Negotiations for a global treaty on reduction in greenhouse gas emissions have stalled. So perhaps more targeted initiatives are needed. In my Green ICT course I teach how to reduce CO2 emissions using computer and telecommunications. These techniques include measures such as smart transport and smart grids, which are particularly relevant to large cities. It would be interesting to see what impact this would have on global emissions if applied to the world's top 100 cities or perhaps just the world's top 20 cities would be sufficient. These cities have a high concentration of wealth making carbon pollution reduction measures more affordable and I suspect they also have high energy use.
Trying to work out what to do for the whole world is difficult to conceive let alone do, action for 20 to 100 cities sounds manageable. There would be fewer governments to deal with (about half the top 20 cities are in the USA). To this perhaps should be added capital cities, such as Canberra.
Table adapted from List of cities by GDP, Wikipedia, 2010.