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Urbanization as a Global Ecological Process
3672 Global Ecology | Urbanization as a Global Ecological Process
Further Reading Adams CC (1935) The relation of general ecology to human ecology. Ecology 16: 316–335. Adams CE, Lindsey KJ, and Ash SJ (2006) Urban Wildlife Management. Boca Raton: CRC Press, Taylor and Francis. Alfsen-Norodom C (2004) Urban biosphere and society: Partnership of cities. Annals of New York Academy of Sciences 1023: 1–9. Blair RB (1996) Land use and avian species diversity along an urban gradient. Ecological Applications 6(2): 506–519. Colding J, Lundberg J, and Folke C (2006) Incorporating green-area user groups in urban ecosystem management. Ambio 35(5): 237–244. Collins JP, Kinzig A, Grimm NB, et al. (2000) A new urban ecology. American Scientist 88: 416–425. Felson AJ and Pickett STA (2005) Designed experiments: New approaches to studying urban ecosystems. Frontiers in Ecology and the Environment 10: 549–556. Kinzig AP, Warren P, Martin C, Hope D, and Katti M (2005) The effects of human socioeconomic status and cultural characteristics on urban patterns of biodiversity. Ecology and Society 10(1): 23. http://www.ecologyandsociety.org/vol10/iss1/art23 (accessed December 2007).
McDonnell MJ and Pickett STA (1990) Ecosystem structure and function along urban–rural gradients: An unexploited opportunity for ecology. Ecology 71: 1232–1237. McDonnell MJ and Pickett STA (1993) Humans as Components of Ecosystems: Subtle Human Effects and the Ecology of Populated Areas, 363pp. New York: Springer. McGranahan G, Marcotullio P, Bai X, et al. (2005) Urban systems. In: Scholes R and Ash N (eds.) Ecosystems and Human Well-being: Current State and Trends, ch. 27, pp. 795–825. Washington, DC: Island Press. http://www.maweb.org/documents/document.296. aspx.pdf (accessed December 2007). Millennium Ecosystem Assessment (2005) Ecosystems and Human Well-being: Synthesis. Washington, DC: Island Press. Pickett STA, Cadenasso MI, Grove JM, et al. (2001) Urban ecological systems: Linking terrestrial ecological, physical and socioeconomic components of metropolitan areas. Annual Review of Ecology and Systematics 31: 127–157. Sukopp H, Numata M, and Huber A (1995) Urban Ecology as the Basis of Urban Planning. The Hague: SPB Academic Publishing. Turner WR, Nakamura T, and Dinetti M (2004) Global urbanization and the separation of humans from nature. Bioscience 54: 585–590.
Urbanization as a Global Ecological Process A Svirejeva-Hopkins, Potsdam Institute for Climate Impact Research, Potsdam, Germany ª 2008 Elsevier B.V. All rights reserved.
Introduction and Definitions Past, Present, and Future of Urbanization Present and Future Dynamics of Urban Areas The City as a Specific Heterotrophic Ecosystem Environmental Effect of Urbanization and Ecological Footprint
Carbon Balance in Urbanized Territories Conclusion Further Reading
Introduction and Definitions
inhabitants are considered to be urban areas in the USA; while 2000 inhabitants living in contiguous housing form an urban area in France, and in the Netherlands it is municipalities with 2000 or more inhabitants. Settlement densities can be orders of magnitude higher than agricultural rates, although residential densities in some urban areas are only marginally higher than the farmland densities in the most intensively cultivated agricultural areas (compare 2500 people per km2 in Los Angeles suburbs with 2000 peasants per km2 of arable land in Sichuan, China). However, maximum residential densities, c. 90 000 people per km2 (the center of Hong Kong), translate into an anthropomass of 36 MJ m2. This is roughly 200 times higher than the density of large herbivorous ungulates in Africa’s richest ecosystem. The definition of urban area in some other regions differs significantly from the above-mentioned one, with the concept of urban area somewhat based on the ancient structure of a city. All this shows a significant uncertainty in the term ‘urban(ized) area’ and which is necessary to take into account in any quantitative estimations.
The most concise definition of urbanization is given by Encyclopaedia Britannica: ‘‘this is the process by which large numbers of people become permanently concentrated in relatively small areas, forming cities and their suburbs.’’ Population, P, and territory, S, of these formations are changing in time, so that urbanization is a dynamic process controlled by these main variables. The degree of people concentration is determined by the ‘population density’, D ¼ P/S, where P is population size and S is the area it occupies. Different national statistics operate with different definitions and meanings of the terms ‘urban’, ‘urban area’, or ‘urbanized territory’. For instance, the United Nations defines all places with more than 20 000 inhabitants living close together as urban, while the US Census Bureau uses ‘urban area’ as a densely populated area (built-up area) with D > 1000 inhabitants per square mile and P > 50 000. Thus, the minimal urban area is equal to 50 square miles. At the same time, settlements with more then 2500
Global Ecology | Urbanization as a Global Ecological Process
Past, Present, and Future of Urbanization
1. the rise of each person’s ability to affect the natural environment through energy sources’ exploitation; 2. the rise of the unevenness of the spatial distribution of people through development of ‘cities’;
Population growth in developed countries
Population (thousands)
3600 3100 2600 2100 1600 1100 600 100 1920 1930 1940 1950 1960 1970 1980 1990 2000 Year Population growth in developing countries 3600
Population (thousands)
Two thousand years ago, there were about a quarter of a billion people living on our planet. The global population doubled to about half a billion by the sixteenth to seventeenth centuries. The next doubling required two centuries (from the middle of the seventeenth century to 1850, when the size of two European cities, Paris and London, exceeded 1 million inhabitants); the following doubling occurred just over the next 100 years, while the last one took only 39 years. The year 1650 is named as the start of ‘the urban explosion’. Generally speaking, beginning from this date, the enormous population growth started. Nevertheless, it is a common practice to perceive the beginning of urbanization to coincide with the start of the agricultural revolution (7000–5000 BCE). It was at this time that nomadic hunters settled down and began to grow their food. A food surplus was created, and the division of labor made it possible to evolve gradually into the complex, interrelated social structures we know now as cities. The first cities were located along the Tigris and Euphrates Rivers (4000–3000 BCE) in contemporary Iraq, with urbanization then occurring also in Egypt, North Africa, India, China, Japan, and Europe – the Americas being the regions of most recent urbanization. Environmental factors were the major driving forces in the development of earlier cities. Fertile soils and easy access to water bodies, as well as adequate water supply, were essential. The first environmental disaster was triggered by the deforestation of the Middle East that led to soil degradation in the area, followed by a collapse of irrigations systems, and, as a consequence, to famine. Ancient cities were extremely dependent on the surrounding ecosystems, in particular, on agricultural lands. In Europe, since the eleventh century, there has been a historical continuing flow of people from the countryside to the cities, although the ‘Black Death’ in the fourteenth century impaired the process of urbanization. Europe recovered from the effect of this pandemic only by the middle of the sevententh century, when the ‘urban explosion’ occurred. Urbanization had also been occurring worldwide for at least two centuries. During the eighteenth century we have seen modern urbanization due to technological development, while earlier the process was driven by the migration of people from rural areas, since they were not needed in farming anymore. However, in the last decades of the last century, we observed the unprecedented global population growth and the accompanying process of urbanization (Figure 1). Generally speaking, this enormous population growth was accompanied by other significant changes:
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Urban Rural
3100 2600 2100 1600 1100 600 100 1920 1930 1940 1950 1960 1970 1980 1990 2000 Year
Figure 1 Comparison of urban and rural population growth in developed and in developing countries (urban is defined as settlements of 20 000 people and above). Source: UN, World Urbanization Prospects, 1990, 1991.
3. migration and travels’ increase, while contacts between cultures also rise. Although only 12% of the world’s population lived in urban centers in 1940, this percentage had risen to 33% by 1980. After World War II, a 2% urbanization rate was observed in the developed world, while it was almost 4% in the developing countries. However, urban growth rates doubled that of the total population. And while the total population growth rate in the developed world has been decreasing, the urban population’s proportion has increased from 55% to 70% of the total population. The major reason is the decline in rural population, as well as the arrival of new immigrants to the cities of some countries. Since the middle of the last century, the following trend is observed in the percentages of urban population: 1950, 29%; 1960, 34%; 1970, 37%; 1980, 40%; 1990, 43.5%, and in 1995 45%. However, the definition of what is urban varies greatly, which is why the estimations differ as well. For example, another source estimates 20% in 1950.
3674 Global Ecology | Urbanization as a Global Ecological Process
Urbanization growth rate has significantly left total population growth behind. From the year 1800 to 1990, the absolute number of city dwellers increased from 18 million to 2.3 billion, a 128-fold increase, while the total population has increased by only 6 times (from 0.9 billion to 5.3 billion). Furthermore, more then 1.4 billion city dwellers live in the less-developed world. If we look at the rates of urban area growth and compare them with urban population growth, we find that the first grows faster than the population, which in turn grows at a faster rate than the total country population, and this is a common phenomenon. In 1990–95, the world’s urban population grew by 2–4% per year, while rural population only grew by 0.7% per year. Urban population increased by 2.1% during the 1995–2000 period, while rural population only by 0.7%. Today, 75% of the world’s population lives in the less-developed countries, and 58% in Asia. In 1999, 19 urban settlements had 10 million or more inhabitants, and 47% of all people lived in cities. The number of ‘megacities’, that is, giant urban agglomerations with a densely settled urban core of the original city, is increasing. Around this core, the satellite cities have grown, either planned or unplanned, linked to the central core by transport, communication, economic interdependence, and political-administrative structures. This tendency is confirmed by the following statistics: from 1950 until 1975, many cities with population of 5 million people have doubled in total urban population, while at the same time, cities with less than 100 000 people declined in their relative importance. In 1992, there were 23 megacities with populations greater than 8 million: 6 in the developed world (Tokyo, New York, Los Angeles, Osaka, Paris, and Moscow), and 17 in the developing world (from which 11 were in Asia). For most Asian cities, the shortage of water will be the most critical issue and is the limiting factor for the further growth of Beijing, Manila, Bangkok, Jakarta, and other cities. Also, while during the nineteenth century water and air pollution were associated with only a few larger cities, they are now becoming a global problem due to the rapid industrialization and the simultaneous concentration of people in cities. While the past and current demographic situations are estimated more or less accurately, future dynamics are forecast with a very high level of uncertainty. The UN vividly illustrates that if current exponential and hyperbolic growth continues in each major region and at the current rates, then the population will increase by more than 130-fold in 160 years, from 5.3 billion in 1990 to 694 billion in 2150. However, eventually, the problem will be how to feed these people, since food and water limitations will certainly arise. The UN also shows that future global population size is very sensitive to the future level of average fertility. Projections of global population dynamics are also uncertain, because external factors such as climate may
change unexpectedly. Furthermore, even if external factors change as expected, the relationship between those factors and demographic rates may change. We have the following hypothetical picture for the next half of the century. The global population will grow by 2–4 billion people, mostly in poor, but not rich, countries. It will also increase less rapidly than before and will become more urban than now. Hence, ‘‘from here on it is an urban world.’’ Most of all, the additional people will be living in cities in poor countries, which can become an epidemiological danger. Population of the more developed countries will decline slightly, but increase substantially in lessdeveloped countries. In this century, global urban population will increase 1.8 times by 2020 (relative to 1990), while the total population will grow by only 1.4 times, and almost all population growth will be associated with cities in the developing countries. By the year 2030, the world urban population will reach 4.9 billion (1 billion in developed countries and 3.9 billion in developing countries). The global rural population will remain constant at 3.2 or 3.3 billion, although in the developed countries the rural population will decline. The trend in the developing countries is that rural population will slowly rise through the next couple of decades, reaching 3.1 billion, and then will slowly decline.
Present and Future Dynamics of Urban Areas In 1985, 43% of the world population lived in cities while urban settlements covered just over 1% of the Earth’s surface. In 1990, 50% of the global population of Homo sapiens inhabited less than 3% of the Earth’s ice-free land area. However in the near future we may expect a further growth of the urban territories’ area. World dynamics of this process is shown in Figure 2. All these prognoses are based on two models: the first is a regression model, connecting urban population and urban area, and giving a minimal estimation, and the second, uses the spatial distribution of population density and gives a maximal estimation. Note that the dynamics of urban areas are significantly different in the major world regions (Figure 3). For instance, the fast growth in African region differs from almost constant dynamics in the highly industrialized countries.
The City as a Specific Heterotrophic Ecosystem From an ecological point of view, any city is a heterotrophic system maintained by external inflows of energy,
Global Ecology | Urbanization as a Global Ecological Process
World’s total relative urban area (%)
7 6 5 4 1st model 2nd model
3 2 1 0 1980
1990
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2010 2020 Year
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Su (%) gamma model
Su (%) gamma model
Figure 2 Dynamics of the relative world urban area (in percentage of the total world area) between 1980 and 2050.
45 40 35 30 25 20 15 10 5 0 1980
HI AsP
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3.5 3 2.5 2 1.5
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1 0.5 0 1980
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Figure 3 Dynamics of the relative urban areas (% of the total regional area) for the three world regions: HI, highly industrialized European; AsP, Asia and Pacific; and Afr, African.
food, water, and other substances. Thermodynamically any city (and generally, any urbanized territory) is an open system that is far from thermodynamic equilibrium. All matter and energy needed for a city’s functioning are collected from external territories that are significantly larger than the area of the city itself and very often are located quite far away. The heterotrophic ecosystem ‘city’ differs very much from a natural heterotrophic ecosystem. In fact, a city has a more intensive metabolism per area unit, requiring a significant inflow of artificial energy. Its consumption per
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urban area unit may be 3–4 orders of magnitude higher than the same for rural area. For instance, the annual subsidies in fuel, fertilizers, labor, etc., required to maintain a lawn in the Madison metropolitan area (Wisconsin, USA) is equal to 22 GJ ha1, which is approximately equal to the artificial energy input for a maize field. During the process of its own metabolism, a city consumes large amounts of various materials: food, water, wood, metals, etc., all that we call ‘gray energy’. Products of city’s metabolism have larger volumes of, and more toxic, substances than the same of natural ecosystems. If we compare cities and natural forests in Wisconsin, USA, we can see that the number of species in a city forest (75 tree and 74 bush species) is more than in natural one (10 tree and 20 bush species). The annual net production and the amount of living biomass (in carbon units) in city’s forests are equal to 500 t C km2 yr1 and 7000 t C km2; while in a natural forest the corresponding values are 400 t C km2 yr1 and 13 000 t C km2. The greater values of species diversity and production are provided in city’s forests at the expense of additional inflow of ‘gray energy’. For instance, the annual import of fertilizers is about 140 t km2. While natural forest is almost a closed system, a city forest is a typical through-flow system with ‘gray energy’ input and output in the form of dead organic matter: about half of the annual accretion is exported from the city to waste treatment plants. This is one possible explanation why the amount of biomass in the city forest is lesser than in the natural ecosystem. The carbon storage in urban forests with their relatively low tree cover (25.1 t C ha1 in average for US) is less than in natural forest stands (53.5 t C ha1). The gross sequestration rate, that is, the fraction of the gross annual production accumulated in wood, in urban forests is equal to 0.8 t C ha1 yr1, which is also less than in natural ones (for instance, 1.0 t C ha yr1 for a 25-year-old natural regeneration spruce–fir forest with 0.1 kg C m2 cover), although the difference is insignificant. However, on a per-unit tree cover basis, carbon storage by urban tree and gross sequestration may be greater than in natural forests, 92.5 t C ha1 and 3.0 t C ha1 yr1, due to a larger proportion of large trees and the more open structure (that leads to the weakness of competition) in urban forests.
Environmental Effect of Urbanization and Ecological Footprint Retrospectively, most humans have lived in small settlements dispersed within larger biomes (Figure 4). We see there is a certain correlation between the type of biomes and the degree of urbanization, some biomes being more preferable for cities.
3676 Global Ecology | Urbanization as a Global Ecological Process
Figure 4 Different types of global vegetation (biomes). Red dots represent the concentration of cities; the green line is the border of the regions. 1, Polar desert, polar tundra; 2, tundra; 3, mountainous tundra; 4, forest tundra; 5, north taiga; 6, middle taiga; 7, south taiga; 8, temperate mixed forest; 9, aspen–birch lower taiga; 10, deciduous forest; 11, subtropical deciduous and coniferous forest; 12, xerophyte woods and shrubs; 13, forest steppe; 14, temperate dry steppe (including mountainous); 15, savanna; 16, dry steppe; 17, sub-boreal desert; 18, sub-boreal saline ‘desert’; 19, subtropical semidesert; 20, subtropical desert; 21, mountainous desert; 22, alpine and subalpine meadows; 23, evergreen tropical rainforest; 24, deciduous tropical forest; 25, tropical xerophyte woodland; 26, tropical savanna; 27, tropical desert; 28, mangrove forest; 29, saline land; 30, subtropical and tropical woodland and Tugay shrubs.
The growth of cities in these ‘patches’, absorbing and transforming nearby natural ecosystems and agricultural lands, modifying energy and matter flows, typical for these ecosystems, negatively influences the local and regional biodiversity, increasing fragmentation of large rural areas and natural zones. This process (and contamination of the atmosphere) leads to the changing of the nature of land surfaces and near-surface atmospheric layer, and therefore its reflection and absorption of solar radiation and aerodynamic properties. This in turn leads to raising urban temperatures and the changing of the local climate, creating so-called ‘urban heat islands’, which is warmer by 1–2 C than surrounding territories. The ‘urban heat island’ effect occurs mainly at night, when the buildings, etc., release heat absorbed during day. In addition, metropolitan agglomerations influence the local and global environment through their consumption of non-native resources and their concentrated production of waste and consumables. The footprint is the quantitative conversion of the material and energy flows required to support human population in cities into the land area required to produce these flows. Although cities occupy a relatively small area on the planet, they are the dominant human ecosystem and the ecological space taken up by humans as a species is much higher. Every city depends (for its existence and
growth) on a globally diffuse productive hinterland up to 200 times the size of a city itself. To illustrate this, let us examine the following case studies. For instance, one person of the USA urban population consumes daily (1) food produced by 0.75 ha of agriculture land, (2) paper and wood by 0.4 ha of forest, (3) water by about 7.5 m3. So, a 1-million-populated city with the population density of 4000 persons per km2 (city area is 250 km2 correspondingly) needs a significantly larger area for its support: 7500 km2 of agriculture land and 4000 km2 of forest. A rather large river watershed (presuming abundant precipitation) can provide inflow of 7.5 million m3 of water daily. If we take into account that the area of such watershed is about 15 000 km2 then the total footprint area will be about 26 500 km2, that is, 106 times the city area. The city of Vancouver (Canada) had in the year 1991 a population of 47 2000 living in an area of 11 400 ha. If we assume that the per capita land consumption rate is 4.3 ha, then the people in this city would require 2.03 Mha of land. Hence, the inhabitants would require a land area 180 times larger then their habitat. Furthermore, adding a marine footprint of 0.7 ha per person, the total area needed to support the city becomes 2.36 Mha, or 200 times larger than the geographic area of the city. For London, the equivalent footprint is 120 times the area of the city itself. The New York metropolitan area annually
Global Ecology | Urbanization as a Global Ecological Process
consumes the equivalent of 800 000 ha of wheat, or approximately the total amount of wheat grown yearly in the state of Nebraska. So, if we presume that the world urban area constitutes 1% of the total land area, and the footprint is 100 times this area (note that these are minimal estimations), even in this case the urban footprint exceeds all the world land area.
Carbon Balance in Urbanized Territories Therefore, we can say that although the total area of urbanized territories is relatively small (1–2% in 1990s), they play an ever-increasing role in global change in general and in the global carbon cycle, the main biogeochemical cycle of the biosphere, in particular. Urban areas emit (in accordance with different estimations) 78–97% of the total anthropogenic carbon emission. Up to 60% of this emission comes from the transportation and building sectors, while the rest are from industry. Of course, all of these emissions are ‘spread’ and mixed in the entire atmosphere over 3–4 months period, but they are generated in particular by urban point sources. Cities transform the natural territories they occupy, partially obliterating vegetation and soil, partially modifying them. Similarly, urbanization changes the structure and function of the local carbon flows within these territories. Note that the process often involves considerably larger territories than the exact city areas. Cities consume a lot of organic carbon in the form of food and other agricultural products, as well as wood, etc., produced, as a rule, far from the urban territories, transforming them into other forms of carbon (feces, exhaled CO2, residues of food processing, dead organics of ‘green zones’, etc.) in the process of urban and purely physiological human metabolism. In other words, cities destroy the spatial entity of the processes of production and decomposition of living matter that is typical for natural ecosystems. Note that this entity provides the closure of any local carbon cycle. Urban territories have more carbon stored per unit area than natural ecosystems. Organic carbon is stored in soils and vegetation of urban territories, and also in people and pets; nonorganic carbon includes carbon transported into the cities and stored in buildings, etc., but most of this carbon is transformed into waste. Processes similar to those in peatlands accompany carbon fluxes from mineralization, incineration (rapid oxidation of carbon), and landfilling of solid waste. For instance, the global input of carbon into solid waste (sludge and industrial waste) is estimated to be 0.16 Gt yr1 (1 Gt ¼ 109 t). Long-term organic carbon in urban territories is accumulated in
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1. Biomass in humans and animals. For the world population of 6 billion, the total amount of carbon is equal to 45 million t of carbon that constitutes about 10% of the total biomass of land animals. A biological metabolism of 6 billion people is accompanied by exhalation of 0.34 Gt C yr1 and secretion of 0.18 Gt yr1 with feces and other discharges that, respectively, give a total 0.52 Gt C yr1. The value is entirely comparable with components of the global carbon cycle. For instance, soil erosion gives 0.98 Gt C yr1. 2. Biomass in trees and other plants. The mean global value of living plant biomass in cities is 3500 t C km2, and the mean net primary production is 500 t C km2 yr1. By taking the global urban area in 1980 as 2 106 km2 and assuming that 50% of it is covered by city vegetation, we find that urban territories contain 3.5 Gt C in living vegetation biomass, while a global figure for net plant assimilation of carbon in urban territories is approximately 0.5 Gt C yr1. 3. Carbon in construction material, furniture, books. Extensive amounts of carbon are accumulated for long time period in building constructions, furniture, books, and other articles made of organic materials. For instance, c. 3 Gt C is fixed in houses in the whole of Europe, North America, Japan, and Australia, and about 0.4 Gt C in other regions. 4. Carbon in solid waste. Most products of forestry and agriculture are turned eventually into waste. Solid waste is either deposited in sanitary landfills or incinerated. Carbon stored in landfills experiences slow decomposition rates and is gradually released due to microbiological activity. The annual world solid-waste production is equal to 170–180 million t of carbon. Approximately 60– 70 million t is released into the atmosphere by burning, while about 110–120 million t is deposited in landfills followed by slow release into the atmosphere. Landfills are often regarded as long-term accumulators of carbon and in this respect can be compared with natural peatland ecosystems (even after 30 years one-third of the organic carbon remains nonmineralized). This carbon is bound in long-lived humus and is not mineralized for a very long time.
Conclusion Urbanized territories dominate the surrounding environment in a number of ways – the growth of cities, and absorbing and transforming nearby natural ecosystems and agricultural lands. This process leads to the changing of the nature of land surfaces. Therefore, we can say that although the total area of urbanized territories is relatively small (1–2% in 1990s), they play an ever-increasing role
3678 Global Ecology | Urbanization as a Global Ecological Process
in global change in general and in the global carbon cycle in particular. We can summarize this influence as the following:
Population Growth; Global Warming Potential and the Net Carbon Balance.
1. Cities transform the natural territories they occupy, partially obliterating vegetation and soil, partially modifying them. By the same token, urbanization changes the structure and function of the local carbon flows within these territories. Note that the process often involves considerably larger territories than the exact city areas. 2. Cities consume a lot of organic carbon in the form of food and other agricultural products, as well as wood, etc., produced, as a rule, far from the urban territories, transforming it into other forms of carbon (feces, exalted CO2, residues of food processing, dead organics of ‘green zones’, etc.) in the process of urban and purely human metabolism. In other words, cities destroy the spatial entity of the processes of production and decomposition of living matter that is typical for natural ecosystems. Note that this entity provides the closure of any local carbon cycle.
Further Reading
See also: Biomass, Gross Production, and Net Production; Biosphere: Vernadsky’s Concept; Carbon Cycle; Climate Change 1: Short-Term Dynamics; Human
Brundtland’s World Commission on Environment and Development (1987) Our Common Future. Oxford: Oxford University Press. Encyclopædia Britannica, Inc. (2005) Encyclopædia Britannica 2005. CD-ROM/Ultimate Reference Suite 2005 DVD. Hauser JA (1992) Population, ecology and the new economics: Guidelines for a steady-state economy. Futures 24(4): 364–387. Heinke G W (1997) The challenge of urban growth and sustainable development for Asian cities in the 21st century. Environmental Monitoring and Assessment 44: 155–171. Miller GT (1988) Living in the Environment, 6th edn. Belmont, CA: Wadsworth. Small C and Cohen JE (1999) Continental physiography, climate and the global distribution of human population. In: Svirezher Yu (ed.) Proceedings of the International Symposium on Digital Earth, pp. 965–971.Beijing: Chinese Academy of Science. Stempell D (1985) Weltbevo¨lkerung 2000. Leipzig: Urania-Verlag. Svirejeva-Hopkins A and Schellnhuber H-J (2006) Modelling carbon dynamics from urban land conversion: Fundamental model of city in relation to a local carbon cycle. Carbon Balance and Management 1: 8. Svirejeva-Hopkins A, Schellnhuber H-J, and Pomaz VL (2004) Urbanised territories as a specific component of the global carbon cycle. Ecological Modelling 173: 295–312. United Nations (UN) (1999) Prospects for Urbanization – 1999 Revision. New York: United Nations (ST/ESA/SER.A/166), Sales No. E.97.XIII.3. United Nations (UN) (2000) The State of the World Cities 2001, 121pp. Nairobi: United Nations Centre for Human Settlements (UNCHS).
Further Reading Adams CC (1935) The relation of general ecology to human ecology. Ecology 16: 316–335. Adams CE, Lindsey KJ, and Ash SJ (2006) Urban Wildlife Management. Boca Raton: CRC Press, Taylor and Francis. Alfsen-Norodom C (2004) Urban biosphere and society: Partnership of cities. Annals of New York Academy of Sciences 1023: 1–9. Blair RB (1996) Land use and avian species diversity along an urban gradient. Ecological Applications 6(2): 506–519. Colding J, Lundberg J, and Folke C (2006) Incorporating green-area user groups in urban ecosystem management. Ambio 35(5): 237–244. Collins JP, Kinzig A, Grimm NB, et al. (2000) A new urban ecology. American Scientist 88: 416–425. Felson AJ and Pickett STA (2005) Designed experiments: New approaches to studying urban ecosystems. Frontiers in Ecology and the Environment 10: 549–556. Kinzig AP, Warren P, Martin C, Hope D, and Katti M (2005) The effects of human socioeconomic status and cultural characteristics on urban patterns of biodiversity. Ecology and Society 10(1): 23. http://www.ecologyandsociety.org/vol10/iss1/art23 (accessed December 2007).
McDonnell MJ and Pickett STA (1990) Ecosystem structure and function along urban–rural gradients: An unexploited opportunity for ecology. Ecology 71: 1232–1237. McDonnell MJ and Pickett STA (1993) Humans as Components of Ecosystems: Subtle Human Effects and the Ecology of Populated Areas, 363pp. New York: Springer. McGranahan G, Marcotullio P, Bai X, et al. (2005) Urban systems. In: Scholes R and Ash N (eds.) Ecosystems and Human Well-being: Current State and Trends, ch. 27, pp. 795–825. Washington, DC: Island Press. http://www.maweb.org/documents/document.296. aspx.pdf (accessed December 2007). Millennium Ecosystem Assessment (2005) Ecosystems and Human Well-being: Synthesis. Washington, DC: Island Press. Pickett STA, Cadenasso MI, Grove JM, et al. (2001) Urban ecological systems: Linking terrestrial ecological, physical and socioeconomic components of metropolitan areas. Annual Review of Ecology and Systematics 31: 127–157. Sukopp H, Numata M, and Huber A (1995) Urban Ecology as the Basis of Urban Planning. The Hague: SPB Academic Publishing. Turner WR, Nakamura T, and Dinetti M (2004) Global urbanization and the separation of humans from nature. Bioscience 54: 585–590.
Urbanization as a Global Ecological Process A Svirejeva-Hopkins, Potsdam Institute for Climate Impact Research, Potsdam, Germany ª 2008 Elsevier B.V. All rights reserved.
Introduction and Definitions Past, Present, and Future of Urbanization Present and Future Dynamics of Urban Areas The City as a Specific Heterotrophic Ecosystem Environmental Effect of Urbanization and Ecological Footprint
Carbon Balance in Urbanized Territories Conclusion Further Reading
Introduction and Definitions
inhabitants are considered to be urban areas in the USA; while 2000 inhabitants living in contiguous housing form an urban area in France, and in the Netherlands it is municipalities with 2000 or more inhabitants. Settlement densities can be orders of magnitude higher than agricultural rates, although residential densities in some urban areas are only marginally higher than the farmland densities in the most intensively cultivated agricultural areas (compare 2500 people per km2 in Los Angeles suburbs with 2000 peasants per km2 of arable land in Sichuan, China). However, maximum residential densities, c. 90 000 people per km2 (the center of Hong Kong), translate into an anthropomass of 36 MJ m2. This is roughly 200 times higher than the density of large herbivorous ungulates in Africa’s richest ecosystem. The definition of urban area in some other regions differs significantly from the above-mentioned one, with the concept of urban area somewhat based on the ancient structure of a city. All this shows a significant uncertainty in the term ‘urban(ized) area’ and which is necessary to take into account in any quantitative estimations.
The most concise definition of urbanization is given by Encyclopaedia Britannica: ‘‘this is the process by which large numbers of people become permanently concentrated in relatively small areas, forming cities and their suburbs.’’ Population, P, and territory, S, of these formations are changing in time, so that urbanization is a dynamic process controlled by these main variables. The degree of people concentration is determined by the ‘population density’, D ¼ P/S, where P is population size and S is the area it occupies. Different national statistics operate with different definitions and meanings of the terms ‘urban’, ‘urban area’, or ‘urbanized territory’. For instance, the United Nations defines all places with more than 20 000 inhabitants living close together as urban, while the US Census Bureau uses ‘urban area’ as a densely populated area (built-up area) with D > 1000 inhabitants per square mile and P > 50 000. Thus, the minimal urban area is equal to 50 square miles. At the same time, settlements with more then 2500
Global Ecology | Urbanization as a Global Ecological Process
Past, Present, and Future of Urbanization
1. the rise of each person’s ability to affect the natural environment through energy sources’ exploitation; 2. the rise of the unevenness of the spatial distribution of people through development of ‘cities’;
Population growth in developed countries
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Two thousand years ago, there were about a quarter of a billion people living on our planet. The global population doubled to about half a billion by the sixteenth to seventeenth centuries. The next doubling required two centuries (from the middle of the seventeenth century to 1850, when the size of two European cities, Paris and London, exceeded 1 million inhabitants); the following doubling occurred just over the next 100 years, while the last one took only 39 years. The year 1650 is named as the start of ‘the urban explosion’. Generally speaking, beginning from this date, the enormous population growth started. Nevertheless, it is a common practice to perceive the beginning of urbanization to coincide with the start of the agricultural revolution (7000–5000 BCE). It was at this time that nomadic hunters settled down and began to grow their food. A food surplus was created, and the division of labor made it possible to evolve gradually into the complex, interrelated social structures we know now as cities. The first cities were located along the Tigris and Euphrates Rivers (4000–3000 BCE) in contemporary Iraq, with urbanization then occurring also in Egypt, North Africa, India, China, Japan, and Europe – the Americas being the regions of most recent urbanization. Environmental factors were the major driving forces in the development of earlier cities. Fertile soils and easy access to water bodies, as well as adequate water supply, were essential. The first environmental disaster was triggered by the deforestation of the Middle East that led to soil degradation in the area, followed by a collapse of irrigations systems, and, as a consequence, to famine. Ancient cities were extremely dependent on the surrounding ecosystems, in particular, on agricultural lands. In Europe, since the eleventh century, there has been a historical continuing flow of people from the countryside to the cities, although the ‘Black Death’ in the fourteenth century impaired the process of urbanization. Europe recovered from the effect of this pandemic only by the middle of the sevententh century, when the ‘urban explosion’ occurred. Urbanization had also been occurring worldwide for at least two centuries. During the eighteenth century we have seen modern urbanization due to technological development, while earlier the process was driven by the migration of people from rural areas, since they were not needed in farming anymore. However, in the last decades of the last century, we observed the unprecedented global population growth and the accompanying process of urbanization (Figure 1). Generally speaking, this enormous population growth was accompanied by other significant changes:
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Urban Rural
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Figure 1 Comparison of urban and rural population growth in developed and in developing countries (urban is defined as settlements of 20 000 people and above). Source: UN, World Urbanization Prospects, 1990, 1991.
3. migration and travels’ increase, while contacts between cultures also rise. Although only 12% of the world’s population lived in urban centers in 1940, this percentage had risen to 33% by 1980. After World War II, a 2% urbanization rate was observed in the developed world, while it was almost 4% in the developing countries. However, urban growth rates doubled that of the total population. And while the total population growth rate in the developed world has been decreasing, the urban population’s proportion has increased from 55% to 70% of the total population. The major reason is the decline in rural population, as well as the arrival of new immigrants to the cities of some countries. Since the middle of the last century, the following trend is observed in the percentages of urban population: 1950, 29%; 1960, 34%; 1970, 37%; 1980, 40%; 1990, 43.5%, and in 1995 45%. However, the definition of what is urban varies greatly, which is why the estimations differ as well. For example, another source estimates 20% in 1950.
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Urbanization growth rate has significantly left total population growth behind. From the year 1800 to 1990, the absolute number of city dwellers increased from 18 million to 2.3 billion, a 128-fold increase, while the total population has increased by only 6 times (from 0.9 billion to 5.3 billion). Furthermore, more then 1.4 billion city dwellers live in the less-developed world. If we look at the rates of urban area growth and compare them with urban population growth, we find that the first grows faster than the population, which in turn grows at a faster rate than the total country population, and this is a common phenomenon. In 1990–95, the world’s urban population grew by 2–4% per year, while rural population only grew by 0.7% per year. Urban population increased by 2.1% during the 1995–2000 period, while rural population only by 0.7%. Today, 75% of the world’s population lives in the less-developed countries, and 58% in Asia. In 1999, 19 urban settlements had 10 million or more inhabitants, and 47% of all people lived in cities. The number of ‘megacities’, that is, giant urban agglomerations with a densely settled urban core of the original city, is increasing. Around this core, the satellite cities have grown, either planned or unplanned, linked to the central core by transport, communication, economic interdependence, and political-administrative structures. This tendency is confirmed by the following statistics: from 1950 until 1975, many cities with population of 5 million people have doubled in total urban population, while at the same time, cities with less than 100 000 people declined in their relative importance. In 1992, there were 23 megacities with populations greater than 8 million: 6 in the developed world (Tokyo, New York, Los Angeles, Osaka, Paris, and Moscow), and 17 in the developing world (from which 11 were in Asia). For most Asian cities, the shortage of water will be the most critical issue and is the limiting factor for the further growth of Beijing, Manila, Bangkok, Jakarta, and other cities. Also, while during the nineteenth century water and air pollution were associated with only a few larger cities, they are now becoming a global problem due to the rapid industrialization and the simultaneous concentration of people in cities. While the past and current demographic situations are estimated more or less accurately, future dynamics are forecast with a very high level of uncertainty. The UN vividly illustrates that if current exponential and hyperbolic growth continues in each major region and at the current rates, then the population will increase by more than 130-fold in 160 years, from 5.3 billion in 1990 to 694 billion in 2150. However, eventually, the problem will be how to feed these people, since food and water limitations will certainly arise. The UN also shows that future global population size is very sensitive to the future level of average fertility. Projections of global population dynamics are also uncertain, because external factors such as climate may
change unexpectedly. Furthermore, even if external factors change as expected, the relationship between those factors and demographic rates may change. We have the following hypothetical picture for the next half of the century. The global population will grow by 2–4 billion people, mostly in poor, but not rich, countries. It will also increase less rapidly than before and will become more urban than now. Hence, ‘‘from here on it is an urban world.’’ Most of all, the additional people will be living in cities in poor countries, which can become an epidemiological danger. Population of the more developed countries will decline slightly, but increase substantially in lessdeveloped countries. In this century, global urban population will increase 1.8 times by 2020 (relative to 1990), while the total population will grow by only 1.4 times, and almost all population growth will be associated with cities in the developing countries. By the year 2030, the world urban population will reach 4.9 billion (1 billion in developed countries and 3.9 billion in developing countries). The global rural population will remain constant at 3.2 or 3.3 billion, although in the developed countries the rural population will decline. The trend in the developing countries is that rural population will slowly rise through the next couple of decades, reaching 3.1 billion, and then will slowly decline.
Present and Future Dynamics of Urban Areas In 1985, 43% of the world population lived in cities while urban settlements covered just over 1% of the Earth’s surface. In 1990, 50% of the global population of Homo sapiens inhabited less than 3% of the Earth’s ice-free land area. However in the near future we may expect a further growth of the urban territories’ area. World dynamics of this process is shown in Figure 2. All these prognoses are based on two models: the first is a regression model, connecting urban population and urban area, and giving a minimal estimation, and the second, uses the spatial distribution of population density and gives a maximal estimation. Note that the dynamics of urban areas are significantly different in the major world regions (Figure 3). For instance, the fast growth in African region differs from almost constant dynamics in the highly industrialized countries.
The City as a Specific Heterotrophic Ecosystem From an ecological point of view, any city is a heterotrophic system maintained by external inflows of energy,
Global Ecology | Urbanization as a Global Ecological Process
World’s total relative urban area (%)
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food, water, and other substances. Thermodynamically any city (and generally, any urbanized territory) is an open system that is far from thermodynamic equilibrium. All matter and energy needed for a city’s functioning are collected from external territories that are significantly larger than the area of the city itself and very often are located quite far away. The heterotrophic ecosystem ‘city’ differs very much from a natural heterotrophic ecosystem. In fact, a city has a more intensive metabolism per area unit, requiring a significant inflow of artificial energy. Its consumption per
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urban area unit may be 3–4 orders of magnitude higher than the same for rural area. For instance, the annual subsidies in fuel, fertilizers, labor, etc., required to maintain a lawn in the Madison metropolitan area (Wisconsin, USA) is equal to 22 GJ ha1, which is approximately equal to the artificial energy input for a maize field. During the process of its own metabolism, a city consumes large amounts of various materials: food, water, wood, metals, etc., all that we call ‘gray energy’. Products of city’s metabolism have larger volumes of, and more toxic, substances than the same of natural ecosystems. If we compare cities and natural forests in Wisconsin, USA, we can see that the number of species in a city forest (75 tree and 74 bush species) is more than in natural one (10 tree and 20 bush species). The annual net production and the amount of living biomass (in carbon units) in city’s forests are equal to 500 t C km2 yr1 and 7000 t C km2; while in a natural forest the corresponding values are 400 t C km2 yr1 and 13 000 t C km2. The greater values of species diversity and production are provided in city’s forests at the expense of additional inflow of ‘gray energy’. For instance, the annual import of fertilizers is about 140 t km2. While natural forest is almost a closed system, a city forest is a typical through-flow system with ‘gray energy’ input and output in the form of dead organic matter: about half of the annual accretion is exported from the city to waste treatment plants. This is one possible explanation why the amount of biomass in the city forest is lesser than in the natural ecosystem. The carbon storage in urban forests with their relatively low tree cover (25.1 t C ha1 in average for US) is less than in natural forest stands (53.5 t C ha1). The gross sequestration rate, that is, the fraction of the gross annual production accumulated in wood, in urban forests is equal to 0.8 t C ha1 yr1, which is also less than in natural ones (for instance, 1.0 t C ha yr1 for a 25-year-old natural regeneration spruce–fir forest with 0.1 kg C m2 cover), although the difference is insignificant. However, on a per-unit tree cover basis, carbon storage by urban tree and gross sequestration may be greater than in natural forests, 92.5 t C ha1 and 3.0 t C ha1 yr1, due to a larger proportion of large trees and the more open structure (that leads to the weakness of competition) in urban forests.
Environmental Effect of Urbanization and Ecological Footprint Retrospectively, most humans have lived in small settlements dispersed within larger biomes (Figure 4). We see there is a certain correlation between the type of biomes and the degree of urbanization, some biomes being more preferable for cities.
3676 Global Ecology | Urbanization as a Global Ecological Process
Figure 4 Different types of global vegetation (biomes). Red dots represent the concentration of cities; the green line is the border of the regions. 1, Polar desert, polar tundra; 2, tundra; 3, mountainous tundra; 4, forest tundra; 5, north taiga; 6, middle taiga; 7, south taiga; 8, temperate mixed forest; 9, aspen–birch lower taiga; 10, deciduous forest; 11, subtropical deciduous and coniferous forest; 12, xerophyte woods and shrubs; 13, forest steppe; 14, temperate dry steppe (including mountainous); 15, savanna; 16, dry steppe; 17, sub-boreal desert; 18, sub-boreal saline ‘desert’; 19, subtropical semidesert; 20, subtropical desert; 21, mountainous desert; 22, alpine and subalpine meadows; 23, evergreen tropical rainforest; 24, deciduous tropical forest; 25, tropical xerophyte woodland; 26, tropical savanna; 27, tropical desert; 28, mangrove forest; 29, saline land; 30, subtropical and tropical woodland and Tugay shrubs.
The growth of cities in these ‘patches’, absorbing and transforming nearby natural ecosystems and agricultural lands, modifying energy and matter flows, typical for these ecosystems, negatively influences the local and regional biodiversity, increasing fragmentation of large rural areas and natural zones. This process (and contamination of the atmosphere) leads to the changing of the nature of land surfaces and near-surface atmospheric layer, and therefore its reflection and absorption of solar radiation and aerodynamic properties. This in turn leads to raising urban temperatures and the changing of the local climate, creating so-called ‘urban heat islands’, which is warmer by 1–2 C than surrounding territories. The ‘urban heat island’ effect occurs mainly at night, when the buildings, etc., release heat absorbed during day. In addition, metropolitan agglomerations influence the local and global environment through their consumption of non-native resources and their concentrated production of waste and consumables. The footprint is the quantitative conversion of the material and energy flows required to support human population in cities into the land area required to produce these flows. Although cities occupy a relatively small area on the planet, they are the dominant human ecosystem and the ecological space taken up by humans as a species is much higher. Every city depends (for its existence and
growth) on a globally diffuse productive hinterland up to 200 times the size of a city itself. To illustrate this, let us examine the following case studies. For instance, one person of the USA urban population consumes daily (1) food produced by 0.75 ha of agriculture land, (2) paper and wood by 0.4 ha of forest, (3) water by about 7.5 m3. So, a 1-million-populated city with the population density of 4000 persons per km2 (city area is 250 km2 correspondingly) needs a significantly larger area for its support: 7500 km2 of agriculture land and 4000 km2 of forest. A rather large river watershed (presuming abundant precipitation) can provide inflow of 7.5 million m3 of water daily. If we take into account that the area of such watershed is about 15 000 km2 then the total footprint area will be about 26 500 km2, that is, 106 times the city area. The city of Vancouver (Canada) had in the year 1991 a population of 47 2000 living in an area of 11 400 ha. If we assume that the per capita land consumption rate is 4.3 ha, then the people in this city would require 2.03 Mha of land. Hence, the inhabitants would require a land area 180 times larger then their habitat. Furthermore, adding a marine footprint of 0.7 ha per person, the total area needed to support the city becomes 2.36 Mha, or 200 times larger than the geographic area of the city. For London, the equivalent footprint is 120 times the area of the city itself. The New York metropolitan area annually
Global Ecology | Urbanization as a Global Ecological Process
consumes the equivalent of 800 000 ha of wheat, or approximately the total amount of wheat grown yearly in the state of Nebraska. So, if we presume that the world urban area constitutes 1% of the total land area, and the footprint is 100 times this area (note that these are minimal estimations), even in this case the urban footprint exceeds all the world land area.
Carbon Balance in Urbanized Territories Therefore, we can say that although the total area of urbanized territories is relatively small (1–2% in 1990s), they play an ever-increasing role in global change in general and in the global carbon cycle, the main biogeochemical cycle of the biosphere, in particular. Urban areas emit (in accordance with different estimations) 78–97% of the total anthropogenic carbon emission. Up to 60% of this emission comes from the transportation and building sectors, while the rest are from industry. Of course, all of these emissions are ‘spread’ and mixed in the entire atmosphere over 3–4 months period, but they are generated in particular by urban point sources. Cities transform the natural territories they occupy, partially obliterating vegetation and soil, partially modifying them. Similarly, urbanization changes the structure and function of the local carbon flows within these territories. Note that the process often involves considerably larger territories than the exact city areas. Cities consume a lot of organic carbon in the form of food and other agricultural products, as well as wood, etc., produced, as a rule, far from the urban territories, transforming them into other forms of carbon (feces, exhaled CO2, residues of food processing, dead organics of ‘green zones’, etc.) in the process of urban and purely physiological human metabolism. In other words, cities destroy the spatial entity of the processes of production and decomposition of living matter that is typical for natural ecosystems. Note that this entity provides the closure of any local carbon cycle. Urban territories have more carbon stored per unit area than natural ecosystems. Organic carbon is stored in soils and vegetation of urban territories, and also in people and pets; nonorganic carbon includes carbon transported into the cities and stored in buildings, etc., but most of this carbon is transformed into waste. Processes similar to those in peatlands accompany carbon fluxes from mineralization, incineration (rapid oxidation of carbon), and landfilling of solid waste. For instance, the global input of carbon into solid waste (sludge and industrial waste) is estimated to be 0.16 Gt yr1 (1 Gt ¼ 109 t). Long-term organic carbon in urban territories is accumulated in
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1. Biomass in humans and animals. For the world population of 6 billion, the total amount of carbon is equal to 45 million t of carbon that constitutes about 10% of the total biomass of land animals. A biological metabolism of 6 billion people is accompanied by exhalation of 0.34 Gt C yr1 and secretion of 0.18 Gt yr1 with feces and other discharges that, respectively, give a total 0.52 Gt C yr1. The value is entirely comparable with components of the global carbon cycle. For instance, soil erosion gives 0.98 Gt C yr1. 2. Biomass in trees and other plants. The mean global value of living plant biomass in cities is 3500 t C km2, and the mean net primary production is 500 t C km2 yr1. By taking the global urban area in 1980 as 2 106 km2 and assuming that 50% of it is covered by city vegetation, we find that urban territories contain 3.5 Gt C in living vegetation biomass, while a global figure for net plant assimilation of carbon in urban territories is approximately 0.5 Gt C yr1. 3. Carbon in construction material, furniture, books. Extensive amounts of carbon are accumulated for long time period in building constructions, furniture, books, and other articles made of organic materials. For instance, c. 3 Gt C is fixed in houses in the whole of Europe, North America, Japan, and Australia, and about 0.4 Gt C in other regions. 4. Carbon in solid waste. Most products of forestry and agriculture are turned eventually into waste. Solid waste is either deposited in sanitary landfills or incinerated. Carbon stored in landfills experiences slow decomposition rates and is gradually released due to microbiological activity. The annual world solid-waste production is equal to 170–180 million t of carbon. Approximately 60– 70 million t is released into the atmosphere by burning, while about 110–120 million t is deposited in landfills followed by slow release into the atmosphere. Landfills are often regarded as long-term accumulators of carbon and in this respect can be compared with natural peatland ecosystems (even after 30 years one-third of the organic carbon remains nonmineralized). This carbon is bound in long-lived humus and is not mineralized for a very long time.
Conclusion Urbanized territories dominate the surrounding environment in a number of ways – the growth of cities, and absorbing and transforming nearby natural ecosystems and agricultural lands. This process leads to the changing of the nature of land surfaces. Therefore, we can say that although the total area of urbanized territories is relatively small (1–2% in 1990s), they play an ever-increasing role
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in global change in general and in the global carbon cycle in particular. We can summarize this influence as the following:
Population Growth; Global Warming Potential and the Net Carbon Balance.
1. Cities transform the natural territories they occupy, partially obliterating vegetation and soil, partially modifying them. By the same token, urbanization changes the structure and function of the local carbon flows within these territories. Note that the process often involves considerably larger territories than the exact city areas. 2. Cities consume a lot of organic carbon in the form of food and other agricultural products, as well as wood, etc., produced, as a rule, far from the urban territories, transforming it into other forms of carbon (feces, exalted CO2, residues of food processing, dead organics of ‘green zones’, etc.) in the process of urban and purely human metabolism. In other words, cities destroy the spatial entity of the processes of production and decomposition of living matter that is typical for natural ecosystems. Note that this entity provides the closure of any local carbon cycle.
Further Reading
See also: Biomass, Gross Production, and Net Production; Biosphere: Vernadsky’s Concept; Carbon Cycle; Climate Change 1: Short-Term Dynamics; Human
Brundtland’s World Commission on Environment and Development (1987) Our Common Future. Oxford: Oxford University Press. Encyclopædia Britannica, Inc. (2005) Encyclopædia Britannica 2005. CD-ROM/Ultimate Reference Suite 2005 DVD. Hauser JA (1992) Population, ecology and the new economics: Guidelines for a steady-state economy. Futures 24(4): 364–387. Heinke G W (1997) The challenge of urban growth and sustainable development for Asian cities in the 21st century. Environmental Monitoring and Assessment 44: 155–171. Miller GT (1988) Living in the Environment, 6th edn. Belmont, CA: Wadsworth. Small C and Cohen JE (1999) Continental physiography, climate and the global distribution of human population. In: Svirezher Yu (ed.) Proceedings of the International Symposium on Digital Earth, pp. 965–971.Beijing: Chinese Academy of Science. Stempell D (1985) Weltbevo¨lkerung 2000. Leipzig: Urania-Verlag. Svirejeva-Hopkins A and Schellnhuber H-J (2006) Modelling carbon dynamics from urban land conversion: Fundamental model of city in relation to a local carbon cycle. Carbon Balance and Management 1: 8. Svirejeva-Hopkins A, Schellnhuber H-J, and Pomaz VL (2004) Urbanised territories as a specific component of the global carbon cycle. Ecological Modelling 173: 295–312. United Nations (UN) (1999) Prospects for Urbanization – 1999 Revision. New York: United Nations (ST/ESA/SER.A/166), Sales No. E.97.XIII.3. United Nations (UN) (2000) The State of the World Cities 2001, 121pp. Nairobi: United Nations Centre for Human Settlements (UNCHS).