Decline of the ozone layer

DEFINITION: Region of the lower stratosphere in which most of the earth’s ozone is found

The earth’s ozone layer protects life on the planet from exposure to dangerous levels of ultraviolet light. Because of the introduction of certain human-made chemicals into the atmosphere, the amount of stratospheric ozone steadily declined during the second half of the twentieth century.

Ozone (O₃) is a molecule made of three atoms of oxygen. It is considered a trace gas because it accounts for only .000007 percent of the earth’s atmosphere. Ozone concentrations are measured in terms of Dobson units (DU), which represent the thickness of all the ozone in a column of the atmosphere if it were compressed—on average only 3 millimeters (0.118 inch) thick. Depending on where it is found in the atmosphere, ozone may have either a positive or a negative impact on life. When ozone is near the earth’s surface, it is a major air pollutant, a chief constituent of smog, and a greenhouse gas. Fuel combustion and other human activities increase the quantities of ozone in this atmospheric region. Ozone that resides in the stratosphere protects earth’s organisms from lethal intensities of solar ultraviolet (UV) radiation. Without this shield of ozone, which is created through naturally occurring processes, life on earth as it is now known would probably cease to exist. Approximately 90 percent of the earth’s ozone is found in the stratosphere.

Ozone concentration peaks in the lower stratosphere, between the altitudes of 20 and 25 kilometers (12 and 16 miles). Within this “ozone layer,” two sets of chemical reactions, both powered by UV radiation, continuously occur. In one reaction, ozone is produced; in the other, ozone is broken down into oxygen molecules and ions. Slightly more ozone is produced than is destroyed by these reactions, so that stratospheric ozone is constantly maintained by nature. Because UV radiation is a catalyst for both reactions, most of this radiation is used up and prevented from ever reaching earth’s surface.

UV radiation is categorized as UVA, UVB, or UVC, depending on wavelength. UVC rays readily kill living cells with which they come into contact. UVB rays, although less energetic than UVC rays, also damage cells. The ozone shield prevents all UVC rays from reaching the surface; however, some UVB radiation does pass through the ozone layer. It is contact with this radiation that causes sunburns, accelerates the natural aging of the skin, and has been shown to increase rates of skin cancer and cataracts. In 2023, the American Cancer Society estimated that more than five million new cases of skin cancer occur each year in the United States, mainly as a result of UV radiation. Some researchers estimate that every 1 percent decline in ozone concentration causes a 2 percent increase in UV intensity at the earth’s surface, resulting in a greater risk of skin cancer, cataracts, and immune deficiencies.

Increased UV radiation also harms plant and animal life. Some studies have suggested that yields from crops such as corn, wheat, rice, and soybeans drop by 1 percent for each 3 percent decrease in ozone concentration. Increased UV radiation in polar regions impairs and destroys phytoplankton, which makes up the base of the food chain. A decrease in phytoplankton would likely cause population reductions at all levels of the ecosystem. Fewer than 1 in every 100,000 molecules in the atmosphere is ozone, a ratio that both underscores and belies the critical role ozone plays in protecting human health and the global environment.

Discovery of the Antarctic Ozone Hole

Satellites placed in orbit during the late 1970s allowed scientists to observe concentrations of stratospheric ozone. Observations showed that during the Southern Hemisphere’s spring (primarily September and October), the ozone layer above Antarctica thinned dramatically, then recovered during November. These findings were initially dismissed as being caused by instrument error; however, the measurements were confirmed by the British Antarctic Survey in 1985. During that spring, the loss of ozone exceeded 50 percent of normal concentrations. Alarmingly, continued satellite measurements indicated that each year throughout the 1980s and 1990s, the ozone concentration over Antarctica dropped to record lows, and the size of the ozone-depleted area (dubbed the “ozone hole”) increased. In 1986, the size of the ozone hole grew larger than the size of the Antarctic continent, and in 1993, the hole was larger than all of North America.

During an intensive Antarctic field program in 1987, extremely high levels of chlorine monoxide (ClO) were found in the stratosphere. This finding was seen by many scientists as evidence that the cause of ozone depletion was chlorofluorocarbons (CFCs), anthropogenic chemicals that make excellent refrigerants, cooling fluids, and cleaning solvents. Since the 1950s, millions of tons of CFCs had been produced in the United States alone. CFC leakage from old refrigerators and automobile air conditioners, combined with a lack of chemical recycling efforts, allowed huge quantities of CFCs to make their way into the atmosphere. Also, one of the CFCs, CFC-11, was used for decades as a propellant in aerosol spray cans until it was banned during the late 1970s.

In the stratosphere, UV radiation breaks down CFCs, causing them to release chlorine (Cl), a gas that readily reacts with ozone. The reaction produces oxygen (O₂) and ClO, which then combine to produce O₂ and Cl. Thus, at the end of these reactions, the chlorine ion is again free to destroy another ozone molecule. It is estimated that a single chlorine ion may reside in the stratosphere for fifty years or longer and destroy hundreds of thousands of ozone molecules.

Most scientists believed the detection of stratospheric ClO was the “smoking gun” that proved that human-made CFCs were the cause of ozone depletion. However, many industrialists and others opposed to what they perceived as overzealous environmental regulations asserted that the chlorine in the atmosphere was a result of natural processes such as volcanic eruptions or sea spray. This theory was finally debunked in 1995 when scientists also found hydrogen fluoride in the stratosphere. Hydrogen fluoride is not produced by any natural process; however, fluoride would be liberated from a CFC molecule when it is broken down by UV radiation. Scientifically, there is no debate that ozone destruction is a direct result of CFCs. Other chlorine-containing compounds have also been implicated in ozone destruction, as have some bromine-containing compounds. While bromine is less abundant in the stratosphere than chlorine, atom for atom it is more effective than chlorine in destroying ozone.

Global Ozone Depletion

Since the initial satellite observations of the 1970s and 1980s, the understanding of the complex science of the formation of the Antarctic ozone hole has greatly improved. Most ozone-destroying compounds are released in the Northern Hemisphere; however, they mix throughout the lower atmosphere in about one year and then mix into the stratosphere in two to five years. A key element that enhances ozone depletion is the presence of polar stratospheric clouds. These clouds form only in the polar regions where temperatures in the stratosphere drop to below −80 degrees Celsius (−112 degrees Fahrenheit). The surfaces of these clouds are sites on which inactive chlorine and bromine are converted into the reactive forms that destroy ozone. The dark Antarctic stratosphere becomes so frigid during winter that these clouds are produced in abundance. As spring begins, the stratosphere is hit with solar UV radiation, accelerating the ozone depletion process. A strong vortex of winds circles the South Pole every winter, keeping the Antarctic stratosphere isolated from neighboring air masses. This allows the ozone-destroying reactions to act on a limited amount of ozone that cannot be replenished by ozone from other latitudes. The result is the appearance of the ozone hole each spring. In late spring the winds weaken, allowing comparatively ozone-rich air from other latitudes to mix into the Antarctic atmosphere.

Ozone concentrations declined globally during the second half of the twentieth century as a result of rising levels of atmospheric chlorine and bromine. It was discovered that an ozone hole also forms over the Arctic in the Northern Hemisphere during the spring, although the hole is smaller in size and higher in ozone concentrations than its Antarctic counterpart. The main reason for the difference between the Arctic and Antarctic is that the winds encircling the Arctic are typically not as strong as those in the Antarctic, thus allowing ozone-rich air to mix constantly into the Arctic. Still, significant ozone depletion occurred throughout the 1990s as Arctic spring concentrations dropped about 30 percent below average levels.

Ozone levels in other parts of the world also declined. Ground stations measured decreasing stratospheric ozone levels at midlatitudes. Satellites indicated a negative trend in ozone concentrations in both hemispheres, with a net decrease of about 3 percent per decade. The depletion increased with latitude and was somewhat larger in the Southern Hemisphere. Over the United States, Europe, and Australia, 4 percent per decade was typical. A new threat to ozone followed the eruption of Mount Pinatubo in the Philippines in 1991, the century’s second most violent volcanic eruption and the largest one to occur since ozone began. Massive amounts of sulfur aerosols were injected into the stratosphere. Volcanic aerosols function like the ice crystals in the Antarctic stratosphere by helping convert chlorine and bromine from their inactive state to a reactive one. During the two years following the eruption, record low levels of annual ozone concentrations were recorded: 9 percent below normal between 30 and 60 degrees north latitude.

As expected, measurements also indicated increased UV radiation reaching the earth’s surface. During the 1990s, summer levels of UVB increased 7 percent annually over Canada, while winter levels increased 5 percent per year. At the same time, the incidence of skin cancer grew faster than that of any other form of cancer, with reported cases doubling between 1980 and 1998.

International Response

As evidence grew during the 1980s that CFC production was responsible for ozone depletion, the international community decided to act through a series of treaties and amendments. The multilateral Vienna Convention for the Protection of the Ozone Layer of 1985 outlined the responsibilities of states to prevent human-driven ozone depletion. The Vienna Convention laid the groundwork for the Montreal Protocol on Substances That Deplete the Ozone Layer, which was adopted in 1987. The Montreal Protocol put into place a framework for the phaseout of CFC production in developed countries beginning in 1993. In the following years, as ozone levels kept falling, amendments added more chemicals to the phaseout, accelerated the phaseout timetable, and called for the developing world’s participation. By 2018, the Montreal Protocol had been ratified by 197 countries, although not all of them had ratified all the subsequent amendments. Under the amended protocol, hydrobromofluorocarbons, CFCs, halons, and carbon tetrachloride have already been phased out; methyl chloroform and methyl bromide have been phased out in developed countries and were generally phased out in developing countries by 2015; and hydrochlorofluorocarbons were scheduled to be phased out by 2020 in developed countries and 2040 in developing countries. According to the EPA, production and import of hydrochlorofluorocarbons were phased out as of 2020. Certain essential-use exemptions have been made for those countries beyond the official phaseout dates.

Since the beginning of the twenty-first century, concentrations of ozone-depleting substances in the stratosphere have been on the decline as a result of the Montreal Protocol. These substances have such a long residence time in the atmosphere, however, that even with a ban on production, their concentrations in the stratosphere are not expected to return to pre-1980 levels until 2050 at the earliest. According to the U.S. Environmental Protection Agency (EPA), Antarctic ozone may return to pre-1980 levels between 2060 and 2075. Factors including volcanic eruptions, solar activity, and changes in atmospheric temperature, composition, and air motion could affect the ozone layer’s rate of recovery.

Some studies predicted that maximum ozone depletion would occur between 2000 and 2020, followed by a slow recovery. UV radiation levels at the earth’s surface were also expected to increase during the early part of the twenty-first century, bringing with them negative health and ecological effects. The largest ozone hole ever recorded occurred in September 2006. However, according to an EPA report, over most of the world, the ozone layer remained relatively stable between 1998 and 2007. In 2022, scientists from NASA reported the ozone hole was making significant strides in recovery, and a United Nations report outlined a timeline of complete recovery within four decades from 2023 if trends continued.

The story of the ozone layer contains both positive and negative lessons regarding human interaction with the environment. Human production of CFCs and other compounds during the latter half of the twentieth century caused significant, long-term harm to the earth’s protective barrier against deadly UV radiation. Human health has been affected, and immeasurable damage has been inflicted on many ecosystems. With the earth in the balance, however, the international community decided to act, and did so forcefully. Without the Montreal Protocol, the fate of the ozone layer and life on earth would have been sealed. It is known that human activity has the power to destroy the global environment, but perhaps human activity may have the power to save it as well.

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