Ozone Hole
Ozone Hole
Introduction
The ozone hole refers to the thinning (but not the complete absence) of the ozone layer of Earth’s atmosphere that occurs over the continent of Antarctica. The thining occurs mainly during the winter.
Ozone absorbs the incoming ultraviolet (UV) portion of sunlight, reducing the amount of this light that reaches Earth’s surface. This natural buffering of sunlight is beneficial, as the high energy UV light waves are able to penetrate into the uppermost layers of skin. The result can be the cell damage that is evident as sunburn and, more ominously, the destruction of portions of the genetic material (deoxyribonucleic acid or DNA) that resides inside every cell. Some DNA damage in skin cells is associated with the development of certain cancers.
The winter human population of the Antarctic consists only of several hundred researchers. The warm winter clothing protects them from the increased UV light, and so the concerns over the health effects of the ozone hole are minimal. However, the existence of the hole, and its increase in size through the 1990s and up until 2006, when the hole was the largest ever recorded, has become a concern for those studying the influence of human activities on the atmosphere, as the basis of ozone destruction is the release of human-generated compounds into the atmosphere. In addition, a similar thinning elsewhere on the globe could have much more serious consequences on climate and human health.
As of early 2008, the size of the ozone hole was smaller than in 2006, as was the measured loss of ozone, according to data from the U.S. National Oceanic and Atmospheric Administration (NOAA). Whether this is the beginning of a longer-term trend that reflects the reduction of the release of ozone-destroying compounds—agreed to by the 191 nations that signed the Montreal Protocol on Substances That Deplete the Ozone Layer—is not yet clear.
Historical Background and Scientific Foundations
Beginning in 1957–1958, which was designated the International Geophysical Year, scientists began to annually record atmospheric measurements, including the concentration of ozone. At first these measurement were taken from instruments placed in weather balloons. With the satellite era, more accurate measurements could be obtained from instruments while in an orbit around Earth.
Through the 1960s and until the late 1970s, ozone levels in the South Pole region were consistently higher in the late spring than in the wintertime. But, in 1978 and 1979, the ozone levels were less at the end of winter than ever before. For the next several years, the late winter ozone level continued to decline.
British researchers reported these findings in 1984. The following year, the U.S. satellite Nimbus-7 confirmed these findings and produced an image of the thinned Antarctic ozone layer. The term ozone hole was coined to describe the phenomenon.
Ozone is a gas that is composed of three oxygen atoms; its chemical formula is O3. The form of oxygen that we breathe consists of a pair of atoms (O2). Although the O2 form of oxygen makes up about 21% of the volume of the atmosphere, ozone occupies only 0.000004% of the atmosphere’s volume, which, if it was present as a discrete layer, would be only about one-eighth of an inch thick.
Ozone is not dispersed all through the atmosphere. Rather, it is confined to the uppermost layer of the atmosphere called the stratosphere, which begins 6 to 12 mi (10 to 19 km) above sea level and extends to nearly 30 mi (48 km) high. The stratosphere is not where clouds normally form and where there is a lot of air movement (these occur in a lower layer called the troposphere). This is advantageous, as the ozone that is concentrated in the lower portion of the stratosphere remains relatively undisturbed. The stratosphere’s thickness is not uniform; rather, it tends to be thinnest above the equator and thickest at both poles.
Ozone forms when the UV portion of sunlight splits an O2 molecule into two oxygen (O) atoms; one of these subsequently forms a chemical bond with another O2 molecule to generate O3. Even though UV light is required to form ozone, once the molecule has formed, it is capable of absorbing incoming UV light. This reduces the amount of UV radiation that passes through the atmosphere to the surface.
The basis for ozone destruction are chemicals called chlorofluorocarbons (CFCs), hydrochlororfluorocarbons (HCFCs, which are very similar in structure to CFCs), bromine-containing hydrocarbons, and nitrogen oxide. CFCs were once widely used as a coolant in air conditioners and refrigerators, and to propel gas out of aerosol cans. Ironically, a main reason for their use was the belief that they were chemically non-reactive, and so safe to use around people. Nitrogen oxide is a by-product of the burning of fuel and is released to the atmosphere in aircraft exhaust and other sources.
As the use of CFCs became more popular, their escape into the atmosphere accelerated. Subsequently, it was discovered that CFCs can persist for up to 100 years in the stratosphere, and that during that time incoming UV light can break the CFC molecule apart. This
WORDS TO KNOW
ATMOSPHERE: The air surrounding Earth, described as a series of layers of different characteristics. The atmosphere, composed mainly of nitrogen and oxygen with traces of carbon dioxide, water vapor, and other gases, acts as a buffer between Earth and the sun.
CLIMATE MODEL: A quantitative method of simulating the interactions of the atmosphere, oceans, land surface, and ice. Models can range from relatively simple to quite comprehensive.
GREENHOUSE GAS: A gas whose accumulation in the atmosphere increases heat retention.
PRIMARY POLLUTANT: Any pollutant released directly from a source to the atmosphere.
releases the chlorine component of CFC, which then can destroy ozone.
The ozone hole would not form if ozone had not been depleted from the stratosphere. The hole forms over Antarctica in the winter months. During this time of the year, strong winds blow around the continent, in effect cutting off the air of Antarctica from the air elsewhere. The ozone-destroying CFCs that are dispersed all through the stratosphere become more concentrated in this trapped region of the atmosphere.
As well, clouds known as Polar Stratospheric Clouds form. Clouds do not usually form in this layer of the atmosphere and, when they do, they concentrate the ozone-destroying compounds still further. The result is an accelerated breakdown of ozone, which produces the ozone thinning over the continent.
As spring returns to the Antarctic and the wind pattern shifts, the atmospheric conditions that triggered the ozone hole disappear, as does the ozone hole. Essentially, the redistribution of ozone restores the ozone to a level that is similar to other parts of the stratosphere.
The reduced amount of ozone in the ozone hole means that less UV light is absorbed. This form of sunlight has enough energy to be able to penetrate into the top-most layers of the skin, and is capable of slicing apart the two strands of DNA that make up the double helix of genetic material inside cells as varied as those of humans, other animals, and microorganisms. When DNA is broken, cell repair mechanisms may be able to restore the structure with minimal effect on the cell. But, damage can be too major to repair; if this damage occurs in regions of the DNA that are critical to the regulation of cell growth and division, the result can be the uncontrolled cell growth that is the hallmark of cancer.
Increased exposure to UV light has been linked to increased rates of skin and other cancers and eye damage in humans. As well, DNA damage has reduced the numbers of microorganisms called phytoplankton in the ocean. Since other marine species that feed on phytoplankton are eaten by additional species that, in turn, are eaten by still other species, a change in the base of this food chain is a serious disruption.
Impacts and Issues
The first report of the thinned ozone layer over Antarctica in 1984 prompted yearly satellite re-examinations of the area as well as the remainder of the atmosphere. While a similar thinning has not so far been detected anywhere else, especially over the other pole, the general depletion of ozone from the stratosphere has been confirmed. Given the sparse number of people in the Antarctic, the ozone hole has not been a health concern. However, the same would not be true if a similar thinning of the ozone occurred over the Arctic. An Arctic ozone hole of the same size as the Antarctic version would expose over 700 million people, plants, and wildlife in the upper Northern Hemisphere to UV levels that have been linked to the development of cancer and eye damage.
From 1985 through 2006, the ozone hole varied in size. Measurements conducted in 2006 revealed that the hole was as big as had been recorded, at 10.6 million square mi (27.5 million square km). This prompted much concern. However, the latest measurements taken in 2007 found that the hole had shrunk by about 30% over the previous year.
Whether this shrinkage will continue is unclear. The atmospheric temperature is an important factor in determining the extent of ozone thinning. A warmer atmosphere tends to diminish the thinning. So, in this sense, global warming—the increasing warming of the atmosphere that is related to human activities—may reduce the severity of the ozone hole.
In addition, as the release of CFCs and other ozone-depleting compounds into the atmosphere has been curbed following the implementation of the Montreal Protocol by the 191 participating nations, ozone depletion could eventually ease. However, given the long lifetime of atmospheric CFCs, the depletion may continue through the 21st century.
See Also Air Pollution; Carbon Dioxide (CO2) Emissions; Greenhouse Effect
BIBLIOGRAPHY
Books
DiMento, Joseph F. C., and Pamela M. Doughman. Climate Change: What It Means for Us, Our Children, and Our Grandchildren. Boston: MIT Press, 2007.
Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. New York: Rodale Books, 2006.
Seinfeld, John H., and Spyros N. Pandis. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. New York: Wiley Interscience, 2006.
Web Sites
NASA. “Ozone Hole Watch.”December 4, 2007. http://ozonewatch.gsfc.nasa.gov/ (accessed April 7, 2008).
Brian D. Hoyle