Cities are also a major source of greenhouse gas emissions. And those on the coasts face the risk of rising sea levels associated with climate change.

城市也是温室气体排放的主要来源之一，而那些沿海的城市也在面临着气候变化所带来的海平面上升的风险。

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That urban growth—roughly half of humanity already lives in cities for the first time in recorded history—may lead to a big increase in greenhouse gas emissions.

有史以来，居住在城市里的人口粗略达到了整体人口的一半，这种城市扩张可能会导致温室气体排放的猛增。

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Find the cheapest gas prices in hundreds of cities across the US. The site lists the average gas price for a city along with the address of specific places which offer prices below the city average.

汽油价格网站，在全美境内数以百计的城市中找到最便宜的加油站，可以显示某市中低于该市平均汽油价格的加油站。

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These are highways that have natural gas fueling stations between cities, just like the one that folks at UPS, South Coast Air and Clean Energy Fuels are opening today between Los Angeles and Salt Lake City.

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And, Qatar-based businessman Hesham Ismail AbdulRahman Al Emadi is leading an ambitious project to create a global network of " Energy Cities" to house the oil and gas industry.

CNN: Doha - round table highlights

Cities are particularly keen to tackle landfill gas, says Mr Blumenfeld of San Francisco's Department of Environment, because it is one of the few sources of emissions over which they have jurisdiction.

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