Ozone
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OVERGRAZING Norm Catto Memorial University of Newfoundland, St. John’s, NL, Canada
Definition Overgrazing is the excessive consumption of vegetation by livestock. Discussion Overgrazing, and the consequent food shortage for livestock, represents a hazard directly for human populations dependent on the livestock. It also is a contributing cause to soil and sediment erosion, as material unprotected by vegetation is subject to removal by wind, running water, and mass movements. Overgrazing can be a major contributing cause of desertification, where formerly arable land or pasture is converted to unproductive terrain. Overgrazing may result in desertification, even though there has been no significant change in the amount of rainfall or the temperature. However, desertification is a complex process resulting from the interplay of numerous factors, and many instances of desertification are not directly linked to overgrazing. Human populations that have increased rapidly due to increased livestock production may be at risk if overgrazing reduces food supply. Overgrazing is a significant risk in areas subject to drought or strong winds, particularly steppes and areas adjacent to deserts, including Australia, Patagonia, northern China, India, Pakistan, the prairies of North America, and drier regions of northern and southern Africa. As overgrazing and its effects can be very localized, detailed vegetation cover and land use assessments should
be conducted, and local and regional factors considered prior to the implementation of changes in agricultural policy. Effective management of a potential overgrazing problem requires careful consideration of the human cultural factors, in addition to physical analysis.
Bibliography Magole, L., 2009. The ‘shrinking commons’ in the Lake Ngami grasslands, Botswana: The impact of national rangeland policy. Development in Southern Africa, 26, 611–626. Williams, M., 2003. Desertification in Africa, Asia and Australia: Human impact or climatic variability? Annals of the Arid Zone, 42, 213–230.
Cross-references Challenges to Agriculture Desertification Disaster Relief Dust Bowl Dust Storm Erosion Erosivity Insurance Land Degradation Universal Soil Loss Equation (USLE)
OZONE Tom Beer Centre for Australian Weather and Climate Research, Energy Transformed Flagship, Aspendale, VIC, Australia
Definition Ozone (O3) is a tri-atomic form of oxygen that constitutes a minor and variable constituent of the atmosphere. Most of the ozone is found in the stratosphere (which is the
P.T. Bobrowsky (ed.), Encyclopedia of Natural Hazards, DOI 10.1007/978-1-4020-4399-4, # Springer Science+Business Media Dordrecht 2013
OZONE
region from approximately 15 to 45 km altitude). It can also be found at ground level.
Urban air quality High ozone concentrations and the resulting photochemical smog are of particular concern in localities with plentiful sunshine that do not control their automobile and industrial emissions. Ozone shows average concentrations that exceed guideline values in cities on all contin
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