Beijing | Shanghai, China's most developed and richest city, has been sinking into the mud for at least 100 years.
It was one of the first cities in China to suffer serious land subsidence, with an average rate of 22.94 mm/year from 1921 to 2007.
That rate of subsidence has stabilised since 2010, but rising sea levels caused by global warming pose a new and very real threat to the city of 24 million.
Although land subsidence is estimated to be relatively stable, the rate of sea level rise is projected to accelerate under climate change in the future. The primary drivers will be oceanic thermal expansion, followed by glacier and ice cap melting. The disintegrating Antarctic ice sheet is projected to contribute most to the broad uncertainty ranges of future sea level rise.
By 2050, the median relative sea level rise is projected to be between 45 cm and 50 cm, with nearly equal contributions from both climatic and non-climatic factors the latter including glacial isostatic adjustment and subsidence.
Due to the threats of atmospheric and ocean warming to sea level rise, computer modelling shows the current 10 and 100-year flood water levels in Shanghai could be exceeded approximately twice more frequently in 2030, 3-5 times more frequently by 2050, and over 50 times more frequently with a 136 cm rise in relative sea level by the end of this century.
Increasing flood water levels, in turn, could increase the risks of dyke failure and associated flooding over time. For example, seawall failures and flood-inundated areas are projected to increase by around 30 percent and 50 percent by 2100 compared to 2010.
Without timely adaptation measures, catastrophic flooding could overwhelm the delta city by the end of this century. Under this scenario, more than 4,200 km/square (62 percent of the total area) of the city could be flooded with an average inundation depth of 1.2 metres.
The most susceptible areas to the magnified flood hazards are Chongming Island and the Huangpu River floodplain including Shanghai city centre. Overall, the implication is that Shanghai, with a population of 24 million, though relatively safe from flooding today, will become increasingly risk-prone due to sea level rise and land subsidence.
Shanghai has been sinking more rapidly than rising sea levels. During the past three decades, the sea level in Shanghai has risen 115 mm, at a mean linear rate of 3.8 mm/year, which was higher than the global average rate.
However, the Shanghai city centre has subsided by more than three metres since the late 19th century. Based on an analysis of very long baseline interferometer (VLBI) data, Shanghai's average rate of tectonic subsidence is estimated to have been nearly 1-1.5 mm/year.
Shanghai's sinking has mainly been caused by tectonic subsidence and compaction of sediments due to natural conditions and human activities. According to monitoring data, this compaction subsidence in Shanghai can be divided into three stages: 1. Rapid subsidence stage from 1921 to 1965 caused by excessive groundwater extraction; 2. Recovery stage from 1965 to 1985 with artificial recharge; and 3. Slow subsidence stage from 1985 to 2007 due to large-scale construction of high-rise buildings and underground projects.
Data estimates show that the average rate of compaction subsidence has stabilised at 6 ± 1 mm/year after 2010.Flood barriers may avert disaster.
Since Shanghai is highly susceptible to sea level rise and coastal flooding, authorities could consider a movable barrier system at the mouth of the Huangpu River (like London's Thames Barrier or Rotterdam's Maeslantkering Barrier) to avert future disasters.
Such an investment might be more economically effective in the long term as the city's 479.7 km floodwalls would need to be raised every decade due to sea level rise in the absence of a storm surge barrier.
Another urgent action is upgrading the current low-level seawalls (typically 5-7 metres) along the coasts of the low-lying and poorly protected areas of Baoshan and Chongming. This, together with the regular maintenance of the high-level seawalls in Pudong and Changxing.
Suitable underground spaces can be identified and used as temporary emergency water storage facilities in the downtown area.
Finally, and most importantly, detailed emergency evacuation planning can be prepared now to deal with all contingencies as urban flood risk management is a complex system and failures are always possible and always costly.