The sinking mega-cities

A bulletin from Lloyd's: As sea levels rise, ground levels in coastal megacities are also falling – with potentially disastrous implications for insurers. Insurers of large property portfolios in the world’s great coastal cities will have factored the effects of climate change into their catastrophe models – including rising sea levels and more frequent storm surges. But what’s often missed is that many of these cities are sinking faster than the water is rising. In some, subsidence outstrips sea level rise by a factor of ten to one.

Together with sea water inundation and flood damage, this can have disastrous consequences for the built environment – and property and business interruption insurers. The surge that overwhelmed New Orleans following Hurricane Katrina, and the subsequent cascading collapse of critical infrastructure, offered a glimpse of the sort of scenario underwriters fear. “We’re going down and the sea is coming up,” confirms Gilles Erkens, of the Deltares Research Institute in Utrecht. “Potential losses could run into hundreds of millions of dollars every year.”

The causes are varied. Foremost is large-scale groundwater extraction for drinking water and industrial processes – although elsewhere, like Los Angeles, it is oil and gas extraction that is to blame. Some urban areas are also constructed on multiple layers of soft soil, which compacts when built on – one of the problems facing New Orleans, for instance, and a feature of megacities that spring up on river deltas, such as Guangzhou in south west China.

One of the most severely affected cities was Tokyo, which grew rapidly in the middle of the last century and sunk over four metres as a result – until drastic remedial measures were put in place in the 1970s to restrict the extraction of groundwater. Since then the subsidence has stabilised. But from Jakarta and Dhaka to Venice, the risks are still all too real…

Jakarta is subsiding faster than any other megacity. The northern part has sunk by nearly four metres in the last 35 years, mainly due to groundwater extraction as the population has mushroomed and former agricultural land has been taken over by massive residential and industrial developments.

...Venice sunk about 120mm in the 20th century due to natural and human causes. In addition, the sea level rose about 110mm. A range of measures – such as restrictions on groundwater extraction – were introduced to stabilise the problem. But recent satellite mapping suggests these may not be enough, as the city is still subsiding by one to two millimetres a year. The causes are two-fold....

A waterside shanty in Jakarta, shot by Thehero, Wikimedia Commons, under the Creative Commons 2.0 license