* Cities can act more easily than nations
* Many big cities have more economic clout than nations
By Alister Doyle, Environment Correspondent
CANCUN, Mexico, Dec 3 (Reuters) - Cities should play a much bigger role in fighting global warming and can act more easily than governments struggling to agree on a U.N. climate accord, the World Bank said on Friday.
"The 10 biggest cities in the world emit more greenhouse gases than Japan," Andrew Steer, the World Bank's special envoy for climate change, told Reuters. He urged reforms including changes to carbon markets to help cities become greener.
A World Bank study said that urban areas, home to just over half the world's population and responsible for two-thirds of greenhouse gas emissions, could help by shifting to greener transport, clean energy or better trash recycling.
"Cities are the most important cause of climate change and cities are the most important potential solution to climate change," Steer said. And they have huge economic power.
The report said that the world's 50 biggest cities had a combined gross domestic product behind only that of the United States, ahead of China. It listed Tokyo and New York as having bigger economies than Canada or Turkey.
"When you have 194 countries in the world it's not always easy to get consensus," he said of U.N. climate talks, which are seeking to agree a modest package of measures to slow global warming at Nov. 29 to Dec. 10 talks in Cancun.
More than 1,000 U.S. mayors, for instance, signed on in 2008 to targets to cut greenhouse gases in line with the U.N.'s Kyoto Protocol, an accord binding almost 40 nations to curb emissions until 2012 but never ratified by Washington.
And many of the world's biggest cities, such as Tokyo, Shanghai, New York or Buenos Aires, are near coasts or rivers and so have compelling reasons to act to limit risks of floods or sea level rise.
He said the World Bank favored an overhaul of a U.N. market mechanism that encourages investments in individual projects in developing nations, such as solar panels in Morocco or hydropower in Honduras, to allow a broader, city-wide scale.
Such a reform of the U.N.'s Clean Development Mechanism (CDM) would allow mayors to get money and allow them to invest in areas ranging from flood barriers to hydrogen-powered buses, rather than getting each individual project approved.
"Our view is that measurement challenges are not overwhelming for cities as a whole," he said. The CDM allows companies to invest in green projects in developing nations and claim credits back home for the averted emissions.
Steer also said that city-dwellers' views of where it was best to live were shifting -- in past decades, when industrial air pollution was high, areas downwind such as the east side of London were home to the poor.
In future, the poor would live in low-lying areas at risk of river floods or rising sea levels. (Editing by Eric Walsh)