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* Most cities of 2030 yet to be built, big chances for companies
* Asia, Africa have big chance to curb greenhouse gas emissions
By Alister Doyle, Environment Correspondent
OSLO, April 14 (Reuters) - The world's urban areas are set to grow by almost twice the size of Manhattan a day until 2030 and the design of future cities in Asia and Africa will be crucial to slow global warming, a U.N. study showed on Monday.
The breakneck expansion means billion-dollar opportunities for companies, ranging from greener construction of homes and offices to improved rail and bus networks, according to a report by the U.N.'s Intergovernmental Panel on Climate Change (IPCC).
"There is a window of opportunity" to enlist urban design to slow global warming, said Karen Seto, a professor at Yale University who co-led a chapter about city planning in a 2,000-page IPCC report about slowing climate change.
A 33-page IPCC summary - with a photo of Shanghai on the cover - was issued on Sunday. It said yet-to-be-built cities could help slow warming but most details are in a 116-page chapter, obtained by Reuters before publication on Tuesday.
In one scenario, urban expansion from 2000 to 2030 will add 1.2 million sq kms (460,000 sq miles) to towns and cities, mostly in Asia and Africa and more than the total urban land in 2000 of about 650,000 sq kms (250,000 sq miles), it says.
That expansion works out at 110 sq kms (42 sq miles) every day for three decades, almost twice the size of Manhattan or 20,000 American football fields.
"20,000 football fields will go from being farms to cities, from being forests to cities, every day," Seto told Reuters. And urban areas account for between 71 and 76 percent of the world's carbon dioxide emissions from energy, it said.
More compact city designs that cut commutes, insulation to save energy, better public transport, cycle lanes and pedestrian areas can all cut emissions, mainly from fossil fuels.
Problems include a lack of planning regulations, especially in developing nations, where opportunities for limiting future greenhouse gases are highest and which could also benefit from curbs on fossil fuels to reduce air pollution.
In 1800, only Beijing had more than a million inhabitants and cities and towns were home to three percent of the world population. By 2010, 449 cities had more than a million inhabitants and 52 percent of all people lived in urban areas.
"Each week the global urban population increases by 1.3 million," the chapter said. By 2050, the urban population is likely to rise to about two-thirds of everyone on Earth.
Once built, cities can be hard to shift to greener policies. "If you live in the average North American city you are a long way from buying a loaf of bread or where you work and you have to use a car," Seto said.
The IPCC's summary said that swifter action is needed to limit global warming to an agreed ceiling of 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial times, seen as a threshold for ever more damaging heatwaves, floods and rising seas.
A previous IPCC report in September said the probability that human emissions of greenhouse gases, rather than natural variations in the climate, were the dominant cause of warming since 1950 had risen to at least 95 percent from 90 percent previously.
A report last week by the Institute for Transportation and Development Policy said that urban projects such as London's Central Saint Giles, Hammarby Sjöstad in Stockholm and Liuyun Xiaoqu in Guangzhou, China, were good examples of low-emissions developments. (Reporting By Alister Doyle; Editing by Tom Heneghan)