What has caused the change in urbanization rates over time?
Historically, human populations have led rural lifestyles as hunters, gatherers, and farmers. However, since the industrial revolution, human societies have been transitioning from a rural lifestyle, to a more urban one, at an exponential rate (Figure 9.1). From 1800 to 1900 the number of people that lived in urban areas increased from less than 5%, to 14% of the total population. Between 1900 and 1950, rates of urbanization more than doubled up to 30% and by the year 2000, 47% of the human population, or 2.8 billion people, lived in urban areas. By 2030, demographers estimate that approximately 65% of humans will live in urban areas.
Although rates of urbanization are increasing worldwide, developing countries are expected to have the greatest urban populations in the future (Figure 9.1). Currently 76% of people in developed countries live in urban areas, whereas 40% of people in developing countries are urbanized. However, it is estimated that in the future more than 90% of all people in developing countries will reside in cities or megacities, which house more than 10 million people (Figure 9.2).
Urbanization over time by income
Urbanization over time for select megacities
http://www.prb.org/Content/NavigationMenu/PRB/Educators/Human_Population/Urbanization2/Patterns_of_World_Urbanization1.htm Source: United Nations, World Urbanization Prospects, The 1999 Revision.
International and rural migration has been the main contributor to rapid urban growth. When dealing with migration there are ‘push’ and ‘pull’ factors that make people want to move away from rural areas and into urban areas, respectively. Social and economic pressures are dominant factors that push people out of rural areas. Example of push factors include government policies that, for the sake of efficiency, turn agricultural production to larger enterprises and minimize government intervention, such as, farm subsidies and cheap credit policies. This scenario leaves farmers to shoulder the burden of farming with all its risks, and eventually may force the farmer to sell his land to foreign investors and move to the city seeking a higher standard of living. On the other hand, there are events taking place in cities that make them attractive to farmers. For example, in order to increase exports, and boost production of cheaper goods, government policy may be geared to reduce the cost of urban labor and urban life (pull factor). This factor makes the move to the city attractive to immigrants.
Environmental degradation and exploitation of the land can
lead to soil erosion, desertification, and deforestation, which cause people to
migrate to cities. In general, damage to the environment is usually done for
the sake of development. An example is
the case of
Unfortunately, there are many problems associated with
rapid urbanization and large cities, specifically related to human health,
environmental degradation, and quality of life issues. Populations in large
cities are often poor, underemployed, lack access to proper healthcare, have
poor sanitation, and are subject to environmental pollution and disease. Rapid
urbanization also tends to lead to the development of urban slums, which are
associated with large-scale social and political problems. In their immediate
vicinity, cities have a variety of impacts, such as: conversion of agricultural
or forest land for urban uses; reclamation of wetlands; quarrying and
excavation of sand, gravel and building materials; indoor and outdoor air pollution
caused by use of biomass fuel; pollution of waterways, lakes and coastal waters
by untreated effluent; altering the hydrology of coasts and their natural
features such as mangrove swamps, reefs, and beaches. Large cities also consume
vast quantities of resources and produce huge quantities of waste, which have
environmental implications for the surrounding region. In order to quantify the
effect of large cities, the concept of ecological footprint has been
introduced. An ecological footprint is the area of productive land and aquatic
ecosystems required to produce the resources used, and to assimilate the wastes
produced by a defined population at a specified material standard of living,
wherever that land may be located. A typical North American city with
a population of 650,000 people would require 30,000 km2 of land, an
area roughly the size of Vancouver Island in
Demographers and environmentalist alike question whether the current trend toward rapid urbanization, especially in the developing world, is sustainable. Much of this future urban population is expected to be poverty-stricken and subject to unemployment, environmental pollution, as well as inadequate food, water, housing, and sanitation, which creates a poor quality of life and the potential to spread disease and radical political ideologies. To avoid many of the social and environmental pitfalls associated with rapid urbanization, governments and other institutions must collaborate to design practical solutions for sustainable cities and communities.
How is the atmosphere impacted by urbanization?
and aerosols that are normal constituents of the atmosphere can become air
pollutants. These substances become
pollutants when their concentration increases to levels that can be harmful to
humans, animals or plants. In the
Aerosols are particles suspended in air and include particles that range in size from a few nanometers to ten micrometers. Large particulates can be observed as smoke, soot and dust; however, small particulate pollution is often not visible and can be extremely damaging to humans and other organisms. Fine particulate matter includes, particulate copper, lead, arsenic, zinc, sulfates, nitrates and asbestos, which can be inhaled and absorbed into the circulatory systems of animals.
Sulfur oxides (SO2 and SO4) are primary pollutants formed from fossil fuel combustion, especially from coal fired power plants. Sulfur oxides can damage the respiratory system in animals and is one of the main components in acid rain, which can damage plants, forests, soils and agricultural systems.
Carbon monoxide (CO) is another primary pollutant and is formed from natural sources like volcanoes, as well as anthropogenic sources such as the incomplete burning of organic compounds. Carbon monoxide is extremely toxic to humans and animals because CO binds very strongly with the hemoglobin in our blood and blocks the binding of oxygen, causing headaches, fatigue, nausea and possibly death from asphyxiation.
Nitrogen oxides (NOx) are another primary urban pollutant, with most NOx being emitted from anthropogenic sources like automobiles and power plants. Although direct exposure to NO and NO2 is only mildly toxic for animals, these compounds are major contributors for the development of smog and acid rain, which can have large-scale detrimental effects on biotic systems.
Volatile organic compound (VOC) is a general term for organic compounds such as hydrocarbons that are used in industrial solvents, and fuels like butane, propane, and methane used for transportation and heating. Many VOC’s are toxic to biotic systems in themselves; however, they are also key components of many chemical reactions that produce toxic secondary atmospheric pollutants.
Carbon Dioxide (CO2) is naturally produced by cellular respiration of organisms, and it is removed through photosynthesis. Other natural sources are forest and brush fires and volcanic activity. The anthropogenic sources are: combustion of fossil fuels (coals, oil and natural gas) for electric power generation, transport and heating. Because of the large quantities in which it is produced, CO2 is the most important of the so called greenhouse gases, responsible for global warming.
Although primary pollutants can be damaging in themselves, secondary pollutants are those, which form from chemical reactions between primary pollutants and other atmospheric compounds; and these can also be extremely detrimental to biotic systems. One of the most damaging secondary pollutants is an oxidant called tropospheric ozone, which causes asthma, eye irritation, and respiratory problems in animals, and also damages leaves and stems of photosynthetic organisms.
Why is one type of ozone considered to be good, and another type to be bad?
The atmosphere on earth is composed of 21% oxygen with the majority of that in the form of diatomic oxygen (O2). However, triatomic oxygen (O3), better known as ozone also exists and is considered to be helpful, or harmful, depending on its location in the atmosphere. Ozone is a powerful oxidant and is thus highly chemically reactive. Therefore, if it is found in the wrong location, ozone can be highly toxic, corrosive, and detrimental to living organisms.
This ‘bad’ ozone is found in the lower atmosphere or the troposphere, near the surface of the Earth. Tropospheric ozone is a powerful secondary pollutant formed from reactions with sunlight, nitrogen dioxide (NO2), volatile organic compounds (VOCs) and oxygen (O2). Ozone forms in sunny urban areas because NOx and other primary pollutants are produced there due to fossil fuel combustion from automobiles, power plants, and other industrial activities. Because ozone is extremely reactive and can be transported long distances, it is extremely detrimental to biological systems. In plants, ozone damages the photosynthetic apparatus and in humans ozone causes respiratory damage (Figure 9.3).
The ‘good’ ozone is chemically the same as the ‘bad’ ozone; however ‘good’ ozone is found higher up in the atmosphere, in the stratosphere, approximately 15 to 40 km above the surface of the Earth. This is the location of 90% of all ozone in our atmosphere, and the location of the ozone layer, which shields life on Earth from harmful UV radiation. The stratospheric ozone layer forms from the photolysis of oxygen by sunlight in the atmosphere (Figure 9.4). Diatomic oxygen (O2) is broken apart into singlet oxygen by the energy from the sun. Singlet oxygen is extremely reactive; therefore, it often reacts with O2 to form O3 or ozone. The ozone layer in the stratosphere is beneficial because is absorbs UV radiation that is harmful to organisms.
In 1985 scientist discovered that the ozone layer,
especially over the
Ozone depletion has contributed to greater rates of skin cancer in humans and other animals, as well as contributed to damage in crops and other plants from high levels of UV radiation. However, the Montreal Protocol and international agreement enacted in 1987 to phase out the use of CFCs has significantly decreased stratospheric ozone depletion. Scientists expect that by eliminating the use of CFC compounds, the stratospheric ozone layer will eventually repair itself by the mid-twenty-first century.
What are temperature inversions?
Once pollutants enter the atmosphere their concentration decreases as they mix with clean air. The rate of dilution depends on atmospheric conditions, such as wind speed and atmospheric stability. The way atmospheric stability plays a role is the following: solar radiation is reflected and absorbed by the surface of the Earth; the absorbed radiation heats the surface, which in turn heats the air near the surface; normally (under atmospheric unstable situations) warm air from near the surface moves upward and is replaced by cold air from above (convection). The fact that the temperature in the troposphere decreases with altitude allows convection to happen. This normal pattern is illustrated in the top of Figure 9.3. In this situation the rate of dilution of pollutants is high.
The temperature in the
troposphere does not always decrease with altitude. This situation, illustrated
in the bottom of Figure 9.5, is referred to as a temperature inversion. In the
presence of a temperature inversion, the air near the surface is trapped and
cannot move upward. In this case gases
emitted at the surface do not dilute, and the concentration of pollutants
increases. Temperature inversions happen in cities that are in valleys such as
How does urbanization put pressure on freshwater resources?
Historically, human populations have flourished where freshwater resources were readily available. However, because the human population has grown at such a rapid pace since the industrial revolution, now many human populations live in regions that lack access to sustainable and adequate freshwater resources. As a result of the lack of safe drinking water, many people (mostly poor) also have major health problems from pollution related diseases, and lack adequate and secure food supplies. For example, more than 5 million people die annually from preventable diseases caused by inappropriate disposal of sewage and other wastes. Concurrent with population growth, the increase in urbanization also has had major impacts on freshwater resources. Currently, 60% of all freshwater consumed globally is used to support urban populations, and this demand is expected to increased steadily in the coming decades. Urbanization concentrates a large number of people in relatively small areas; therefore, urban development alters the flow, storage, and quality of freshwater. Specifically, urbanization changes the runoff patterns of water, and increases erosion and pollutant loads in freshwater systems.
Urbanization impacts freshwater resources by altering the imperviousness of the landscape, or the ability of the land to absorb water, and thus alters the pattern of water movement into streams, rivers, and lakes. Urban land area has greater quantities of impervious surfaces in which water cannot percolate, such as concrete, roads, and buildings. Impervious surfaces cause rainwater to run off up to ten times faster than runoff from soil or unpaved surfaces. This rapid runoff carries high concentrations of pollutants and debris. Urbanization also causes land use change and deforestation, and this increases erosion, decreases evapotranspiration from the land, and further increases stream-flow during storm events. These changes in runoff, imperviousness, and evapotranspiration contribute to a much greater flood frequency and volume in urban areas. Impervious surfaces and high runoff rates also contribute to high levels of bank erosion, turbidity, and pollution where urban runoff is channeled into local streams and rivers. Changes in evapotranspiration and the presence of concrete surfaces in urban areas are also responsible for the ‘heat island effect’, which results in an increase in temperature in the city over the surrounding areas.
Urban populations currently generate 75% of all waste on the planet, and scientists estimate that by 2025 urban waste will increase by another 375%. Urban water pollution includes point source pollution, or that which is discharged from a fixed point such as a chemical or industrial plant effluent pipe; and non-point source pollution, which includes pollutant laden surface runoff occurring after a precipitation event. Non-point source pollution is responsible for the greatest amount of urban water pollution, and this is partly due to the difficulty in regulating urban, suburban and agricultural surface runoff pollutants. Urban water pollution can be caused from organic and non-organic chemicals, sediments, as well as bacteria and other disease causing organisms. Heat can also be considered a non-point source urban water pollutant because surface runoff from urban areas is often very warm due to the urban heat island effect. Hot surface runoff causes stress to organisms that reside in the streams, lakes, and rivers that receive the urban surface runoff.
Currently, growing human populations, particularly those in urban areas, are exerting the greatest pressure on water resources globally. In addition to the greater demand on freshwater resources from growing populations, freshwater resources are being consumed in greater quantities around the world, which is increasing pressure on scarce resources. Freshwater resources are not only being used for household uses like cooking, but also for agricultural irrigation, and for industrial production processes (Figure 9.6).
Water use by sector over time
Scientists estimate that freshwater will become even scarcer in the future as the population continues to grow (Figure 9.7). Furthermore, global warming and climate change are likely to heavily impact the availability of freshwater worldwide. Therefore, appropriate management of freshwater resources must be employed today to conserve the freshwater resources we have, and to repair the pollutant damage that human activities have caused, for a sustainable future.
Urbanization over time for select cities
http://www.prb.org/Content/NavigationMenu/PRB/Educators/Human_Population/Urbanization2/Patterns_of_World_Urbanization1.htm Source: United Nations, World Urbanization Prospects, The 1999 Revision.
Why is transportation infrastructure critical to urban sustainability?
A sustainable society is one that neither depletes nor destroys the resources in a system for current or future generations: or a society that is in balance with the natural world and does not exceed its sustainable ecological footprint. Since urban areas now house the majority of the humans on the planet and are where most natural resources are consumed, the future sustainability of the human population depends on sustainable urban areas. Transportation infrastructure is critical to sustainable urban areas because without strong transportation infrastructure within urban areas, cities tend to sprawl into low-density suburban areas, which have wide-scale environmental and social costs.
For example, urban areas with high amounts of sprawl consume much more fossil fuels than those with sustainable transportation infrastructure. Sprawling cities leads to residents that must commute long distances and also increases individual ownership of automobiles. Sprawling cities also reduces the incentive for residents to walk (Figure 9.8), which contributes to health problems including obesity, heart disease, and high blood pressure. Sprawling urban areas end up costing much more than more compact cities with sustainable transportation infrastructure due to the cost of maintaining and expanding roads and highways. Cities with high amounts of sprawl also tend to have less mass-transit facilities, a higher risk of fatal traffic accidents, and high levels of traffic congestion.
Average trips by population density
Furthermore, the combustion of fossil fuels and high levels of traffic congestion produces large quantities of air pollution because approximately 80% of air pollution in cities is caused by vehicle emissions. Roads and highways, which increase with urban sprawl, are also detrimental to freshwater systems because a great deal of non-point source pollution originates from road surfaces. These surfaces collect motor oil, debris, litter, and dust from tire wear, which eventually runs off into waterways. Urban sprawl also contributes to land use change as sprawl consumes land that would otherwise be used for agriculture or wild-lands. Urban sprawl is one of the main contributors to habitat fragmentation and is responsible for the loss of countless species of plants and animals.
How do we create more livable and sustainable cities?
Sustainable and livable cities can only be realized with the cooperation between government, business, nongovernmental organizations, and the public. Cooperation between these bodies is crucial for lifestyle changes that will allow urban areas to function effectively and be attractive, enjoyable, and aesthetically designed for the population, while being environmentally sound. There are many ways that urban areas can become more environmentally sustainable and livable for it’s residents. Three key aspects of urban sustainability are environmental planning and management, social equity, and economic efficiency.
Environmental planning for sustainable urban areas should strive to reduce automobile traffic and have greater reliance on foot and public transportation. These steps are the backbone to reducing energy consumption, pollution, and creating a local economy that is strong and efficient. Governments and city planners have a responsibility to create and maintain more livable and sustainable cities by implementing urban renewal projects, infilling or building within existing space in urban areas, and building vertically to make cities more compact and use the urban space efficiently. Governments, developers, businesses, and the individual can all contribute to urban sustainability by using green building technologies, consuming environmentally friendly products, and installing solar and wind power generators to decrease energy consumption and fossil fuel pollution. In addition to green technologies for energy use and consumption, waste, which is a critical problem in all cities, must be considered for a city to be sustainable, healthy, and attractive. Thus, comprehensive solid waste recycling programs and efficient waste management facilities need to be utilized to decrease waste and increase efficiency.
The most livable cities generally have safe, clean, and affordable housing options, as well as a strong sense of community, utilizing community centers and other centers of learning that allow residents to learn new skills or better themselves. Livable and sustainable cities also depend on reliable, affordable and efficient public transportation, good public health facilities, and strong governmental institutions that control crime, prevent corruption and clean up pollution and solid waste. To realize sustainable and livable urban areas, communities must strive to have a creative and diversified economy, as well as enjoyable and diverse amenities such as museums, parks, urban greenways, entertainment, and the arts. Communities must also develop programs that give citizens a sense of place and feelings of pride in their community. Governments and city planners can facilitate this process by creating pedestrian only streets and other safe social spaces geared toward the local community, which create social and economic bases for businesses, jobs, entertainment and activities.
Individuals can make a substantial impact on creating and maintaining sustainable cities by utilizing urban and rooftop gardens, which reduce dependence on imported foods, recycle compost and wastewater, grow useable products like fruits and vegetables, and in the process reduce greenhouse gases and the urban heat island effect. Individuals can also contribute to developing more sustainable cities by smart and informed individual consumptive choices, political involvement, and participation in nongovernmental and environmental organizations. Globally, the number of people living in cities and urban areas is expected to increase dramatically in the future. Therefore, it is imperative the governments, businesses, organizations, and individuals alike do their part to develop and maintain sustainable and livable urban spaces.