That means keeping the growth of gasoline-powered vehicles in check and developing land use strategies that cut down on urban sprawl and allow for efficient transport, city leaders said on the sidelines of a climate change meeting this week in Seoul.

But advice is about all these cities can offer because their limited budgets and pressing demands make it difficult for them to offer any monetary help, even though they are at the front-line of the climate change battle.

"Cities are capable of moving further and faster. If mayors work together and cities work together, they can move even faster," Boris Johnson, mayor of London, told Reuters at the C40 Large Cities Climate Summit in Seoul.

Urban areas, home to just over half of the world's population, are key to attacking global warming because they account directly for 50-60 percent of human greenhouse gas emissions, according to U.N. Habitat.

According to a UN report, the urban areas of the world are expected to absorb all the population growth expected over the next four decades. Most of the population growth expected in urban areas will be concentrated in cities and towns of the less developed regions.

Asia, in particular, is projected to see its urban population increase by 1.8 billion, Africa by 0.9 billion, and Latin America and the Caribbean by 0.2 billion. Among the world's cities, Tokyo is expected to be the world's most densely populated with 36 million inhabitants in 2025, followed by Mumbai with 26 million people and Delhi with 23 million people.

Johnson, who has pushed to make the streets of London more friendly to hybrid and electric vehicles, said his city could share its experience on implementing low-carbon initiatives. "I want cities from around the world to get together to reduce the costs of these technologies," he said.

"They (developing cities) have to learn the lessons of the many failures that modern cities made over the past 40 or 50 years in areas such as transportation systems and land use planning," Toshi Noda, a director for U.N. Habitat, said.

The UN agency that helps manage housing has launched projects in four developing cities including Kampala in Uganda to import green technologies used in developed cities such as less polluting transport and drainage systems to halt urban flooding. Delhi Mayor Kanwar Sain wants more global assistance to help his city tackle a global problem.

"Developed nations ... should come forward for collaborative projects," Sain said. Roads jammed with hybrid cars with an efficient subway system underneath are a world away from cities in the developing world such as Addis Ababa, where a sizable percentage of the population uses charcoal, wood and cow dung for fuel.

Addis Ababa, with few resources of it own, has worked with international agencies to replant and protect forests as a way to battle climate change. Representatives from these cities said they are protecting forests to benefit in carbon trading schemes and win support from global donors but they would not mind receiving technology abandoned by richer cities as they become more climate friendly.

"Every city must find its own way," said Katrin Lompscher, a senator from the city of Berlin. "And not make the same mistakes."