In China more than half the 1.3 billion-strong population lives in a city. Meanwhile, across much of Europe and the US, nearly 80% of the population are urban-dwellers. Not surprisingly, these cities account for a large proportion of a country's energy usage – 75% of energy demand in China comes from cities – and produce a significant fraction of its carbon dioxide emissions. But who, or what, is guzzling the energy?

To answer this question, Cui Shenghui from the Chinese Academy of Sciences in Xiamen and his colleagues totted up the carbon footprint of Xiamen – a typical sub-provincial Chinese city, situated on the south-east coast. In recent years the conurbation, which has a population of over 3.5 million, has seen an economic boom, with GDP growing by more than 10% each year.

"Much of this growth has come from the manufacturing in the city, including electrical, rubber and plastic industries," said Shenghui.

Shenghui and his co-workers created a model of carbon flow for Xiamen, taking into account the emissions associated with the import of raw materials and power generation, and the emissions linked to the export of products to other regions and countries.

In 2010 the total urban-related emissions of Xiamen City were 55.2 Mt of carbon dioxide per year. Shenghui and the team found that 59% of these emissions were associated with imports and 65% associated with exports (which includes the value added to exports by manufacturing processes in Xiaman).

"A large percentage of the import emissions comes from the electricity used by manufacturing industries, which is imported from other regions of China," Shenghui explained. Mainland China and Taiwan were the regions with the largest values for net carbon imported-from while North America and Europe had the highest figures for net carbon exported-to.

"This shows that trade contributes more to carbon emissions than the emissions produced inside the city boundaries, and it shows that we really need to pay attention to what flows in and out of a city," said Shenghui, whose findings are published in Environmental Research Letters (ERL).

Many other cities in China are likely to have a similar emissions profile to Xiamen; the scientists hope that their study will prompt other cities to scrutinize their supply chains.

"Indeed, studies in many world cities have shown that cities often import key infrastructure-related flows, for example, cities often import electricity, water, petro-fuels and construction materials," said Anu Ramaswami from the University of Minnesota, US, who contributed to the research. "This is the first study to really look at the full range of upstream supply chains supporting cities, including both infrastructure and non-infrastructure components such as metal and manufactured parts used in factories. The information we have gathered could be used by businesses to reduce their emissions by imposing standards on their supply chains for example, like Walmart did in the US."

Meanwhile, at the other end of the chain, educating consumers and households on the environmental impact of their purchasing choices is likely to have the most impact on export emissions. "We've shown that you can't just look at what happens inside the city boundary and that cities need to take a supply-chain approach when calculating carbon footprints," said Ramaswami.

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