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A megacity is pegged as any city with more than 10 million residents. Urban sprawl or suburban sprawl mainly refers to the unrestricted growth in many urban areas of housing, commercial development, and roads over large expanses of land, with little concern for urban planning. In addition to describing a particular form of urbanization, the term also relates to the social and environmental consequences associated with this development. In the postindustrial era, sprawl has entailed no clear-cut disadvantage, such as the loss of protection, nor resulted in a uniquely identifiable cost such as investment in new fortification, however its disadvantages and costs include increased travel time, transport costs, pollution, destruction of countryside, and so on. In Continental Europe, the term peri-urbanisation is often used to denote similar dynamics and phenomena, although the term urban sprawl is currently being used by the European Environment Agency. There is widespread disagreement about what constitutes sprawl and how to quantify it. For example, some commentators measure sprawl only with the average number of residential units per acre in a given area. But others associate it with decentralization (spread of population without a well-defined centre), discontinuity (leapfrog development, as defined below), segregation of uses, and so forth.
The term urban sprawl is highly politicized and almost always has negative connotations. It is criticized for causing environmental degradation, intensifying segregation, and undermining the vitality of existing urban areas and is attacked on aesthetic grounds. Due to the pejorative meaning of the term, few openly support urban sprawl as such. The term has become a rallying cry for managing urban growth.
Urban sprawl is associated with a number of negative environmental outcomes.
One of the major environmental problems associated with sprawl is land loss, habitat loss and subsequent reduction in biodiversity. A review by Czech and colleagues finds that urbanization endangers more species and is more geographically ubiquitous in the mainland United States than any other human activity. Urban sprawl is disruptive to native flora & fauna and introduces invasive plants into their environments. Although the effects can be mitigated through careful maintenance of native vegetation, the process of ecological succession and public education, sprawl represents one of the primary threats to biodiversity.
Regions with high birth rates and immigration are therefore faced with environmental problems due to unplanned urban growth and emerging megacities such as Kolkata.
Other problems include: flooding, which results from increased impervious surfaces for roads and parking (see urban runoff) increased temperatures from heat islands, which leads to a significantly increased risk of mortality in elderly populations. At the same time, the urban cores of these and nearly all other major cities in the United States, Western Europe, and Japan that did not annex new territory experienced the related phenomena of falling household size and, particularly in the U.S., "white flight", sustaining population losses. This trend has slowed somewhat in recent years, as more people have regained an interest in urban living.
Due to the larger area consumed by sprawling suburbs compared to urban neighborhoods, more farmland and wildlife habitats are displaced per resident. As forest cover is cleared and covered with impervious surfaces (concrete and asphalt) in the suburbs, rainfall is less effectively absorbed into the groundwater aquifers. This threatens both the quality and quantity of water supplies. Sprawl increases water pollution as rain water picks up gasoline, motor oil, heavy metals, and other pollutants in runoff from parking lots and roads.
Sprawl leads to increased driving, which in turn leads to vehicle emissions that contribute to air pollution and its attendant negative impacts on human health. In addition, the reduced physical activity implied by increased automobile use has negative health consequences. Sprawl significantly predicts chronic medical conditions and health-related quality of life, but not mental health disorders.[39] The American Journal of Public Health and the American Journal of Health Promotion, have both stated that there is a significant connection between sprawl, obesity, and hypertension.
In the years following World War II, when vehicle ownership was becoming widespread, public health officials recommended the health benefits of suburbs due to soot and industrial fumes in the city center. However, air in modern suburbs is not necessarily cleaner than air in urban neighborhoods. In fact, the most polluted air is on crowded highways, where people in suburbs tend to spend more time. On average, suburban residents generate more per capita pollution and carbon emissions than their urban counterparts because of their increased driving.
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