This morning I attended the first part of the Centre for Environment’s Research Day, and I’m glad I did, because I caught the talk by Chris Kennedy from the Sustainable Infrastructure Group in the Dept of Civil Engineering, on “Greenhouse Gases from Global Cities”. He talked about a study he’s just published, on the contribution of ten major cities to GHG emissions. Chris points out that most of the solutions to climate change will have to focus on changing cities. Lots of organisations are putting together greenhouse gas inventories for cities, but everyone is doing it differently, measuring different things. Chris’s study examined how to come up with a consistent approach. For example, the approach taken in Paris is good at capturing lifecycle emissions, London is good at spatial issues, Tokyo is good at analyzing emissions over time. Each perspective useful, but the differences make comparisons hard. But there’s no obvious right way to do it. For example, how do you account for the timing of emissions release, e.g. for waste disposal? Do you care about current emissions as a snapshot, or future emissions that are committed because of waste generated today?

The IPCC guidelines for measuring emissions take a pure producer perspective. They focus only on emissions that occur within the jurisdiction of each territory. This ignores, for example, consumer emissions when the consumer of a product or service is elsewhere. It also ignores upstream emissions: e.g. electricity generation is generally done outside the city, but used within the city. Then there’s line loss in power transmission to the city; that should also get counted. In Paris, Le Bilan Carbon counts embodied emissions in building materials, maintenance of vehicles, refining of fuels, etc. but it ignores emissions by tourists, which is a substantial part of Paris’ economy.

In the study Chris and colleagues did, they studied ten cities, many iconic: Bankok, Barcelona, Cape Town, Denver, Geneva, London, Los Angeles, New York, Prague and Toronto. Ideally they would like to have studied metropolitan regions rather than cities, because it then becomes simpler to include transport emissions for commuting, which really should be part of the assessment of each city. The study relied partially on existing assessments for some of these cities and analyzed emissions in terms of electricity, heating/industrial fuels (lumped together, unfortunately), ground transport, aviation and marine fuels, industrial processes, and waste (the methodology is described here).

For example, for electricity, Toronto comes second in consumption (MWh per capita), after Denver, and is about double that of London and Prague. Mostly, this difference is due to the different climate, but also the amount of commerce and industry within the city. However, the picture for carbon intensity is very different, as there is a big mix of renewables (e.g. hydro) in Toronto’s power supply, and Geneva gets its power supply almost entirely from hydro. So you get some interesting combinations: Toronto has high consumption but low intensity, whereas Cape Town has low consumption and high intensity. So multiply the two: Denver is off the map  at 9 t eCO2 per capita, because it has high consumption and high intensity, while most others are in the same range, around 2-3 t eCO2 per capita. And Geneva is very low:

Climate has to be taken into account somehow, because there is an obvious relationship between energy used for heating and typical temperatures, which can be assessed by counting heating degree days:

Aviation is very interesting. Some assessments exclude it, on the basis that local government has no control. But Chris points out that when it comes down to it, local government has very little control over anything, so that argument doesn’t really wash. The UNFCC says domestic flights should be included, but this then has a small country bias – small countries tend to have very few domestic flights. A better approach is to include international flights as well, so that we count all flights taking off from that city. Chris’ methodology assesses this as jet fuel loaded at each airport. For this then, London is way out in the lead:

In summary, looking at total emissions, Denver is way out in front. In conversations with them, it was clear they had no idea – they think of themselves as a clean green city up in the mountains. No surprises that the North American cities all fare the worst, driven by a big chunk of emissions from ground transportation. The real surprises though are Bangkok and Cape Town, which compare with New York and Toronto for total emissions:

Chris concluded the talk with some data from Asian cities that were not included in the above study. In particular, Shanghai and Beijing are important in part because of their sheer size. For example, if Shanghai on its own was a country, it would come it about #25 in the world for total emissions.

One thing I found interesting from the paper that Chris didn’t have time to cover in the talk was the obvious relationship between population density and emissions from ground transportation fuels. Clearly, to reduce carbon emissions, cities need to become much denser (and should all be more like Barcelona):


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  2. Gustavo Alcala

    I have read the article related to Greenhouse Gases from Global Cities. I do not see any methodology in it but rather interesting methods.

  3. Amanda van Daal

    Yes. Although the title promises a methodology what is delivered is far from being that, if we consider what a methodology should be. If adopted by some cities the results will not be comparable because many issues in the proposed methodology need to be further developed. Good methods…I agree

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