By 2030, more than 60 percent of the world's population would live in cities, up from almost half now and just a athird in 1950, Secretary-General Kofi Annan said. The growth poses huge problems, ranging from clean water supplies to trash collection.
"Already, one of every three urban dwellers lives in a slum," he said in a statement. "Let us create green cities," he said, urging better planning and investment in everything from sanitation to public transport.
Annan said city planning was often haphazard, especially in poor nations where urban growth was likely to be fastest. Unless city planning improved, the U.N. goal of halving poverty by 2015 would not be met, he said.
Activists around the world mark June 5, the date of the first environmental summit in Stockholm in 1972, as the U.N.'s World Environment Day.
In San Francisco, the main host of the event, mayors from more than 50 cities including Shanghai, Kabul, Buenos Aires, Sydney, Phnom Penh, Jakarta, Rome and Istanbul plan to sign up for a scheme setting new green standards for city planning.
Among other plans on Sunday, a group in Sri Lanka will plant trees to help build up the coastline after the devastating Dec. 26 tsunami. In Greece, the port of Zakynthos planned to ban cars and allow free public transport.
GAZA BEACH
In the Gaza Strip, a beach clean-up was planned
And from Brisbane, Australia, to Ocho Rios, Jamaica, activists would plant trees. A town cleanups was planned for Bindura, Zimbabwe. The theme for 2005 is "Green Cities: Plan for the Planet" with rallies in many towns.
"The battle for sustainable development, for delivering a more environmentally stable, just and healthier world, is going to be largely won and lost in our cities," said Klaus Toepfer, head of the U.N. Environment Programme.
Managed well, cities can help protect the environment by reducing pressure on rural areas where humans can threaten the habitats of animals and plants.
The mayors' meeting in San Francisco would set goals including a cut in their emissions of heat-trapping gases from cars, factories and power plants by 25 percent by 2030.
That is more ambitious than under the U.N.'s Kyoto protocol, which seeks to cut emissions from developed nations by 5.2 percent below 1990 levels by 2008-12. President George W. Bush pulled the United States out of Kyoto in 2001, saying it was too expensive and wrongly excluded developing nations.
Other targets for the cities will include ensuring that residents would not have to walk more than 500 metres in 2015 to reach public transport or an open space.
Cities would be ranked from zero to four stars according to compliance with a set of 21 targets.
"Cities are prolific users of natural resources and generators of waste. They produce most of the greenhouse gases that are causing global climate change," Annan said.
"They often degrade local water quality, deplete aquifers, pollute the marine environment, foul the air and consume the land, thereby devastating biological diversity," he said.