|Make your work easier and more efficient installing the rrojasdatabank toolbar ( you can customize it ) in your browser.|
|World indicators on the environment||World Energy Statistics - Time Series||Economic inequality|
World Resources 1996-97 (A joint publication by The World Resource Institute, The United Nations Environment Programme, The United Nations Development Programme, and the World Bank) (Data edited by Dr. Róbinson Rojas)
3. Urban Impacts on Natural Resources LAND CONVERSION
Although cities represent a dramatic transformation of the natural landscape, the total amount of land dedicated to urban uses is small--just 1 percent of Earth's total land surface, according to several estimates (21). Overall, far more natural lands are lost to agricultural activities, forestry, and grazing than to urbanization.
Urbanization may actually help to reduce the scale and severity of environmental impacts on land from population growth. High residential densities can reduce land pressures in countries experiencing rapid population growth. Households in villages and rural areas in the Republic of Korea, for example, consume six times more land per capita for residential purposes than households in Seoul (22).
Although the amount of land converted to urban uses may be small globally, a trend is emerging in both developed and developing countries: cities from Los Angeles to Jakarta, Indonesia, are rapidly expanding outward, consuming ever greater quantities of land (23). This urban sprawl, characterized by low-density development and vacant or derelict land, leads to the wasteful use of land resources, higher infrastructure costs, and excessive energy consumption and air pollution because of the greater use of motorized transport. Many criticize urban sprawl for aesthetic reasons as well.
The United States provides an apt example. Urban population growth there has slowed to less than 1.3 percent per year (24), yet urban development continues to encroach on surrounding lands as residents abandon inner cities and move to the suburbs. The total amount of land dedicated to urban uses increased from 21 million hectares in 1982 to 26 million hectares in 1992. In one decade, 2,085,945 hectares of forestland, 1,525,314 hectares of cultivated cropland, 943,598 hectares of pastureland, and 774,029 hectares of rangeland were converted to urban uses (25).
Land pressures are even greater in developing countries with high rates of urban population growth. The physical size of cities in developing countries is expected to double between 1980 and 2000 (26). A round certain cities, urban land expansion is occurring even faster. In Sao Paulo, Brazil, the urban core grew from an area of 180 square kilometers in 1930 to more than 900 square kilometers in 1988. (See Figure 3.1.) The metropolitan region is even larger, covering an astounding 8,000 square kilometers (27). Prime agricultural land and forestland have been converted to urban uses, and development is beginning to move onto steep slopes, which include some of the region's last remaining reserves of natural vegetation (28). Urban expansion is also threatening the local watershed: an estimated 1 million squatters now live in protected watershed areas, and wetlands located next to rivers are covered by streets and housing (29).
The example of Sao Paulo shows that it is not necessarily the scale of urban land conversion that is so important as the type of land being lost. Where a city is located is a major determinant of its environmental impact, and--for historical reasons--cities are often located on prime agricultural land or valuable ecosystems near rivers, lakes, or coasts.
Estimates suggest that 476,000 hectares of arable land in developing countries is being transformed annually to urban uses (30). To compensate for this loss of land, crop production on the remaining lands may become more intense and potentially more environmentally damaging (31). Urban development can push agriculture to less suitable land, with unintended results. In Canada, for instance, replacing the food production from 1 hectare of prime Ontario farmland lost to urban growth requires about 3 hectares of prairie land, thus increasing the rate of land conversion overall (32). As agriculture moves farther from the city, the transportation of food to the city incurs added energy and pollution costs (33). For countries with little unused arable land, such as China and Egypt, the loss of productive farmland may threaten food security (34). For most rapidly developing cities, however, the loss of agricultural land may not constitute a serious problem, because in part it reflects the lower value of agricultural land relative to that of urban land (35).
A greater long-term threat than the loss of agricultural land may be urban encroachment onto fragile ecosystems as a result of intense land use pressures and poor land use planning (36). Squatter settlements are commonly found on steep hillsides vulnerable to landslides and soil erosion, on water catchment areas, on protected lands, and on land subject to flooding or tidal inundation. Although the poor are aware that their settlements lie in precarious locations, the lack of alternative land for housing leaves them no choice. The impacts are twofold: damage to the local environment and threats to the health and well-being of the residents who live there. In Rio de Janeiro, Brazil, favela dwellers (residents of squatter settlements) who live on the steep slopes surrounding the city disturb the natural vegetation and destabilize the hillside soils, leading to intensified mudslides that claim hundreds of lives and that leave thousands of people homeless each year (37).
Natural areas are also sacrificed to formal urban development, such as residential estates, industry, and tourism (38). Forests, wetlands, and other ecosystems serve important functions--for instance, species habitat or flood control--but these benefits can seem "intangible" compared with the benefits of urban development.
Urban land use planning can help guide urban development away from vulnerable ecosystems. Such policies will not work, however, unless cities also provide adequate land for housing, for industries and commerce, and for public infrastructure and buildings. Given the scarcity of urban land, important trade-offs will have to be made in land allocation to balance environmental protection with economic development and to meet the basic needs of the poor (39).
Converting the Coasts
Coastal ecosystems, including wetlands, tidal flats, saltwater marshes, mangrove swamps, and the flora and fauna that depend on them, are especially threatened by urban land conversion (40). (See Table 3.1.) Already, coastal urban centers are home to almost 1 billion people worldwide and are experiencing unprecedented growth (41). Much of this growth will take place in developing nations; even now, growth rates in many coastal cities in the developing world substantially exceed those in surrounding rural regions (42). Even in developed countries such as the United States, some of the highest levels of urban growth are occurring in small coastal cities (43). Accordingly, urban impacts along the coasts stand to increase markedly in the years ahead.
In coastal cities, the higher value placed on shoreline locations increases the economic incentives to develop there. Thus, as coastal cities grow and expand, original coastal habitat is increasingly converted to other uses. Land conversion activities range from draining and filling of marshes and other wetlands to constructing homes or resorts on beaches or dunes, to building seawalls, to undertaking large-scale reclamation projects that extend the shoreline into the sea.
According to a recent study by the World Resources Institute, roughly half of the world's coasts are threatened by development- related activities (44). In Singapore, for instance, demand for land is so great that the island nation has added 6,000 hectares to its land area by filling along the shoreline, increasing its area some 10 percent from what it was three decades ago (45). Along the San Francisco Bay, the most highly urbanized estuary in the United States, filling has reduced the areal extent of the bay by one third in the past 150 years (46).
In addition to habitat loss, shoreline development can intensify coastal erosion, alter the hydrology of estuaries, and otherwise disrupt natural processes. For instance, beaches, sand dunes, marshes, mangrove swamps, barrier islands, and reefs all act as physical barriers against storm damage. When these buffers are lost to development, the coast is more vulnerable to erosion. The protective structures typically built to dissipate storm energy in turn can disrupt the normal processes by which sand is replenished, leading to further erosion in adjacent areas (47) (48) (49). Expansion of the port in Tangiers, Morocco, has altered the coastal profile and modified local current patterns; as a result, nearby beaches are now eroding at a rate of 5 meters per year (50).
Cities have been a particular locus of coastal wetland conversion because marshes, mangroves, or other wetlands are apt to line the estuaries and bays where most coastal cities are located. Of the estimated 80,940 hectares of coastal marshes that originally fringed San Francisco Bay, for instance, 80 percent have been lost to development (51).
Wetlands serve as spawning grounds for many aquatic species as well as habitat for waterfowl and other wildlife. In addition, they filter out many waterborne pollutants and provide extensive flood protection. Often, however, the importance of urban wetlands is apparent only after their services have been lost. In East Calcutta, India, 4,000 hectares of lagoons and swamps that had been used to raise fish were filled to provide home sites for 100,000 middle-class families, resulting in an estimated annual loss of 25,000 metric tons of fish and contributing to local flooding problems (52) (53). Even if wetlands are not immediately filled or drained, they frequently suffer from proximity to developed areas, acting as a receptacle for city wastes and runoff that may overwhelm their natural absorptive capacity or disturb their hydrology.