A noted marine geologist warned yesterday that land subsidence remains a major issue for Shanghai, the country's financial hub.

"As the local economy booms, the widespread construction of skyscrapers has become a new challenge for land stability," said Wang Pinxian, an academic at the Chinese Academy of Sciences.

It has been estimated that every millimeter of subsidence costs Shanghai as much as US$20 million.

Wang, a professor of marine geology at the Shanghai-based Tongji University, issued the warning at the second session of summer school for 30-plus doctorate students from both home and abroad. The theme of the meeting was Sustainable Use of Water Resources.

Wang said over-exploitation of ground water beneath the city and the density of high-rise buildings were causing subsidence in coastal cities.

According to data from the State Key Laboratory of Marine Geology at Tongji University, more than 90 medium and large cities on the mainland suffer from land subsidence.

The land in Shanghai subsided at a rate of 5-7 mm per year from 2001 to 2006.

In 2004, the city's land sank 8 mm, causing alarm and spurring the local government to act.

Shanghai has restricted the pumping of ground water, particularly downtown.

Wang said that people who pump groundwater have also been told to replace it.

"People who use ground water are requested to pump in the same amount of water during the winter that they pumped out in summer," Wang said.

Meanwhile, the city government has shifted most of the industrial use of ground water from downtown to the suburbs.

Wang said such measures have proven successful in controlling the speed of land subsidence, but more needed to be done.

"We have established a network of experts to monitor subsidence and ground water levels," Wang said. "A research centre will be set up soon."

The professor said he believes that by combining all the measures mentioned above, the city will keep the pace of subsidence at less than 5 mm per year by 2010.

Land subsidence in Shanghai became a real hazard in the 1950s when the city's ground water resources were extensively exploited for cooling during the summer by the city's newly developed industrial sector.

Land subsidence hit a pace of 38 mm per year in the late 1950s and early 1960s. The city sank by 110 mm per year during the period from 1957-61.

The rapid pace of subsidence during that period caused cracks to form in the land, buildings to slant and tidal flooding.

(China Daily August 21, 2007)