Land Cover Changes
Rapid expansion of urban areas changes land cover and causes habitat loss. Chapter 4 noted how the combination of urban population growth, decreasing densities and peri-urbanization could convert large chunks of valuable land to urban uses in coming decades.
The environmental challenges posed by the conversion of natural and agricultural ecosystems to urban use have important implications for the functioning of global systems. How serious they are depends on where and how urban localities will expand. They depend even more on the patterns of consumption that city populations impose.
“Urban footprints” spread well beyond the immediate vicinity of cities, particularly in developed countries. Rising incomes and consumption in urban areas lead to increasing pressure on natural resources, triggering land-use and land-cover changes in their zones of influence, sometimes over vast areas. This typically causes much greater losses of habitat and ecosystem services than urban expansion itself.
For example, tropical forests in Tabasco were razed to provide space for cattle, in response to rising demand for meat in México City, 400 km away. Rising demand for soybeans and meat in China’s urban areas, added to the demand from Japan, the United States and Europe, is accelerating deforestation in the Brazilian Amazon.(6)
The “urban footprint” concept, which has been used to describe this expansion of the perimeter of urban consumption, is now quite familiar.(7)But many people take it to mean that urban concentration itself is the problem, rather than consumption by a large number of more or less affluent people. Evidently, urban centres in poor countries do not have the same footprint as those in developed countries.
The concept of the environmental transition brings out the differences between cities in high- and low-income countries.(8)In cities of poorer countries, environmental problems are local and mostly concern health, such as inadequate water and sanitation, poor air quality (inside houses as well as outside) and limited or no waste disposal. As average incomes increase, these immediate problems are not so pressing, but changes in productive activities and consumption patterns increase the impact on surrounding rural areas. In more affluent cities, local and regional impacts have usually declined through extended environmental regulation, investment in waste treatment and pollution control, and a shift in the economic base from industry to services. But affluence increases impact on global environmental burdens such as climate change.
The issue of water is particularly relevant in this discussion. The dependence of cities on a guaranteed supply of water makes significant demands on global fresh water supplies. Cities already compete with the much larger demands of agriculture for scarce water resources in some regions such as the south-western United States, the Middle East, southern Africa, parts of Central Asia and the Sahel. In extreme cases—for instance, the Cutzmala system supplying México City—whole communities are flooded or relocated to make way for water supply infrastructure. This will be seen on a monumental scale should China complete the South-North Water Diversion.(9)Ultimately, cities outbid rural and agricultural users for available water supplies.(10)
Urban areas can affect water resources and the hydrological cycle in two other main ways: first, through the expansion of roads, parking lots and other impervious surfaces, which pollute runoff and reduce the absorption of rainwater and aquifer replenishment; and, secondly, through large-scale hydroelectric installations that help supply urban energy needs.(11)
These examples illustrate the complexities of addressing the impacts of cities on the biophysical system and highlight the need for a broad and integrated perspective.