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Understanding Ozone

The Ozone Balance

Over Earth's lifetime, natural processes have regulated the balance of ozone in the stratosphere. Scientists are finding that ozone levels change periodically as part of regular natural cycles such as seasons, periods of solar activity, and changes in wind direction. Concentrations are also affected by isolated events that inject materials into the stratosphere, such as volcanic eruptions.

Polar regions reflect the greatest changes in ozone concentrations, especially the South Pole. The topography of Antarctica is such that a stagnant whirlpool of extremely cold stratospheric air forms over the region during the long polar night. The air stays within this polar vortex all winter, becoming cold enough to allow the formation of polar stratospheric clouds.

Polar stratospheric clouds speed up the natural process of ozone destruction by providing ice crystal surfaces on which the destructive reactions take place. After the long polar winter, ozone within this extremely cold vortex is very vulnerable to the arrival of sunlight. As spring arrives, major ozone losses occur. In the Southern Hemisphere, the area of most severe ozone depletion is localized above Antarctica and is generally referred to as the ozone hole. The hole appears in the southern spring, following the continent's coldest season and polar night.

Ozone depletion over the Arctic is not as well defined as in Antarctica. The rugged topography results in an air circulation pattern that is quite different from that of the South Pole, but expeditions have shown that the atmospheric chemistry of the two polar regions is very similar. In the Northern Hemisphere, the polar vortex is not as strong. It can break up and reform several times during the course of winter. One air mass after another enters the polar darkness and soon emerges back into low sunshine. Thus, a bit of ozone is lost from each parcel of air, rather than a large amount from one parcel as in the Southern Hemisphere.

Ozone Over the Poles
Ozone Over the Poles. (Click for a larger image.)
The end result is that we are losing ozone in both hemispheres. Ozone levels in the atmosphere have been monitored from the ground since the 1950s and by satellite since the 1970s. Regional total ozone levels measured from satellites over Antarctica have decreased 30-50% since their monitoring began.

Since ozone is created and destroyed by solar UV radiation, there is some correlation of ozone concentration with 11-year sunspot cycles. Sunspots emit high levels of electromagnetic radiation. The increased UV radiation contributes to ozone production. Sunspot variations only account for 2 to 4% of the total variation in ozone concentrations. Natural cycles in ozone variation are also associated with the quasi-biennial oscillation in which tropical winds switch from easterly to westerly every 26 months. This cyclic change in wind direction accounts for approximately 3% of the natural variation in ozone concentration.

Global agreements, regulations and social considerations

In 1973, two scientists from the University of California at Irvine, Mario Molina and F. Sherwood Rowland, discovered that man-made substances called chlorofluorocarbons (CFCs) could play a major role in the destruction of stratospheric ozone. Their findings were published in the journal Nature in June 1974. Since that time there has been much controversy surrounding the subject of ozone depletion. Researchers have struggled to understand the nature and severity of the problem through numerous scientific studies. Nations from all over the world have come together and agreed to establish international industrial regulations in hope of protecting the ozone layer.
Click to see a timeline of events related to ozone depletion.

Because of uncertainty about how global environmental systems work, and because the people affected will probably live in circumstances very much different from those of today and may have different values, it is difficult to predict how present-day actions will affect future generations. To project or forecast the human consequences of global change at some point in the relatively distant future, one would need to know at least the following:

  • The future state of the natural environment
  • The future of social and economic organization
  • The values held by the members of future social groups
  • The proximate effects of global change on those values
  • The responses that humans will have made in anticipation of global change or in response to ongoing global change

International Agreements

Even the value systems and technological advancements of present day nations are extremely different. Nonetheless, efforts to predict and protect are underway. Despite their differences, the international community has made significant progress in addressing ozone depletion as a serious global environmental problem. Through the 1985 Vienna Convention for the Protection of the Ozone Layer, the 1987 Montreal Protocol on substances that deplete the ozone layer, and the 1990 London Amendments to the protocol, members from nations around the world have committed to phasing out the production and consumption of CFCs, and a number of related chemicals, by the year 2000.

Ozone depletion control started in the early 1970s, when the United States, along with a handful of other Western countries, expressed concern over emissions from supersonic transport (SST) aircraft and aerosol spray cans. Environmental groups organized opposition to the development of the SST and to the extensive use of aerosols. Public response led to a sharp drop in the sales of aerosol products. The U.S. Congress, prodded by government studies supporting the CFC ozone depletion theory and its links to skin cancer, approved the Toxic Substances Control Act of 1976, which gave the Environmental Protection Agency (EPA) authority to regulate CFCs. In 1978, the United States became the first country to ban the nonessential use of CFCs in aerosols. However, the EPA ruled that other uses of CFCs, such as refrigeration, were essential and lacked available substitutes.

Ozone depletion emerged as a major international issue in the 1980s. This occurred primarily as a result of initiatives by the United Nations Environmental Programme and actions of the international scientific and environmental communities. A United Nations Environment Program to protect the ozone layer was signed in Vienna in 1985, and a protocol outlining proposed protective actions followed. The Vienna convention of 1985 embodied an international environmental consensus that ozone depletion was a serious environmental problem. However, there was no consensus on the specific steps that each nation should take. The Montreal Protocol, signed in September 1987, stated that there would be a 50% cut back in CFC production by 2000. The United States ratified the Montreal Protocol in 1988. The 1990 London Amendments to the protocol state that production of CFCs, CCl4, and halons will be completely halted by the year 2000. The phaseout schedule for other compounds was accelerated by 4 years by the 1992 Copenhagan agreement.

All human activity potentially contributes directly or indirectly to global change. Earth's atmosphere consists of a delicate balance of gases essential to life. Throughout the history of the planet, the atmospheric gases have been influenced by Earth processes and by the living organisms from both the oceans and land, and natural changes have occurred in the type of gases and their concentrations. Anthropogenic activities are now believed to be causing rapid changes in atmospheric composition on an accelerated time scale. Due to extended human life expectancies and greater population densities, the influence of humans will continue to grow.

Scientists are now confident that stratospheric ozone is being depleted worldwide. However, how much of the loss is the result of human activity, and how much is the result of fluctuations in natural cycles, still needs to be determined. To understand global atmospheric changes, we need to understand the composition and chemistry of Earth's atmosphere and how they are affected by human activity. To create accurate models, scientists must account for all of the factors affecting ozone creation and destruction, and conduct simultaneous, global studies over the course of many years.

Text, images and videos courtesy of Distributed Active Archive Center at NASA's Goddard Space Flight Center.

Related to chapter 4 in the print guide.
Related Materials

Learn more about ozone through the Graphic Stratospheric Ozone activity.

See the Layers of the Atmosphere to learn more about protective ozone.

Glossary Terms

Click for the definitions of the following words which are used on this page:(Definitions appear in a pop-up window.)

Dobson units
ultraviolet radiation

View the full, printable version of the glossary.

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