Protocol on Substances That Deplete the Ozone Layer, including the United States which was the largest single producer and consumer of the suspect chemicals. The treaty agreed to a 50 percent reduction in important ozone destroyers, such as the chlorofluorocarbons designated as CFC-11 and CFC-12, below 1986 levels by mid-1998.
The ready acceptance of the Montreal Protocol was undoubtedly eased by the development of substitutes for CFCs in refrigerators, air conditioners, foaming, and cleaning solvents. In fact, the biggest producer, Du Pont unilaterally stopped making the chemicals even before the Protocol required it.
The protocol was strengthened in 1990, at a meeting in London, when it was agreed to a complete phase-out of CFC-11 and C FC-12 by the year 2000. The phase-out schedules of other ozone depleting substances were accelerated at another meeting in Copenhagen in 1992, including the halons, which are another type of chlorofluorocarbon. At a meeting in Vienna in 1995 further controls were implemented. A total of 155 countries had ratified the Montreal Protocol by 1996, including the vast majority of both the producers and consumers of the dangerous substances.
The phase-out schedules differed for the rich, industrial countries and the poor, undeveloped ones. The industrial countries had to eliminate halon consumption as of 1 January 1994 and CFC consumption as of 1 January 1996. Developing countries were given a grace period, but had to complete their phase-out by 1 January 2010. Several of these countries will complete their phase-out much before this date. A multilateral fund was also established to assist the developing countries in meeting their goals.
urther amendments were made at meetings in Montreal in 1997 and in Beijing in 1999, all aimed at reducing and eventually eliminating the emissions of all kinds of man-made ozone depleting substances. As an example, a recent amendment, that took effect on 1 January 2001, requires participating countries to freeze the production of hydrochlorofluorocarbons (HCFCs) still used in refrigeration and cooling equipment.
The accord has accomplished far more than halting the production of dangerous synthetic chemicals. The Montreal Protocol was the first international agreement to protect the global environment. The treaty also marked the first time that the governments of the industrial nations agreed to help developing countries with environmentally safe substances and technology. It was further hoped that the precedent would pave the way for international agreement on global warming.
Although production of ozone-destroying substances has been substantially curtailed under international agreement, more than 20 million tons of them have already been dumped into the atmosphere, and this damage cannot be undone. Because of their long lifetime and slow diffusion into the stratosphere, the synthetic chemicals that are already in the air will keep on destroying the ozone layer for about a century.
The springtime hole in the ozone layer above Antarctica has, in fact, shown no signs of closing up, even though many of the chemicals producing them have long been banned. In the year 1990 the ozone hole was about as wide as the North American continent, covering approximately 14 million million (1.4 x 1013) square meters. A decade later, in the year 2000, the ozone hole had grown to its largest size yet, measuring 28 million million (2.8 x 1013) square meters despite the near abolishment of the CFCs.
The chemicals that were already in the air were apparently still wafting upward into the stratosphere. Hopefully their abundance in the ozone layer peaked at the end of the 20th century. After that, ozone loss should be stabilized and the trends reversed as the chemicals are gradually removed by slow washing-out processes.
The ozone layer will not regain full strength until well into the latter half of the 21st century when it should then recover to the natural levels that existed before the ozone hole was discovered. In the meantime, scientists will continue to monitor the ozone layer using a series of NASA satellites, while also keeping a close eye on the Sunís varying ultraviolet output which modulates ozone production by amounts comparable to human destruction of it.