By Timothy Gardner

NEW YORK, May 16 (Reuters) - A leading United Nations climate adviser said on Wednesday the world's largest cities should get independent scientific guidance about every two years to help them fight global warming.

The U.N.'s Intergovernmental Panel on Climate Change produces a series of reports every five or six years. Drawn on the work of 2,500 scientists, they assess the causes of climate change, describe its impacts and ways to fight it.

But large cities are emerging as a force in sharing ideas on cutting heat-trapping gases and may need more frequent scientific assessments to gauge how well their actions are working, Cynthia Rosenzweig, head of climate impacts at New York's NASA Goddard Institute for Space Studies, said in a interview.

"Cities are efficient, they take things on more quickly," she said.

Urban areas consume 75 percent of the world's energy and produce 80 percent of its greenhouse gas emissions.

Rosenzweig, who is also a lead author of the IPCC's impacts assessment, said she recently helped form the Urban Climate Change Research Network with representatives from cities on each continent. The network hopes to publish a scientific assessment of city efforts against climate change.

"Cities are just taking climate change on board, it's extremely new," she said on the sidelines of the second C40 Large Cities Climate Summit in New York, an event that was first held in London in 2005.

The urban climate network is in talks with C40 to produce the report to coincide with future meetings of the large cities group, which is expected to occur about every two years.

Rosenzweig said cities are well placed to cut greenhouse gases in ways that also help people adapt to the expected rise in heat waves, flooding and droughts that could be brought about by heat-trapping gases already emitted.

Roofs covered with vegetation instead of steel or blacktop that are popular in Chicago, Berlin and Portland, Oregon, are an example of something that can both cut emissions and help people cope with climate change, she said.

Widespread so-called green roofs could help combat the urban heat island effect that makes cities several degrees warmer in summer and would also cut emissions by reducing the need for air conditioning.

Cities in developed countries could also learn from ones in developing countries, she said. Cities in Bangladesh, which are at greater risk of flooding from climate change, have already taken more action than coastal cities like New York, she said.

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