Excessive extraction of underground water in a number of seaside cities has considerably lowered local ground water levels and caused land to cave in, facilitating sea water invasion.

And dykes, mostly built using outdated engineering standards, have in the past three years become vulnerable as sea levels reach record high, said the bureau’s 2000 Bulletin on Sea Levels, the first of its kind.

Affected by global weather, sea levels around China, as in other areas, have risen over the past 50 years.

But those levels increased faster between 1998 and 2000, rising on average 2.5 millimeters a year, instead of between 1 and 3 millimeters.

The reason for this, explained the bulletin, was severe floods in the Yangtze River basin in 1998 and 1999, forcing sea levels in the Yellow Sea and the East China Sea to rise considerably.

Although a drought then lowered sea levels in the north in 2000, especially in the Bohai Sea, the level that year was 51 millimeters up on the average world sea level between 1975 and 1986.

While sea levels in northern Chinese regions, such as Tianjin, was only 1 millimeter higher than the world’s average sea level between 1975 and 1986 - the internationally accepted criterion for calculating changes in sea level - that of southern Chinese regions like Hainan was 110 millimeters higher.

Sea levels are not expected to rise as quickly over the next three years, though the average level in 2003 is still estimated to be 31 millimeters higher than the standard world average between 1975 and 1986.

However, Chen Manchun, environment division director of the State Marine Information Center, said the authorities should not relax their efforts, because rising sea levels off certain areas, such as Shanghai and Hainan, might still be as high as 76 millimeters and 71 millimeters above the average world level between 1975 and 1986.

With sea levels set to continue their rise, China needs to try to stop the invasion of salt sea water by building higher dykes and extracting less ground water.

“All this takes time and work, and we have to do it immediately because we cannot promise the sea will not rise,” he said.

Since most Chinese seaside cities are less than four meters above sea level, with the lowest even less than one meter, Chen believes this problem will remain a threat long into the future.

(China Daily 04/12/2001)