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The world’s cities are going to have to move aggressively to curb their greenhouse-gas emissions, or the whole planet is going to pay for it.

That’s the word in a new report from the United Nations Human Settlement Program, or UN-HABITAT. The report is called “Hot Cities: Battle-Ground for Climate Change,” (you can find a summary and links to purchase the full report here). It paints a dire picture of how an increasingly urban and wealthy global population could mean “potentially devastating effects of climate change on urban populations”:

Urban centres have become the real battle-ground in the fight against climate change and cities will neglect their role in responding to this crisis at their peril. Not just their own peril but that of the world. This is the tough and urgent message of UN-HABITAT’s new “Cities and Climate Change: Global Report on Human settlements 2011.” According to the report, the world’s cities are responsible for up to 70 per cent of harmful greenhouse gases while occupying just 2 per cent of its land. What goes on in cities, and how they manage their impact on the environment, lies at the core of the problem. It is the combination of urbanization’s fast pace and the demand for development that poses the major threat. “Cities are responsible for the majority of our harmful greenhouse gases. But they are also places where the greatest efficiencies can be made. This makes it imperative that we understand the form and content of urbanization so that we can reduce our footprint,” said Joan Clos Executive Director of UN-HABITAT. “Understanding the contribution of cities to climate change will help us intervene at the local level. With better urban planning and greater citizen participation we can make our hot cities cool again.”

The report compares cities from around the world on their carbon emissions. Not surprisingly, the United States doesn’t come out looking so good, in large part because of reliance on fuel-inefficient cars for transportation:

In comparison to North American cities, the contribution of urban areas in Europe to climate change is relatively low. European urban areas tend to be more compact. They tend to have lower car ownership and car usage rates, smaller, more fuel-efficient cars, reducing emissions from private transportation. They tend to have more effective public transportation networks, which are deemed socially acceptable to a broader range of individuals. Furthermore, urban areas in Europe have higher levels of densification and lower levels of sprawl in comparison to North American cities.

As the developing world becomes more prosperous, the report suggests, emissions could skyrocket if effective strategies aren’t formulated now:

The few detailed emission inventories from the developing world show much lower emissions than cities in the developed world, but with a wide gap between the rich and the poor in each country.

A recent study in India showed that the average greenhouse gas emissions of the wealthiest 1 per cent of the Indian population are 4.52 tonnes CO2 equivalents per annum, or more than four times as much as the 1.11 tonnes CO2 equivalents per annum generated by the poorest 38 per cent of the population.

Cities may be the source of the problem. The U.N. report makes it clear that they’ll have to be the source of the solutions as well.