A new scientific report that ranks 10 world cities for their contribution to global warming has a surprising conclusion: Denver is the most-polluting city. By a long shot.
Denver's per capita greenhouse gas emissions were twice the median, and four times as much as the lowest-emitting city on the list, Barcelona.
True, the list of cities in the running was not comprehensive, but the study, published in Environmental Science & Technology revealed Denver as a polluter about 50% more profligate as notoriously smoggy Los Angeles (No. 2 on the list) and twice as polluting as Bangkok.
It's not because mile-high pollution counts more (though, if airplane pollution is any guide, it might). One of the University of Toronto study's conclusions is that a city's climate has a lot to do with its impact on the climate: The more heating degree days -- those cold winter days when we turn up the thermostat to stay warm -- the more emissions. Wealth and employment also makes a big difference, because wealthy cities keep the lights on longer and power plants humming to manufacture goods in local factories.
The researchers based their conclusions on seven criteria:
Electricity: Denver and Toronto had the highest rates of electricity usage (though Denver's usage is still 40% below the U.S. average), but even Cape Town, with low electricity usage, has high emissions because it relies on coal. The fuel used to power electric generating plants makes a big difference. (Geneva, for instance, uses hydropower, with very low emissions.)
Heating and industrial fuels: Denver and Toronto, relatively cold compared to other cities, topped the list for most greenhouse gases from these sources ... but even Los Angeles and New York were well above average, possibly because of the relatively large size of individual houses.
Industrial processes: As with electricity and heating, the source of fuel for local industries, and their relative need for power, make all the difference.
Ground transportation: The density of a city -- how many people live per square mile -- has a big influence on emissions, with the more efficient cities being more tightly packed. Fuel efficiency of automobiles, and the quality of public transportation also play an important role. Double density and you reduce emissions 40%.
Aviation: "Gateway" cities, like Denver, New York, London, Bangkok and Geneva, are more likely to have higher emissions associated with airports, and transportation to and from airports. In Denver, 22% of the fuel loaded at Denver International Airport is associated with trips made to or from the central city. But London and New York ranked No. 1 and 2 on this list.
Marine: Similarly, the presence or absence of a busy port dictated how much marine transportation and shipping contributed to a city's emissions. Cape Town, a refueling port between the Atlantic and Indian oceans, was associated with the most marine pollution.
Waste: The biggest factor in emissions from landfills is, simply, whether a city burns methane -- a greenhouse gas more potent than carbon dioxide -- to generate heat or electricity. Overall, the researchers concluded that waste had a small contribution to a city's emissions.
So which cities were the most and least polluting? Because the authors compared the cities in units that are unfamiliar to most people -- carbon dioxide equivalents -- we're presenting the cities, from least to most polluting, along with how they compare to the median emissions, expressed in percentage difference.
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