Three river deltas in China are sinking due to global warming and excessive extraction of underground water, leaving millions of people with an increasing risk of floods, a recent scientific report showed.

The Pearl, Yangtze and Yellow river deltas in China are among the 33 major deltas studied, with 24 of them found to be sinking, according to the report released by the University of Colorado. The three river deltas in China cover about 160,000 sq km and nearly 100 million people live in the areas surrounding these river deltas.

"Most of the at-risk river basins are in the developing countries of Asia, but there are several in developed nations as well, including the Rhone in France and the Po in Italy," Altert Kettner from the university told British Broadcasting Corporation (BBC).

Researchers calculate that 85 percent of major deltas have seen severe flooding in recent years. In the next four decades, the areas of land vulnerable to flooding will increase by about 50 percent. The land is sinking and sea level is rising due to climate change.

The researchers took data from various space satellites, and combined it with historical records and measurements of the rise in sea levels to get a comprehensive report.

"This study shows there are a host of human-induced factors that already cause deltas to sink much more rapidly than could be explained by sea level alone," he said in the BBC report.

Chinese scholars don't seem surprised by the report, since the results adhere to previous domestic studies.

"Besides the three river deltas mentioned above, the Liaohe and Haihe river deltas are also on our watch list. They are all facing the serious situation of land sinking and sea level rising," Han Mukang, a retired professor in Peking University who has studied the issue for decades, told China Daily.

Tianjin in the Haihe River Delta is the most affected place due to over extraction of underground water and its geological structure. Climate change does not have much impact, Han said.

"Yellow River Delta is not in an urgent situation," Han said.

However some experts don't believe that the sinking land in the three river deltas is dangerous enough to be included in the high-risk list.

Liu Shouqi, of the Shanghai-based Geological Society of China, told Chinanews that the data from satellites is not as exact as observations on the ground.

Statistics show that whenever the ground of a city sinks 1 mm, the city will face economic losses of about 200 million yuan (US$29.4 million), the Chinanews website reported yesterday.

By the end of 2003, five areas in Beijing were sinking. About 2,815 sq km of the land sank by more than 5 cm, Chinanews reported. Some land sank by more than 72 cm.

Land sinking has cost Shanghai about 280 billion yuan since the 1960s and about 95 cities have faced the risks of land sinking in the past two decades, the website reported.

Experts called for a reduction on extraction of underground water and the construction of higher flood banks.

In the past 30 years, sea levels in China have increased by 92 mm and the sea temperature increased by almost 1 C, according to statistics from the State Oceanic Administration.

The Mekong Delta and Pearl River Delta near Hong Kong are places where floods or hurricanes are likely in future, the researchers said.

(China Daily September 23, 2009)