Further Reading

Bacon, Francis. Novum Organum (The New Organon, or True Directions Concerning the Interpretation of

Nature, translated by Basil Montague.) Philadelphia, Pa.: Parry and MacMillan, 1854. Miyashiro, Akiho, Keiti Aki, and A. M. Celal Sengor. Orogeny. Chichester, England: John Wiley & Sons, 1982.

Suess, Eduard. Das Antlitz der Erde (The Face of the Earth, translated by Hertha B. C. Sollas.) Oxford; Clarendon Press, 1909.

-. Die Entstehung der Alpen (The origin of mountains). Ann. Acad. Sci. Fennicae, Ser. A, III. Geol.-Geogr., No. 97, 28 p., 1875. Wegener, Alfred, Die Entstehung der Kontinente and Die Entstehung der Kontinente und Ozeane (The Origin of the Continents and the Oceans), 1915; 1929. Wilson, J. Tuzo. "A New Class of Faults and Their Bearing on Continental Drift." Nature 207 (1965): 343-347.

ozone hole Ozone (O3) is a poisonous gas that is present in trace amounts in much of the atmosphere, but reaches a maximum concentration in a stratospheric layer between 9 and 25 miles (15-40 km) above the Earth, with a peak at 15.5 miles (25 km). The presence of ozone in the stratosphere is essential for most life on Earth, since it absorbs the most carcinogenic part of the solar spectrum with wavelengths between 0.000011 and 0.0000124 inches (280 and 315 nm). If these ultraviolet rays reached the Earth they would cause many skin cancers, and possibly depress the human immune system. These harmful rays would greatly reduce photosynthesis in plants and reduce plant growth to such an extent that the global ecosystems would crash.

Ozone naturally changes its concentration in the stratosphere, and is also strongly affected by human or anthropogenically produced chemicals that make their way into the atmosphere and stratosphere. Ozone is produced by photochemical reactions above 15 miles (25 km), mostly near the equator, and moves toward the poles, where it is most abundant and where it is gradually destroyed. The concentration of ozone does not vary greatly in equatorial regions, but at the poles tends to be the greatest in the winter and early spring. The formation of a strong vortex that isolates the stratospheric air over the pole characterizes stratospheric circulation during the night in the Antarctic winter.

Atmospheric and stratospheric flow dynamics can change the distribution of ozone, solar flare and sunspot activity can enhance ozone, and volcanic eruptions can add sulfates to the stratosphere that destroy ozone. In the 1970s scientists realized that some aerosol chemicals and refrigerants, such as chlorofluorocarbons (CFCs), could make their way into the stratosphere, where ultraviolet light broke them down, releasing ozone-destroying chlorine.

Sep 17 2001

Satellite data shows the Antarctic ozone hole, 9/17/01. The hole is roughly the size of North America (NASA Goddard Space Flight Center [NASA-GSFC])

Environmental regulations subsequently curtailed the use of CFCs, but the aerosols and chlorine have long residence times in the stratosphere, and each chlorine ion is capable of destroying large amounts of ozone. Since the middle 1980s, a large hole marked by large depletions of ozone in the stratosphere has been observed above Antarctica every spring, its growth aided by the polar vortex. The hole has continued to grow, but the relative contributions to the destruction of ozone by CFCs, other chemicals (such as supersonic jet and space shuttle fuel), volcanic gases, and natural fluctuations is uncertain. In 1999 the size of the Antarctic ozone hole was measured at more than 9,650,000 square miles (25 million km2), more than two and half times the size of Europe. The appearance of ozone depletion above Arctic regions has added credence to models that show the ozone depletion being largely caused by CFCs. Many models suggest that the CFCs may lead to a 5-20 percent reduction in global ozone, with consequent increases in cancers and disease and loss of crop and plant yield.

See also atmosphere; climate change; electromagnetic spectrum; global warming; meteorology.

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