A central concept in the scientific study of cities is that of urban metabolism. The metabolism of a city can be interpreted either primarily in terms of energy flows or more broadly including a city’s flows of water, materials, and nutrients. Through studies of urban metabolism, scientists have developed an understanding of phenomena such as ecosystem appropriation by cities; the accumulation of toxic materials in the urban building stock; historical growth in the transportation of materials; and economies of scale for urban infrastructure systems.

A key issue for urban ecology, however, is the lack of reliable, published data on comprehensive energy use in cities. Data for some components are available, e.g., for urban transportation or electricity. Reviews by Decker et al. and Kennedy et al. found a paucity of data on overall urban energy consumption. The first challenge of our work in comparing GHG emissions between cities was to establish consistent data on energy use by cities.

—Kennedy et al. 2009

The researchers determined the global warming potential, expressed in carbon dioxide equivalents (t e CO₂), for seven components of urban inventories: electricity, heating and industrial fuels, industrial processes, ground transportation, aviation, marine, and waste.

They calculated emissions for ten cities (or metropolitan regions), which vary in population from 432,000 to 9,519,000; comparisons are in per capita terms. The cities/metro areas studied were: Los Angeles County; Greater Toronto; Geneva Canton; Greater Prague; Cape Town; Denver City and County; New York City; Greater London; Barcelona; and Bangkok.

They presented the GHG emissions from an end-use perspective; for emissions that only occur within the borders of the cities; and in a broader measure including the upstream lifecycle emission for fuels used in the cities.

The total end use emissions for the ten cities range between 4.2 and 21.5 t e CO₂/cap. With high population density, low heating requirements, and relatively clean electricity, Barcelona has the lowest per capita emissions. Whereas Denver, having the highest per capita emissions for electricity, heating/industrial fuels, and ground transportation is, not surprisingly, the top emitter. The next two highest cities are also both North American: Los Angeles (13.0 t e CO₂/cap.) and Toronto (11.6 t e CO₂/cap.). Other than Geneva at 7.8 t e CO₂/cap., the other cities all have emissions fairly close to 10 t e CO₂/cap. These values for end use emissions are typical of those reported by municipal governments.

...If emissions are to be attributed to cities based on consumption activities, however, then a fuller lifecycle perspective should be taken. Beyond the GHGs emitted during the combustion of fossil fuels, for example, there are emissions produced from the extraction, processing, and transportation of these fuels to cities...The life-cycle emissions are between 7% and 24% higher than the direct emissions for the cities. The greatest changes in life-cycle emissions are observed for the North American cities, since they have higher upstream GHG intensity factors.

—Kennedy et al. 2009

Per capita GHG emissions from ground transportation fuels are inversely related to population density. Credit: ACS, Kennedy et al. 2009. Click to enlarge.

Transportation. GHG emissions for the transportation in each of the cities were calculated based on fuel sales data, vehicle kilometers traveled, or by scaling from a regional level; these three approaches were found to differ by less than 5%.

Overall, the team found that GHG emissions from ground transportation fuels are inversely proportional to population density—a finding that correlates with some studies of urban transportation energy use.

By including emissions from air and marine travel, this study goes beyond the activities that occur in the cities and considers travel between urban centers that happens on a global scale...The GHG emissions for air and marine generally reflect each city’s gateway status. London has the highest emissions for air transportation at 3.12 t e CO₂/cap., with New York City also high. The relatively high emissions for Geneva (1.72 t e CO2/cap.) might reflect its role as an international organizational center. Of the port cities, Cape Town has the highest GHG emissions for freight, at 2.92 t e CO₂/cap. This likely reflects its key location at the Cape of Good Hope for refuelling of ships passing between the Atlantic and Indian Oceans.

—Kennedy et al. 2009

Authors of the study included:

• Christopher Kennedy, University of Toronto
• Julia Steinberger, University of Klagenfurt
• Barrie Gasson, University of Cape Town
• Yvonne Hansen, University of Cape Town
• Timothy Hillman, University of Colorado
• Miroslav Havránek, Charles University Environment Center
• Diane Pataki, University of California, Irvine
• Aumnad Phdungsilp, Dhurakij Pundit University
• Anu Ramaswami, University of Colorado
• Gara Villalba Mendez, Autonomous University of Barcelona

Megacity Urban Metabolism

Separately, at the 238^th National Meeting of the American Chemical Society in August, Dr. Charles Kolb from the Center for Atmospheric and Environmental Chemistry and the Center for Aerosol and Cloud Chemistry of Aerodyne Research Inc, presented a report on the urban metabolism model of megacities (also referenced by Kennedy et al.).

The concept of urban metabolism views large cities as living entities that consume energy, food, water, and other raw materials, and release wastes. The releases include carbon dioxide; air pollutants, sewage and other water pollutants; and even excess heat that collects in vast expanses of concrete pavement and stone buildings. Humans directly produce a significant share of this waste, but emissions from industrial, power generation and transportation systems respire the largest quantities of greenhouse gases and other air pollutants. Other urban metabolizers include sewage systems, landfills, domestic pets and pests like rats, which in some cities outnumber people.

During the last five years, this body of knowledge has drawn into sharper focus the hazards of poor air quality in megacities, not just on the large local populations but also on population centers, agricultural activities and natural ecosystems located downwind from these sprawling areas, Kolb said.

More than half the world’s population today lives in cities, and the world’s largest urban areas are growing rapidly. The number of megacities—metropolitan areas with populations exceeding 10 million—has grown from just three in 1975 to about 20 today.

Controlling urban growth in the developing world is key to improving the world’s air quality, Kolb said. Urban pollutant levels in poor countries will remain high, with increased emissions expected as the city populations and economic activities increase. Until megacities are rich enough to devote significant funds to reduce their emissions, two factors will invariably increase the stresses on their environment—increasing vehicular traffic and industrial growth.

We need to start with low-hanging fruit. In some cities in Asia and Africa, they still have lead in their gasoline. In the developed world, we can institute emissions controls on diesel vehicles, which create hazardous fine particles, and we can also reduce pollution by using more rail-based mass transport or setting up specialized bus routes.

—Charles Kolb

Resources

• Christopher Kennedy et al. (2009) Greenhouse Gas Emissions from Global Cities. Environ. Sci. Technol., Article ASAP doi: 10.1021/es900213p