Urban Ecosystems

The environmental impacts of urbanization are complex and affect both local and regional ecosystems, as well as the global climate. The interactions between urbanization and environment vary across regions, involve both natural and man-made landscapes, and are affected by various social, economic and institutional factors.
Urbanization is driven by natural increase (a higher rate of births than deaths) and by rural-urban and international migration. Urbanization also results from reclassification of rural areas as urban. Cities are projected to grow by more than the total global population increase between 2007 and 2050. Between 2007 and 2050, the UN projects that the global population will increase by 2.5 billion (from 6.7 to 9.2 billion), while the global urban population will increase by 3.1 billion (from 3.3 billion to 6.4 billion). This increase will be concentrated in the developing world (Figure 1 ).

Figure 1: Distribution of the world’s urban population: 2007 and 2050

Environmental concerns of urbanization

Cities have long been constructed along rivers and deltas for easy access to water sources, and these water sources have been modified to meet the needs of cities. Water is used for domestic purposes, and is intricately linked to industrial processes, sanitation and protection from natural disasters. Changes in landscape in constructing cities and to support the demands of urban populations greatly alter the biophysical condition of the local areas. The design of most cities is such that accumulated pollutants are channeled from buildings, roadways and parking lots into streams. Concentration of transportation and industry in urban areas also modifies biogeochemical cycles, or the flow of chemical elements. Cities become concentrated sources of CO2 and other greenhouse gases and pollutants, which influence neighboring exposed ecosystems.

The urban heat-island affect, which results in higher temperatures in cities than in their surroundings, affects local climate, water resources, biodiversity, air quality and ecosystem functioning. Urbanization and other land-use changes accounted for half of the observed reduction in daytime temperature ranges and an increase in average air temperature of 0.27 degree Celsius in the US during the past century.

The magnitude of population growth is an important variable affecting urban environmental problems because it directly impacts the spatial concentration of people, industry, commerce, vehicles, energy consumption, water use, waste generation and other environmental stress. The larger the city, it is assumed, the greater the per capita environmental costs or damages.

However, sheer magnitude of pollution discharge is only one side of the story; more importantly, what ultimately counts is the capacity of cities to be designed and managed such that they eliminate more pollution than they emit.  

Rapidly developing stress

Most developing countries experiencing rapid urbanization do not have the resources to either detect many modern chemicals or to establish facilities and sites to treat hazardous wastes. Many cities have inadequate waste-management policies and practices, even though solid waste management accounts for 20 to 50 percent of local government expenditure in developing countries. Likewise, the sanitation systems provided by most municipalities are generally inadequate and do not produce the anticipated public health and environmental benefits.

Informal settlements and slums are often unconnected to even basic sanitation, and in some African cities, around nine in 10 people in urban areas live in slum settlements. Air pollutants emitted from vehicles, industrial and domestic sources in cities throughout the developing world exceed the capacity of natural ventilation systems to disperse and dilute these emissions to non-harmful exposure levels.  

Population growth and energy consumption

While urbanization is often implicated in local and regional environmental degradation, increasingly there is a more balanced view on the wider national and global environmental impact of urbanization, including the role of cities in economic growth and increasing economies of scale, technical innovation, access to information, efficient land and energy use, better living conditions, provision of clean water and access to health care services.

Urbanization can also have an effect on population growth. As women move to cities, they tend to adopt the lower fertility patterns of urban women and have fewer children than if they had stayed in rural areas. In global terms, population growth is actually slowed by the growth of cities, a pattern that helps to lessen population pressure on limited natural resources.

As rural people are urbanized, their consumption patterns and lifestyles change with rising income, and negative environmental consequences are inevitable, which may offset the positive impact from lower population growth.

Cities are clearly the hot spots of production, consumption and waste generation. Already, according to the UN, cities are responsible for 75 percent of global energy consumption and 80 percent of greenhouse gas emissions, and a disproportionate share of resource use, including, for example, food, timber and steel.

On the other hand, continuous economic growth and technological advancement could eventually bring down the adverse environmental impacts after crossing the threshold embedded in the Environmental Kutznets Curve assumption, which is that pollution increases with income to a certain threshold before decreasing.

According to the Environmental Kuznets Curve assumption, urbanization and accompanied and induced economic growth in developing countries should contribute to pollution abatement. However, some authors argue that urbanization does not necessarily induce economic growth.

For instance, the highly correlated urbanization-GDP growth could be due simply to the effect of GDP growth on urbanization, with no reversed effect; or it could be due to a common factor that affects both urbanization and GDP growth. More importantly, adding environment into this equation, one could argue that growth in income and consumption may cause adverse environmental consequences and at the same time provide resources for environmental preservation. In that case the link between urbanization and environmental changes would not be significant and both could be driven by economic growth.

To address this issue, we investigate the interactions between urbanization and energy consumption/carbon emission, accounting for the effect of income for 237 countries/areas for the period of 1980-2005. Figure 2-a clearly shows the positive cross-spatial and cross-temporal relationship between the level of urbanization (i.e. the urban share of the population) and per capita energy consumption and carbon emission of the countries/areas. However, the speed of urbanization (i.e. the annual growth rate of the level of urbanization) displays a significantly different relationship with energy intensity (energy used to produce per unit of $ GDP output) and carbon intensity (carbon emitted from per unit of energy consumed): as urbanization speeds up, energy intensity goes down, but carbon intensity increases first then levels off later (Figure 2-b).

While the scatter plots imply an interaction, there are clearly other economic, political and cultural factors that affect the relationship between urbanization and energy use/carbon emission.

Figure 2: Relationship between urbanization and energy consumption/carbon emission, controlling for income, for 237 countries/areas: 1980-2005

(a) Urbanization and per capita energy consumption/dioxid emission

 

 

 

(b) Urbanization and energy/dioxid intensity

 

 

 Table 1 shows results from a partial correlation analysis of the same data, in which we controlled for economic growth (using the variable per capita GDP), contextual specificity (countries/areas) and a period effect (years). After accounting for income level and other contextual effects, the partial correlation analysis shows that annual urban growth rate is negatively associated with urbanization level, not surprisingly since urban growth is faster when it is occurring in largely rural countries but slows when more people already live in urban areas.

Table 1: Partial correlation analysis of urbanization and energy use/carbon emission

 

More importantly, the analysis points out that larger urban population share is associated with more per capita energy consumption and more per capita carbon emission. However, countries or areas with higher annual urban growth rates have lower per capita energy use and lower per capita carbon emissions, since those countries/areas are usually less industrialized and use less energy in their economies to produce the same amount of GDP output.

Moreover, faster urbanization also changes the composition of energy sources from more traditional biomass to more efficient modern fossil fuels, which contributes to lower energy intensity but higher carbon intensity. On the other hand, when a country reaches a relatively higher urbanization level, people rely on energy in their work places, but emit less carbon in producing the same amount of GDP output through technological advancement, cleaner energy sources (oil, natural gas vs. coal) and non-fossil fuel sources (nuclear, wind and solar power).  

Conclusion

Urbanization is a process that will continue, and future population growth will almost exclusively occur in developing countries, particularly in cities that lack significant economic growth to keep pace with rapid urban expansion, resulting in increasing urbanization of poverty and deteriorating urban environment.

From this analysis, we argue that: (1) the relationship between urbanization and environment, particularly urbanization and energy consumption and carbon emission, is complicated; (2) the study of the impact of urbanization on global climate change should not be isolated from analysis of other socioeconomic, cultural and institutional factors; (3) countries/areas with various urbanization patterns and at various urbanization stages face different challenges in protecting the environment while improving standards of living.

While developing countries with relatively high urban growth rates will increase carbon intensity from more commercial energy use to alleviate energy poverty and develop their economies, urban areas of these countries, serving as the engine of economic growth, may achieve significant energy efficiency. Developed countries that have been largely urbanized are in a better position to achieve low carbon intensity by adopting new energy technologies, even while they consume more energy and emit more carbon per capita.

Global leaders should cooperatively identify efficient and environmental-friendly technologies and economic policies to help developing countries make progress towards developing their economies and improving living standards while protecting local and regional as well as the global environment.