As human activity modifies global land cover, Earth's biogeochemical, hydrological and energy cycles are shifted away from their natural state. The implications of these disturbances are greatly debated, but there is no longer widespread doubt that human action is altering biospheric properties and processes. While there have always been natural phenomena that have altered Earth's surface, anthropogenic (human caused) changes are increasing and their effects have altered the biosphere significantly.
Chapter 3 discussed some issues of human impact on the flow of energy and materials in the biosphere, and Chapter 7 has noted how land cover change and associated land use affects human populations. In this section we focus more specifically on the human dimension of global environmental change.
The change patterns of land cover vary throughout the world with socioeconomic, cultural, and demographic factors such as population density, per capita income, foreign debt, transportation infrastructure, and education. While land cover analysis gives indications of status in the simple manner of observed stability or change, the more direct measures of change momentum often entail the description of land use. Land cover simply refers to a straightforward physical description of what exists on the surface, for example trees, buildings, water, or roads. Land use refers to the human activities on a parcel of land, inferring the use of the surface environment. It can include such labels as orchard, pasture, industrial, residential, irrigated cropland, or sheep ranching. It is not unusual for land cover and land use to be confused and intermingled, given how interrelated they are.
Classification of types of land use can provide insight into how use of the land is likely to affect biospheric properties and processes, as well as the time scale on which change operates. For example, an urban area is unlikely to return to forest in the foreseeable future, and spreading urban areas change local surface temperatures and modify carbon dioxide production and utilization.
Land transformation is often guided by social and political forces for a particular region. In some areas socioeconomic forces (labor supply) may encourage farmers to practice slash-and-burn agriculture, and in other areas strong opportunity may encourage investors to convert naturally vegetated areas into suburban neighborhoods. Below, we explore land use patterns in rural and urban areas.
1.1.1 Rural areas -- In the rural areas of nonindustrialized nations, land use patterns are often dominated by agriculture. In industrialized nations, agriculture, housing, and recreation are the most common motivations for land conversion. Frequently this conversion follows a pattern from forest to agriculture, then agriculture to suburban or urban development. This process often takes place on some of the most productive land, such as river corridors and valleys, due to the rich soils, gentle terrain and access to transportation.
In developing nations, high demand for increased agricultural acreage comes from two major factors. First and foremost, the burgeoning populations consume more and more food from both subsistence and market sources. Second, there are economic and political pressures to increase exports of agricultural produce for financial leverage and generating foreign exchange. To meet these demands, local farmers tend to develop small parcels of land until successive plantings can no longer be sustained, whereupon the land is then abandoned and a new patch of land is prepared for planting. This type of land use, called swidden agriculture, creates a patchwork of land cover consisting of land being converted for cultivation, cultivated land, and abandoned land (see Figure 8.01). This type of land use is generally productive for the smallholder farmers and is sustainable only over short periods of time. When bigger projects clear large uniform tracts of land for grazing or agriculture (see Figure 8.01), productivity is frequently not sustainable without intensive management (including use of fertilizers, pesticides and herbicides).
Another pattern of land use common in developing nations occurs after a forested area has been cleared to put in a road for transport, exploration, or logging. After access through the forest is improved, people follow these routes and develop the land next to the road. This type of development can grow quickly and cover a large area (see Figure 8.02).
1.1.2 Urban areas -- Because cities tend to be centers of economic activity, there is a natural tendency for people to move to cities for employment. In urban areas, nearly all cover change is in the direction of more urbanization, converting internal vestiges of natural vegetation or agricultural land to urban development. "Urban sprawl" describes an urban expansion out from the center of a city. This is evident around most large cities and often results in decay at the city core unless economic incentives are provided to reverse this trend. It has been shown that the process of urban expansion is very similar to the growth of tumors, wherein new resources are consumed at a rapid rate out from the center and new growth is spread along corridors (roads or veins). In satellite imagery over several different dates, urban sprawl is seen as a growing ring of developed land, the urban halo, with dendritic features showing growth along well-defined corridors (see Figure 8.03).