On the eastern tip of Chongming, the world's largest alluvial island in the mouth of the Yangtze River, birdwatchers wait patiently to glimpse an occasional crane or plover rising from the wetlands' reeds.

A few kilometers to the southwest, in an area of fishponds, marshes and farmland, developers are plotting out a city for up to 400,000 people that they hope will be a model of ecological harmony, powered entirely by renewable energy.

Shanghai's Dongtan Eco-city has a lofty ambition: to become the world's first carbon-neutral city.

But the project has been marred by delays and faces rising doubts over whether it will be a model for China's rapid urbanization, or just a posh community for wealthy commuters eager to flee the smog and traffic of Shanghai.

"The so-called zero-emission city is pure commercial hype," said Dai Xingyi, a professor at the department of environmental science and engineering at Shanghai's Fudan University. "You can't expect some technology to both offer you a luxurious and comfortable life, and save energy at the same time. That's just a dream."

Ten wind turbines already stand at the boundaries of the city, which will run on energy from sources including wind, solar power and biogas extracted from municipal waste.

"The idea is that China is moving from an industrial age to an ecological age," said Roger Wood, an associate director of Arup, a consulting firm headquartered in London that was selected to design the Dongtan project.

Arup also worked on some iconic venues for the Beijing Olympics, including the National Stadium, popularly known as the "Bird's Nest", where the opening ceremony and track and field events will be held.

Price of zero emissions

Some dismiss the eco-city plan as too costly to be feasible.

"True 'zero-emissions' comes with a big price tag. I doubt anyone would be willing to pay for it," said Fudan University's Dai.

Generating electricity from wind would be at least twice as much as expensive as using coal. Electricity from solar power could be 10 times more expensive.

Arup has declined to disclose the cost of the eco-city project, but an official at its partner, the State-owned Shanghai Industrial Investment Corporation (SIIC), said the construction costs could be at least 30 or 40 percent more than for a typical property development of the same size.

Those costs would be offset in the long term, Arup's Wood argued, when the city becomes self-sufficient in energy.

Environmental friendliness must be practical, he said, not just an image to splash over a "business-as-usual" development. "We don't want 'green-wash'. It's got to be real."

Construction of the first phase of the eco-city has been postponed to the beginning of 2009 from 2006, while the projected population for that phase was reduced to 5,000 and the primary focus narrowed to building an environment-related research institute.

The project's supporters applaud it for combining existing energy-saving technologies. "Dongtan is exploring a new way of urbanization," said Zheng Shiling, a professor at the architecture department of Tongji University in Shanghai. "It would not be realistic if we continued to build cities the way we've been doing."