Countries vary across the globe in releases of greenhouse gases (GHG). For example, eight countries -- USA (23.5), Canada (22.6), Czech Republic (13.7), UK (10.6), Spain (10.1), Switzerland (7.3), South Africa (9.0), and Thailand (5.6) -- released in 2005 their portion of per capita carbon dioxide emissions in metric tons as shown inside the parentheses. The data are from the World Resource Institute and exclude bunker-fuel emissions and land-use change. Just USA, EU, Canada, Czech Republic, Switzerland, South Africa, and Thailand contributed on the same basis 43.4% of the total global emissions in 2006 per U.S. Department of Energy.

More than half of us live in urban. What is the contribution of towns in GHG generation? Earlier studies have reported an urban GHG inventory range between 3 and 22 t eCO₂ per capita. Notice this range is similar to the country portions listed above.

A simple urban model. (Credit: Florida State University)

A global study was completed recently regarding how and why emissions differ among ten cities in eight countries I listed above. The results as reported in the October 1 issue of ACS Environmental Science&Technology apply specifically to Bangkok (10.7), Barcelona (4.2), Cape Town (11.6), Denver (21.5), Geneva (7.8), London (9.6), Los Angeles (13.0), New York (10.5), Prague (9.4), and Toronto (11.6). These numbers are for total end-use emissions in t eCO₂ per capita. Now compare them with those of their country of origin. Some cities like Denver, Geneva, and London are close representatives of their country behavior in GHG emissions. Others differ for a variety of reasons which appear abundantly in Greenhouse Gas Emissions from Global Cities by Christopher Kennedy et al. 

However, there are general indicators to assist in reductions in urban GHG emissions. Some are quoted below in four categories.

Electricity: With relatively high electricity consumption and a high intensity (792 t eCO₂/GWh), Denver has GHG emissions that are almost a factor of 3 higher than the next city. Emissions from seven of the ten cities fall within the narrow range of 2.46 to 3.38 t eCO₂ per capita. Toronto’s high consumption is mediated by low intensity, while Cape Town’s low consumption is mediated by high intensity. With 92% of South Africa’s electricity generated from combustion of coal, Cape Town has the highest intensity of 969 t eCO₂/GWh. Access to nearby hydropower results in the lowest electricity-related emissions for Geneva at 0.35 t eCO₂ per capita, a factor of 26 less than Denver.

Heating and Industrial fuel use: While heating degree days largely explain the differences in fuel consumption between cities, there is some variability, likely associated with differences in industrial fuel consumption and building characteristics. Denver and Toronto have the greatest consumption at 73.5 and 58.9 GJ per capita, respectively, whereas Cape Town and Barcelona each consumes less than 16 GJ per capita. Notably, all three U.S. cities lie above the best-fit line, perhaps due to larger house sizes or the quality of building envelopes. Also above the line is Bangkok, which has zero heating degree days below 18.0 °C. Its relatively high fuel consumption in this category is primarily for industrial processes; only 5% of Bangkok’s 28.4 TJ per capita is for residential use.

Transportation: Three lower density North American cities (excluding New York) have substantially higher GHG emissions than the other cities. Denver’s per capita emission from ground transportation fuels of 6.31 t eCO₂ is a factor of 8 higher than those of Barcelona at 0.77 t eCO₂. The GHG emissions for air and marine generally reflect a city’s gateway status. London has the highest emissions for air transportation at 3.12 t e CO₂ per capita, followed by New York. The relatively high emissions for Geneva (1.72 t eCO₂ per capita) might reflect its being an international organizational center. Of the port cities, Cape Town has the highest GHG emissions for freight, at 2.92 t eCO₂ per capita. The Cape of Good Hope is the spot for refuelling of ships passing between the Atlantic and Indian Oceans.

Waste: Denver’s GHG emissions from waste have alternatively been calculated as −0.3 t e CO₂ per capita, i.e., the landfill is seen as a sink, in contrast to a value of +0.59 t eCO₂ per capita by the method employed in this global study. The contribution of waste is thus uncertain in a city’s GHG inventory, although a minor contributor to the emissions from most cities.

Emissions are not static. A positive finding is that cities try to reduce their emissions wherever they can. In general, "a balance of geophysical factors (climate, access to resources, and gateway status) and technical factors (power generation, urban design, and waste processing) determine the GHGs attributable to cities. By including upstream emissions from fuels, GHG emissions attributable to cities exceed those from direct end use by up to 25%." Having identified differences among ten cities depending on extent of public transit and heating (building codes, size of dwelling), the researchers recommend further research.

Lexicon:
bunker-fuel: fuel oil used aboard ships;
eCO₂: carbon dioxide equivalent;
t: metric ton