Modeling the carbon cycle of urban systems

e c o l o g i c a l m o d e l l i n g 2 1 6 ( 2 0 0 8 ) 107–113 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/ecolmodel Modeling the carbon cycle of urban systems Galina Churkina a,b,∗ a The University of Michigan, School of Natural Resources and Environment, Dana Building, 440 Church Street, Ann Arbor, MI 48109-1041, USA b Max-Planck Institute for Biogeochemistry, Hans-Knöll str. 10, 07745 Jena, Germany a r t i c l e i n f o a b s t r a c t Article history: Although more than 80% of carbon dioxide emissions originate in urban areas, the role of Published on line 20 April 2008 human settlements in the biosphere evolution and in global carbon cycling remains largely neglected. Understanding the relationships between the form and pattern of urban develop- Keywords: ment and the carbon cycle is however crucial for estimating future trajectories of greenhouse Carbon cycle gas concentrations in the atmosphere and can facilitate mitigation of climate change. In this Urban system paper I review state-of-the-art in modeling of urban carbon cycle. I start with the properties Modeling of urban ecosystems from the ecosystem theory point of view. Then I discuss key elements City of an urban system and to which degree they are represented in the existing models. In conclusions I highlight necessity of including biophysical as well as human related carbon fluxes in an urban carbon cycle model and necessity of collecting relevant data. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Arguably more than 80% of carbon dioxide emissions originate in urban areas (Grubler, 1994; O’Meara, 1999), which occupy less 2.4% of land mass globally (Potere and Schneider, 2007). The role of human settlements in the biosphere evolution and in global carbon cycling remains however largely neglected. Understanding the relationships between the form and pattern of urban development and the carbon cycle is however crucial for estimating future trajectories of greenhouse gas concentrations in the atmosphere and can facilitate mitigation of climate change. A widely accepted statement is that human settlements occupy a small proportion of the landmass (Potere and Schneider, 2007) and therefore play an insignificant role in the changes of global carbon cycle. Most modeling studies focusing on the carbon cycle of land (Running and Coughlan, 1988; Parton et al., 1993; McGuire et al., 2001; Brovkin et al., 2002; Svirezhev, 2002; Churkina et al., 2003; Stich et al., 2003; Bondeau et al., 2007), use models of different complexity to estimate carbon fluxes through forests, grasses, and croplands, but completely omit urban areas from their scope. Recent studies show that the urban areas are not as small as it was originally thought and are continuously increasing. The worldwide rate of migration towards the cities is three times the rate of population growth (UN, 2006). By 2030 it is expected that the proportion of the urban population will increase to 70% worldwide. The proportion of urban land is growing rapidly as more cities expand into natural ecosystems and agricultural lands (Milesi et al., 2003; Brown et al., 2005). This expansion involves replacement of natural vegetation or of agricultural fields by artificial surfaces made of concrete and asphalt or by turf grasslands and garden plants. The impervious surfaces – buildings, roads, parking lots, roofs, etc. – constitute an important manifestation of urbanization. The total impervious surface area of the USA was estimated to be just slightly smaller than the state of Ohio (Elvidge et al., 2004). The urban grasses in the continental US occupy an area which is three times larger than that of any irrigated crop (Milesi et al., 2005). In the European Union, more than a quar- Correspondence address: Max-Planck Institute for Biogeochemistry, Hans-Knöll str. 10, 07745 Jena, Germany. E-mail address: [email protected]. 0304-3800/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ecolmodel.2008.03.006 ∗ 108 e c o l o g i c a l m o d e l l i n g 2 1 6 ( 2 0 0 8 ) 107–113 ter of its territory has been directly affected by an urban sprawl (EEA, 2006). Effects of urban areas on the environment extend much further than the city’s boundaries, so that biogeochemistry of significantly larger areas is affected. Urban areas are too small to be self sufficient in producing resources needed and absorbing pollutions emitted. Urban areas have a footprint extending to distant and remote places, arising from the influence on climate, transport of air pollution originating in the cities, as well as cities’ demands for energy and material goods. Although most studied aspects of urbanization are its effects on local climate and air temperature (Oke, 1988; Bottyan et al., 2005 #1163; Jauregui, 1991 #1164; Jin et al., 2005), the latest studies point to its importance for the continental climate. A recent modeling study showed that urbanization of Europe reduced the continental diurnal temperature range, redistributed precipitation in winter and reduced it in summer (Trusilova et al., 2008). The comparison of differences between trends in observed and reconstructed surface temperatures suggested that land use change including urbanization played an important role in the continental climate warming in the US (Kalnay and Cai, 2003). Air pollutants originating in a city and transported outside of its limits can adversely affect regional climate and atmospheric chemistry (Rosenfeld, 2000; Crutzen, 2004) as well as the vegetation in that region. Vegetation productivity in the affected area may decrease or increase depending on the type of pollution and its effect on vegetation productivity. Although increased nitrogen deposition may act as fertilizer and enhance vegetation growth (Oren et al., 2001; D’Antonio and Mack, 2006; Churkina et al., 2007), increased ozone concentrations may reduce plant production by 10–35% (Chameides et al., 1994; Gregg et al., 2003). City not only pours its waste products into the countryside, but it depends on this same countryside to provide almost all of its life-supporting resources. The material flows through urban areas are estimated to be so huge that the areas of ecosystems, which are required to assimilate and process these flows, are 400–1000 times larger than the size of the cities themselves (Decker et al., 2000). Recent review (Kennedy et al., 2007) of eight metropolitan regions across five continents showed that the metabolism of cities is increasing. Most regions exhibit increasing per capita metabolism with respect to water, wastewater, energy, and materials, although one city showed increasing efficiency for energy and water over 1990s. This increasing metabolism implies greater loss of forest, species diversity, and more fossil fuel burned. Most emissions of carbon dioxide are originated in urbanindustrial areas from burning fossil fuels – coal, oil, and natural gas – natural products produced in bygone geological ages with high fraction of carbon in it. Burning fossil fuels is necessary to satisfy very high demand of urban population for energy. The amount of energy consumed per unit of area per year of an urban area is 1000 or more times greater than that of a forest (Odum, 1997). Unlike natural ecosystems fueled by solar energy, production of energy in urban areas is closely connected with emissions of carbon. Therefore urban systems play a prominent role in the evolution of the global carbon cycle. Accurate modeling of urban carbon cycle and its feedbacks with other ecosystems and atmosphere is going to be crucial for predictions of future trajectories of atmospheric CO2 concentrations and global climate change. In this paper I review state-of-the-art in modeling of urban carbon cycle. I start with the properties of urban ecosystems from the ecosystem theory point of view. Then I discuss key elements of an urban system and to which degree they are represented in the existing models. I conclude with suggestions for urban model development. 2. Ecosystem theory In ecosystem theory urban areas are classified as technoecosystems (Odum and Barrett, 2005), which have entirely new arrangements than natural ecosystems. Natural ecosystems such as forests or prairie are self-maintaining. They operate without energetic or economic flows directly controlled by humans. They depend on sunlight for energy as well as other natural drivers like rainfall and wind. In contrast, urban areas are maintained by high inflows of artificially produced energy as well as externally produced food and high outflows of pollution and heat. Urban systems are energetic hotspots which require large areas of natural and semi-natural ecosystems to maintain their demands and are fueled by energy mostly produced by fossil fuel burning. Urban areas create strong urban–rural gradients of temperature, vegetation composition, build-up density, etc. From a thermodynamic point of view the existence for these gradients causes the emergence of energy fluxes in the direction of gradient (Jørgensen, 1992). Entropy production of an urban ecosystem as an ecosystem under anthropogenic stress has never been estimated. Most likely energy required to compensate for environmental degradation will be higher than for agricultural systems where three fold increment of already high-artificial energy inflow was required (Svirezhev, 2000). 3. Elements of an urban system An urban system (Fig. 1) includes urban sprawl and urban footprint, the latter is the area required to meet demands of urban population in terms of consumption and waste accumulation and the area affected by urban pollution and changes in climate. Most of the carbon and energy used by a city come from outside the city boundaries or from urban footprint. The pathway of carbon through the city tends to be linear as opposed to cyclic in natural ecosystems. The flow paths of carbon into the city are longer than flow paths out. The key elements of carbon budget in an urban system include the following: • Driving forces and urban matrix, • Vertical fluxes of carbon, • Horizontal fluxes of carbon. 3.1. Driving forces and urban matrix People and climate are the major driving forces behind morphology, development, and expansion of an urban area as well as associated carbon fluxes and pools. Location and boundary of a natural ecosystem are determined by climate, soils, and e c o l o g i c a l m o d e l l i n g 2 1 6 ( 2 0 0 8 ) 107–113 109 Fig. 1 – Urban system. The vertical and horizontal carbon fluxes are shown with black arrows. disturbance regime. In contrast, location of an urban system is defined by people’s priorities (e.g. fertile land, proximity to the trade roads, esthetical value of a location) and to a smaller degree by climate (e.g. people prefer to live in warmer climates). Dietary preferences of people determine how much of carbon in food is transferred from agricultural into the urban areas. Construction preferences and climate determine how much organic carbon (e.g. wood) is incorporated into the buildings and furniture and enters pools with long-term storage. Fraction of carbon in waste depends also on consumption patterns of people and increases with economic development of society (Bramryd, 1980). Urban matrix encompasses spatial and structural patterns of urban areas in different parts of the globe. The description of structural patterns includes extents and properties of impervious areas and vegetation growing in cities as well as their management practices. Changes in land occupied by urban settlement are not linearly related to population density. When wealth of population reaches a certain threshold, spatial growth of an urban area supersedes increase in population. These trends have been documented in Europe (EEA, 2006) as well as in the USA (Brown et al., 2005). reduction of household size results in increases in the number of dwellings, vehicles per capita, and energy use. One has to determine relationships between urban pattern and form, on one hand, and corresponding vertical fluxes of carbon, on the other. Carbon fluxes of ecosystems in the urban footprint can be affected by deposition of pollutants transported from an urban center. 3.3. Horizontal fluxes of carbon are mostly driven by human activities. These fluxes include transfers of food and fiber from agricultural fields and forests into urban systems and flows of trash from area of urban sprawl into landfills located usually in the urban footprint. The footprint of urban areas is not necessarily adjacent to urban area. In some cases it can be located hundreds kilometers away (Folke et al., 1997). 4. Current models to estimate urbanization effect on carbon cycle 4.1. 3.2. Vertical fluxes of carbon The vertical fluxes of carbon in urban areas have natural and anthropogenic origins. Fluxes of natural origins or of vegetation include ecosystem photosynthesis and respiration. The vertical fluxes of anthropogenic origin are produced from fossil fuel burning, decomposition of waste, and human breath. Although energy production statistics provide the first good estimate of fossil fuel emissions (Marland et al., 2005), many aspects of urban matrix and growth influence the magnitude of fossil fuel emissions and their rate of change. For instance, Horizontal fluxes of carbon Urbanization effect on vegetation and soils Vertical carbon fluxes of urban vegetation and soils can be estimated using ecosystem process models. Ecosystem process models simulate carbon, water, and sometime nitrogen cycles of soils and vegetation of different ecosystem of the world. Although most of these models have been developed for natural ecosystems, they have been successfully applied to urban vegetation and soils as well. Milesi et al. (2005) used BIOME-BGC model to simulate carbon balance of turf grasses of the US. Imhoff et al. (2004) estimated effect of urban land cover change on NPP of the USA using CASA model. Changes in biomass production of turf grass associ- 110 e c o l o g i c a l m o d e l l i n g 2 1 6 ( 2 0 0 8 ) 107–113 ated with management were studied with CENTURY model (Bandaranayake et al., 2003; Qian et al., 2003). The ecosystem process models are able to estimate vertical carbon fluxes of vegetation and soils with detailed representation of ecosystem processes. They can simulate responses of vegetation in the cities and in the urban footprint to urban pollution, enhanced levels of atmospheric CO2 , and to changes in urban climate. The strength of the abovementioned models is that they can be applied over larger areas and provide regional to global estimates. These models however completely omit horizontal or vertical carbon fluxes associated with human activities. Gross and net primary productivity of urban vegetation can be also estimated with a light use efficiency model (Monsi and Saeki, 1953; Monteith, 1977) based on remotely sensed data. Zhao et al. (2007) used this model to analyze changes in land-cover and annual GPP over an urban–rural gradient in Michigan, USA. This method is mostly diagnostic. It detects only recent changes in vertical carbon fluxes in response to climate and land-cover conversion and has no predictive capacity. Moreover it is not suitable for detection of carbon emissions. A statistical model driven by changes in urban population (Svirejeva-Hopkins et al., 2004) was used to study effects of shifts in urban land use on carbon balance of different economic regions of the world. The authors also accounted for horizontal fluxes of carbon associated with transport of organic matter outside of urban areas. Carbon balance in this model was however insensitive to climate or vegetation management. One-sided assessments (focusing on only one effect) of urban carbon fluxes can lead to controversial results. For instance, Imhoff et al. (2004) pointed out that urban land transformations had a disproportionally large negative impact on the net primary productivity of the US. In contrast Zhao et al. (2007) indicated that low-density exurban development, characterized by large proportions of vegetation, can be more productive in the form of GPP than the agricultural land it replaces. Trusilova (2007) showed that environmental effects such as changes in climate, atmospheric CO2 concentrations (CO2 dome), atmospheric nitrogen deposition accompanying urbanization can partially compensate each other. In the latter study urbanization driven changes in climate reduced land carbon uptake of Europe, while increased nitrogen deposition and atmospheric CO2 enhanced carbon uptake. The effect of all above-mentioned factors together was a small increase in uptake of carbon by land. An integrated assessment of most prominent effects of urbanization is required for estimation of urbanization effect on carbon cycle. 4.2. Urban metabolism (analysis of human related material and energy flows) Carbon fluxes related to human activities can be estimated using models developed in industrial ecology. More than 50 years ago Abel Wolman developed the urban metabolism concept (Wolman, 1965). He quantified the metabolism of a hypothetical American city quantifying the overall fluxes of energy, water, materials, and waste into and out an urban region of one million people. In urban metabolism models the physical and biological processes of converting resources into useful products and wastes are like the ecosystem’s metabolic processes. In these models input into a biological system must pass through and the amount of waste is therefore dependent on the amount of resources required. A balance sheet of inputs and outputs can be created. One can manage the waste produced, but energy is required to turn it into anything useful. Ultimately all materials will eventually end up in waste. All carbon products will end up as CO2 and it is not possible to recycle them any further without enormous energy inputs that in themselves have associated wastes. This is an entropy factor in urban metabolism. Urban metabolism studies (Decker et al., 2000; Kennedy et al., 2007) illuminate basic trends in human energy and material fluxes. Increase in city metabolism implies larger burden on the environmental resources in the footprint of urban area or increase of this footprint. The concept of urban metabolism has been applied to materials (Brunner and Rechberger, 2004; Klee and Graedel, 2004), energy (Newman and Kenworthy, 1991; Newman, 1999; Sahely et al., 2003), food (Bohle, 1994), nutrients (Baker et al., 2001), and water (Wolman, 1965; Sahely et al., 2003). Although carbon balance has never been addressed as an urban metabolic process, some processes studied are closely related to carbon fluxes. Anthropogenic carbon dioxide emissions correspond quite closely with energy inputs (Kennedy et al., 2007). This correlation can however weaken once cities start exploiting renewable energy sources such as photovoltaics or energy recovery from wastewater. Carbon inputs into cities are also related to demand for timber, food, or nutrients. Carbon outputs are related to composition and treatment of waste products. Stochiometric ratios can be used to derive fluxes of carbon from nutrients’ fluxes, e.g. nitrogen. Although a lot about carbon cycling through an urban system can be learned using urban metabolism approach, its applicability to a region or a continent is problematic. The existing studies typically cover one city (Wolman, 1965; Hendrics et al., 2000) or a greater metropolitan area (Huang, 1998; Sahely et al., 2003) including rural or agricultural fringes around urban centers. This approach has not been applied to a region or a country including multiple cities. In a recent review of cities’ metabolism Kennedy et al. (2007) points to concerns about commensurability of data from different cities collected in different countries and in different decades. In addition urban metabolism approach provides little insight into vertical carbon fluxes through vegetation and soils of urban sprawl or footprint. 5. Can we model effects of urbanization on carbon cycle? For a comprehensive assessment of urban system’s impact on global carbon cycle, the model has to include not only physical and biological properties, but also human component of an urban system. In this paper I suggest a modeling approach where biophysical and human dimensions will be combined within one model (Fig. 2). This will lead to a signifi- e c o l o g i c a l m o d e l l i n g 2 1 6 ( 2 0 0 8 ) 107–113 111 Fig. 2 – Carbon inputs, pools and outputs of urban ecosystems. Pools and fluxes related to human activities where formal modeling framework does not exist are highlighted by italic font. cant progress in ecological modeling, because such approach currently does not yet exist even though theoretical basis for understanding urban system and their carbon cycle has been developed (Odum and Barrett, 2005). The current generation of carbon cycle models are able to simulate only carbon fluxes through vegetation-soil component of an urban system (Bandaranayake et al., 2003; Qian et al., 2003; Imhoff et al., 2004; Svirejeva-Hopkins et al., 2004; Milesi et al., 2005). Formal framework for simulations of human related carbon fluxes through cities, although can be partially based on urban metabolism approach, has not been yet developed. Modeling studies attempting to integrate biophysical and socio-economic characteristics of resource flows of landscape (Matthews, 2006) remain at a community level and are not transferable to a larger scales. Development of an urban carbon cycle model will require observations of both biophysical and human related fluxes as well as their interactions. There is a growing number of data sets (Pataki et al., 2006) such as energy use and traffic related emissions or carbon gain and losses in vegetation and soils following urbanization (Kaye et al., 2006; Pouyat et al., 2006), which are necessary for developing of an urban carbon cycle model. There are however still very few interdisciplinary studies of socioeconomic and biophysical factors that influence urban carbon cycle. A recent review of research on urban biogeochemistry (Kaye et al., 2006) highlighted the complexity of urban biogeochemistry and suggested that developing a model encompassing myriad influences of people on biogeochemistry is one of the grand challenges for urban ecologists. I add that this challenge is in the ability of urban ecologists to cooperate with economists and sociologists. The challenge is in finding common tools for observing and understanding structure and functioning of an urban system. Acknowledgements This review was written while the author had a research fellowship from German Research Foundation (Deutsche Forschungsgemeindschaft, DFG). I thank Kristina Trusilova for fruitful discussions about modeling approach and helpful editorial comments. I am grateful to Natalia Ungelenk and Cristina Milesi for help with literature search and to Silvana Schott for assistance with graphics. I thank an anonymous reviewer for constructive review comments. references Baker, L.A., Hope, D., Xu, Y., Edmonds, J., Luaver, L., 2001. Nitrogen balance for the Central Arizona-Phoenix (CAP) ecosystem. Ecosystems 4, 582–602. Bandaranayake, W., Qian, Y.L., Parton, W.J., Ojima, D.S., Follett, R.F., 2003. Estimation of soil organic carbon changes in turfgrass systems using the CENTURY model. Agron. J. 95, 558–563. Bohle, H.G., 1994. Metropolitan food system in developing countries: the perspective of urban metabolism. GeoJournal 34, 245–251. Bondeau, A., Smith, P.C., Zaehle, S., Schaphoff, S., Lucht, W., Cramer, W., Gerten, D., Lotze-Campen, H., Muller, C., Reichstein, M., Smith, B., 2007. Modelling the role of agriculture for the 20th century global terrestrial carbon balance. Global Change Biol. 13, 679–706. Bottyan, Z., Kircsi, A., Szegedi, S., Unger, J., 2005. The relationship between built-up areas and the spatial development of the mean maximum urban heat island in Debrecen, Hungary. Int. J. Climatol. 25, 405–418. Bramryd, T., 1980. Fluxes and accumulation of organic carbon in urban ecosystems on a global scale. In: Bornkamm, R., Lee, J.A., Seaward, M.R.D. (Eds.), Urban Ecology. Blackwell Scientific Publications, Oxford, pp. 3–12. Brovkin, V., Bendtsen, J., Claussen, M., Ganopolski, A., Kubatzki, C., Petoukhov, V., Andreev, A., 2002. Carbon cycle, vegetation, and climate dynamics in the Holocene: experiments with the CLIMBER-2 model. Global Biogeochem. Cycles 16, 1139, doi:10.1029/2001GB001662. Brown, D.G., Johnson, K.M., Loveland, T.R., Theobald, D.M., 2005. Rural land-use trends in the conterminous United States. Ecol. Appl. 15, 1851–1863. Brunner, P.H., Rechberger, H., 2004. Practical Handbook of Material Flow Analysis. CRC Press, Boca Raton, FL. Chameides, W.L., Kasibhatla, P.S., Yienger, J., Levy II, H., 1994. Growth of continental scale metro-agro-plexes, regional ozone pollution, and world food production. Science 264, 74–76. Churkina, G., Vetter, M., Trusilova, K., Dentener, F.J., 2007. Contributions of nitrogen deposition and forest re-growth to land carbon uptake. Carbon Balance Manage. 2, 5. Churkina, G., Tenhunen, J., Thornton, P.E., Elbers, J.A., Erhard, M., Falge, E., Grünwald, T., Kowalski, A.S., Rannik, Ü., Sprinz, D.F., 2003. Analyzing the ecosystem carbon dynamics of four European coniferous forest using a biogeochemistry model. Ecosystems 6, 168–184. Crutzen, P.J., 2004. The growing urban heat and pollution island effect. Impacts on chemistry and climate. Atmos. Environ. 38, 3539–3540. 112 e c o l o g i c a l m o d e l l i n g 2 1 6 ( 2 0 0 8 ) 107–113 D’Antonio, C., Mack, M.C., 2006. Nutrient limitation in a fire-derived, nitrogen rich Hawaiian grassland. Biotropica 38, 458–467. Decker, E.H., Elliot, S., Smith, F.A., Blake, D.R., Rowland, F.S., 2000. Energy and material flow through the urban ecosystem. Annu. Rev. Energy Environ. 25, 685–740. EEA, 2006. Urban Sprawl—The Ignored Challenge. European Environment Agency, Copenhagen, 60 pp. Elvidge, C., Milesi, C., Dietz, J.B., Tuttle, B.T., Sutton, P.C., Nemani, R.R., Vogelmann, J.E., 2004. U.S. constructed area approaches the size of Ohio. Eos 85, 233–240. Folke, C., Jansson, Å., Larsson, J., Costanza, R., 1997. Ecosystem appropriation by cities. AMBIO 26, 167–172. Gregg, J.W., Jones, C.G., Dawson, T.E., 2003. Urbanization effects on tree growth in the vicinity of New York City. Nature 424, 183–187. Grubler, A., 1994. Technology. In: William, B.M., Turner I.I., B.L. (Eds.), Changes in Land Use and Land Cover: A Global Perspective. Cambridge University Press, Cambridge, pp. 287. Hendrics, C.R., Obernosterer, R., Muller, D., Kytzia, S., Baccini, P., Brunner, P., 2000. Material flow analysis: a tool to support environmental policy decision making. Case studies on the city of Vienna and the Swiss lowlands. Local Environ. 5, 311–328. Huang, S.-L., 1998. Urban ecosystems, energetic hierarchies, and ecological economics of Taipei metropolis. J. Environ. Manage. 52, 39–51. Imhoff, M.L., Lahouari, B., DeFries, R., Lawrence, W.T., Stutzer, D., Tucker, C.J., Ricketts, T., 2004. The consequences of urban land transformation on net primary productivity in the United States. Remote Sens. Environ. 89, 434–443. Jauregui, E., 1991. Influence of a large urban park on temperature and convective precipitation in a tropical city. Energy Buildings 15–16, 457–463. Jin, M., Dickinson, R.E., Zhang, D.L., 2005. The footprint of urban areas on global climate as characterized by MODIS. Am. Meteorol. Soc., 1551–1565. Jørgensen, S.E., 1992. Integration of Ecosystem Theories: A Pattern. Ecology and Environment. Kluwer Academic Publishers, Dordrecht, 420 pp. Kalnay, E., Cai, M., 2003. Impact of urbanization and land use change on climate. Nature 423, 528–531. Kaye, J.P., Groffman, P.M., Grimm, N.B., Baker, L.A., Pouyat, R.V., 2006. A distinct urban biogeochemistry? Trends Ecol. Evol. 21, 192–199. Kennedy, C., Cuddihy, J., Engel-Yan, J., 2007. The changing metabolism of cities. J. Ind. Ecol. 11, 43–59. Klee, R.J., Graedel, T.E., 2004. Elemental cycles: a status report on human or natural dominance. Annu. Rev. Environ. Nat. Resour. 29, 69–107. Marland, G., Boden, T., Andres, R.J., 2005. Global, Regional, and National Fossil Fuel CO2 Emissions, Trends: A Compendium of Data on Global Change. Carbon Dioxide Analysis Center. Oak Ridge National Laboratory, Oak Ridge, TN. Matthews, R., 2006. The people and landscape model (PALM): toward full integration of human decisionmaking and biophysical simulation models. Ecol. Modell. 194, 329–343. McGuire, A.D., Sitch, S., Clein, J.S., Dargaville, R., Esser, G., Foley, J.A., Heimann, M., Joos, F., Kaplan, J.O., Kicklighter, D.W., Meier, R.A., Melillo, J.M., Moore III, B., Prentice, I.C., Ramankutty, N., Reichenau, T., Schloss, A., Tian, H., Williams, L.J., Wittenberg, U., 2001. Carbon balance of the terrestrial biosphere in the twentieth century: analysis of CO2 , climate and land use effects with four process-based ecosystem models. Global Biogeochem. Cycles 15, 183–206. Milesi, C., Elvidge, C., Nemani, R.R., Running, S.W., 2003. Assessing the impact of urban land development on net primary productivity in the southeastern United States. Remote Sens. Environ. 86, 401–410. Milesi, C., Running, S.W., Elvidge, C., Dietz, J.B., Tuttle, B.T., Nemani, R.R., 2005. Mapping and modeling the biogeochemical cycling of turf grasses in the United States. Environ. Manage. 36, 426–438. Monsi, M., Saeki, T., 1953. Über den Lichtfaktor in den Pflanzengesellschaften und seine Bedeutung für die Stoffproduktion. Jpn. J. Bot. 14, 22–52. Monteith, J.L., 1977. Climate and efficiency of crop production in Britain. Philos. Trans. R. Soc. 281, 277–294. Newman, P.W.G., 1999. Sustainability and cities: extending the metabolism model. Landsc. Urban Plann. 44, 219–226. Newman, P.W.G., Kenworthy, J., 1991. Cities and Automobile Dependence: An International Source Book. Avebury, Aldershot, UK, 388 pp. Odum, E.P., 1997. Ecology: A Bridge Between Science and Society. Sinauer Associates, Sunderland, xv + 336 pp. Odum, E.P., Barrett, G.W., 2005. Fundamentals of Ecology. Brooks/Cole, Belmont, 598 pp. Oke, T.R., 1988. Boundary Layer Climates. Routledge, London, 450 pp. O’Meara, M., 1999. Reinventing Cities for People and the Planet, vol. 147. Worldwatch, Washington. Oren, R., Ellsworth, D.S., Johnsen, K.H., Phillips, N., Ewers, B.E., Maier, C., Schafer, K.V.R., McCarthy, H., Hendrey, G., McNulty, S.G., Katul, G.G., 2001. Soil fertility limits carbon sequestration by forest ecosystems in a CO2 -enriched atmosphere. Nature 411, 469–472. Parton, W.J., Ojima, D.S., Schimel, D.S., Kittel, T.G.F., 1993. Development of simplified ecosystem model for applications in Earth system studies: the CENTURY experience. In: Ojima, D.S. (Ed.), Modeling the Earth System. UCAR/Office for Interdisciplinary Earth Studies, Boulder, CO, pp. 291–302. Pataki, D.E., Alig, R.J., Fung, A.S., Golubiewski, N.E., Kennedy, C.A., McPherson, E.G., Nowak, D.J., Pouyat, R.V., Romero Lankao, P., 2006. Urban ecosystems and the North American carbon cycle. Global Change Biol. 12, 2092–2102. Potere, D., Schneider, A., 2007. A critical look at representations of urban areas in global maps. GeoJournal 69, 55–80. Pouyat, R.V., Yesilonis, I.D., Nowak, D.J., 2006. Carbon storage by urban soils in the United States. J. Environ. Qual. 35, 1566–1575. Qian, Y.L., Bandaranayake, W., Parton, W.J., Mecham, B., Harivandi, M.A., Mosier, A.R., 2003. Long-term effects of clipping and nitrogen management in turfgrass on soil organic carbon and nitrogen dynamics: the CENTURY model simulation. J. Environ. Qual. 32, 1694–1700. Rosenfeld, D., 2000. Suppression of rain and snow by urban and industrial air pollution. Science 287, 1793–1796. Running, S.W., Coughlan, J.C., 1988. A general model of forest ecosystem processes for regional applications. I. Hydrologic balance, canopy gas exchange and net primary production processes. Ecol. Modell. 42, 125–154. Sahely, H.R., Dudding, S., Kennedy, C.A., 2003. Estimating the urban metabolism of Canadian cities: GTA case study. Can. J. Civil Eng. 30, 468–483. Stich, S., Smith, B., Prentice, I.C., Arneth, A., Bondeau, A., Cramer, W., Kaplan, J.O., Levis, S., Lucht, W., Sykes, M.T., Thonicke, K., Venevsky, S., 2003. Evaluation of ecosystem dynamics, plant geography and terrestrial carbon cycling in the LPJ dynamic vegetation model. Global Change Biol. 9, 163–185. Svirejeva-Hopkins, A., Schellnhuber, H.J., Pomaz, V.L., 2004. Urbanized territories as a specific component of the global carbon cycle. Ecol. Modell. 173, 295–312. Svirezhev, Y.M., 2000. Thermodynamics and ecology. Ecol. Modell. 132, 11–22. e c o l o g i c a l m o d e l l i n g 2 1 6 ( 2 0 0 8 ) 107–113 Svirezhev, Y.M., 2002. Simple spatially distributed model of the global carbon cycle and its dynamic properties. Ecol. Modell. 155, 53–69. Trusilova, K., 2007. Urbanization Impacts on the Climate in Europe. PhD Thesis. University of Hamburg, Hamburg, 77 pp. Trusilova, K., Jung, M., Churkina, G., Karstens, U., Heimann, M., Claussen, M., 2008. Urbanization impacts on the climate of Europe: numerical experiments with the PSU/NCAR Mesoscale Model (MM5). J. Appl. Meteorol. Climatol. 47. 113 UN, 2006. World Urbanization Prospects: the 2005 Revision. United Nations, New York. Wolman, A., 1965. The metabolism of cities. Sci. Am. 213, 179–190. Zhao, T., Brown, D.G., Bergen, K.M., 2007. Increasing gross primary production (GPP) in the urbanizing landscapes of Southeastern Michigan. Photogrammetr. Eng. Remote Sens. 73, 1159.